BACKGROUND OF THE INVENTION

This invention s in the field of high-safety control circuits for alternating current electrical heating pads and other electric appliances and devices. Alternating current heating pads and appliances usually involve higher voltages than d.c. appliances.

Heating pads and electric blankets are appliances that, by their nature, conduct high current electrical power in close proximity to the user. Besides the obvious danger of electrifusion as from any electrical appliance, a health concern exists regarding the prolonged exposure to the electromagnetic radiation. Heaters of the PTC type 2 (using positive temperature co-efficient material for the heating element) are known to be configured so as to virtually eliminate the magnitude of the electromagnetic fields thought to be harmful. The safe operation of the PTC heating elements is of great importance. The invention is useful in controls for such heating elements, but its safety characteristics are such that the invention is useful also in controls for negative and zero temperature coefficient electric appliances and

devices .

PTC materials used for heating elements have the added safety of limiting the current draw as the temperature approaches the design limit. With this in mind a heater can be designed without the need for an additional temperature limiting device, such as is disclosed in Crowely, U.S. Patent No. 4,271,350. Due to the nonlinear response of temperature with current, sufficient temperature control can be achieved by proportioning power to the heater. The only condition that subverts the inherent safety of the PTC heating element is when one of the conductors, in intimate contact with the PTC material, breaks and arcing occurs. To prevent this condition from continuing and possibly causing fire, a safety circuit is commonly used that will detect the condition, and then generate a current surge designed to blow the power input fuse, so that the unit is thereby disabled. Carlson, U.S. Patent No. 4,436,986, teaches the idea of sensing voltage changes and conducting sufficient current to disable the unit when neon bulbs exceed their breakdown voltages. Carlson goes further and incorporates three electrodes within a neon lamp forming a triode that breaks down at a single predetermined voltage, thus reducing the effect of breaks down voltage tolerance Carlson uses a current limiting resistor

to blow the fuse in a predetermined period of time. It is necessary for the current limiting resistor to be rated at a higher power than the fuse to provide a safe open circuit. The fuse, however, must be sized to handle currents of two to three times the continuous current rating of the heater to accommodate the inrush associated with the start up characteristic of the Positive Temperature Coefficient material. The fuse is also relied upon in Carlson's invention to deactivate the unit in all possibilities of short circuits. The development of safety circuits for PCT heaters, however, suggests the possibility of safety circuits in the control of other types of electric heaters or electric appliances.

Typically an adjustable bimetallic control switch is used to provide differing heat settings for the PTC heating. As the current flows through the bimetallic element, it heats up causing the element to bend due to the differential expansion of the metals that comprise the elements. The deflection causes the contacts to open and interrupt the current to the heater and the small bimetallic element to cease bending. The bimetallic element then cools down until contact is again made and the cycle repeats. The deactivation of this type of electromechanical control, for safety reasons, has

best been accomplished by blowing a fuse that is in series with the switch, unless a back-up triac is used.

Modern electrical power controls use solid state switching devices such as silicon control rectifiers, power transistors, solid state relays and triacs. Edwin Mills, U.S. Patent No. 4,315,141 uses a pair of solid state switches biased by a temperature sensitive capacitive element as a temperature overload circuit for conventional electric blankets. In these control systems, a small signal controls switching of lager load currents. Integrated circuits or micro processors can be used to provide the control signal required to operate high speed solid state switching. Micro circuits of this type are capable of operating at speeds many times the 50 or 60 HZ. commonly used in a.c. electrical power sources. This capability makes it possible to control each a.c. cycle. In fact, the switching can occur as the a.c. waveform crosses zero. This type of control can lower the noise generation associated with a.c. switching and makes the most efficient use of a.c. power.

Recent advances in microwatt power control has improved the reliability of integrated circuits (ICs) by assuring the proper voltage input to the

micro controller. Jamieson and Weiss, U.S. Patent No. 5,196,781, teach an extremely low power voltage detection and switching circuit to provide power input to an IC within a narrow voltage band when only a low power and variable supply is available. Watchdog timing circuits can be incorporated within an IC to perform the task of periodically resetting the IC and to avoid a prolonged lockup or ambiguous operation resulting from power faults and voltage spikes often associated with a.c. power.

SUMMARY OF THE INVENTION

The present invention utilizes an electrical feedback circuit and a semiconductor switching system to control power to an electric appliance that requires a safe operation condition in the event of an open or short circuit. Although the invention is not limited to heating pads, it is described herein in terms of high-safety controls for heating pads using a PTC heating element. An integrated circuit is used to signal a solid state switch to time the ON and OFF proportion of the a.c. electric power to a heating element in order to control the temperature. If the heating element is a PTC element, the natural effect of increasing temperature is to throttle down and limit the current draw. The ability to control the temperature of the heater, by current control or

time proportioned power control is thereby enhanced. The power control level can be set externally by a heat scale setting via up-down key pad or rotary potentiometer and internally by a feedback safety circuit.

In accordance with the invention both return lines from the heater provide positive switching and an IC is not relied on to compare an input with a voltage or with another input. To accomplish that objective a neon tube breakdown element provides positive switching at one IC input and for the other input the bias of a transistor makes positive switching possible. In general, one input to the microcontroller IC is a pulse signal at the a.c. frequency and another input has a d.c. characteristic. For 220 VAC heaters the control circuit's neon tube breakdown element can be in cascade with one or two additional neon tubes for providing positive switching in the return line from the ungrounded side of the heater. When only one additional neon tube is in the cascade it is desirable to add one or more reversed non light- emitting diodes (LED) in the cascade. The LED does not emit light and provides a voltage drop of about 7 volts. When it is so used it is referred to as a non lighting LED or NLED. The use of one or more NLED in cascade with only one neon tube is also

useful in special cases to control the required voltage to turn ON the neon lamp.

In one embodiment of the high-safety control circuit the control circuit contains a fuse that is sized for the normal current draw. The start up cycle, controlled by a microcontroller IC is designed to provide a limited duty cycle within the first few seconds and to raise the PTC material temperature and thus raise the electrical resistance in order to reduce the average inrush current. In this way the fuse, a slow blow type, avoids even the design restraint associated with high inrush current typical with the PTC heating elements. With the fuse rated close to the normal current draw a current surge resulting from a short circuit quickly disables the power. After the initial cycle, designed to reduce the inrush current, a preheat cycle starts that raises the temperature of the pad to a certain temperature by allowing a 100% duty cycle for period of time corresponding to an initial setting. This preferred embodiment can be provided in two variations: one for nominal 220 volt alternating current and another for nominal 110 volt a.c.

In a further development of the invention, the possibility that a triac heater switch might lock up

in the ON position is counteracted by adding a second Triac in series, also controlled by the microcontroller IC.

The response to a fault is so quick that less than a quarter of a second is an ample allowance for detecting a fault. In consequence, the first response, or the first few fault responses, to a fault may be allowed to interrupt the ON condition of the heater switch for a period no longer than a second, so that a second response will come quickly if the fault persists, before a final switch-off is provided.

Instead of the second triac being simply in series with the heater switch triac, the second triac may be connected to de-energize the heater after enough fault detections (for example 10 of them or more generally from 3 to 16 of them) to make it clear that the heater switch triac is locked in its "ON" position.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described, by way of illustrated examples, with reference to the annexed drawings in which:

Fig. 1 is a circuit diagram of a first embodiment of a 220 volt heater control circuit of the invention, utilizing 3 neon breakdown tubes in the primary return line;

Fig. 2 is a circuit diagram of a second embodiment of a 220 volt heater control circuit of the invention utilizing, in the primary return line, 2 neon breakdown tubes and a non lighting LED used in the reverse breakdown voltage polarity;

Fig. 3 is a circuit diagram of a third embodiment of 220 volt heater control circuit of the invention utilizing, in the primary return line, 2 neon breakdown tubes and also a transistor to switch the neon tubes ON when the control circuit is switched on;

Fig. 4A is a circuit diagram of a first embodiment of a 110 volt heater control according to the invention utilizing, in the primary return line, a single neon breakdown tube and incorporating an optional second triac in the secondary return line;

Fig. 4B is a diagram of a variant of the optional second triac of Fig. 4A;

Fig. 5 is a circuit diagram of a second embodiment of a 110 volt heater control according to the invention utilizing, in the primary return line, a single neon tube having a breakdown voltage somewhat higher than the breakdown tube of Fig. 4A;

Fig. 6 is a block diagram of a microcontroller;

Fig. 7 is a timing diagram of possible pulsed control signals in the connection 12 of the microcontroller IC of Fig. 5 with reference to an a.c. input over the connection 20;

Fig. 8A is a timing diagram showing safe operation;

Fig. 8B is a timing diagram representing a break in a heater ground conductor;

Fig. 8C is a timing diagram for a break in the heater's 110 VAC conductor;

Fig. 8D is a timing diagram for the case of when the heater is not connected to its microcontroller;

Fig. 9 is a program diagram for the routine of checking the IC inputs 22 and 23 of the IC of Fig. 1

and Fig. 6 ; and

Fig. 10 is a program diagram for the same routine as in Fig. 9 with reference to the inputs 22 and 23 of the IC 34 of Fig. 4A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a control circuit according to the invention used to control an electric heater operating on 220 volts alternating current, for example a positive temperature coefficient (PTC) heater.

On the hot side of the 220 VAC source there is a fuse 5. On the protected side of that fuse there is a low current connection to supply the IC 14 with a source of alternating current for timing and also (by means not shown in Fig. 1) to provide a source of 5.6 volts d.c. as well as a connection to the heater through the terminal 1. The terminals 1, 2, 3 and 4 identify the conductors connected to the heater. The heating current goes through the heating element PTC and returns to ground through a triac Tl which is controlled by the IC 14 through the control connection 12. The conductor 2 connected to the ungrounded side of the triac Tl is connected to a ground heater conductor for connection with the grounded side of all of the

individual heating elements of the heater PTC and ultimately to a conductor 3. Similarly, on the high voltage side of the heater PTC the conductor 1 becomes an a.c. feeder conductor for all of the individual heater elements in parallel and then proceeds to connect with a conductor 4.

The conductors 3 and 4 provide return lines used for control purposes. The circuit connected to the conductor 4 may be referred to as the primary return line circuit and the circuit connected to conductor 3 may be referred to as the secondary return line circuit, since it receives energization only after the current has passed through the heater.

In the primary return line circuit there is first a diode Dl for providing half wave rectification, to reduce the power and heat dissipated in the primary return line circuit (this may be omitted in 110 VAC appliances) . The principal components of the primary return line circuits are the three gas discharge tubes, typically neon or other rare gas discharge tubes, each having a breakdown voltage of 55 to 65 volts, thus providing a turn-on threshold between 165 to 195 volts of half wave rectified alternating current.

The signal to fire the triac Tl is provided by the IC 14 and is of the zero crossing type. The a.c. signal for determining the a.c. phase angle is sent to the IC through the current limiting resistance R9 and is clamped to +5 volts through diode D5. The return lines provide a safety circuit that does not depend on precise voltage comparison by the IC. Positive switching is provided on both the 220 VAC and ground return signals for avoiding signal level determination by the IC 14. During the heating cycle the conductor between the terminal 2 and the terminal 3 is connected to ground and that conductor has a resistance of 7 ohms, so that the a.c. voltage at terminal 3 is low even for 220 volts energization. At this level the voltage drop through the voltage divider R3, R4 half-wave rectified through the diode D2 and stabilized by C2 and R6 is not sufficient to bias off the transistor Ql, so that conduction between its emitter and collector provides a 5 volt signal to the IC 14 at its input 23. If the conductor between terminals 2 and 3 breaks and arcing occurs, the a.c. voltage at terminal 3 goes high and the signal to the base of Ql blocks conduction, so that the signal at input 23 of the IC goes to ground through R13. If the conductor between the terminals 2 and 3 breaks near the end of the heater or beyond the end, a resistor R10 between the primary and secondary return lines

will provide the a.c. signal at terminal 3.

The primary return line from terminal 4 in 220 VAC service is connected to a cascade of neon lamps Nl, N2 and N3, this cascade being grounded by the resistor Rll. The a.c. signal at the junction 8 is connected to the input 22 of the IC 14 through a resistance R12 and is clamped to 5 volts by means of the diode D12 during the positive half cycle and to ground by means of the diode Dll during the negative half cycle. The IC reads its input 22 at a phase angle of 90° looking for 5 volts. If a break any place along the conductor connecting the terminal 1 to the terminal 4 occurs, the voltage across the cascade of neon tubes drops below the breakdown voltage and opens the circuit between the junctions 8 and 6. If arcing occurs across the break the combined voltage drop across he arc plus the required breakdown voltage across the neon lamps is greater than the input voltage, so that the neon amp cascade turns OFF. That drops the voltage at input 22 of the IC to ground through resistor Rll. The corresponding program of the IC microcontroller is to be discussed in connection with Figs. 6 and 9. Typical values for resistors shown in Fig. 1 are tabulated in Table 1 at the end of this specification.

The circuit of Fig. 2 differs from the circuit of Fig. 1 only in the components connected between the junctions 6 and 8. In that branch of the circuit there are only two neon tubes selected for breakdown voltage as in the case of Fig. 1 but they are in cascade with a reversed LED (light emitting diode) 60, which is non-lighting when used in the reverse breakdown voltage polarity. A series of NLEDs can also be included to boost the input voltage required to operate the neons. Each NLED in the cascade increases the required voltage by about 7 volts. With a single neon tube, the circuit of Fig. 2 can be used for 110 VAC service for relatively high current loads.

The circuit of Fig. 3 is similar to the circuit of Fig. 2, but adds a second transistor Q2 which is connected to switch the neon tubes ON when the controller 14 is turned ON. In this case the signal at the input 22 of the controller 14 is read only when the unit is on, and at 90° phase angle, and only when the triac Tl is triggered. If a signal at input 23 indicates that the heat is ON and, however, the trigger signal to Tl is not present, then the triac Tl must be shorted and the signal at 24 to operate the neons can be used to turn OFF the neons or flash them to alert the user that the safety circuit is not operational.

Referring now to Fig. 4A which illustrates a 50 watt heater powered by 110 VAC, the IC 34, used for the control of power switching by sending a signal to the triac Tl, is powered by a nominal 5 volt power supply shown as 5.6 volts. More generally the power supply has a stabilized voltage of from 4 to 7 volts. As shown in Fig. 6, the heat desired is set by using momentary switches 15, 16 and 17 for ON- OFF, up and down control. The setting status is displayed by a liquid crystal display 18. In addition to displaying the heat setting, the display can indicate an abnormal operating mode by flashing or activating a segment of the display that instructs the user to discontinue use. Similarly an audible alarm can be used to alert the user.

The IC 34 function used to control the temperature of the PTC heater is by time- proportioning the power. For a low temperature setting, for example, the ON time may be 2 seconds with a 28 second dwell or OFF time in a 30 second cycle. The middle setting would have a 15 second ON time and a 15 second OFF time. The highest setting accordingly would provide a 30 second ON time or continuous heating. The lowest setting, in this case 2 seconds on, could be much lower because of the quick reaction in time of the breakdown tube 55.

In Fig. 4A, when arcing occurs, resulting from a conductor break, the high local temperature of the arc or spark will cause the PTC material to burn and form carbon in the area surrounding the arc. The carbon path created by the arcing condition can conduct current that will produce a lower level a.c. signal at terminal 4. To prevent this lower voltage signal from generating the pulsed signal at junction 25 and to prevent input to the IC at 22, a voltage breakdown device 55 is placed in series with the 110 VAC return conductor between terminal 4 and RIO. Resistor R15 provides a current path to ground and is sized to allow the current and voltage across 55 to be just over its threshold voltage when the input a.c. voltage is at the lowest rated voltage, as for example 100 volts. When the breakdown device 55 conducts the junction 25 and the IC 34 input 22 is clamped at 5.6 volts by the diode D7 during alternate half-waves of the a.c. cycle and at ground potential the rest of the time. The breakdown device 55 shown in this embodiment is a miniature neon bulb having a minimum breakdown voltage and a turn-ON voltage of 80 volts. Xenell Co. in Oklahoma, Part #A1E, can be used for this purpose. With the current limiting resistor R15 attached to the ground return, the rare gas lamp 55 is ON only when the triac Tl is firing and will act as a heat indicator light. If the current limiting resistor

R15 were attached to the common ground, the lamp would be ON during the OFF cycle as well as the ON cycle and can have the dual function of providing backlighting for the display 18 connected to the microcontroller IC 34. Table II at the end of this specification shows typical values of some of the resistors of Fig. 4A.

A second triac T2 can be used to disable the controller permanently in the event of the control Tl failing in a short circuit condition, as shown in Figs. 4A and 4B. The second triac circuit of Fig. 4A for the control disablement is of the common crowbar type that for a short duration connects ground across the fuse 5. A current limiting resistor (not shown) can be used to prevent the surge current from causing damage to the internal wiring of a house, or else the firing sequence for triac T2 can be time-proportioned to limit current and accomplish the blowing of the fuse 5 within a specific period of time, for example, 200 milliseconds.

The control of the triac through its control connection 12 is produced by sequences of unidirectional current pulses respectively bridging zero crossing instants of the a.c. wave form, resulting in continuous conduction of the a.c. wave

form through the triac so long as the sequence of control pulses is not interrupted. As noted below, in response to the safety circuit the control pulses, instead of being stopped entirely, may be interrupted for 800 milliseconds only.

Once a fault condition is detected, the signal to fire the triac is delayed. After the initial fault is detected, a triac delay time of 800 milliseconds may pass and another signal is sent to the triac requiring 200 milliseconds and if another fault is detected the triac firing is again delayed. At this time, a third fault test sequence can be enacted or the triac signal can be disabled for the entire operation. More generally those periods can be greatly shortened safely because of quick reaction of the breakdown tube 55.

To further assure the detection of the fault 200 milliseconds after the next cycle begins the IC reads the safety circuit input signal and if the fault is detected, the triggering of the triac is again by-passed and the drive for the LCD display is switched into a blinking mode. To avoid further hazardous use especially in an unattended situation, a repetitive fault occurrence would cause the unit to turn off. Repetitive interaction by the user to turn OFF and turn ON would only cause the display to

blink since the fault condition can be stored in memory. At this time the only way power would go to the faulty heater is if the user disconnected the power cord from the receptacle and again inserted the power plug into the receptacle. By repowering the controller, the first 200 milliseconds of heating would again look for the fault and the detection cycle would repeat, again disabling the power to the heater.

As shown in Fig. 4A and 4B, redundant safety control can be achieved by including a second switching device in series with the triac. Fig. 4A shows one circuit in which, even if a triac fails in the closed position, complete control is achieved by the second triac T2. Both switching devices are simultaneously fired by either the same signal or respectively by separate signals sent by the IC 34. In Fig. 4A a separate line 55 controls the triac T2.

Fig. 4B shows both triacs Tl and T2 on the ground side of the a.c. supply, in which case the line 18 opens the triac Tl instead of closing it to blow the fuse 5.

Fig. 5 shows a circuit similar to Fig. 4A without the feature of the second triac. The 110 VAC primary return line produces an a.c. power

frequency pulse for input to the IC at its input 22. The ground return safety link conductor 103 is connected to the junction of R110 and a voltage divider R113-R114. When the ground conductor through the heater is continuous then this conductor shorts that junction to ground through the triac Tl and the secondary safety link signal to the IC at 23 is ground potential. The safe operating signal thus obtained configuration is the same as in Fig. 8A, as further explained below. In the event of a broken or open ground heater conductor, the junction between resistors R113 and R114 is no longer shorted to ground, the voltage divider becomes active and the junction voltage is 5 volts. The error condition for the signal input to port 23 of the IC 114 is now 5 volts (in pulses) and the safe condition is ground potential, as in the previous example of Fig. 4A.

In Fig. 5, the PTC heating strands are powered through the input conductors 101 and 102. Conductor 101 is attached to 110 VAC through the fuse 105 and conductor 102 is switched to ground by the triac T101. The signal to fire the triac is provided by the IC 114 and is of the zero crossing type. The a.c. signal for determining the a.c. phase angle is sent to the IC through R109 and clamped to +5 volts through D105. Again the conductors 103 and 104 are

returned to a safety circuit but do not require precise analysis by the IC. Positive switching is provided on both the 110 VAC and ground safety return links 104 and 103, thus avoiding signal level determination by the IC.

During the heating cycle the conductor between 102 and 103 is connected to ground, the conductor has a resistance of 7 ohms, and the a.c. voltage at 103 is low, 2 to 10 volts. At this level, the subsequent voltage drop through the voltage divider R113 and R114, followed by halfwave rectification through D102 and stabilization by C102 and R106, is not sufficient to bias off the transistor Q101, so that conduction between the emitter and collector provides a 5 volt a.c. signal to the IC microcontroller 114 at 123. If the conductor breaks between 102 and 103 and arcing occurs, the a.c. voltage at 103, now connected through the PTC resistive material to the 110V side, goes high and the signal to the base of Q101 blocks conduction and the signal at 123 goes to ground through R113. If the conductor 102-103 breaks near the end of the PTC or beyond the end, a resistor R110 connected between the return conductors will provide the a.c. signal at 103.

The 110 VAC safety return at 104 is connected through a neon lamp N101 and a series resistor Rill to ground. The neon lamp is selected for a breakdown voltage of 75 to 85 volts. The a.c. signal at 108 is connected to input 122 of the IC through R112, is clamped to 5 volts with D112 during the positive half cycle and to ground with Dill during the negative half cycle. The IC reads 122 at a phase angle of 90° looking for 5 volts. If arcing or a break any place along the 110 VAC conductor

101, 104 occurs, the voltage across N101 drops below the breakdown voltage, opens the circuit between 106 and 108 and drops the voltage at 122 to ground through Rill. Table I for the corresponding components of Fig. 1 can be used for Fig. 5, except that R103 of Fig. 5 would have half the resistance of R3 of Fig. 1 because Fig. 1 is for 220 VAC while Fig. 5 is for 110 VAC.

The microcontroller IC, Fig. 6, includes a read-only-memory, "ROM" 29, where the algorithms and instruction set that comprise the program to control the heating and the display are stored. The instructions from ROM 29 are processed within the arithmetic logical unit, ALU 30, and the resulting values are decoded and stored in a data register random access memory RAM 31, to be used as input to the program. The input signals, a.c. in, and safety

circuit inputs 22 and 23 are received through the data bus 32. The program determines when power is to be supplied, based on the input from the safety circuit and the control status. The triac firing is coordinated with the a.c. wave form at the a.c. input 20 to trigger the triac at the zero crossing. A program counter, PC 32, is required to keep track of the program steps and serves to index to the next program instruction.

A timing circuit 21, shown as outside the microcontroller IC, serves to control the clock speed at which the program operates. It is made up of a conventional RC oscillator. A crystal oscillator can also be used. Typically the clock speed is of the order of 1 to 2 million cycles per second. The watchdog timer is set to overload periodically and thereby to initiate a device reset. Upon reset the program is initialized and starts from the beginning. The watchdog timer 28 intermittently times out the microprocessor operation for a preset period, adjustable between .01 and 3 seconds. The time counter 33 and program counter 32 are also reset. If a lock up occurs, the watchdog timer, having its own internal oscillator, will continue to countdown and then reset the program. The timeout mode is also enacted upon power-up to assure the proper voltage is put into

the microprocessor, thus allowing the power circuit time to stabilize. The watchdog timer is important to guarantee the processing of the safety circuit signal. It may also reset the microprocessor in all situations involving noise pulses that may corrupt memory or cause a lock-up. While in the heating cycle, the IC produces an output signal on a connection 12 or 112 that triggers the triac connecting a.c. to the heater. The output signal at 12, 112 controls the firing of the triac. OKI Co. device number MSM64162 is one example of a microcontroller IC that can perform the functions as stated above for a 110 volt control circuit.

The description of the microcontroller of Fig. 6, although it refers particularly to a 110 VAC power source, is explained in a manner that will be sufficiently clear for those skilled in the art to understand how a corresponding microprocessor for a system operating with a 220 VAC power source can be selected. For convenience and brevity the microcontroller of Fig. 5 is described only as it applies to the 110 volt control circuits of Figs. 4 and 5.

The a.c. power input and the triac trigger signal for the embodiment of Fig. 5 are shown in the Fig. 7 timing diagram. The signal time period is 60

Hz for 10 cycles. The same situations apply when the a.c. power is at 50 Hz. For the same time frame, Figs. 8A-8D show the possible combination of signals that will be at input of the IC for determination of safe operation. The 60 Hz pulse at IC input 22 and ground at 23, shown in Fig. 8A, make up the signal combination required for safe operation. Fig. 8B shows the signals at 22 and 23 when a break in the heater ground conductor has occurred. Fig. 8C is the signal combination resulting from a break in the heater 110V conductor. The signal combination of Fig. 8D would be expected when both the 110 VAC and the ground heater conductors are open: this typically occurs if the heater is not connected to the controller. The signal analysis of one of Figs. 8B, 8C and 8D could result in the interruption of the triac trigger signal over the connection 12 shown in Fig. 5 and thus the interruption of the 110 VAC power to the heater. In the case of Figs. 8B and 8C, this power interruption will prevent the PTC material from arcing and causing a fire. The open circuit condition of Fig. 8D occurs when the user has not yet plugged in the heater.

The program, stored in the ROM section 29 of the IC 114, has a routine to analyze the safety link signals 22 and 23, as shown in Fig. 9. Referring to

the routine flow chart of Fig. 9, the first instruction looks for the safe signal, Fig. 8A. If the pulse signal is detected at port 22 and ground is detected at 23, then the result at the first stage is YES and the next instruction is to verify that the heating cycle is on. If the triac has failed in a short circuit condition and heating is in the OFF mode then a No answer to the heating status indication routes the program to the heat status counter "HSC" sub-routine. The HSC sub¬ routine adds one to the HSC value then compares the value to 10. If the count is over 10 then 10 consecutive cycles indicate triac failure and the sub-routine is routed to fault protection and an alarm routine. If the Heating Status Counter shows that normal operation is occurring, the Error Counter and the Heating Status Counter are set to zero and the routine goes back to the main program. At the first stage, if a pulse is not detected at 22 or a pulse exists at 23 then the answer is No and if the Heating Status is ON and an error condition exists that would indicate an unsafe operating condition. At this point the error counter is incremented by one and in ten cycles, approximately 87 milliseconds, the subroutine is routed to the fault safety routine for disabling the triac, flashing the display, flashing an indictor light or sounding an alarm. The error count is set at 10,

for example, to react to a fault in 87 milliseconds. The count can be smaller if a quicker reaction is required. The count should not be as small as one, in order to prevent nuisance failures that may result from power fluctuation. It is quite practical for the HSC subroutine error count threshold to be only 5.

The program routine shown in Fig. 10, is the same as Fig. 9 for the sequence stages 36 through 48. Stage 49 compares the Heat Status Counter to 10 as in stage 47 only this time if it is over 10 then the next program sequence is the triac 2 firing routine that disables the unit. If at stage 49, the HSC is not greater than 10, then the error condition is not a result of the triac Tl failing closed and the routine is directed to the stage 50 that will flash the display after disabling the triac 1 signal.

The circuits of Figs. 1-6 and the diagrams of Figs. 7-10 illustrate the principles of the invention and it will accordingly be recognized that in the application of the principles of the invention various modifications are possible in many respects. In particular, the use of a second triac can be applied to the 220 volt control circuits (Figs. 1-3) in the manner illustrated and described

with reference to a 110 VAC control circuit (Fig.

4) , of course with the expected modifications of the controller of Fig. 6 for use with 220 VAC.

TABLE I (Relates to Fig. 1)

Tl TRIAC

Ql PNP TRANSISTOR VCE>35VOLTS

Nl, N2, N3 NEON LAMP BDV AC 75-85 VAC R3 200 K RESISTOR 1/8 WATT

R4 100 K RESISTOR 1/8 WATT

R6 470 K RESISTOR 1/8 WATT

R9 1 M RESISTOR 1/8 WATT

R10 220 K RESISTOR 1/8 WATT

Rll 20 K RESISTOR 1/8 WATT

R12 1 K RESISTOR 1/8 WATT

R13 20 K RESISTOR 1/8 WATT

D2, D5, Dll, AND D12...IN 4001 DIODES

TABLE II (Relates to Fig. 4A)

R9 1 megohm

R10 1 megohm

R13 5 K ohms

R14 50 K ohms

R15 20 K ohms