Provisional Application No. 60/119, 704, filed February 11,1999.

The present invention relates to paving screeds used with paving vehicles, and more particularly to heating systems for such paving screeds.

The quality of an asphalt mat is affected by the temperature of the asphalt material during the paving process. One method for ensuring that the asphalt is at a sufficiently high temperature is to use a heated screed, such that thermal energy (i. e., heat) is transferred from the screed to the asphalt while the material is being leveled by the screed. Systems for heating a paving screed include one or more heaters located inside the housing of the screed and configured to transfer thermal energy to the screed plate (the portion of the screed that actually levels the asphalt).

Such heating systems include gas burners, usually for diesel or propane gas, in combination with"blower"fans, electrical resistance heaters, etc.

In general, the control of such screed heating systems is merely a simple"on-off"switch that requires the screed operator to start the heating system and then the system operates until such time as the operator decides to shut the system off. If the operator does not properly monitor the temperature of the screed,

excessive heating of the screed, causing poor asphalt mat finish or damage to the screed components. Further, if the screed operator shuts down the heating system and then forgets to re-start system, the quality of the asphalt mat is diminished due to asphalt particles adhering to the"cold"screed.

Therefore, it would be desirable to have an automatically regulated heating system that operates safely and ensures that the heating system does not over- heat or insufficiently heat the screed.

SUMMARY OF THE INVENTION In one aspect, the present invention is an automatic heating system for a screed assembly of a paving vehicle, the screed assembly including a screed plate. The heating system comprises an electric heater connected with the screed assembly and having at least one heating element disposed proximal to the screed plate. The heater is configured to apply thermal energy to the screed plate when current flows through the heating element. An electric power supply is mounted on either the vehicle or the screed assembly. The power supply is connected with the heater and configured to generate electrical current flow through the heating element.

Further, a control system is connected with the screed assembly and electrically connected with the heater. The control system is configured to automatically adjust current flow through the heating element so as to regulate temperature of the screed plate.

In another aspect, the present invention is also an automatic heating system for a screed assembly of a paving vehicle, the screed assembly including a screed plate. The heating system comprises an electric heater connected with the screed assembly and configured to transfer thermal energy to the screed plate. A temperature sensor is connected with the screed assembly and is configured to sense temperature of the screed plate and to generate electrical signals proportional to sensed temperature. A current actuator is operably connected with the heater and configured to adjust thermal output of the heater. Further, a microprocessor is electrically connected with the sensor and with the actuator and has a programmable memory circuit configured to automatically operate the actuator so as to regulate sensed temperature of the screed plate.

BRIEF DESCRIPTION OF THE DRAWING FIGURES The description of the invention below will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, which are diagrammatic, embodiments that are presently preferred.

It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: Fig. 1 is a schematic view of an automatically regulated control system in accordance with the present invention; Fig. 2 is a plan view of a paving screed showing portions of the heating system;

Fig. 3 is a schematic diagram of an actuator assembly box; Fig. 4 is a plan view of a circuit board and attached components, showing electrical connection paths between control system components; and Fig. 5 is a plan view of a controller housing.

DETAILED DESCRIPTION Referring now to the drawings in detail, wherein like numbers are used to indicate like elements throughout, there is shown in Figs. 1-5 a presently preferred embodiment of an automatic heating system 10 for a screed assembly 1 of a paving vehicle (not shown).

The screed assembly 1 includes at least one screed plate 5 and preferably a plurality of screed plates 5, as discussed below. The heating system 10 comprises an electric heater 12 mounted on the screed assembly 1 and having a heating element 13 disposed proximal to the screed plate 5. The heating element 13 is configured to apply thermal energy (i. e., heat) to the screed plate 5 when current flows through the heating element 13.

Preferably, the heating system 10 includes a plurality of heaters 12, one for each screed plate 5, as discussed below. An electric power supply 14 is mounted on either the paving vehicle (not shown) or the screed assembly 1 and is electrically connected with each heater 12. The power supply 14 is configured to generate electrical current flow through the heating element 13 of the heater 12.

Further, a control system 15 is mounted on the screed assembly 1 or the paving vehicle (not shown) and

is electrically connected with each electric heater 12.

The control system 15 is configured to automatically adjust current flow through the heating element 13 of each heater 12 so as to regulate temperature of the screed plate 5. More specifically, the control system 15 monitors the temperature of each screed plate 5 and automatically adjusts the electric current flowing through the associated (i. e., proximal) heating element 13 so as to maintain the temperature of the screed plate 5 either at about a desired temperature value or within about a desired range of temperature values ("temperature band"). Each of the above-described elements of the automatic heating system 10 is described in further detail below.

Referring to Fig. 2, the heating system 10 is preferably used with a screed assembly 1 that includes a main screed having left and right screed halves 2A, 2B and left and right extendible screeds 4A, 4B, each movably connected with a proximal half 2A, 2B, respectively, of the main screed. Each screed section 2A, 2B, 4A, 4B includes a screed plate 5 having a lower surface 5b used for leveling of paving material and an upper surface 5a to which certain portions of the heater 12 are attached, as described below. Thermal energy is transferred from the heating element 13 of each heater 12 to the proximal plate 5, and is then generally transferred through the plate 5 to paving material contacting the lower surface 5b (Fig. 2) of the plate 5.

However, the heating system 10 of the present invention may be utilized with a screed assembly 1 of any

other appropriate structure. For example, the main screed may have"bolt-on"extensions fixedly mounted to the main screed, the screed assembly 1 may have only the two screed halves 2 without any screed extensions or the screed assembly 1 may be a main screed may be single frame member and screed plate as opposed to two movable screed halves (none depicted). It is within the scope of the present invention to use the heating system 10 with any type of paving screed, with any appropriate modifications necessary to adapt the automatic heater system 10 to particular paving screed assembly 1.

Referring specifically to Fig. 1, the electric heater 12 is preferably an electrical resistance heater 19, such that the heating element 13 is the electrical resistor (s) 21 of the resistance heater 19. As the preferred screed assembly 1 includes four screed sections 2A, 2B, 4A and 4C, the automatic heating system 10 preferably includes four resistance heaters 19 each having an electrical resistor 21 disposed proximal to a separate one of the screed plates 5, and most preferably disposed on the upper surface 5a of each plate 5. All four of the resistance heaters 19 are electrically connected with the electric power supply 14, such that the power supply 14 generates current flow separately through each resistor 21, and are connected with the control system 15, as discussed below. Preferably, each heater 19 is capable of heating the associated screed plate 5 to at least within a range of about two hundred twenty-five (225) degrees Fahrenheit ("°F") and about four hundred seventy-five (475) °F.

As is well known, the electrical resistance heaters 19 basically function by passing electrical current through the resistor 21, typically provided by one or more non-insulated wires formed of a material having a high electrical resistance, such that significant RI electrical energy losses occur and generate substantial thermal energy within the resistor 19. At least a portion of the thermal energy in the resistor 21 is transferred from the resistor 21, preferably by conduction, to the screed plate 5 such that the temperature of the screed plate 5 is either increased or at least maintained about a particular desired temperature or within a desired range of temperatures.

Preferably, the resistor 21 of each resistance heater 19 is formed of a grid of wires disposed or "embedded"within a quantity of a thermally conductive material 20 so as to form an electric heating pad 22. By having the resistors 21 formed into a heating pad 22, thermal energy or heat is dispersed through the pad material 20, which is formed to have a relatively wide, continuous surface area in contact with the plate 5, so that thermal energy is more evenly and effectively distributed to the associated screed plate 5. Most preferably, the heating pads 22 are each a commercially- available heating pad manufactured by Watlow Electric Manufacturing Co. of St. Louis, Missouri, USA and marketed as a"Silicon Rubber Flexible Heater with Wound Wire Element"product.

As best shown in Fig 2, a separate heating pad 22 is disposed on the upper surface 5a of the screed plate 5 of

each of the four screed sections 2A, 2B, 4A and 4B. When current generated by the power supply 14 flows through each of the heating pads 22, thermal energy generated by the resistor 21 is conducted through the surrounding conductive pad material 20 to the upper surface 5a of the associated screed plate 5. Thus, the temperature of each screed plate 5 is increased (or maintained) by the transfer of thermal energy from each resistor 21 within the pad 22 disposed on the upper surface 5a of the screed plate 5. However, the resistors 21 of the electrical resistance heaters 19 may alternatively be formed as a one or more separate wires, or a grid of such wires, directly attached to the plate 5 and without any pad material 20 (not depicted).

Furthermore, it is within the scope of the present invention to construct the electric heaters 12 as any other type of electrical heater device. For example, the heaters 12 may be an induction heating device with an appropriately constructed inductor coil providing the heating element 13 and disposed proximal to the screed plate 12 (not shown). With an induction heating device, current flowing through the coil will generates eddy currents in the screed plate 5, which due to the resistance of the material of the screed plate 5, will directly generate heat in the scree plate 5 itself (as opposed to being conducted to the plate 5). The automatic heating system 10 of the present invention embraces any appropriate electric heating device for the electric heater 12.

Referring to Fig. 1, the electric power supply 14 is a preferably a commercially available hydraulic generator 24 mounted on the screed assembly 1 or elsewhere on the paving vehicle (e. g., the chassis) (not shown) and is most preferably of the type commonly referred to as a "Gen Set". Preferably, the generator 24 is configured to generate alternating current at a high voltage, most preferably of at least two hundred forty (240) volts alternating current ("VAC"). Further, the generator 24 is preferably driven by a hydraulic pump in a conventional hydraulic circuit (neither shown) that is typically included on known paving vehicles.

A hydraulic generator 24 is preferred for the power supply 14 due to the large amount of thermal energy required to sufficiently heat all four of the screed plates 5 in the preferred application. More specifically, with the preferred screed structure having four separate screed sections 2A, 2B, 4A, and 4B, the screed sections collectively comprise a significant mass of material. As such, the 12-volt or 24-volt electrical systems typically provided on known paving vehicles do not have sufficient electrical power to generate the thermal energy required to heat several screed plates 5.

However, it is within the scope of the present invention to use any other type of electrical power supply, such as for example the 12-volt or 24 volt systems incorporated in a standard paving vehicle, particularly if the screed plate mass is less substantial, or a separate gas-powered generator (neither shown) as long as the power supply 14 is capable of

generating sufficient electrical current through the heaters 12 necessary to heat the mass of the screed plates 5.

Referring again to Figs. 1-5, the control system 15 basically includes at least one temperature sensor 26, at least one electric current actuator 28 and an electronic controller 16. The temperature sensor 26 is connected with the screed assembly 1 and is configured to sense or measure the temperature of the screed plate 5. The current actuator 28 is electrically connected with the heater 12 and is configured to adjust current flowing through the heating element 13. Further, the controller 16 has an electrical logic circuit 17 electrically connected with the temperature sensor 26 and with the current actuator 28. The controller 16 is configured to operate the current actuator 28 in accordance with sensed temperatures of the screed plate 5, i. e., as measured by the sensor 26 and inputted into the controller 16, as discussed in further detail below.

Preferably, the logic circuit 17 of the controller 16 is provided by a microprocessor 23. The microprocessor 23 has at least one input channel 25 electrically connected with the temperature sensor 26 and at least one output channel 27 electrically connected with the current actuator 28. Further, the microprocessor 23 has a programmable memory circuit programmed or otherwise configured to analyze input signals from the temperature sensor 26 and to generate and transmit a control signals to the actuator 28 in order to adjust current flow through the heating element

13 according to the sensed temperature (i. e., by the sensor 26) of the screed plate 5. The microprocessor 23 thereby adjusts the thermal output of the heaters 12, as discussed in further detail below.

Preferably, the microprocessor 23 has at least four input channels 25 (one indicated) and four output channels 27 (one indicated). Each input channel 27 is electrically connected with a separate one of four temperature sensors 26 through a path on a printed circuit board 45, as discussed below. Further, each output channel is electrically connected with a separate one of four current actuators 28 through the circuit board 45, as also discussed below. The programmable memory circuit (not indicated) of the microprocessor 23 is configured to separately analyze input signals from each temperature sensor 26 and to separately generate and transmit control signals to independently operate each actuator 28, such that the temperature of each. screed plate 5 is separately and independently regulated, as discussed below.

Preferably, the microprocessor 23 is a commercially available microprocessor, most preferably a model# PIC16C74 Microcontroller manufactured by Microchip Technology, Inc. of Phoenix, Arizona, USA. Although a microprocessor 23 is preferred, the logic circuit 17 may alternatively be configured as any other appropriate type of electrical logic circuit, either digital or analog, integrated or comprised of discrete electronic components, as long as the controller 16 is capable of functioning as described in the present application.

Referring to Fig. 5, the controller 16 preferably includes a housing 30, the microprocessor 23 being disposed within the housing 30. Further, the controller 16 includes appropriately configured input and output ports (not depicted) disposed on the housing 30 and electrically connected with separate input and output channels 25,27, respectively, of the microprocessor 23, which provide readily disconnectable means to connect the temperature sensors 26 and actuators with the microprocessor 23.

Further, the controller 16 also preferably includes means for inputting information into the programmable electric memory circuit of the microprocessor 23, such as one or more buttons 34 each connected with an input channel of microprocessor 23.

Further, the electronic controller 16 also preferably includes a display screen 36 for displaying information such as controller settings and any information being entered into the controller 16 by the operator. The buttons 34 and the display 36 are mounted to the housing 30 and are of a type generally known in the electronic arts. Using the input buttons 34, the operator may enter any desired temperature or temperature band into the microprocessor 23. Furthermore, the controller 16 also preferably includes a visual indicator (not shown), such as an ordinary indicator light-bulb, mounted to the controller housing 30 and connected with the generator 24. Such an indicator is configured to produce different visual signals, such as turning lights on and off or using different colored lights, to enable

the paver operator to readily determine whether or not the generator 24 is operational (i. e., generating voltage) simply by viewing the indicator.

Referring to Fig. 1, the control system 15 preferably includes four temperature sensors 26, each sensor 26 being electrically connected with a separate channel of the microprocessor 23. Each temperature sensor 26 has a sensing element 26a disposed proximal to a separate one of the screed plates 5 of the four screed sections 2A, 2B, 4A, and 4B. The temperature sensors 26 each measure or sense the temperature of the screed plate 5 where the sensing element 26a is proximally located and transmits a signal proportional to the screed plate temperature to a separate channel of the microprocessor 23.

Preferably, the temperature sensors 26 are each an electrical temperature transducer, most preferably a thermocouple 29 with a sensing junction 29a disposed on the upper surface 5a of the screed plate 5. The thermocouple junction 29a generates voltage signals proportional to the temperature of the screed plate 5, which is sent to the microprocessor 23 for comparison with a desired temperature value or temperature band.

The thermocouples 29 are preferably commercially- available thermocouple 29, although specially made thermocouples 29 may alternatively be used.

Referring to Figs. 1 and 3, the control system 15 preferably includes four electrical current actuators 28.

Each actuator 28 is electrically connected with the

heating element 13 of a separate heater 12, i. e., resistor 21 of one of the heating pads 22, with a separate channel of the microprocessor 23 and with the generator 24. The actuators 28 each adjust the current flowing through the resistor 21 of the associated heating pad 22 to thereby adjust the flow of thermal energy to the screed plate 5 on which the heating pad 22 is disposed.

Preferably, each actuator 28 is an electric switching device, most preferably a thyristor or solid- state relay ("SSR") 31. A preferred SSR 31 is a part# 2440D manufactured by Crydom Corp. of San Diego, California, USA. Each SSR 31 is electrically connected with a separate one of the channels of the microprocessor 23, such that the microprocessor 23 sends separate signals to each SSR 31 independently of the other SSRs 31. Thereby, each SSR 31 independently adjusts current flowing through the associated heating pad 22 when necessary to increase or decrease the temperature of the screed plate 5 on which the heating pad 22 is disposed.

In this manner, the microprocessor 23 separately operates each SSR 31 to independently maintain the temperature of each of the four screed plates 5 either about a desired temperature or within about a desired range of temperature values (the"temperature band"), as discussed in further detail below.

More specifically, when the temperature of a screed plate 5 is above a desired temperature or a maximum value of a temperature band, the microprocessor 23 sends a first control signal to SSR 31 connected with the heating

pad 22 on the particular plate 5, such that the SSR 31 switches to an open state. By switching to an open state, the SSR 31 interrupts or prevents current flow through the resistor 21 of the connected heating pad 22 and thus disrupts the flow of thermal energy or heat to the screed plate 5. Alternatively, when the screed plate temperature is below the desired temperature or a minimum band temperature, the microprocessor 23 sends a different or second control signal to the associated SSR 31 such that the SSR 31 switches to a closed state to permit current flow through the heating pad 22, thus initiating and/or increasing the flow of thermal energy to the screed plate 5.

Referring specifically to Fig. 3, the four SSRs 31 of the heater system 10 are preferably contained within a housing 46 to form an actuator assembly box 33 configured to be disconnectably engaged with the electronic controller 16, with the power supply 14 and with each of the heating pads 22 to disconnectably electrically connect the SSRs 31 with the controller 16, the power supply 14 and the four heating pads 22. More specifically, the housing 46 includes a generator input socket 47, a controller input socket 49 and four heater output sockets 51, as discussed below. The four SSRs 31 are disposed within the housing 46 and are preferably coupled in two pairs to form two double-switch units 35.

Further, each SSR 31 is connected with a separate one of four electro-mechanical contactors 48 mounted on a printed circuit board ("PCB") 45 disposed within the housing 46.

The controller input socket 47 includes four leads 47a separately connectable with the output channels 25 of the microprocessor 23 by means of a plug 60 connected with the microprocessor 23. The controller leads 47a are each connected to a separate one of the SSRs 31 through the PCB 45 and through the connected contactor 48.

Further, the generator input socket 49 is mounted on the housing 30 and is connectable with the generator 24. The generator socket 49 is electrically connected to the PCB 45, such that the generator 24 is electrically connected with each of the four SSRs 31 through circuit paths (not indicated) on the PCB 45 and the associated contactor 48.

Furthermore, four heater output sockets 51 disposed on the housing 30 are each connectable with a separate one of the heaters 12 (heating pads 22) by means of separate plugs 64 and are electrically connected with a separate one of the SSRs 31 by means of wires 78. In addition, the microprocessor 23 is connected with each thermocouple 29 by a separate circuit path on the PCB 45 (not indicated) between certain contact 76-lead 47a pairs of the controller socket 47 and a separate contact 72-lead wire 70 pair of one of the heater sockets 51, with the heater plugs 64 also connecting thermocouple lines 29b to the actuator assembly box 33.

The above-recited contactors 48 are each configured to close when the generator 24 is electrically connected with SSRs 31 and to thereafter open to interrupt current flow to the SSR 31 if the SSR 31 remains idle for a certain time interval, preferably an interval of about ten minutes. The contactors 48 therefore prevent overheating of each of the screed plates 5 in the event

that the SSRs 31 controlling current flow to the heater 12 on a particular plate 5 becomes short-circuited and continues to supply current to the resistor 21 without regard to plate temperature.

By being disposed within a housing 30 having sockets 47,49 and 51, the actuator assembly box 33 is readily connectable and disconnectable with the microprocessor 23, the heating pads 22 and the generator 24. Therefore, when one of more SSRs 31 become damaged after a period of use, replacement of the damaged actuators 28 is performed simply by removing plugs 60,62 and 64 connected with the microprocessor 23, the generator 24 and the four heating pads 22, respectively, from the housing sockets 47,49 and 51, respectively, removing the damaged actuator assembly box 33 and replacing it with another actuator assembly box 33.

Although SSRs 31 are preferred for the actuators 28, it is within the scope of the present invention to use any other type of current actuator controllable by a logic circuit 17 and capable of adjusting current flow through an electrical heater 12. For example, the actuators 28 may be another type of electric switching device, such as an electro-mechanical contactor switch (i. e., contactor) (not shown), configured to adjust thermal output of the heating pads 22 by alternatively permitting and interrupting current flow through the pads 22 as necessary to control screed plate temperature.

Further for example, the actuators 28 may each be provided by a variable resistance type of current actuator, such as a rheostat (not shown), whereby the

actuator 28 does not alternatively interrupt and permit current flow through a heater 12, but rather varies the magnitude or"strength" (i. e., amperage) of the current flowing through the resistor 21 by controlled variation of the actuator resistance.

As yet another example, the actuators 28 may be another electrically controllable device, such a solenoid valve (not shown) controlling hydraulic flow through the generator 24. With such a solenoid valve, either the rotational speed of the generator 24 may be varied to adjust the voltage output of the generator 24 and thereby adjust the amperage of the current flowing through the heaters 12, or alternatively may be operated to turn the generator 24 on and/or off to respectively permit and interrupt current flow through the electric heaters 12.

The automatic heating system 10 of the present invention embraces these and any other alternative structures and/or configurations of the actuators 28 that are operable by the electronic controller 16 to adjust current flow through the heaters 12 (heating pads 22).

Referring to Fig. 5, the control system 15 preferably further includes a power"on/off"electrical switch 38 electrically connected with the generator 24 so as to alternately start and stop operation of the generator 24. The switch 38 may be mounted to the electronic controller 16 (as shown) or at another appropriate location on the screed assembly 1, such as directly on the housing of the generator 24. The power- on switch 38 is connected with an electrically-actuated valve, preferably a solenoid valve (not shown), that

regulates the flow of hydraulic fluid to the generator 24, although the controller 16 may include any other known, appropriate means for starting and/or stopping the functioning of the generator 24.

Referring generally to Figs. 1-5, in operation of the heating system 10, the microprocessor 23 receives electrical input signals from each thermocouple 29, each signal being proportional to the temperature of the associated screed plate 5 as measured by the thermocouple 29. The microprocessor 23 compares the input signal to a desired temperature value or the values of the temperature band as stored in the programmable memory of the microprocessor 23. If the measured temperature is less than the desired temperature or below the temperature band, the microprocessor 23 sends a control signal to the particular SSR 31 such that the SSR 31 increases the current flowing from the generator 24 to the heating pad 22 to increase the temperature of the associated screed plate 5. If the measured temperature of a screed plate 5 is at or above a desired temperature or within or above the desired temperature band, the microprocessor 23 does not send any signal to the SSR 31, such that the SSR 31 then prevents current from flowing to the associated heating pad 22. Without current, the heating pad 22 no longer generates and transfers heat to the associated screed plate 5, such that heat within the screed plate 5 dissipates, causing the temperature of the plate 5 to decrease.

Electric current is disrupted or prevented from flowing to the particular heating pad 22 (i. e., by the connected SSR 31) until such time as the screed plate

temperature (as measured by thermocouple 29) drops sufficiently below the desired value/minimum band value, at which time the microprocessor 23 sends a signal to the SSR 31 to enable current flow through the heating pad 22.

The microprocessor 23 controls each of the four heating pads 22 independently of the other pads 22 such that each heating pad 22 is cycled on and off as needed. Thus, each screed plate 5 of the four screed sections 2A, 2B, 4A and 4B are each independently maintained at a desired temperature or within a desired temperature band.

Further, the controller 16, specifically microprocessor 23, may be configured to maintain the plates 5 of each of the four screed sections 2A, 2B, 4A and 4B at a particular temperature or temperature band that is greater or lesser than that the temperature/temperature band of the other four screed sections. For example, the screed plates 5 of the screed extensions 4A, 4B may each be maintained at one temperature, for example two hundred ninety (290) °F, and the screed plates 5 of the main screed halves 2A, 2B may be maintained at a different temperature, such as two hundred seventy-five (275) °F. Further for example, the four screed plates may each be maintained at a different screed temperature so that a varying temperature profile or gradient of temperatures may exist across the width of the screed assembly 1. It is within the scope of the present invention to configure or program the microprocessor 23 to maintain any desired temperature profile of two or more screed plates 5 with an automatic heating system 10 used with a screed assembly 1 having a plurality of screed plates 5.

Alternatively, the controller 16 may be configured to operate such that current is always flowing to the four heating pads 22, but is increased or decreased according to the temperature of the associated screed plate 5. In other words, the preferred microprocessor 23 may alternatively be configured/programmed so that when the measured temperature of a plate 5 is greater than the desired value/maximum band temperature, the microprocessor 23 sends a control signal to the actuator 18 to decrease, but not disrupt, the current flow through the resistance heaters 19 to thereby reduce screed plate temperature. Further, the microprocessor 23 is configured/programmed to send another control signal to the actuator 18 to alternatively increase current flow through the resistors 21 when the screed plate temperature is below a desired temperature/minimum band temperature.

However, with such a configuration of the controller 16, the actuator 18 must be of a type other than an SSR or other electrical switching device, for example a variable resistor electrically connected with the heating pad 22 or a valve controlling hydraulic flow through the generator (as described above). This is due to the fact that switching devices (e. g., an SSR) function only to alternatively permit or interrupt current flow through an electrical circuit and not to adjust voltage or resistance, as is well known.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described

above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.