Abstract: A foil element for the electric heating of objects comprises a resistance heating coil placed between insulating plastic foils and a sensor coil placed between the plastic foils beside the heating coil. The sensor and heating coils are formed of the same metallic material. The sensor coil is spaced sufficiently far from the heating coil to assure that the sensor coil is heated mainly by heat flowing to or from the object being heated and only slightly by heat directly from the heating coil. The electric resistance of the sensor coil is sensed to determine the temperature thereof. The heating coil is controlled in accordance with the temperature of the sensor coil.

Abstract: A power control switching arrangement for controlling energization of a load in response to an external controller, in which a first semiconductor switch is connected in parallel with the series combination of a first normally open thermally actuated switch and a second normally closed thermally actuated switch, to couple the power signal to the load. A first heater for the first thermal switch is connected in parallel with the load. A second heater for the second thermal switch is connected in series with a second semiconductor switch, across the power supply. The second semiconductor switch, when switched into conduction by the controller, enables energization of the second heater. To energize the load, the controller switches the first semiconductor switch into conduction enabling energization of the load and the first heater. Energization of the first heater closes the first thermally actuated switch, thereby shunting the first semiconductor switch.

Abstract: A temperature regulating control unit and method of making the same are provided, the unit comprising a RTD temperature sensor, a unit for applying electrical signals to the sensor, and a microcomputer for receiving digital signals from the sensor in relation to the temperature being sensed by the sensor, the unit for applying electrical signals to the sensor applying a varying voltage to the sensor.

Abstract: A programmable heater which can be electronically programmed to provide an intended heat profile and intended heater power for application of heat only to an intended area. The heater is especially adapted for use in electronic circuit board repair systems. The heater comprises a matrix of electrical heater elements arranged in row and column configuration, each element being disposed within a respective tube terminating at a nozzle plane. Individual heater elements are selectively energized and the power applied to the heating elements is controlled to provide an intended heat level. Gas or air is supplied to the heater housing and flows through the tubes and out of the nozzle openings. Only the gas flowing through the tubes containing the energized heating elements is heated to provide a selected heat profile for application to a work surface.

Abstract: Method and device for supplying electric current to ceramic heaters comprising heating resistors embedded in ceramic sintered bodies, being characterized in that direct current is supplied from a direct current supply circuit as the first current supply step and the direction of the direct current supplied to the heating resistors at the first current supply step is reversed by a switching circuit or alternate current is supplied as the second current supply step in order to eliminate or considerably reduce cracking of the ceramic sintered bodies of the ceramic heaters when the heaters are repeatedly used for a long time at high temperatures.

Abstract: In an oven calibration system, a value is stored corresponding to a predetermined temperature, and a calibration value is also stored corresponding to an offset. The calibration value is variable. A temperature sensor produces an output that is compared with a predetermined combination of the stored values. The calibration value may include a fixed increment and a variable multiplier for combination with the fixed increment.

Abstract: A temperature controller and alarm device for use with a cartridge heater of the general type providing heat within a mold in a plastic injection molding apparatus includes an amplifier for sensing voltage signals developed by a thermocouple within the cartridge and which signal is representative of the temperature produced by the heater. The heater temperature is compared to a desired temperature and a signal proportional to the difference in temperature is coupled to a comparator which produces an output signal when the magnitude of a triangular signal exceeds the magnitude of the difference signal to cause the comparator to operate switches which connect and disconnect a DELTA configured three-phase voltage source to and from both sides of the heating element. Visual and audible alarms are provided to indicate the occurrence of a thermocouple open circuit fault condition, a heating element open circuit fault condition and a heating element short circuit fault condition.

Abstract: A temperature control apparatus for controlling, e.g., a fixing heater of a copying machine has a fuse resistor connected to the fixing heater through a power relay, a thermistor for detecting the surface temperature of a fixing roller heated by the fixing heater, abnormality timers for counting the ON time of the fixing heater and having different preset counts corresponding to different modes of the copying machine, and a control circuit for disconnecting the fuse resistor when the output from the thermistor indicates overheating or the ON time of the heater exceeds the preset count of the timer. Overheating of the heater is prevented with high reliability.

Abstract: A power control system for operating a heating element in a cooking appliance in response to the selection of one of a plurality of user selectable power settings. Approximate heating element temperature information is provided by a heater energy counter. Each power setting has associated with it a corresponding steady state power level and a corresponding predetermined steady state energy count. The steady state energy count is approximately proportional to the steady state operating temperature of the heating element when operated at its corresponding steady state power level. The energy counter is selectively incremented and decremented to approximately track the heating element temperature when the count is respectively less than or greater than the steady state count for the applied power level. At least certain ones of the power settings have corresponding minimum threshold counts less than the steady state counts and maximum threshold counts greater than the steady state counts.

Abstract: Integral sensor controller for an electrical resistance heater, where the heater is constructed from materials such as nickel, balco, platinum, alumel, or like materials which have an appreciable, positive temperature coefficient of resistivity. The resistance versus temperature characteristic of the heater acts as the temperature sensor. A low level D.C. current provides a sensor voltage which is compared to a set point voltage for switching the heater power through a transistor. The relationship of the sensor voltage to the set point voltage is compared by a comparator which is subsequently used to toggle flip flops for switching of the heater power. Circuitry is provided for protection against heater short circuits.

Abstract: A microcomputer control system for a furnace is disclosed which provides accurate control of the temperature of the furnace such that the temperature can be quickly increased to a predetermined desired temperature without overshooting the desired temperature. The control system determines the tuning constants of the furnace as a function of the system's response to a predetermined input and, thereafter, uses the determined tuning constants to accurately control the temperature of the furnace. The control system employs a novel method for increasing the temperature of a furnace wherein full power is applied to the furnace heat source until the microcomputer makes the determination, based upon the furnace tuning constants, that power supplied to the furnace must be reduced to prevent the temperature of the furnace from overshooting the desired temperature.

Abstract: An electric power control device is composed of plural resistor loads (1.sub.0 -1.sub.4) the heat-generating powers of which are different from one another, switch means (3.sub.0 -3.sub.4) connected in series to each of the resistor loads and adapted for the ON-OFF control of an ac power (2), and means (4.sub.0 -4.sub.4) for ON-OFF controlling the switch means so as to turn on or turn off the ac power and also for selectively controlling the individual resistor loads so as to adjust the heat-generating powers of all of the resistor loads in accordance with their corresponding desired heat-generating powers.

Abstract: A heating control apparatus for exciting the heating element of a kiln or the like. The heating control apparatus includes an interval timer that produces a pulse of electricity having a predetermined duration of time, a fixed timer adapted to open and close an electrical path, and a power relay connected to the heating element and which is adapted to excite the heating element for selected periods of time in response to the operation of the fixed and interval timers.

Abstract: In the case of an electric cooker with at least two electric hotplates having several heating resistors and controlled via separate power control devices, one hotplate has a high power additional heating resistor for an initial cooking operation. While intermediately connecting a temperature switch and an additional contact of the associated power control device, the additional heating resistor is connected to the electric mains via a further additional contact. The further additional contact is a component of the power control device of the other electric hotplate and is only closed if its power consumption drops below a predetermined switching value.

Abstract: An electronic controller for a predetermined temperature coefficient heater provides solid state control of the application of power to the heater. In particular, a positive temperature coefficient glow plug wherein the heater is plated on a ceramic substrate glow plug is controlled by contactless solid state controller. The temperature of the glow plug is sampled by a clock signal and compared with a predetermined temperature value. Pulsed power is supplied to elevate and maintain the temperature of the heater to the predetermined value. In a complete motor vehicle engine wherein one glow plug is used in each cylinder, one glow plug is sampled and controls the remainder of the plugs.

Abstract: An apparatus for controlling and adjusting heat output during the heating phase of a cooking vessel. The cooking vessel is heated by an electric heating element and the temperature inside the cooking vessel is measured. The heating element is provided with a control circuit and two program memories which operate as a function of the temperature increase characteristics during the simmer or warming cycles, whereby in the simmer cycle, both the first and second program memories are operative whereas in the warming cycle only the second program memory is operative in a similar manner as during the simmer cycle and over the same temperature differential but at different temperature values.

Abstract: A triac and a mechanical relay, in series with the electrical heater of an incubator, together switch heater current on and off at the same set point temperature if the triac fails short. Similarly, a second mechanical relay in parallel with the triac switches heater current around the triac, should the triac fail open.

Abstract: The heating device for an appliance for gluing sheet-like textile articles has several heating regions (23, 24), the electrical heaters (25) of which can be switched on or off individually. To achieve the best possible setting of the temperature of the heating regions, the individual heaters (25) can, in addition, be changed over to different power inputs, preferably between a series connection and a parallel connection. Also, the individual heaters (25) can each have two or more heating wires which can be changed over between a series connection and a parallel connection in each heater.

Abstract: The specification discloses a vehicle electrical control system capable of switching the alternator output between a uniform standard voltage and a uniform higher voltage. The control system includes a supplemental voltage regulator and a switching circuit for selectively switching the supplemental regulator into the system to regulate the conventional voltage regulator. Preferably, the control system further includes a timing circuit for powering a plurality of electrically heated windows in a timed sequence.

Abstract: A dual temperature curling iron has a plastic handle and a metal barrel connected thereto. An electric heater positioned within the metal barrel and is arranged to rapidly heat the barrel and is selectively controlled by either a low temperature thermostat or a high temperature thermostat located within the barrel to provide heat to the metal barrel. A hair clamp is pivotally mounted on the metal barrel, and when opened, causes a switch to generate an in-use signal which is fed to a safety timing circuit. The timing circuit is reset each time it receives the in-use signal. In the event that no further in-use signals are received, the timing circuit, after a preselected interval, provides a signal to a silicon controlled rectifier which is connected to the electric heater causing the silicon controlled rectifier to interrupt electric power to the electric heater. The curling iron is provided with a visual signal for indicating that has reached the selected temperature and is ready for use.

Abstract: A dual stage thermostat for an electrical space heating system in which there are two microswitches connected in parallel and operated by a common bimetallic strip. The first switch when closed connects a heating element to a power supply through a diode which reduces by half the power applied to the element. The second switch when closed connects the heating element directly to the power supply. The temperature at which the second switch opens is set about 1.5.degree. F. lower than that at which the first switch opens. Thus, as the room temperature approaches the desired temperature, the heater element is switched from full to half power, producing less temperature overshoot and saving energy. The thermostat may be used with any standard two wire heater.

Abstract: A control system and control device for controlling a heating unit and method of making the control device are provided, the control device having a rod and tube temperature sensing unit for providing linear motion of a part thereof along a substantially straight linear path in relation to the temperature being sensed by the temperature sensing unit and having a potentiometer unit provided with an electrical output producing unit operatively interconnected to the part of the temperature sensing unit so as to produce a variable electrical signal in relation to the temperature being sensed by the temperature sensing unit, the electrical output unit being movable along an arcuate linear path by the part of the temperature sensing unit. The potentiometer unit has a member provided with an arcuate surface, the electrical output unit being disposed in sliding engagement with the arcuate surface whereby the arcuate surface defines the arcuate path of movement of the electrical output unit.

Abstract: Power-processing unit uses AC buses (30, 32) to supply all current dependent needs such as connections (54, 56) to an ion thruster through an inductor (88) and the primary of a transformer (90), to assure limited currents to such loads. Where temperature control is also required, such as to the main discharge vaporizer heater connection (36, 38), switches (100, 102) are serially connected with inductor (96) and the primary of transformer (98). Temperature sensor (104) controls the switches (100, 102) for temperature regulation.

Abstract: A programmable multi-function feedback cooking apparatus senses the temperature of food substance or a liquid cooking inside a vessel. The analog voltage produced by a temperature transducer is translated into a digital signal and is used by a central processing means which automatically adjusts the amount of heat energy that is applied by the heat source to the cooking food substance. In preferred embodiments, the generating means is a conventional gas or electric stove, oven or microwave. The programmable electronic feedback cooking apparatus comprises a temperature transducer which is placed inside a cooking vessel, an analog-to-digital converter, a microprocessor unit, and a control panel. The apparatus includes in a preferred embodiment, a remote control feature whereby the operator may program the electronic feedback cooking apparatus from a remote location, using a telephone.

Abstract: A heating power source device includes a pair of loads connected to a secondary side of a power source transformer by way of four thyristor units. Each of the thyristor units includes pair of thyristors connected in anti-parallel relationship to each other and is controlled so as to connect the loads in parallel relationship to each other at the beginning of energization and then to connect the loads in serial relationship to each other after the resistance of the loads has been reduced below a predetermined level.

Abstract: An adaptive glow plug controller provides a tracking model temperature means for controlling the application of power to one or more glow plugs in response to the present temperature of the glow plugs. Several control circuits control the maximum time that the power is supplied to the glow plugs and the use of the glow plugs at various temperature levels of the environment and/or the engine.

Abstract: A temperature control apparatus has a first comparator circuit for comparing the voltage corresponding to thermal data provided by a thermal sensor with a first reference voltage generated in synchronism with the zero crossings of an AC power source. The first comparator circuit generates a pulsative output voltage which is synchronous with the zero crossings of the AC power source. This output voltage causes a second reference voltage to be applied to a second comparator circuit in the form of a pulsative voltage. The second comparator circuit compares the thermal data voltage with the second reference voltage and generates a pulse signal. This pulse signal drives a trigger circuit which triggers a thyristor connected to an electric heater.

Abstract: An electric apparatus involves a temperature sensor and a controlled circuit connected to an A.C. source to be sequentially controlled in accordance with the output from the temperature sensor. The control device comprises an auxiliary power source, a first control circuit for controlling the controlled circuit, and a second control circuit for supplying first control signals to the first control circuit, each being connected between control lines. The second control circuit involves an oscillator for producing clock pulses, a generator for generating reset pulses synchronized with the frequency of the A.C. source, a counter which counts the clock pulses and is reset upon receipt of the reset pulses, a control signal generator which processes an output signal from the temperature sensor to produce the first control signals, and circuit arrangements which, when a momentary power failure takes place in the A.C.

Abstract: In a circuit arrangement for controlling the operating temperature of the heating coil of an electric soldering iron, the resistance variation of the heating coil itself is to be evaluated at lowest possible cost of circuitry and doing away with a thermocouple sensor that used to be arranged in the region of the soldering bit or of the heating coil, at high mechanical cost. To this end, a precision resistor is inserted in the heating circuit and a TRIAC switching element connected between the latter and the heating element, which is switched open by a comparator circuit as a function of the voltage drop at the precision resistor or which lets through only the pulses of a pulse generator circuit required for measuring the switch-on threshold, which pulse generator circuit is synchronized via a null detection circuit for null passage of the AC voltage.

Abstract: A step controller for a water heater includes a plurality of individual electrical elements to maintain a predetermined minimum temperature in accordance with a sensed temperature signal and a preset desired temperature signal in combination with a dead band signal. The heating elements are turned "on" and "off" in a random sequence whenever a change is required, and in accordance with a binomial distribution so as to substantially equalize the operating period of the heating elements. The controller includes a microprocessor unit to periodically sense the signals of a sensor and a preset potentiometer, and also operates to store the on-off status of the heating elements, each of which is assigned a binary load indentifying number. The microprocessor includes a psuedo-random binary number generator for generating the load identifying numbers.

Abstract: An apparatus for monitoring the cooking process in a cooking vessel placed on an electric heating element, particularly a stovetop burner, comprising signal transmitter controlled by the temperature in the cooking vessel, a signal receiver in communication with the signal transmitter by a transmitter for activating a control circuit to adjust the heat output of the heating element, and a switch for turning the apparatus on and off. During the entire cooking operation and when the heating element is set at full heating capacity, a periodical surveillance is effected to determine whether the temperature varies, remains steady or decreases. If a temperature increase is measured during a monitoring cycle, the heating element is not turned off, but if the temperature remains constant or falls, the heating element is turned off.

Abstract: An electrical heating system for buildings is described in which the output voltage to a resistance heater is varied in accordance with outside temperature. The disclosure includes system means for compensating for solar radiation and wind effects. Also, night setback and morning start up are provided. The heat present on the interior of the building is provided in accordance with the building characteristics and without regard to interior temperatures.

Abstract: An electrical heating apparatus protected against an overheating condition is disclosed. It includes an electrical heater and an elongate heat sensor disposed in a thermally responsive relationship. The heat sensor consists of a single conductor having positive temperature coefficient electrical impedance characteristics continuously along its entire length. The electricity flowing through the electrical heater is controlled in response to a signal from the heat sensitive conductor.

Abstract: An apparatus and method for controlling a domestic hot water heater system in which the actual temperature of the water in the storage tank of the system is continuously measured and the measured water temperature is continuously displayed at a location remote from the water heater. There are control means located at a remote location for actuating energizing means at the water heater for heating the water. The energizing means is actuated by the person intending to use the heated water on the basis of the measured temperature of the water displayed at the continuous display at the time of measurement and the intended use of the water.

Abstract: In a temperature control system having a heating element and a temperature sensor for sensing the temperature generated by the heating element and providing signals effective to control electric energy to be supplied to the heating element, both the heating element and the temperature sensor are implemented with a single element structure for example a power transistor, which is enabled as a heating element during a first period of time and as a temperature sensor during a second period of time. Thus, the heating/temperature sensing element is enabled on a time-shared basis, compensating for heat resistance incurred during transmission of heat from the heating element to the temperature detecting element. Such a temperature control is useful with precision devices such as an ink jet printer.

Abstract: A heating power supply apparatus for polycrystalline semiconductor rods, including a power transformer and anti-parallel thyristor unit groups connected to respective portions of the secondary of the power transformer, balance transformer groups connected to the anti-parallel thyristor groups and combined to make constant a secondary current of each of the balance transformers, and semiconductor rod connecting terminals connected to the secondaries of the balance transformer groups, respectively. The balance transformer groups are divided into a group of plural balance transformers of small current and high voltage for exclusive use during the starting and a group of plural balance transformers for taking over operation from the group of balance transformers of small current and high voltage and continuing subsequent operations.

Abstract: The invention relates to a circuit arrangement for controlling the temperature of an electric heating element (10), where the resistance of the element is utilized as a measure of the temperature. When the element is switched on, the current flowing through the element is scanned by a small measuring resistor (16). The voltage drop (U.sub.1) arising over this resistor is compared to a reference value (U.sub.2) for interrupting the effect supply to the element when the element has achieved the desired temperature. Thereafter a weak test current is fed through the element, and a voltage drop (U.sub.3) dependant on the element temperature is compared to an additional reference value (U.sub.4) for again connecting the effect feed when the element temperature has dropped to a predetermined level. By means of the invention, which can be used for the temperature control of, for example, electrically heated car seats, both continuous temperature control and adjustable on/off-hysteresis are obtained.

Abstract: A control circuit for controlling a magnetron of a microwave oven includes a microcomputer output control means for controlling the energy output of the microwave oven, and a means for controlling the magnetron in response to a time signal and a food temperature signal. The control circuit includes a back-up circuit to de-energize the magnetron when cooking is complete in the event of a malfunction of the microcomputer.

Abstract: An electrical system for providing an energy load regulator for use with electric clothes dryers and electric hot water tanks. The load regulator cycles the heating element within the dryer or hot water tank for predetermined periods while the appliance remains in an operative stage throughout the on and off cycles of the heating element.

Abstract: The bushing temperature measurement device is a microprocessor based sensor which determines the temperature across a multi-segment electrical resistance heated glass fiber producing bushing. The temperature change is determined by calculating the change in resistance of each segment of the bushing by sensing the voltage drop across that segment and dividing by the current flowing in that segment. All measurements of voltage and current are taken simultaneously and averaged over several cycles. The microprocessor, through appropriate output circuitry, alerts other process controls to eliminate any change in temperature.

Abstract: A circuit for controlling the voltage applied to a glow plug used to assist the starting of a diesel engine, the circuit including a circuit network including a normally-closed relay contact connected in parallel with a serially connected voltage-dropping resistor and a normally-open relay contact, the circuit being serially connected with the heating element of the glow plug and with a power supply for producing the voltage, and a timer connected to the power supply. The normally-closed relay contact is opened when the temperature of the heating element exceeds a preset value which is below a steady-state heating temperature, and the normally-open relay contact is closed by the timer a predetermined time after voltage is applied to the glow plug, whereby the glow plug is heated rapidly. In the event of an abnormal rise in power supply voltage during the rapid heating of the glow plug, the supply of current to the glow plug is interrupted temporarily to prevent glow plug burn-out.

Abstract: An automatic temperature control device includes a series-connected temperature setter, a comparison unit whose second input is connected to an actual temperature transducer of the controlled entity, a deviation signal amplifier, and a regulating unit. The device also includes a unit used to check the circuit of the transducer for continuity, said unit being connected to the output of the latter, and an electronic switch whose control input is connected to the output of the check unit to apply to the input of the actuator a periodic pulse sequence generated while the circuit of the transducer is open.

Abstract: In a method and apparatus for detecting an open circuit in a glow plug group for combination with a heating control circuit of the glow plug group mounted on an internal combustion engine, an output signal generated from a detecting element connected in series with the glow-plug group while electric power is supplied to the glow plug group is applied to a first comparator to detect a change of the composite resistance of the glow plug group due to a temperature rise caused by its heating. At this time, if an open circuit failure has occurred in the glow plug group, the state of the resistance change of the glow plug group varies greatly. The first comparator produces a time measurement signal indicating a time when the composite resistance of the glow plug group has reached a predetermined value and supplies the time measurement signal to a glow plug open circuit detecting circuit.

Abstract: A glow plug heating control device for a diesel engine includes a detector for detecting the heat variable resistance of the glow plug, and a device effecting insertion of a resistor in series with the glow plug and the power source at a given point so as to vary the manner of current application to and the heating of the plug in an advantageous non-linear manner.

Abstract: A device for incorporation into the power supply circuit of an electrical apparatus comprising (a) a switch which is movable from one condition to another on application of power to the switch and which is held in the moved condition, as long as power is applied to the switch; (b) a signal emitter adapted to emit a power signal of short duration and being connected to the switch so that on emission of the power signal the switch will be moved to the other condition for the duration of the power signal; and (c) a current detector capable of detecting when a current flows in a line and being connected to the switch to supply power thereto when current flows in the line. The current detector may be a light emitting diode (L.E.D.) and a light detector arranged to transmit current to a relay switch coil which is electrically connected to the line by a current transformer.

Abstract: Heating apparatus for thermally bonding thermoplastic foil layers to produce a longitudinally welded seam. The heating apparatus comprises an elongated jaw element having a work surface for receiving the thermoplastic foils, at least two heating elements located within the jaw element and at least one temperature sensor associated with each of the heating elements. A control device is coupled to each of the heating elements and its respective temperature sensor to maintain the temperature of the jaw element at a value which will result in bonding of the foil layers. A regulating device is coupled to the control devices and to the temperature sensors for selectively operating the control devices to switch the heating elements in accordance with the temperature detected by the temperature sensor.

Abstract: A liquid temperature control apparatus controllably heats a liquid from a temperature before a control to a desired reference temperature by varying the magnitude and/or application time period of a power for energizing a heater on the basis of a difference between a voltage corresponding to the actual temperature before a control and a reference voltage corresponding to the reference temperature. The parameter on which the magnitude and/or application time period of the heater power is to be based may be the sum of a first voltage differential between a voltage corresponding to the actual temperature before a control and the reference voltage and a second voltage differential between a voltage corresponding to an actual temperature after the start of a control and the reference voltage.

Abstract: A control system for selectively regulating the operation of water heating tank in accordance with anticipated hot water demand thus eliminating storage heat losses during periods when hot water is not required includes a remote control unit, a relay unit and first and second thermostats, all electrically connected selected circuits with an electric heating element located within the tank. The first and second thermostats are arranged to sense the temperature of the tank water and the operating temperature (115.degree. F.) of the first thermostat is lower than that of the second thermostat (150.degree. F.). The remote control unit includes a three-position switch in circuit with the relay unit and first and second thermostats to permit continuous energization of the heating element under control of either the first thermostat in the first switch position or the second thermostat in the second position to provide a continuous supply of hot water at either 115.degree. F. or 150.degree. F.

Abstract: A protective control arrangement for an apparatus incorporating one or more heating units of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material enclosed within a metallic sheath energized in accordance with a duty cycle power control scheme, which prevents the rupture or separation in the sheath caused by an arc at a discontinuity in the heating element from propagating along a substantial portion of the sheath.In one embodiment, a microprocessor controlled relay associated with each heating element selectively couples and decouples its associated heating element and the power supply. The microprocessor is preprogrammed to control the duty cycle of each heating element by periodically opening the associated relay switch, thereby decoupling the heating element from the power supply for idle periods of predetermined duration. A timer monitors the time between power decouplings.

Abstract: A controlled temperature oven provided with a pyrometer having a temperature sensitive resistance, an electrical heating element, a nonlinear heat sensing element within the oven chamber, and a variable power supply electrically connected to the heating element, the variable power supply being controlled in response to the nonlinear heat sensing element, and a switch assembly having a plurality of pairs of single pole double throw switches with a common actuation member for each pair of switches, one switch of each pair being connected in a first cascade of switches and the other switch of each pair being connected in a second cascade of switches, the switches of the first cascade of switches connecting one of a plurality of manually adjustable resistors to the variable power supply to control the output thereof, and the switches of the second cascade of switches connecting one of a plurality of variable resistors to the pyrometer to indicate the temperature selected by the switches of the first cascade of sw