CONTROLS

Field of the Invention:

The present invention generally concerns improvements relating to thermally-responsive controls and more particularly concerns controls for electrical apparatuses including domestic appliances such as washing machines, tumble driers, dishwashers and central heating or air conditioning apparatus and other industrial and business premises equipment where it is commonplace to protect the equipment against overheating by provision of thermal sensors at strategic locations within the equipment and arranging for the apparatus to be disabled or an alarm indication otherwise given in the event of a sensed over-temperature situation.

Background of the Invention:

As abovementioned, it has been commonplace to provide thermal sensors in all manner of domestic and other electrical equipment, and it has not been unusual to provide more than one such sensor in an appliance with each sensor monitoring the temperature condition at a specific potential hot spot in the appliance. Thus, for example, in a domestic tumble

drier there might be one or more sensors associated with the heating element of the drier and a further sensor might be associated with the tumble drive motor. The problem has arisen, however, that such sensors are generally provided in positions which are inaccessible to a greater or lesser degree and, for safety reasons, are often of a non-automatically resetting type. This means that a householder or business operator may be put to considerable inconvenience and expense in a situation where a sensor operates simply on account of a transient condition arising in the apparatus and without there being any significant fault in the apparatus.

Objects and Summary of the Invention: The principal object of the present invention is to enable the abovedescribed problem of conventional arrangements to be overcome or at least substantially reduced.

In order to achieve the abovementioned object, the present invention in one of its aspects provides a thermal relay comprising a thermally-responsive electrical switch having stable on and off conditions between which it can be operated manually, particularly for resetting the switch, and a heater associated with a thermally-responsive actuating

element of the switch such as to cause the switch to adopt its off condition after a time delay following energization of the heater.

Such a thermal relay might for example be utilized in a situation where the operation of the heater is determined by an automatically resetting thermostatic switch located within an apparatus at a position which is inaccessible for manual resetting. With such an arrangement, and with the thermally- responsive switch connected to determine the functioning of the apparatus, an overtemperature situation arising within the apparatus such as to cause the thermostatic switch to operate will result in the heater being energised so as to cause the thermally-responsive switch to go open circuit. The apparatus will then remain inoperative until such time as the thermostatic switch cools and resets and the thermally-responsive switch is manually reset. A plurality of thermostatic switches might be provided at different potential "hot spot" locations in the apparatus, such thermostatic switches being connected electrically in series. The thermally-responsive switch can be located at any convenient position to be accessible for manual operation, and may advantageously be arranged such that its status is clearly visible and/or otherwise noticeable.

One such application is in domestic laundry equipment for example, where automatically resetting thermal cut-outs may be situated at the most advantageous positions within the apparatus without regard to the need to provide a manual reset facility. A plurality of such thermal cut-outs may be connected in series, with the heating element connected overall in parallel with the series connected cut-outs, such that when any one of the cut-outs opens its contacts a voltage will be applied to the heating element so as to energize the same and, after a time delay, cause the thermally-responsive switch to operate. In a typical washer/dryer, for example, the water heater, air heater, and all electric motors provided in the appliance may be individually protected by suitable auto-reset thermal cut-outs, and operation of any one of the thermal cut-outs would cause the thermally- responsive switch to operate, such switch typically constituting the main on/off switch of the apparatus. The apparatus would be resettable simply by manual operation of the thermally-responsive switch after an appropriate cooling down period (and after correction of any fault condition) , the thermal capacity of the system, and particularly of the heater and the actuator of the thermally-responsive switch, ensuring that a suitable time delay occurs before the apparatus

can be switched on again.

In an alternative application, advantage might specifically be taken of the delay which occurs between energization of the heating element and consequent operation of the thermally-responsive switch. For example, in a coffee filter machine, or the like, the subject thermal relay might provide a timed switch-off in the event of the machine being left switched on overnight. The heating element of the thermally-responsive switch might be connected in parallel with the contact thermostat which typically controls the temperature of the coffee pot heater plate. The cycle time of this thermostat is typically of the order of 15 to 30 minutes and the temperature setting is typically somewhat above 100°C. The thermostat normally operates if the coffee pot is removed, or is empty or nearly so. The heating element associated with the thermally-responsive switch would be energized when the thermostat contacts open. If this resulted from removal of the coffee pot for pouring, then replacement of the pot would cause the thermostat contacts to reclose relatively rapidly in which case the heater associated with the thermally-responsive switch would not be energized for a sufficient period of time to cause the thermally- responsive switch to operate. If, however, the pot

were to be replaced empty, or not replaced at all, then the contacts of the thermostat would remain open at least for a sufficient time to allow the heating element to operate the thermally-responsive switch so as safely to de-energize the coffee machine.

Other applications for a thermal relay of the described kind might for example comprise the protection of a number of zones in a convection heater to protect against partial blockage of the heater vents by clothes, or the protection of central heating boilers where thermal cut-outs may be required to be positioned at several inaccessible areas and the provision of a manual reset facility is conventionally difficult. An advantageous form of thermally-responsive electrical switch that may be utilized in the practice of the present invention by modification to incorporate a heating element is described in our British Patent Specification No. GB 2212664A with reference to Figs. 3A to 3M or Figs. 9A and 9B. This switch is marketed by Otter Controls Ltd. as their Jl switch and is intended primarily for use as a steam sensor in a water boiling appliance, particularly though not exclusively in conjunction with the XI or Gemini element protector unit which is also manufactured and marketed by Otter Controls Ltd.. It

is a simple matter to assemble a heating element, constituted for example by a thin film resistor, in close heat transfer relationship with the bimetallic switch-actuating element of the Jl switch. The operating characteristics of such a thermally- responsive switch/heating element combination could be arranged to be adjustable to suit particular applications, by setting the operating temperature of the thermally-responsive switch, and/or by selecting the power of the heating element, and/or by adjusting the physical positioning of the heating element relative to the thermal actuator of the switch so as to determine the heat transfer relationship therebetween. In the Jl switch, the biassing of a bimetallic switch actuator is adjustable by adjustment of a rotary cam and this adjustment can be utilized to determine the operating characteristics of the switch. Of course, the present invention is not restricted to use of the Jl switch. The present invention extends additionally to a thermal relay as defined hereinbefore in combination with one or more thermally-responsive or other switches arranged to determine the state of energization of the heating element, and also extends to an apparatus or appliance incorporating one or more such combinations.

The above and further features of the invention are set forth in the appended claims and will further be explained in the following by reference to an exemplary embodiment which is illustrated in the accompanying drawings.

Brief Description of the Drawings:

Fig. 1 is a schematic circuit diagram illustrating a situation wherein potential hot spots in an appliance are protected by thermal cut-outs which determine the operation of a thermal relay according to the present invention;

Fig. 2 is a perspective view of a thermally- responsive electrical switch in accordance with the teachings of British Patent Specification No. GB 2212664A aforementioned, the switch as is shown schematically having a heating element mounted proximate its bimetallic actuator;

Fig. 3 is an exploded perspective view of the switch of Fig. 2 from the opposite side thereof and illustrating an exemplary form of carrier for the heating element;

Fig. 4 is a perspective view similar to Fig. 3 but showing the heating element and carrier assembled with the switch; and Figs. 5A, 5B and 5C are schematic circuit

diagrams illustrating the use of a thermally- responsive switch as described in our British Patent Application No. 9208264.3 (Serial No. ) in an arrangement according to the present invention.

Detailed Description of the Embodiments:

Referring to Fig. 1, the circuitry of an electrically powered appliance 1 is protected by a plurality of series-connected, auto-reset thermal cut¬ outs 2 which can be of any convenient type such as a simple auto-reset bimetallic switch for example. A thermally-responsive switch 3 is connected in series with the cut-outs 2 and a heating element 4 is connected in parallel with the cut-outs 2 and is arranged in heat transfer relationship with the thermally-responsive actuator of the switch 3. In normal operation of the appliance, the cut-outs 2 and switch 3 will all be in a contacts-closed condition and the heating element 4 will be short-circuited by the cut-outs 2. On the occurrence of a hot spot in the appliance such as to cause one or more of the cut¬ outs 2 to operate and adopt a contacts-open condition, the heating element 4 will be energized and will begin to heat the thermally-responsive switch 3 which, after a delay, will adopt its contacts-open condition. It will be noted that the appliance will continue to be

powered via the heating element 4 after one or more of the cut-outs 2 have gone open circuit and until the switch 3 operates, but any such continued powering of the appliance will be at a reduced level and will be only temporary.

The thermally-responsive switch 3 having operated, its condition will be readily noticeable by virtue of its being provided with a conspicuous ON-OFF knob for example. The appliance will then remain inoperative until such time as the one or more cut¬ outs 2 responsible for the shut-down of the appliance cool and reset and until the switch 3 is manually reset. If there is a fault condition in the appliance, then the reconnection of the appliance would once again cause a cut-out to operate thereby once again switching off the appliance, and this would be the case until the fault was corrected.

Figs. 2, 3 and 4 are perspective views of a modified form of the Jl switch which is described in GB 2212664A aforementioned, and a full understanding of the construction and operation of such a switch can be obtained by reading GB 2212664A. For the present purposes it is sufficient to describe the switch as comprising a plastics moulding 10 defining spaced- apart mountings for an overcentre arrangement consisting of a bimetal blade 11 and a trip lever 12,

the bimetal blade 11 at its upper end (as viewed in Fig. 2) being retained in a V-groove formed in a metal spring 13 which is retained in the moulding 10 and at its lower end being retained in a V-groove formed in the upper end of the trip lever 12, and the lower end of the trip lever 12 being pivotally mounted within a V-groove formed in the moulding 10 as is described in British Patent Specification No. GB 2235257A. As is fully explained in GB 2212664A aforementioned, the bimetal 11 and trip lever 12 constitute a bridge which is movable overcentre between two stable positions defined by abutments provided in the moulding, and an extension of the trip lever 12 projects into a switch compartment of the moulding within which there is mounted a set of switch contacts arranged to be opened and closed in dependence upon the condition of the overcentre bridge. A further portion 14 of the trip lever 12 serves as a convenient mounting for an on/off knob which not only provides for manual operation of the switch but also provides an indication of the status of the switch.

Also shown in Fig. 2 is a heating element 4 which can take any convenient form, but may for example comprise a thin film resistor which may be mounted in a moulding (not shown) adapted to be fitted to the main switch moulding 10 and advantageously

incorporating spring terminals whereby the resistor can be readily changed for one of a different heat output. In addition to the facility to select the thermal output of the heating element, it may also be arranged that the physical relationship between the heating element and the thermal actuator of the switch (i.e. the bimetal) is adjustable.

Fig. 3 shows the switch of Fig. 2 viewed from the opposite side and illustrates an exemplary form of carrier 20 adapted to receive therein a holder 25 for a thin film resistor and to be fitted onto the top of the switch (as viewed in the figures) so as to place the thin film resistor adjacent to the bimetal blade

11. The carrier 20 is constituted by a simple plastics moulding having a recess 21 for receiving the holder

25 and is configured to fit over the top of the switch as shown in Fig. 4. As mentioned hereinabove, the holder 25 has spring terminals (not shown) for holding the thin film resistor, and these connect to terminal pins 26 for connection to electrical leads 27.

Accordingly, there has been described in the foregoing a thermally-responsive control in which a thermally-responsive electrical switch is arranged at a location remote from the thermal source which is to be responsible for switch operation, a heating element which is controlled in dependence upon the condition

of the thermal source being arranged to simulate to the thermally-responsive switch actuator the temperature change to which the control is required to respond. The thermally-responsive electrical switch is capable of manual actuation, and a component of the switch provides a clear indication by virtue of its movement, position or state of the status of the switch contacts. There is a time delay between energization of the heating element and operation of the switch which may be substantially predetermined, and the duration of this time delay may be adjusted by setting the temperature at which the switch operates (for example by selection of the bimetal and/or of its biassing conditions in the described Jl switch) , and/or by selecting the power output of the heating element, and/or by selecting the heat transfer relationship between the heating element and the thermally-responsive switch actuator.

The control as described has useful application in a variety of fields, as has been mentioned hereinbefore, and is of course susceptible to modification and variation of the arrangement specifically described. For example, whereas in the arrangement of Fig. 1 the proposal is to employ bimetallic switches as the thermal cut-outs 2, it is conceivable that for some applications elements having

a positive temperature coefficient of resistance (PTC elements) could be used as thermal cut-outs, such elements exhibiting a dramatic increase in electrical resistance above a predetermined temperature. Referring now to Figs. 5A, 5B and 5C these illustrate the utilization of a switch as described in our British Patent Application No. 9208264.3 (Serial No. ) in an arrangement according to the present invention. The switch described in GB 9208264.3 is a modified form of the Jl switch described in GB 2212664A with reference to Figs. 3A to 3M or Figs. 9A and 9B thereof, and as compared to the basic Jl switch this modification has an additional pair of switch contacts and a manually-operable selector which cooperates with the Jl trip lever and interacts furthermore with the additional switch contacts so as to accommodate the normal ON-OFF movements of the trip lever and additionally be capable of opening and closing the additional switch contacts. The manually- operable selector of the thus modified Jl switch as described in GB 9208264.3 (Serial No. ) is configured as a rocker switch having a central position in which one of the associated pairs of switch contacts is open irrespective of the condition of the other pair, a first alternative position in which only the abovementioned one of the pairs of

switch contacts is closed while the other is open, and a second alternative position in which both pairs of switch contacts are closed, and these three positions are illustrated in Figs. 5A, 5B and 5C respectively where the abovementioned one pair of switch contacts is designated 51 and the other pair is designated 52. With the manually-operable selector in either of its non-central positions, the application of heat to the bimetallic switch actuator will cause the switch to revert to its central position whereat switch 51 is open circuit irrespective of the condition of switch 52.

In application of the modified Jl switch of GB 9208264.3 to the present invention, the two switches 51 and 52 can be connected so as to determine the power supplies to two heating elements, for example, so that neither, only one or both of the elements are powered depending on the condition of the switch. In Figs. 5A, 5B and 5C the two heating elements are designated 53 and 54 and might, for example, be differently powered heating elements of a laundry drier or a room heater, or might be boil and simmer elements of a beverage making machine. In accordance with the teachings of the present invention, such an appliance might be thermally protected by provision of one or more thermal cut-outs or PTC elements connected

in series with the switch 51 (say, between the locations X and Y indicated in Figs. 5A, 5B and 5C) and with a thin film resistor or other heater connected in parallel with the cut-out(s) in the manner indicated in Fig. 1 and arranged in heat transfer relationship with the bimetallic actuator of the switch. As with the arrangement in Fig. 1, the result would be obtained that, in operation of the appliance, irrespective of whether only one or both of the heating elements 53, 54 was powered, the development of a hot-spot in the appliance such as to cause an associated cut-out to go open circuit or an associated PTC element to go high resistance would cause the switch 51 to be rendered open circuit as a result of heating of the bimetallic actuator by heat developed in the thin film resistor.