This is a continuation-in-part of copending U.S. patent application Serial No. 839,296 filed March 13, 1986 entitled "Cover And Support Plate Arrangement For Wall Mounted Devices".

This application is also related to the subject matter disclosed and claimed in the following commonly assigned, copending patent applications:

Design patent application Serial No. 844,297 filed March 13, 1986, entitled "Push Button Switch";

Design patent application Serial No. 844,296, filed March 13, 1986, entitled "Rotary Dimmer Switch";

Design patent application Serial No. 844,298, ^' filed March 13, 1986, entitled "Rotary Dimmer Switch"; and

Design patent application Serial No. 844,299, filed March 13, 1986, entitled "Slide Dimmer Switch " .

Background Of The Invention

The present invention is directed generally to a cover and support plate arrangement for a wall mounted device, such as a wall switch, wall mounted display, etc. In particular, as disclosed herein, but not limited thereto, the present invention is directed to a cover plate and support plate apparatus for a wall switch such as a dimmer or dimmer switch, wherein the support plate is affixed to a wall box embedded in a wall and the cover plate is juxta-

posed over the support plate. According to one aspect of the invention, there is provided a support plate and cover plate arrangement for wall mounted apparatus of the type having a controllably conductive device in thermal connec¬ tion with the support plate and in series with a controlled load and wherein the cover plate appears to float from the wall at wide angles of view. According to a second aspect of the invention, there is provided a low profile cover plate and support plate arrangement for standard size dimmer switches. The dimmer switches meet Underwriters Laboratories operating temperature standards for loads that draw up to about 11.67 amps ( 1400W at 120VAC) accord¬ ing to one embodiment and up to 12.5 amps (1500W at 120VAC) according to another embodiment. Neither one of the embodi¬ ments requires the use of septa on the support plate, which may be of standard size. According to a third aspect of the invention, the support plate or cover plate includes a pair of separately formed, separately attached rails that may be utilized to affix the cover plate to the sup¬ port plate without the use of tools and further, in the case of a slide type dimmer switch, may be used to capture . and guide the slider. According to a fourth aspect of the invention, a slide type dimmer switch includes a slideable operating member operatively coupled to a cam in contact with a viscous lubricant to provide drag and give the slider an improved "feel". According to a fifth aspect of the invention, there are provided means for maintaining the maximum temperature rise of various parts of a low profile dimmer switch having a smooth and continuous dim¬ ming range of up to at least 95% of full voltage within the standards specified by Underwriters Laboratories. By "smooth and continous", it is meant that a controllably conductive means (such as a triac) , and not a mechanical break switch, carries the load current throughout the specified dimming range.

U.S. Patent 3,746,923, incorporated herein by refer¬ ence, discloses a support plate for a dimmer switch wherein

the support plate is provided with a series of spaced, out¬ wardly facing septa. These septa are the individual "fins" which project outwards from the flat surface of the support plate. The septa increase the ability of the support plate to dissipate thermal energy generated by a dimmer switch. Such type of dimmer switch shall be referred to herein as a "septa dimmer switch". (The word "septa" is used herein in lieu of the "fins" referred to in the '923 patent.) According to the '923 patent, a pair of septa is also utilized as a guide for a slider that is connected to the operating member of a linear potentiometer. A cover plate snaps over the septa and has an elongated slot through which access to a handle integral with the slider is provided.

U.S. Patent 3,735,020, also incorporated herein by reference, discloses support plates for dimmer switches, including the septa type support plates of the '923 patent, that may be gang mounted onto ganged wall boxes each having a width A. The support plates of the '020 patent's dimmer switches each have a width of either 1.5A or 2.5A. For simplicity, these are referred to herein as " 1.5A" and "2.5A" dimmmer switches. In commercial practice, 1.5A septa dimmer switches of the type disclosed in the '923 patent measure approximately 2-3/4 inches wide by 4-1/2 inches long and have an overall surface area ( including that of the septa) of approximately 54 square inches. Also, in commercial practice, 2.5A finned dimmer switches of the type disclosed in the '923 patent measure approx¬ imately 4-1/2 inches square and have an overall surface area (including that of the septa) of approximately 91 square inches. In commercial practice, the support plates of both 1.5A and 2.5A septa dimmer switches have been constructed of aluminum.

It is known that- the amount of heat generated by a dimmer circuit is related to the forward voltage drop across the dimmer circuit's triac when operating at a specified current, i.e., the lower the voltage drop, the

less heat generated by the triac. Triac selection, however, has not been a major design consideration in septa dimmer switches because such dimmer switches are capable of accom¬ modating a relatively wide range of triacs (i.e., triacs wit a relatively high forward voltage drop) while still meeting Underwriters Laboratories standards.

Known 1.5A septa dimmer switches are capable of operating loads that draw currents of up to about 8.33 amps while meeting Underwriter's Laboratories (U.L.) operating temperature standards. Thus, 1.5A septa dimmer switches have been commercially embodied as two separate models, one designed for loads that draw up to 5 amps (600W at 120 VAC) and another designed for loads that draw up to 8.33 amps (1000W at 120VAC). Both have utilized Jedec TO220 type triacs. The 5 amp capacity, 1.5A septa dimmer switch has utilized 25 amp continuous current rated triacs having a forward voltage drop as low as about 1.1 volts at 5 amps forward current. The 8.33 amp capacity, 1.5A septa dimmer switch has utilized 25 amp continuous current rated triacs having a forward voltage drop as low as about 1.2 volts at 8.33 amps. In a 5 amp capacity, 1.5A septa dimmer switch, the triac is mounted on a strip of heat conducting, elec- trically nonconductive glass tape interposed between the - triac and the rear surface of the aluminum support plate. The glass tape and aluminum surface are baked together in well-known manner and the triac is mounted to the glass tape with room temperature vulcanizing rubber such as RTV ^® . In an 8.33 amp capacity, 1.5A septa dimmer switch, a small copper strip measuring about 1.23 inches by 0.54 inch is eyeleted to the rear surface of the aluminum sup¬ port plate (a thermally conductive grease such as Dow Corning 340 Silicone Heat Sink Compound is first applied to the contacting surfaces), one side of a beryllium oxide disk is soldered to the copper strip and the triac is soldered to the other side of the beryllium oxide disk.

Similarly, known 2.5A septa dimmer switches are capable of operating loads that draw currents of up to

about 16.67 amps (2000W at 120VAC) while meeting U.L. operating temperature standards. Again, 2.5A septa dimmer switches have been commercially embodied as two separate models, one designed for loads that draw currents of up to about 12.5 amps (1500W at 120VAC) and another designed for loads that draw currents of up to about 16.67 amps (2000W at 120VAC). The commercial embodiment of a 12.5 amp capacity, 2.5A septa dimmer switch has utilized press fit type 25 amp continuous current rating triacs having a forward voltage drop as low as about 1.25 volts at 12.5 amps forward current. The commercial embodiment of a 16.67 amp capacity 2.5 septa dimmer switch has utilized press fit type 40 amp continuous current rating triacs having a forward voltage drop as low as about 1.1 volts at 16.67 amps forward current. In the case of the 12.5 amp capacity 2.5A septa dimmer switch, the triac is mounted in the same manner as in the 8.33 amp capacity 1.5A septa dimmer switch, i.e. , with a small copper strip measuring about 1.23 inches by 0.54 inch, thermally conductive grease and a beryllium oxide disk. In the case of the 16.67 amp capacity 2.5A septa dimmer switch, triac mounting is the same except that it is known to use a large copper pad measuring about 2.8 inches by 1.7 inches instead of a "s ^' mall copper strip to improve thermal performance.

The above described septa dimmer switches have been manufactured and sold by the assignee of the present appli¬ cation under names such as NOVA and CENTURION. A desirable feature of such septa dimmer switches is that they are capable of continuous and smooth dimming up to at least 95% full voltage while meeting U.L. standards. Septa dimmer switches have also been manufactured and sold by Prescolite Controls, Carrolton, Texas under the names PRESET and PRESET n-Touch. The devices manufactured by Prescolite Controls are said to be covered by U.S. Patent No. 4,455,546.

A non-septum dimmer switch having a support plate less than 1.5A in width for controlling loads that draw up

to 8.33 amps (1000W at 120VAC) while meeting U.L. standards is known and is manufactured by Power Controls Corporation, San Antonio, Texas and sold under the name "Tog-L-Dim-R" . Such dimmer switch, however, is not capable of full range dimming but instead utilizes a mechanical on-off switch to carry full load current when the dimmer switch has been set above 63% of full voltage. The manufacturer claims continuous dimming "through 90%" but tests, on. actual com¬ mercial units show that continuous dimming occurs only up to the above mentioned percentage of full voltage. Thus, a drawback of such dimmer switch is that it is not capable of smooth and continuous dimminσ over substantially full range. The device manufactured by Power Controls Corporation is said to be covered by U.S. Patents 3,990,033 and 4,085,399

Another non-septum dimmer switch for controlling loads that draw up to 8.33 amps is manufactured by the assignee hereof under the designation D1000 and is capable of sub¬ stantially full range dimming. However, such dimmer switch has been manufactured with a 2.5A width support plate (having an overall surface area of approximately 42 square inches) to enable it to meet U.L. operating temperature standards. Thus, this dimmer switch suffers from the drawback of requiring a large support plate.

Heretofore, it was believed that a large surface area provided by either a large non-septum support plate or deep septa disposed on the support plate, was required for 8.33 amp and higher capacity dimmer switches to sufficiently dissipate the maximum heat generated by the dimmer circuitry and meet U.L. standards. A major portion of the heat is generated by the triac in the dimmer circuit and, as men¬ tioned, the amount of heat generated by the triac is relate to the triac's forward voltage drop at operating current.

A disadvantage of the cover plate and support plate arrangement disclosed in the '923 patent is that, due to the high outwardly projecting septa, the cover plate is distantly spaced from the support plate and appears to float from the wall at only very narrow angles of view. When viewed

spaced from the support plate and the wall by a signficant distance and the metal septa on the exterior sides are clearly visible. The assignee hereof has manufactured and sold low profile wall mounted apparatus including a support plate with a cover plate that appears to float from the wall at wide angles of view under the names AURORA and VERSAPLEX. However, neither such device houses circuitry that generates substantial amounts of heat. Moreover, neither such device comprises a chimney, as herein described.

Experimentation with novel heat transfer techniques has shown that a large surface area on the support plate is not required for operating loads that draw currents of up to about. 12.5 amps while meeting U.L. standards. In fact, on 1..A and 2.5A width support olates the septa may be removed entirely. The present invention takes advantage of this finding to provide a low profile support and cover plate arrangement that may be used in connection with dimmer switches and other circuitry and that meets U.L. operating temperature standards.

The present invention also takes advantage of find¬ ings relating to the use of rails in wall mounted apparatus generally and to the use of viscous lubricants in slide type dimmer switches. The '923 patent teaches septa, including a pair that serve as rails for a slider, integ¬ rally extruded from a support plate. In commercial prac¬ tice, however, integrally extruded rails have proven to be problematic, primarily due to difficulties in aligning the septa and maintaining the septa and rails within prescribed tolerances. Often, it has been necessary to include an additional costly manufacturing process to align the septa within specified tolerance. A slide dimmer switch manufac¬ tured by Leviton Manufacturing Co., Little Neck, N.Y. under the name "Decora" utilizes a pair of non-extruded plastic rails to guide a slider. But, the rails are integ¬ ral with each other - they are part of a molded plastic unit comprising two rails, two connecting members and an

integral backplate. Thus, the Leviton unit does not permit easy adjustment of rail spacing during manufacture and hence suffers from problems similar to those associated with extruded rails.

It has also been known to grease the rails of a slide dimmer switch to reduce friction between the slider and rails, but this approach is also problematic. One such problem is that the greased rails accumulate dust and other particles, thus defeating the purpose of the grease. Another problem is that the installer may contact the grease and inadvertently rub it into the cover plate, thereby leaving an unsightly appearance. The present invention overcomes these and other problems associated with prior art dimmer switches.

Summary Of The Invention

Apparatus for wall mounted devices comprises a rec¬ tangular support plate for mounting to a wall box embedded in a wall and a rectangular cover plate for mounting to the support plate. The support plate has overall surface dimensions greater than corresponding dimensions of the wall box, a generally flat rear surface and a front surface having only a pair of upstanding portions adjacent vertical side edges thereof. The upstanding portions extend gener¬ ally perpendicularly outward from the front surface of the support plate. The cover plate is juxtaposed over the front surface of the support plate and has front and rear surfaces and overall surface dimensions greater than cor¬ responding dimensions of the support plate and includes first portions that extend laterally over the vertical side edges of the support plate. The cover plate may also have second portions that extend generally perpendicularly outward from the first portions so that they overlay the upstanding portions of the support plate, but this is not necessary. The ratio of (i) the distance between the rear surface of the support plate and the rearmost surface of the cover plate to (ii) the distance by which the first portions of the cover plate overlay the vertical side

-9-

edges is less than approximately two to one. The rear surface of the cover plate is spaced from the front surface of the support plate by a distance so as to define an air gap therebetween. Openings into the air gap are provided at opposing horizontal side edges of the support plate. The combination of the pair of upstanding portions, the air gap and the openings into the air gap define a chimney between the support plate and cover plate. The chimney permits air to flow between the support and cover plates, thus dissipating some of the heat generated by circuitry in thermal connection with the support plate. For example, such circuitry may include a controllably conductive device in thermal connection with the support plate and in series with a controlled load.

In the preferred embodiment, the apparatus may also comprise means for invisibily affixing the cover plate to the support plate. As used herein, the term "means for invisibily affixing" refers to a means disposed on the rear surface of the cover plate and on the front surface of the support plate for affixing the cover plate to the support plate that is not apparent to a casual observer. According to the disclosed structure for invisibly ^' affixing -the cover plate to the support plate, no tools are required to install or remove the cover plate.

The upstanding portions of the support plate are visible only when viewed at extreme angles with respect to an imaginary line perpendicular to the plane of the support plate. In the disclosed embodiment, the upstanding por¬ tions of the support plate are visible only when the appar¬ atus is viewed at angles in excess of 25° with respect to said imaginary line. Further, the cover plate casts a shadow along the vertical side edges of the support plate, thereby giving the appearance that the cover plate is floating from the wall. This shadow also serves to mask the interface between the support plate and the wall.

According to one embodiment"of the invention, the support plate/cover plate arrang_ement_ disclosed herein is used in connection with a slide type dimmer switch. A

first elongated slot is generally centrally disposed in the support plate and a second elongated slot is generally centrally disposed in the cover plate and is in general alignment with the first slot. A linearly adjustable potentiometer is disposed adjacent the rear surface of the support plate and has an operating member extending through and being slidable along the first slot. (As used herein, the term "potentiometer" is meant to include any variable resistor and is not limited to variable resistors of the potentiometer type). A slider is sandwiched between the cover and support plates and is connected to the oper¬ ating member. The slider has a handle protruding through the second opening so that the slider is slideably moveable to cause a corresponding sliding movement of the operating member. A control means such as a dimmer circuit is opera- tively connected to the potentiometer for varying an output signal according to the linear setting of the operating member. A cam is operatively connected to_ the operating member adjacent the rear surface of the support plate. The cam is disposed within a housing greased with a viscous lubricant so as to create a viscous damping or positive drag on the operating member to provide an improved "feel". -The viscous lubricant within the housing is not permitted to escape to any outside parts of the dimmer switch. The cam slidingly contacts a member associated with an on-off switch and opens the switch when the operating member has been moved to an extreme end of its travel.

A guide means may be attached to either the front surface of the support plate or the rear surface of the cover plate for guiding the sliding movement of the slider. The guide means comprises a pair of parallel spaced apart rails separately formed from each other and from the cover plate and support plate. In the preferred embodiment, the rails are affixed to the front surface of the support plate but may be affixed to the rear surface of the cover plate. Each rail is disposed on an opposite side of the slot and receives an opposing side of the slider. The

rails serve to both guide the sliding movement of the slider and support the slider. In this embodiment, the means for invisibly affixing the cover plate to the support plate comprises tab means disposed on the rear surface of the cover plate (or if the rails are on the rear surface of the cover plate, then the tab means are disposed on the front surface of the support plate.) The tab means snap onto corresponding portions of the rails to hold the cover plate in juxtaposed relationship over the support plate.

Alternatively, the operating member may extend through both the first and second slots and a handle may be connec¬ ted thereto adjacent the front surface of the cover plate, whereby the slider is not required.

According to the invention, the rails have application to various wall mounted devices and not solely to slide type dimmer switches. Thus, the separately formed rails may be used in connection with rotary dimmer switches, wall mounted push button switches or any other wall mounted device where it is desired to invisibly affix a cover plate to a support plate.

In another embodiment of the invention, the support plate/cover plate arrangement is used in connection with a rotary dimmer switch. In this embodiment, there is a generally centrally disposed first opening in the support plate and a corresponding generally centrally disposed second opening in the cover plate in substantial alignment with the first opening. The first opening receives a rotatable operating member associated with a rotary potenti¬ ometer disposed adjacent the rear surface of the support plate. Preferably, the second opening is round and only slightly larger in diameter than a circular control knob that extends through the second opening and is connected to the operating member. Alternatively, the operating member may extend through both the first and second openings and a control knob may be connected thereto adjacent the front ^~ surface of the cover plate. The control knob is rotatable to cause a corresponding rotary movement of

the operating member. A control circuit is operatively coupled to the potentiometer for varying an output signal according to the rotary setting of the operating member.

In still a third embodiment, there is a generally centrally disposed first opening in the support plate and a corresponding generally centrally disposed second opening in the cover plate in substantial alignment with the first opening. The first opening accommodates an operating member associated with a switch disposed adjacent the rear surface of the support plate. (As used herein, the term "switch" , as opposed to "dimmer switch" , is used to conno¬ tate any type of maintained contact, momentary contact, or mechanical break switch, including, but not limited to a push button on-off switch.) A depressable, spring loaded push button may be disposed in the second opening and is operable, when depressed, to depress the operating member and thereby open and close the switch. Alternatively, the operating member may extend through the secpnd opening and the push button may be connected thereto adjacent the front surface of the cover plate.

The present invention also has application to wall mounted displays, wherein the display is mounted in or adjacent the "floating" cover plate.

A low profile cover plate and support plate arrange¬ ment for a dimmer circuit of the type described above is made possible by applying the results of experimentation with heat transfer techniques. Such experimentation has shown that 1.5A width dimmer switches for operating loads that draw up to 8.33 amps, and that 2.5A width dimmer switches for operating loads that draw up to 12.5 amps, do not require a large surface area on the support plate to meet U.L. operating temperature standards. Thus, a low profile septumless support plate, or low profile, low height septum support plate may be provided. The following performance standards for dimmer switches are specified by U.L. :

1. That the maximum temperature rise on field wiring terminals or on the conductors of the leads intended for field wiring not be more than 30°C (54°F);

2. That the maximum temperature rise of any part of the dimmer switch that can contact the field wiring not be more than 35°C (63°F); and

3. That the maximum temperature rise of any part that is exposed on the outside of the dimmer switch, other than those indicated in paragraph 2 not be more than 65°C (117°F).

According to the invention, an 8.33 amp capacity 1.5A width low profile dimmer switch comprises an aluminum sup¬ port plate and a 35 amp continuous current rating triac having a forward voltage droo of no greater than about 1.0 volts at a forward current of anproximately 8.33 amps. The dimmer switch has a smooth and continuous dimming range up to at least 95% of full voltage. In the preferred embodiment, the support plate has no septa and the overall surface area of the support plate preferably measures approximately 26 sσuare inches. The triac is in thermal connection with the support plate. The triac may have a low thermal resistance from junction co case. In one embodiment, a small copper strip measuring about 1.23 ^* inches by 0.54 inch is eyeleted to the rear surface of the support plate (thermally conductive grease is applied to the mating surfaces of the copper strip and aluminum support plate) and a beryllium oxide disk is interposed between the triac and the copper strip. A connecting lug is soldered to the base of the triac before mounting. In this embodiment, the triac is the only circuit element disposed on the copper strip. In another embodiment, a large copper pad measuring about 2.8 inches by 1.7 inches is eyeleted to the rear surface of the support plate and both the triac and remaining dimmer circuitry are disposed adjacent the copper pad. Again, a thermally conductive grease is applied to the mating surfaces of the copper pad and aluminum support plate, a beryllium oxide disk is interposed between the triac and the copper pad and a

connecting lug is soldered to the base of the triac before mounting. In both embodiments, the lead wires from the dimmer circuitry to the outside of a housing surrounding the dimmer circuitry are AWG 12 stranded wire, instead of AWG 16 stranded wire utilized in the prior art 8.33 amp capacity 1.5A septa dimmer switches. (Note - the invention is not limited to the use of stranded wires, and solid wires may also be used.) In the first embodiment, it has been observed that, when operating under maximum speci¬ fied load conditions (8.33 amps), the maximum temperature rise on field wiring terminals or on the conductors of the lead wires intended for field wiring is no greater than 30°C, that the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any other part of the dimmer switch that is exposed on the outside is no greater than 65°C. In the second embodiment, it has been observed that, when operating under maximum specified load conditions (8.33 amps) , the maximum tempera¬ ture rise on field wiring terminals or on the conductors of the lead wires intended for field wiring is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any other part of the dimmer switch that is exposed on the outside is no greater than 65°C. Hence, in both cases, these maximum temperature rises are less than those specified by U.L.

According to the invention, a 11.67 amp capacity 2.5A width low profile dimmer switch comprises a copper support plate and a 35 amp continuous current rating triac having a forward voltage drop of no greater than about 1.05 volts at a forward current of approximately 11.67 amps. The triac is in thermal connection with the support plate. The triac may have a low thermal resistance from junction to case. The dimmer switch has a smooth and continuous dimming range up to at least 95% of full voltage. In the Dreferred embodiment, the support plate has no septa and

an overall surface area of approximately 44 square inches. Preferably, the copper support plate is painted black or has a matt finish to improve its heat radiating ability. The thermal performance of this dimmer switch is further improved by a novel lead wire arrangement that is provided at least within the dimmer switch housing. A pair of AWG 12 stranded lead wires is connected in parallel to each side of the dimmer circuit (i.e., two AWG 12 stranded wires are connected in parallel to a "hot junction" on the supply side and two AWG 12 stranded wires are connected in parallel to a "hot junction" on the load side). (Note - the invention is not limited to the use of stranded wires, and solid wires may be used instead.) In one embodiment, all four wires extend through the housing for connection to the field wiring. In another embodiment, each pair of parallel wires is connected to an AWG 10 stranded wire within the housing and the pair of AWG 10 stranded wires extend through the housing for connection to the field wiring.

In the preferred embodiment of the 11.67 amp capacity 2.5A width low profile dimmer switch, a large copper pad measuring about 2.8 inches by 1.7 inches is eyeleted to the rear surface of the copper support plate and both the - triac and remaining dimmer circuitry are disposed adjacent the copper pad. A thermally conductive grease is applied to the mating surfaces of the copper pad and the copper support plate and a beryllium oxide disk is interposed between the triac and the copper pad. A connecting lug is soldered to the base of the triac before mounting. In this embodiment, it has been found that, when the dimmer switch is operating at maximum specified load (11.67 amps at 120VAC, i.e., 1400W) , the maximum temperature rise on field wiring terminals or on the conductors of the leads intended for field wiring is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any part that is exposed on the outside of the dimmer is no greater than

65°C. These maximum temperature rises are thus less than those specified by U.L.

A 12.5 amp capacity 2.5A width low profile dimmer switch, according to the present invention, meets U.L. performance standards without the use of a copper support plate, without the use of a large copper pad eyeleted thereto for triac mounting, and without the four lead wire arrangement discussed above. Moreover, the 12.5 amp version meets U.L. performance standards while providing a smooth and continuous dimming range up to at least 95% of full voltage.

According to the preferred embodiment, a 12.5 amp capacity 2.5A width low profile dimmer switch comprises an aluminum support plate _.nd a 35 amp continuous current rating triac having a forward voltage drop of no greater than about 1.05 volts at forward current of approximately 12.5 amps. The triac is in thermal connection with the support plate. The triac may have a low thermal resistance from junction to case. As mentioned, the dimmer switch has a smooth and continuous dimming range up to at least 95% of full voltage. Preferrably, the support plate has no septa and an overall surface area of no greater than about 44 square inches. The aluminum support plate need not be painted black but is preferably provided with a matt finish to meet U.L. performance standards. The lead wires from the dimmer circuitry to the outside of a housing surrounding the dimmer circuitry are AWG 10 stranded wire, but the invention is not limited to the use of stranded wires - solid wires may also be used.

The 12.5 amp capacity version of the 2.5A width low profile dimmer switch employs a first means for thermally isolating the interior of a housing covering the dimmer circuit (i.e. , the space within the housing) from the support plate and a second means for thermally isolating the wall box (to which the dimmer switch is mounted) from the support plate. In the preferred embodiment, the first means comprises a plastic shield interposed between the

housing and the support plate and suitably sized so as to isolate both the interior of the housing and the housing itself from all parts of the support plate (except for contact provided by mounting eyelets). Preferably, an air gap is provided between the housing (and its interior) and the support plate by the plastic shield to increase the thermal resistance between them. All dimmer circuitry, except the triac, is disposed on the plastic shield. The triac is in thermal connection with the support plate by means of a small copper strip measuring about 1.23 inches by 0.54 inch eyeleted to the rear surface of the support plate and a beryllium oxide disk interposed therebetween. Preferably, the second means comprises a piece of fishpaper such as Ar ite manufactured by Spaulding Fibre Company, Tonawanda, New York, disposed on the rear surface of the suDport plate and suitably sized so as to insulate all parts of the support plate from all parts of the wall box (except for the mounting screws). It has been found that when the above described dimmer switch is operating at maximum specified load (12.5 amps at 120VAC i.e., 1500W) , the maximum temperature rise on field wiring terminals or on the conductors of the leads intended for field wiring is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any part that is exposed on the outside of the dimmer switch is no greater than 65°C.

In sum, as embodied as a dimmer switch, the present invention permits construction of a dimmer switch for controlling loads that draw up to 8.33 amps on a 1.5A width low profile support plate and construction of a dimmer switch for controlling loads that draw up to 12.5 amps on a 2.5A width low profile support plate. Septa are not required on the support plate to meet U.L. operating temp¬ erature standards. Thus, installed prior art septa dimmer switches may be substituted with low profile dimmer switches of the present invention on a one for one basis. No wall boxes need to be added or removed. The new installation

will have a more aesthetically pleasing appearance due to its low profile and floating appearance. Still further, the present invention permits production of a family of low profile, standard size 5 amp, 8.33 amp and 11.67 or 12.5 amp capacity U.L. approved dimmer switches, which heretofore has not been possible. Thus, plural low profile dimmer switches of varying load size (e.g., 600W, 1000W, 1400W and/or 1500W) may be ganged in the manner disclosed in the '020 patent.

Further advantages of the invention will become appa¬ rent to those skilled in the art upon particularly des¬ cribing a preferred embodiment of the invention.

For the purpose of illustrating the invention, there is shown in the drawings a form which is presently pre¬ ferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumen¬ talities shown.

Brief Description Of The Drawings

Figure 1 is a perspective view of a slide dimmer switch constructed according to the teachings of the present invention.

Figure 2 is an exploded view of the slide dimmer switch illustrated in Figure 1.

Figure 3 is a top plan view of the slide dimmer switch illustrated in Figures 1 and 2.

Figure 4 is a perspective view of a rotary dimmer switch constructed according to the teachings of the present invention.

Figure 5 is an exploded view of the rotary dimmer switch illustrated in Figure 4.

Figure 6 is a perspective view of a push button wall switch constructed according to the teachings of the present invention.

Figure 7 is an exploded view of the wall switch illus¬ trated in Figure 6.

Figure 8 is a cross section view taken along line 8-8 of Figure 1.

Figure 9 illustrates alternative means for invisibly affixing the cover plate to the support plate.

Figure 10 is a viewing analysis of a prior art dimmer switch and illustrates the narrow angle of view at which the cover plate appears to float from the wall.

Figure 11 is a viewing analysis of a support plate/ cover plate constructed according to the teachings of the present invention and illustrates the wide angle of view at which the cover plate appears to float.

Figure 12 illustrates details of another prior art dimmer switch.

Figures 13, 14 and 15 illustrate various embodiments of a cover plate according to the present invention.

Figure 16 illustrates a wall box embedded in a wall for mounting the cover and support plate arrangement of the present invention.

Figure 17 is a cross section of an on-off switch associated with a slide dimmer switch of the present inven¬ tion.

Figure 18 is a cross section taken along line 18-18 of Figure 2.

Figure 19 is an exploded isometric view of the rear side of one embodiment of a- 8.33 amp capacity 1.5A width non-septum dimmer switch according to the present invention.

Figure 20 is an exploded isometric view of the rear side of another embodiment of a 8.33 amp capacity 1.5A width non-septum dimmer switch according to the present invention.

Figure 21 is an exploded isometric view of the rear side of a 11.67 amp capacity 2.5A width non-septum dimmer switch according to the present invention.

Figure 22 is a perspective view of the rear side of a 12.5 amp capacity 2.5A width dimmer switch according to the present invention and illustrates in phantom the dimmer switch disposed in a wall box.

Figure 23 is an exploded isometric view of the rear side of a 12.5 amp capacity 2.5A width rion-septum dimmer switch according to the present invention.

Figure 24 is a cross section taken along lines 24-24 of Figure 22.

Figure 25 is a schematic diagram of a dimmer circuit that may be used in the practice of the present invention.

Detailed Description Of The Preferred Embodiment Of The Invention

Referring now to the drawings, wherein like numerals represent like elements, there is illustrated in Figures 1 and 2 a cover and support plate arrangement labeled gener¬ ally 10. The arrangement 10 comprises a generally rec¬ tangular support plate 14 and a generally rectangular cover plate 16. The support plate 14 is adapted to be mounted to a conventional wall box 30 embeded in a wall 12. See Figures 1 and 16. Shallow pan head screws 44 communicate with recessed holes 42 for securing support plate 14 to the wall box 30 by threading engagement with aligned threaded holes 32, 84 in the wall box 80. The use of shallow pan head screws 44 and recessed holes 42 pro¬ vides a small clearance between the head of screw 44 and a slider, as will become evident hereinafter. See Figure 13.-

As illustrated in Figure 16, wall box 30 has a width A and a length 3. As is known in the art, commercially available wall boxes of the type shown measure about 3-9/32 inches long by about 1-31/32 inches wide. The support plate 14 has overall surface dimensions including a length and a width which are greater than the corresponding length B and width A of the wall box 80, i.e., the overall surface dimensions of the support plate 14 are greater than corres¬ ponding dimensions of the wall box. Preferably, the width of support plate 14 is either 1.5A or 2.5A and the length of support plate 14 is no greater than about 3A. Support plate 14 also includes offset mounting holes 86, 88 for the reasons more fully set forth in U.S. Patent 3,735,020. Mounting holes 86, 88 need not be recessed.

As illustrated, the support plate 14 has a generally flat rear surface 23. Preferably the front surface 21 is generally flat (i.e., no septa) and has a pair of upstanding

portions 22 disposed along opposing vertical side edges 29 thereof. See Figures 13, 14 and 15. As shown, the upstan¬ ding portions 22 extend generally perpendicularly outward from the front surface 21 of support plate 14. Preferably, the distance between the rear surface 23 of support plate 14 and the top of upstanding portions 22 is no greater than about 0.2 inches. If desired, support plate 14 may be provided with septa (not shown) on the front surface 21, provided that the height of the septa do not exceed that of upstanding portions 22 and further provided that the relationships disclosed herein are maintained. Septa, however, are not required in the practice of the invention.

As illustrated in Figure 2, the cover plate 16 is generally rectangular and is normally juxtaposed over the front surface 21 of support plate 14. The cover plate 16 has overall surface dimensions, including a length and a width, greater than the corresponding length and width of the support plate 14, i.e., the overall surface dimensions of the cover plate 16 are greater than corresponding dimen¬ sions of the support plate 14. As best illustrated in Figures 8, 11 and 13, the cover plate 16 has at least a pair of first portions 27 that extend laterally over the vertical side edges 29 of support plate 14. If desired, the cover plate may also be provided with second portions 24 that extend generally perpendicularly outward from the periphery of the first portions 27 so that the second portions 24 overlay the upstanding portions 22 as best illustrated in Figures 14 and 15. The second portions 24 are not necessary, but may be provided to improve the aesthetic appearance of the invention. For convenience only, the invention disclosed herein is shown as including the second portions 24 and their disclosure should not be construed as a limiting feature, except as otherwise noted. When second portions 24 are provided, it is prefer¬ able that the depth of second portions 24 be less than the depth of upstanding portions 22, as illustrated in Figures 8, 11, 14 and 15. Preferably, as illustrated in Figure

13, when no second portions 24 are provided, the ratio of (i) the distance "a" between the rear surface 23 of the support plate 14 and the rear surface 19 of the first portion 27 to (ii) the distance "b" by which the first portions 27 extend laterally over the vertical side edges 29 is less than approximately two to one. Similarly, as illustrated in Figures 11, 14 and 15, when second portions 24 are provided, the ratio of (i) the distance "a" between the rearmost portion 31 of second portion 24 and the rear surface 23 of support plate 14 to (ii) the distance "b" by which the first portions 27 extend laterally over the vertical side edges 29 is less than approximately two to one. (As used herein, the term "rearmost sur ace", as opposed to "rear surface" , when used in connection with cover plate 16, refers to the surface 19 of the embodiment . illustrated in Figure 13 and to the portion 31 of the embodiment illustrated in Figures 14 and 15.) Figure 11 illustrates that, according to one practice of the inven¬ tion having second portions 24, a dimension for "a" is .128" and a dimension for "b" is .062".

Generally, in the case of both embodiments, it can be said that the ratio of (i) the distance between the rear surface of the support plate (surface 23) and the rearmost surface of the cover plate (surface 19 in Figure 13; surface 31 in Figures 11, 14 and 15) to (ii) the dis¬ tance by which the first portions overlay the vertical side edges should be less than approximately two to one.

Also, as illustrated in Figures 11, 13, 14 and 15, when the above mentioned ratio is maintained, the upstanding portions 22 of support plate 14 will be visible only when viewed at extreme angles with respect to an imaginary line 60 that is perpendicular to the plane of the support plate

14. With the dimensions specified in Figure 11 (i.e., a = .128", b = .062"), the upstanding portions 22 will be visible only when viewed at an angle 0 ^" in excess of 25° with respect to the imaginary line 60. The first portions 27 of cover plate 16 cause a shadow 46 to be cast around

the periphery of the support plate 14 (Figure 1) thereby giving the appearance that the cover plate 16 is "floating" from the wall within an angle of view that exceeds 50°, i.e. , plus or minus 25° from the imaginary line 60. The shadow 46 also masks imperfections in the portion of the wall surrounding the support plate 14.

If desired, cover plate 16 may also be provided with portions 35, 33 that extend perpendicularly outward from upper and lower horizontal edges thereof. See Figure 2. Portions 35, 33 serve to self-align the cover plate with respect to the support plate, as will become evident here¬ inaf er.

Figure 10 illustrates one present commercial embodi¬ ment of the slide dimmer switch disclosed in the '923 patent. As illustrated therein, the dimension a = .506" and the dimension b = .010". With such dimensions, the angle at which the upstanding portions 22' ' ' appears to be invisible is less than 2° with respect to imaginary line 60'. That is, the overall field of view at which cover plate 16' ' ' appears to "float" from the wall is 4°, i.e. , plus or minus 2° from imaginary line 60'.

Turning now to Figure 3, it will be seen that there is an air gap 62 between the rear surface 19 of cover plate 16 and the front surface 21 of support plate 14. See also Figures 13, 14 and 15. Preferably, the width of the air gap (i.e. , the distance between the rear surface 19 of cover plate 16 and the front surface 21 of support plate 14) is no greater than about 0.15 inch. Openings into the air gap 62 are located between the cover plate 16 and the support plate 14 at upper and lower horizontal edges of the support plate 14 and define inlets to and outlet from the air gap 62, as best illustrated in Figure 3. The combination of the pair of upstanding portions 22, the air gap 62 and the inlets and outlets define a chimney through which air currents may flow to dissipate heat generated by a voltage control means in thermal connection with the support plate 14. For example, a dimmer circuit

may be disposed within a housing 59 adjacent the rear surface 23 of support plate 14 and may have a triac in thermal connection with the support plate 14. The support plate 14 is preferably constructed of a heat conductive metal such as aluminum or copper to facilitate heat dissi¬ pation. Moreover, the front surface 21 of support plate 14 and the rear surface 19 of cover plate 16 may be painted black or provided with a matt finish to improve the thermal performance of the device.

The discussion thus far applies to all embodiments of the invention disclosed herein. 'Thus, the support plate/ 4over plate arrangement discussed thus far has application, for example, to slide dimmer switches, rotary dimmer switches and wall mounted push button switches, as well as any other type of wall mounted devices including wall mounted displays and other types of v/all mounted control devices. Application of the teachings of the present invention to examples of a slide dimmer switch, rotary dimmer switch and wall mounted push button switch will now be discussed. Except as noted in the appended claims, these examples are not intended as limiting in any respect.

Figures 1, 2, 3, 8, 17 and 13 apply to a slide dimmer switch embodiment of the present invention. Figures 4 and 5 apply to a rotary dimmer switch embodiment of the present invention and Figures 6 and ^' 7 apply to a wall mounted push button switch embodiment of the present invention. Figures 19, 20 and 21 are applicable to any dimmer switch application of the present invention.

A slide dimmer switch embodiment of the present invention will be discussed first.

Generally centrally disposed in the support plate 14 is an elongated slot 20. In substantial alignment therewith is a generally centrally disposed elongated opening 18 in the cover plate 16. The width of the opening 18 is greater than the width of slot 20 for reasons that will become apparent hereinafter. As disclosed in the '923 patent, and as shown in Figures 2 and 8, a linearly adjustable

potentiometer 90 is disposed adjacent the rear surface 23 of the support plate 14 and has an operating member 28 extending through the slot 20 and being slidable therealong. The potentiometer is operatively coupled to a well known voltage control means such as a dimmer circuit disposed within housing 59, as also disclosed in the '923 patent and in a manner disclosed herein. Housing 59 is affixed to the rear surface 23 of support plate 14 by low profile eyelets 40. The holes in which eyelets 40 are disposed may contain recesses so that the heads of eyelets 40 are flush with the front surface 21 of support plate 14 to facilitate uninhibited sliding motion of a slider 30 that is guided by guide means discussed hereinafter. Addition¬ ally, mounting holes 42 disposed at upper and lower ends of support plate 14 are each surrounded by recessed regions 43 disposed on the front surface 21 of the support plate 14 so as to provide counterbored portions into which heads of screws 44 may seat. When fully inserted into holes 42, the heads of screws 44 are preferably substantially flush with front surface 21. See Figure 13.

The slider 30 is operatively coupled to the operating member 28 and has a handle 32 that protrudes through the opening 18 in the cover plate 16. The slider 30 is slide- - ^* ably moveable along the guide means to cause a corresponding sliding movement of the operating member. The voltage appearing at the output of wires 25 thus varies according to the linear setting of the slider 30 (and hence operating member 28) as is well-known in the art.

According to the preferred embodiment of the inven¬ tion, the guide means comprises a pair of parallel spaced apart rails 36 separately formed from and attached to the support plate 14. Each rail 36 is secured to the front surface 21 of the support plate 14 by means of three waxed eyelets 38. The eyelets 38 are positioned so that they are aligned with tabs 54 disposed on the rear surface 19 of cover plate 16 for reasons described herein. As illustrated in Figure 8, the portion of the eyelets 38 that protrude through the rear surface 23 of support plate 14 are recessed

so that rear surface 23 will sit flat against wall 12. Each rail 36 is disposed on an opposing side of slot 20 and is arranged to accommodate slider 30. In particular, each rail 36 engages an opposite side of the slider 30, as illustrated in Figures 2, 3 and 18 so that the slider 30 is both guided and supported by rails 36. Rails 36 may be constructed of a metal such as aluminum or of a resilient plastic material such as nylon. The use of separately attached rails allows adjustment of spacing tolerance during manufacture thus eliminating the problems associated with extruded rails used in the prior art. The use of separately attached rails also permits selection of rail material without altering the manufacturing process.

As illustrated in the drawings (e.g. Figure 2), the rails 36 extend the full length of the support plate 14 and overhang the horizontal edges of the support plate 14 by a small amount. Preferably, the rails 36 overhang the support plate 14 by a small distance so that opposing horizontal portions 35, 33 of cover plate 16 cause the cover plate 16 to be evenly aligned (in a vertical direction) with respect to support plate 14. In the preferred embodi¬ ment, the support plate 14 is approximately 4-1/2 inches (more particularly, 4.44 inches) in length, the inside ' ^' distance between horizontal portions 35, 33 is approximately 4.515 inches and the rails 36 overhang the horizontal edges of support plate 14 by approximately .02 inch at each end. Overhanging the rails 36 in this manner has been found to be particularly beneficial when plural dimmer switches are ganged since the cover plates 16 do not slide vertically and thus remain aligned with one another.

In the preferred practice of the invention, the height of the rails 36, i.e., the distance from the top of rails 36 to the front surface 21, is no greater than about 0.15 inches. The rails are grooved on their inside edges as best shown in Figures 8 and 18 to define a track in which slider 30 rides. Moreover, the rails 36 are grooved on their outside edges, as also shown in Figures 8 and 18, to facilitate enαaσement with tabs 54.

Slider 30 may have a plurality of prongs 34 for main¬ taining slider 30 in sliding engagement with the grooves in rails 36, as disclosed in the '923 patent. Preferably, the distance between slider 30 and the front surface 21 is no greater than about 0.045 inches. This small clearance is facilitated by the counterbored mounting holes 42 and low profile pan head screws 44, as best illustrated in Figure 18. This arrangement also permits the overall depth of the dimmer switch to be kept to a minimum.

As also disclosed in the '923 patent, operating member 28 has a cam 56 associated therewith. See Figure 8. The cam 56 is located adjacent the rear surface 23 of the support plate 14 and is operatively connected to the operating member 28. The cam 56 is- provided with a cam tab 57 that has a portion that extends away from the rear surface 23 of the support plate 14, as shown. Unlike the dimmer switch of the '923 patent, however, a viscous lubri¬ cant is provided within a housing in which the cam 56 and cam tab 57 ride. The cam tab 57 moves (i) between housing surface 55 and a strip of Teflon ^® tape 48 disposed on the rear surface 23 of support plate 14 and (ii) in contact with a viscous lubricant 58. See Figures 8 and 17. Similarly, cam 56 moves between housing surface 55 and viscous lubricant 58, as shown. Viscous lubricant 58 is disposed on the housing surface 55 and on the surface of Teflon ^® tape 48 at all points where cam tab 57 and cam 56 may contact either the housing surface 55 or Teflon ^® tape 48. Since viscous lubricant 58 is contained within internal parts of the dimmer switch, it does not cause the problems hereinbefore described. More importantly, however, the viscous lubricant is not added to reduce friction, as in the prior art. Instead, the viscous lubricant 58 provides a viscous damping of the sliding movement of the operating member 28 so as to provide an improved "feel" as slider 30 is moved from one extreme end of its travel to the other. Any suitable grease such as that produced by the Nye Company under the name Nyogel 865 may be used for viscous lubricant 58.

As disclosed in the '923 patent, an on-off switch 92 may be associated with the voltage control means 26. See Figure 17. The cam tab 57 slidingly contacts a member 94 associated with the on-off switch and opens the switch when the operating member has been slid to an extreme end of its travel. This feature is fully disclosed in the '923 patent.

As best shown in Figure 8, the slider 30 is sand¬ wiched between the cover plate 16 and support plate 14 and the handle 32 protrudes through opening 18. Alternatively, however, the operating member 28 could extend through both the slots 13, 20 and a handle could be affixed to the operating member 23 adjacent the front surface 17 of the cover plate 16, wherebv slider 30 is not required. (See Figure 1).

According to the embodiment of the slide dimmer dis¬ closed herein, the means for invisibly affixing the cover plate 16 to the support plate 14 comprises a plurality of tabs 54 disposed on and integral with the rear surface 19 of the cover plate 16. As illustrated in Figure 8, the tabs 54 engage with corresponding portions of rails 36 so that the cover plate 16 snaps thereon. Figure 7 also illustrates the use of tabs (54") to engage rails (36") disposed on the support plate. As previously mentioned, the eyelets 38 and tabs 54 are preferably positioned so that they are in align¬ ment when cover plate 16 is affixed to the rails 36. More particularly, it is preferred that there be three equidis¬ tantly spaced eyelets 38 affixing each rail 36 to support plate 14 and three corresponding, equidistantly spaced tabs 54 on each side of surface 19 of cover plate 16 in align¬ ment with eyelets 38. It has been found that positioning the eyelets 38 and tabs 54 in this manner provides strength and stiffness to the rails 36 in the direction of the shear force applied by the tabs 54.

The present invention is not limited to rails disposed on the support plate. Those skilled in the art will readily recognize that the rails 36 could be affixed to the rear

surface 19 of the cover plate 16 and the slider 30 disposed therebetween as before described. In such case, tabs 54 would be disposed for cooperative engagement with rails 36 on the front surface 21 of support plate 14. The present invention is also not limited to the use of rails with slide type dimmer switches. As described below, the use of separately attached rails has application wherever it is desired to affix a cover plate to a support plate invisibly and without the use of tools.

As illustrated in Figure 8, both the cover plate 16 and the support plate 14 may have notched portions 50, 52 respectively. As explained in U.S. Patent 3,735,020, the notched protions facilitate "ganαing" of a plurality of switches by breaking off portions of the support plate 14 and cover plate 16 at the notches.

Referrinq now to Figures 4 and 5, a rotary dimmer switch constructed according to the teachings of the present invention will be explained. Except as noted, the above description is applicable to the rotary dimmer switch.

As before, the rotary dimmer switch comprises a support plate 14' and a cover plate 16'. The cover plate 16' is juxtaposed over the support plate 14' and casts a shadow 46' around the periphery of the support plate 14' so as to give the appearance that cover plate 16' is "floating" from wall 12', as previously explained. Gene¬ rally centrally disposed in the support plate 14' is a first opening 20'. The first opening 20' accommodates a rotary operatinq member 28' protruding therethrouqh. Cover plate 16' has a corresponding generally centrally disposed second opening 18' in substantial alignment with the first opening 20*, as shown. Preferably, the second opening 18' is round and has a diameter slightly larger than the diameter of a control knob 78 that protrudes therethrough. A voltage control means disposed within housing 59' and having a triac in thermal connection with support plate 14' comprises a rotary potentiometer (not shown) disposed adjacent the rear surface of the support

plate 14'. The rotary potentiometer is operatively con¬ nected to the rotary operating member 28' extending through the opening 20". The control knob 73 is rotatable to cause a corresponding rotary movement of the operating member 28' and thereby vary the output signal appearing at the output of wires 25', as is well-known in the art.

In the embodiment illustrated in Figure 5, control knob 78 has a portion 79 that is sandwiched between the cover plate 16' and support plate 14' while the remainder extends through opening 18'. Alternatively, however, the operating member 28' could extend through both the openings 18', 20' and the circular control knob could be affixed to the operating member 28' adjacent the front surface 17' of cover plate 16', whereby the portion 79 is not required.

As before, the rotary dimmer switch illustrated in Figures 4 and 5 may.be equipped with a pair of rails 36' on either the front surface 21' of support plate 14' or the rear surface 19' of cover plate 16' for locking engage¬ ment with tabs disposed on the other of the rear surface 19' of cover plate 16' or front surface 21' of the support plate 14'. Alternatively, the means for invisibly affixing cover plate 16' to support plate 14' may comprise a pair of metallic strips 74 disposed on the front surface of the support plate 14' and a pair of magnetic strips 76 disposed on the rear surface of the cover plate 16', as illustrated in Figure 9. Additionally the metallic strips 74 may be disposed on the rear surface of the cover plate 16' and the magnetic strips 76 disposed on the front surface of the support plate 14'.

A wall mounted push button switch embodiment of the present invention will now be explained. Except as noted, the description of the slide dimmer embodiment is applicable to the wall mounted push button embodiment.

As illustrated in Figures 6 and 7, the wall mounted switch embodiment comprises a cover plate 16*' and a support plate 14'', as before. Generally centrally disposed in the support plate 14'' is a first opening 20'' which may

be elongated, round, square, etc. , as desired to accommo¬ date the operating member 28' ' of a switch disposed ad¬ jacent the rear surface of the support plate 14' '. Gen¬ erally centrally disposed in the cover plate 16' ' is second substantially rectangular opening 18' ' which acc :<om- modates a push button 64. Push button 64 is maintained in the opening 18' ' by means of integral brackets 66 having resilient spring tabs 68 extending therefrom, as shown. The rear portion 65 of the push button 64 has a greater length and width than the length and width of the opening 18' ', while the length and width of the front portion 67 is slightly less than the length and width of the opening 18' '. Thus, a portion of the push button 64 protrudes through the opening 13' ' , but the larger rear portion 65 is captured behind the cover plate 16' ' and prevents the push button 64 from falling through the front surface of the cover plate 16' '. The spring tabs 68 urge the push button 64 toward the front surface of the cover plate 16' '. Thus, the push button 64 is depressable when pres¬ sure is applied to the front surface thereof to depress the operating member 28' ' and thereby open and close the switch.

Alternatively, the operating member 28" may extend through both openings 18", 20" and a push button may be connected to the operating member 28" adjacent the front surface of the cover plate 16".

As before, the means for invisibly affixing the cover plate 16' ' to the support plate 14' ' comprises tabs 54' ' for engagement with rails 36' '. Alternatively, the means for invisibly affixing the cover plate 16 to support plate 14 could comprise magnetic strips 76 and metallic strips 74, as illustrated in Figure 9.

Figure 12 illustrates a known slide dimmer switch 100 having a cover plate 106, a slider 110 and a handle 102. The slider 110 is disposed between a pair of guide means 108 integrally disposed on the rear surface 107 of cover plate 106. An operating member 104 is associated

with the slider and operates a linear potentiometer (not shown) operatively coupled to a voltage control means (also not shown). A pair of retaining means 112 are integ¬ ral with and extend from the rear portion 107 of cover plate 106. The retaining means 112 permit the cover plate 106 to be snapped onto another plate associated with the dimmer switch.

It has been found that 8.33 amp capacity 1.5A width, 11.67 amp capacity 2.5A width and 12.5 amp capacity 2.5A width low profile dimmer switches, of the type hereinbefore described, may be constructed to meet U.L. operating temperature standards. The following design considerations have been found to be relevant in contructing such dimmer switches to meet U.L. standards: triac selection (minimum forward voltage drop at maximum operating current) ; selec¬ tion of wire diameter comprising the RFI choke that is common to most dimmer circuits; selection of lead wire diameter; lead dress; lead wire arrangement; and selection of type and size of heat transfer materials. Figures 19 through 24 illustrate various embodiments of the herein¬ before described low profile dimmer switches that incorporate these design considerations. The present invention is use¬ ful with any well-known dimmer circuitry, such as that 'schematically illustrated in Figure 25. The present inven¬ tion has particular application to dimmer circuits such as that shown in Figure 25 that have a smooth and continuous dimming range up to at least 95% of full voltage.

Turning now to Figure 19, there is illustrated one embodiment of a 8.33 amp capacity, 1.5A low profile dimmer switch labeled generally 113. Dimmer switch 113 comprises a support plate 14 having a length 1 and a width w. Preferably the width of support plate 14 is 1.5A and the length is no greater than 3A. More particularly, it is preferred that the width w measure no greater than about 2-3/4 inches and the length 1 measure no greater than about 4-1/2 inches. It is also preferred that the support plate be constructed of aluminum and that its

front surface be substantially flat (excepting rails 36), although, as mentioned, short septa not exceeding the height of upstanding portions 22 may be provided, if desired. In the preferred embodiment, the thickness of support plate 14 is approximately 0.055 inch and the overall surface area of support plate 14 measures approximately 26 sguare inches (including the area contributed by the upstanding portions 22). If dimmer 113 is a slide dimmer switch, as is illustrated in Figure 18, then a cradle 114 may be riveted to the rear surface of support plate 14. Cradle 114 houses linear potentiometer 90 (see Figure 8). A pair of lead wires 130, 132 is electrically connected to the load and supply sides of the dimmer circuit.

As shown, the dimmer circuit includes a choke 120 that is part of a RFI circuit. The choke 120 employed in the 8.33 amp capacity 1.5A width low profile dimmer switch preferably has an inductance of 30 μti (nominal) and is constructed of twenty turns of AWG 14 magnet wire (nylon solvar polyurethane insulated) wound on a MSS iron powder core. The core preferably has an external diameter of approximately 0.9 inch and an internal diameter of approxi¬ mately 0.55 inch. A suitable core is availabe from The Arnold Engineering Co., 300 West Street, Marengo, Illinois ^{• '} 60152, part number A4-12055. The choke should exhibit a rise time characteristic (di/dt) not exceeding 5 amps μsec (1500W load, 90° conduction angle, and 120VAC source).

The circuitry of dimmer switch 113 comprises a triac 140 (with attached connecting lug) in thermal connection with support plate 14. More particularly, a small copper strip 144 is eyeleted to the rear surface of support plate 14 (a thermally conductive grease such as Dow Corning 340 Silicone Heat Sink Compound is first applied to the contac¬ ting surfaces of the copper strip and the aluminum support plate), one side of a beryllium oxide disk 142 is soldered to the copper strip 144 and the triac 140 is soldered to the other side of the beryllium oxide disk 142. Preferably, copper strip 144 measures about 1.23 inches by 0.54 inch

and is approximately 0.062 inch thick. Triac 140 is preferably a triac having a 35 amp continuous current rating and a forward voltage drop of no greater than about 1.0 volt at 8.33 amps (the dimmer switch's maximum specified operating current), such as a Sanrex model SSG 35B30F.

In the 8.33 amp capacity 1.5A septa dimmer switch hereinbefore described it is known to use AWG 16 lead wires. It has been found, however, that improved thermal performance is provided if AWG 12 stranded wire is utilized for lead wires 130, 132. The thermal performance of dimmer switch 113 is further improved by dressing the lead wires ("lead dress") in known fashion. Thus, the lead wires 130, 132 are preferably bent or curved in a substantially U shaped pattern as shown at 134, 135 in close vicinity to the dimmer circuitry. Preferably, the length of curve (e.g., the distance from 136 to 138 on wire 132) should be about 2.5 inches.

Preferably, dimmer switch 113 has a continuous and smooth dimming range up to at least 95% of full voltage and the triac, and not a mechanical break switch, carries the full load current at all times when the dimmer switch is on. Dimmer circuits that provide such a dimming range are well known in the art. One such circuit is illustrated in Figure 25. When dimmer switch 113 has been constructed as above described, it has been observed that when opera¬ ting under maximum specified load conditions (8.33 amps at 120VAC) , the maximum temperature rise on field wiring terminals or on the conductors of lead wires 130, 132 is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any other part of the dimmer switch that is exposed is no greater than 65°C. Thus, the thermal performance of dimmer 113 meets requirements specified by U.L.

Illustrated in Figure 20 is another embodiment of a

8.33 amp capacity 1.5A low profile dimmer switch according to the present invention and labeled generally 115. Dimmer switch 115 is identical in all respects to the dimmer

switch 113 of Figure 19 previously described except that dimmer switch 115 utilizes a large copper pad 146 instead of a small copper strip 144. Preferably, copper pad 146 measures approximately 2.8 inches by 1.7 inches and is about 0.063 inches thick. As shov/n, triac 140 and cradle 114 are both disposed over copper pad 146. As before, it is preferred that triac 140 have a 35 amp continuous current rating and a forward voltage drop of no greater than about 1.0 volts at 8.33 amps, such as provided by a Sanrex model SSG 35B30F triac.

Preferably, dimmer switch 115 has a continuous and smooth dimming ranqe up to at least 95% of full voltage and the triac, and not a mechanical break switch, carries the full load current at all times when the dimmer switch is on. When dimmer switch 115 has been constructed as above described, it has been observed that when operating under maximum specified load conditions (8.33 amps at 120VAC) , the maximum temperature rise on field wiring terminals or on the conductors of lead wires 130, 132 is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact the field wiring is no greater than 35°C and the maximum temperature rise of any other exposed part of the dimmer switch is no greater than 65°C.

Turning now to Figure 21, a 11.67 amp capacity 2.5A low profile dimmer switch according to the present inven¬ tion will be described. A 11.67 amp capacity 2.5A dimmer switch 117 according to the present invention comprises a support plate 14 having a width w and length 1. Unlike the support plate 14 of dimmers 113 and 115, the support plate 14 of dimmer 117 is preferably constructed of 0.062 inch thick copper to improve the dimmer switch's thermal performance. Additionally, it is preferred that the copper support plate be painted black or have a matt finish to further improve its ability to radiate heat.

In the preferred practice of the invention, the width w is about 2.5A and the length 1 is about 2.5A.

Still further, it is preferred that the front surface of support plate 14 of dimmer 117 be substantially flat, although low septa may be provided, as before. It is also preferred that the width w measure about 4-1/2 inches and that the length 1 measure no greater than about 4-1/2 inches. The overall surface area of support plate 14, including that contributed by upstanding portions 22, preferably measures about 44 square inches.

As with dimmer switches 113 and 115, if dimmer switch 117 is a slide dimmer, as is the dimmer illustrated in Figure 21, a cradle 114 is riveted to the rear surface of support plate 14. A copper pad 154 is interposed between the cradle 114 and support plate 14 as hereinafter des¬ cribed.

The dimmer circuitry associated with dimmer switch 117 includes a choke that is part of a RFI circuit. The choke 120 employed in the 11.67 amp capacity 2.5A width low profile dimmer switch preferably has an inductance of 5 _H (nominal) and is contructed of eight pairs of spaced turns (for a total of sixteen turns) of AWG 14 magnet wire (nylon solvar polyurethane insulated) wound on an enamel coated MSS iron powder core. The core preferably has an external diameter of approximately 0.9 inch and an internal diameter of approximately 0.55 inch. A suitable core is available from The Arnold Engineering Co., 300 West Street, Marengo, Illinois 60152, part number A4-12055. The choke should ex¬ hibit a rise time characteristic (di/dt) not exceeding 6 amps μsec (2000W load, 90° conduction angle and 120VAC source) .

A triac 150 (with attached connecting lug) associated with the dimmer circuitry is in thermal connection with support plate 14. In particular, copper pad 154, preferably measuring about 2.8 inches by 1.7 inches and being about 0.063 inch thick, is riveted to the rear surface of the copper support plate 14 (a thermally conductive grease such as Dow Corning 340 Silicone Heat Sink Compound is applied to the mating surfaces of the copper pad and copper

support plate). One side of a beryllium oxide disk 152 is soldered to the copper pad 154 while triac 150 is soldered to the other side of beryllium oxide disk 152. Use of the copper pad facilitates construction of the dimmer.

Preferably, triac 150 s a triac having a 35 amp continuous current rating and forward voltage drop of no greater than about 1.05 volts at 11.67 amps, such as a Sanrex model SSG 35B30F.

It has been observed that, in the operation of most dimmer switches, the following temperature relationship exists:

Tr LUG >τ _A > ^• LEAD

where: Tr_,rG i- ^s the temperature of the hottest lug, e.g., lug 177 (Figure 21), to which a lead wire is connected in the dimmer circuit;

T^. is the ambient temperature within the housinq 59 covering the dimmer circuit; and

^T LE _A D ^- ^s the temperature of the hottest conductor of the lead wires measured at the point where the lead wires exit the housing 59.

Since both THJ and Tjs. exceed T gAD' heat from the lugs and the housing interior is transferred to the lead wires within the housing 59. The lead wires thus serve to trans¬ fer heat away from the dimmer circuit and out of the housing, Unless precautionary measures are taken, T _L g^Q _(max) , i.e., the maximum temperature rise on the conductors of the lead wires measured at the point where the lead wires exit the housing, may exceed that specified by U.L. This is partic¬ ularly true in a low profile 11.67 amp capacity 2.5A non- septum dimmer switch. Conventional design methods cannot

solve this problem. According to the invention, however, there is provided a novel lead wire arrangement for main¬ taining T _L EAD ₍ max ₎ wi hin U.L. specifications. Although described herein as for use with a 11.67 amp capacity 2.5A low profile dimmer switch, this lead wire arrangement has application to any circuit where heat transfer to lead wires extending through a housing is a problem.

According to the invention, two parallel wires from each "hot junction" of the dimmer circuit are provided instead of a single wire from each "hot junction". (As used herein, the term "hot junction" refers to the lug or other portion of the dimmer circuit to which a lead wire is connected.) The wire sizes must be chosen so that they may safely carry full load and surge currents, but generally it may be stated that the diameter of each of the parallel wires should be smaller than that of a single wire having a diameter sufficient to carry the same full load and surge currents. For example, if a single AWG 10 stranded wire is required to carry full load current, this may be replaced by two parallel AWG 12 stranded wires or by two parallel AWG 14 stranded wires. The following unexpected and surprising result has been found. While the two para¬ llel wires, e.g., AWG 12 or AWG 14, collectively have a greater heat transfer area CTl ^* D^ ^* L ^* 2, where D-^ is the wire diameter and L is length of wire within the housing) than a single larger diameter wire (_< D2*L, where D ₂ is the wire diameter of the larger wire) e.g., AWG 10, the temperature TLE D (max) measured on any one of the wires in the parallel wire arrangement is less than that measured in the single wire arrangement. One would expect ^T LEAD (max) to be greater in the parallel wire arrangement, since it has greater heat transfer surface area, but this is not the case.

The advantages of this novel lead wire arrangement are numerous. For example, a dimmer circuit that generates more heat may still be constructed to meet U.L. specificatio Thus, less expensive and/or more effective RFI chokes may

be used in the dimmer circuit. A less expensive triac with a higher forward voltage drop at a specified current may be used. Also, the power rating of the dimmer circuit may be increased without altering the size or profile of the support plate.

Application of the novel lead wire arrangement will now be explained in connection with the example of the 11.67 amp capacity 2.5A low profile dimmer switch. As illustrated in Figure 21, a pair AWG 12 stranded lead wires is connected in parallel to each side of the dimmer circuit. In particular, a pair of AWG 12 stranded lead wires 156, 158 is connected in parallel to a "hot junction" of the supply side of the dimmer circuit and a pair of lead wires 150, 162 is connected in parallel to a "hot junction" of load side of the dimmer circuit. In the embodiment illustrated in Figure 21, lead wires 156, 158 are soldered together at 164 (preferably within the housing 59) and to an AWG 10 stranded wire 168 that extends through the housing 59 for connection to field wiring. Similarly, the pair of parallel lead wires 160, 162 are soldered together at 166 (again, preferably within the housing 59) and to an AWG 10 stranded wire 170 that extends through the housing 59 for connection to field wiring. It has been found that this arrangement reduces TLEAD (max) (measured on the AWG 10 stranded wires extending through the housing) by about 1-2°C over the use of only a single AWG 10 stranded wire directly connected to each hot junction.

According to another embodiment of the invention, wires 168, 170 are not provided, but instead wires 156, 158, 160 and 162 extend through the housing 59 for direct connection to field wiring. It has been found that this arrangement reduces TL _E J-.T_. (max) (° ^{n eacn} °f the AWG 12 stranded wires extending through the housing) by about 3-5°C over the use of only a single AWG 10 stranded wire connected to each hot junction.

Experiments have also been performed with parallel AWG 14 stranded lead wires instead of parallel AWG 12

lead wires. Such experiments have shown that this arrange¬ ment can reduce T^E^O ₍ max ₎ ^y ^{as muc} h as 2°C over the use of a single AWG 10 stranded wire connected to the hot junction.

As before, the lead wires 156, 158 160, 162 are preferably dressed by bending or curving into a generally U shaped pattern in the vicinity of the dimmer circuit, as shown at 178, 179. This lead dress further enhances the thermal performance of 117. Preferably, the length of curve, (e.g. , the distance between points 176 and 178 of the lead wires 172, .174) is about 2.5 inches.

Preferably, dimmer switch 117 has a continuous and smooth dimming range up to at least 95% of full voltage and the triac, and not a mechanical break switch, carries the full load current at all times when the dimmer switch is on. Dimmer circuits that provide such a dimming range are well known in the art. One such circuit is illustrated in Figure 25. When dimmer switch 117 has been constructed as above described, it has been observed that when opera¬ ting under maximum specified load conditions (as described 11.67 amps), the maximum temperature rise on field wiring terminals or on the conductors of the lead wires 156, 158, 160, 162, 168, 170, is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any other part of the dimmer switch that is exposed is no greater than 65°C.

Turning to now to Figures 22, 23 and 24, a 12.5 amp capacity 2.5A width low profile dimmer switch according to the present invention will be described. Due to novel "thermal isolating" means utilized in the practice of the 12.5 amp capacity 2.5A width dimmer switch described herein, the use of means such as a copper support plate, a large copper pad for triac mounting, and the four lead wire arrangement discussed in connection with the 11.67 amp capacity 2.5A width dimmer switch is not required to meet U.L. standards. It should be understood that, although des¬ cribed in connection with a 12.5 amp capacity 2.5A width

dimmer switch, the "thermal isolating" means described below has application to any wall mounted dimmer, switch or other circuit where temperature must be controlled.

A 12.5 amp capacity 2.5A dimmer switch 119 accord¬ ing to the present invention comprises a support plate 14 having a width w and a length 1. Preferably the width w is no greater than about 2.5A and the length 1 is no greater than about 2.5A. Still further, it is preferred that the front surface of support plate 14 of dimmer switch 119 be substantially flat, although low septa may be provided. It is also preferred that the width w measure no greater than about 4-1/2 inches and chat the length 1 measure no greater than about 4-1/2 inches. The overall surface area of support plate 14, including that contributed by upstanding portion 22, preferably measures no greater than about 44 square inches. It is also preferred that the support plate be constructed of aluminum having a thickness of approximately 0.060 inch. Preferably, the support plate has a matt finish to improve its ability to radiate heat.

If dimmer switch 119 is a slide dimmer switch, as is the dimmer switch illustrated in Figure 23, then a cradle 114 may be riveted to rear surface of support plate 14 (means 180 and 182 are interposed between the cradle 114 and the rear surface of support plate 14 for reasons des¬ cribed hereinafter). A pair of AWG 10 lead wires 184, 186 is electrically connected to the load and supply sides of the dimmer circuit and are dressed in a substantially U-shaped pattern as shown at 216, 218. Preferably, the dimmer circuit has a continuous and smooth dimming range up to at least 95% of full voltage and the triac 150, and not a mechanical break switch, carries the full load current at all times when the dimmer switch is on. A dimmer cir¬ cuit such as that illustrated in Figure 25 may be used. As shown, the dimmer circuit includes a choke 120 that is part of a RFI circuit. The choke 120 employed in the 12.5 amp capacity 2.5A width dimmer switch should be the same

as that described in connection with the 11.67 amp capacity 2.5A dimmer switch.

Triac 150 (and attached connecting lug) associated with the dimmer circuit is in thermal connection with support plate 14. In particular, copper strip 144, prefer¬ ably measuring about 1.23 inches by 0.54 inch and being approximately 0.062 inch thick, is riveted to the rear surface of support plate 14 (a thermally conductive grease such as Dow Corning 340 Silicone Heat Sink Compound is applied to the mating surfaces of the copper strip 144 and support plate 14). One side of a beryllium oxide disk 152 is soldered to the copper strip 144 while triac 150 is soldered to the other side of beryllium oxide disk 152. Triac 150 is preferably a triac having a 35 amp continuous current rating and a forward voltage drop of no greater than about 1.05 volts at 12.5 amps, such as a Sanrex model SSG 35B30F.

As has been mentioned, support plate 14 is adapted to mount to a wall box 80 (see Figure 22). Thus, screws 44 secure support plate 14 to wall box 80 by means of threaded ears 194. As is known, wall box 80 comprises knock-outs 131 that are removed when it is desired to insert field wiring into wall box 80. Thus, as illustrated in Figure 22, field wiring 187, comprising conductors 183, 185, extends into wall box 80 for connection to lead wires 184, 186 of dimmer switch 119. Conductors 183, 185 are electrically coupled to the conductors of lead wires 184, 186 by means of wire connectors 196, 198 in well known manner. As is also well known, wall box 80 comprises internal clamps (not shown) and associated screws 191 for securing field wiring 187 to the wall box 80.

As best shown in Figure 22, housing 59 comprises a plurality of holes 188, 190. Three holes 188 are provided at one end of housing 59 while two holes 190 are provided at the other end of housing 59. The diameter of holes 188 is smaller than the diameter of holes 190. Holes 190 are provided to accommodate the larger diameter AWG 10 wire.

The smaller holes 188 are utilized in connection with the practice of the 8.33 amp 1.5A dimmer switch wherein smaller diameter lead wires may be utilized.

It has been found that if the interior of housing 59 is thermally isolated from the support plate 14, and further if the wall box 80 is thermally isolated from the support plate 14, the maximum temperature of the conductors of lead wires 184, 186 will remain far within acceptable levels when the dimmer switch 119 is operating at or near its full load current of 12.5 amps. This is true even if the parallel lead wire arrangement previously described is not utilized. It is also true,if an aluminum support plate rather than a copper support plate is utilized. Thus, the thermal isolation means described herein provides a simple and inexpensive means of producing a U.L. listed 12.5 amp capacity 2.5A width low profile dimmer switch.

The thermal isolation means utilized in the practice of the present invention comprises a first means for ther¬ mally isolating the interior of housing 59 from the support plate 14 and a second means for thermally isolating the wall box 80 from the support plate 14. The first means comprises a thermal isolating means 180 disposed between the housing 59 and the rear surface of the support plate ^' Ϊ4. Similarly, the second means comprises a thermal iso¬ lating means 182 disposed between wall box 80 and the rear surface of support plate 14.

Preferably, the first means 180 is constructed of a molded plastic such as Valox ^® 420SEO as manufactured by General Electric Company, although any suitable high tem¬ perature plastic that has thermal insulating qualities will suffice. As best illustrated in Figure 24, the plastic heat shield 180 is preferably hollow so as to provide an air gap 200 between the rear surface of the support plate 14 and the housing 59 (and its interior). It is believed that the air gap 200 provides increased thermal resistance. As shown in Figure 23, the plastic heat shield 180 is inter¬ posed between the cradle 114 and support plate 14. Cradle 114 and housinq 59 are eyeleted to the support plate 14,

through the plastic heat shield 180, by means of holes 210 provided in the plastic heat shield 180. Plastic heat shield 180 is also provided with a slot 202 that is in substantial alignment with the slot 20 in support plate 14 to permit sliding movement of the operating member 28. See Figures 23 and 24. Preferably, slot 202 is provided with shoulders 203. See Figure 24. Similarly, it is preferred that holes 210 be provided with shoulders 204, as also illustrated in Figure 24. Preferably, plastic heat shield 180 is provided with an opening 206 configured to accommodate triac 150.

As shown, the length and width of plastic shield 180 is only slightly larger than the corresponding length and width of housing 59 so as to prevent any part of housing 59 from coming into contact with support plate 14. Thus, housing 59 is completely thermally isolated from support plate 14 (except for the eyelets affixing housing 59 to support plate 14). In the preferred practice of the inven¬ tion, the thickness of plastic heat shield 180 is about 0.22 inch.

The second thermal isolating means 182 preferably comprises a piece of fishpaper 182 such as Armite manu¬ factured by Spaulding Fibre Company, Tonawanda, New York, disposed on the rear surface of support plate 14. The size and shape of the piece of fishpaper 182 is not criti¬ cal - any size and/or shape will suffice provided that the fishpaper insulates all parts of the wall box from the support plate (except for the screws 44 affixing support plate 14 to wall box 80). The precise thickness is also not critical. Thicknesses of 0.015 inch to 0.030 inch have been found to provide satisfactory results. In the pre¬ ferred practice of the invention, the piece of fishpaper 182 is rectangular in shape and is provided with four elongated holes 208 that are in substantial alignment with the mounting holes 42, 88 in support plate 14. Fish¬ paper 182 is also provided with holes 212 that are in alignment with holes 210 in the plastic heat shield 180.

Still further, the fishpaper 182 is provided with an open¬ ing 214 that accomodates both the copper strip 144 and the operating member 28. See Figures 23 and 24. As shown, when the dimmer switch is fully constructed as hereinbefore described, the piece of fishpaper 182 is sandwiched between the support plate 14 and the plastic heat shield 180 and thus is secured to the dimmer switch 119 thereby.

When dimmer switch 119 has been constructed as above- described, it has been observed that when operating under maximum specified load conditions (12.5 amps), the maximum temperature rise on field wiring terminals or on the con¬ ductors of the lead wires 184, 186 is no greater than 30°C, the maximum temperature rise of any part of the dimmer switch that can contact field wiring is no greater than 35°C and the maximum temperature rise of any other part of the dimmer switch that is exposed is no greater than 65°C. Thus, the dimmer switch 119 meets UL operating temperature standards.

There has been described a novel low profile cover and support plate arrangement that has application to wall mounted dimmer switches, push button switches, wall mounted displays, etc. For use in connection with dimmer circuitry, there has also been described novel construction techniques ^' for enhancing the thermal performance of the dimmer such ^' that septa are no longer required to meet U.L. temperature performance standards.

The present invention may be embodied in other speci¬ fic forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.