Control device and safety circuit for heating pads with PTC heater

• US 5,422,461 A
• Filed: 12/15/1992
• Issued: 06/06/1995
• Est. Priority Date: 12/15/1992
• Status: Expired due to Term

• Current Assignee: Sunbeam Products, Inc. d/b/a Jarden Consumer Solutions (Newell Brands Inc.)
• Original Assignee: Micro Weiss Electronics, Inc.
• Inventors: Weiss, John, Lin, K. Y.
• Primary Examiner(s): Paschall, Mark H.
• Application Number:
  US07/990,530
• Time in Patent Office: 903 Days
• Field of Search: 219/212, 219/497, 219/492, 219/494, 219/501, 219/505, 219/504, 219/508-510, 219/481, 323/235, 323/236, 323/901, 323/319, 323/908, 307/117
• US Class Current: 219/501
• CPC Class Codes: G05D 23/2401 using a heating element as a sensing element

Abstract

Heating pads using positive temperature coefficient (PTC) resistance material are subject to fire risk if one of the conductor wires between which the PTC material extends should break and produce an electric arc. Protection by fuse and a fuse-blowing circuit responsive to fire detection must allow for an immense inrush of current when the cold pad is turned on. A heat setting control using a microprocessor can reduce the fuse rating by chopping the a.c. heating current for a short start-up period following with full-on feed until the heat setting is reached. The presence of a microprocessor allows response to a safety circuit that detects a break in a heater feed or return conductor before much excess heat develops, so that the microprocessor can turn off the heater switch. That response is so quick that it can be confirmed by repeated detection after very short pauses before the heater switch (a triac) is turned off. The safety circuit producing the fault detection signal may be external to the microprocessor chip, or most of it can be built into the microprocessor chip, which then receives two inputs from a smaller circuit connected to the heating pad. A second triac can be used to shut off the heater if the heater switch malfunctions by locking in its "on" position.

Claims

1. A safety circuit for an electric alternating-current heater, said heater having a heating element provided by a web of PTC electroresistive material extending between first and second heater feed conductors, said first heater feed conductor (1) being connectable through a protecting fuse (5) to an ungrounded pole of a source of alternating electric current and said second heater feed conductor being connectable through a heater switch (T1) to a grounded pole of said source of alternating electric current, said first and second heater feed conductors, at their respective ends remote from said fuse and from said heater switch, being respectively connected to first and second safety link return conductors leading respectively to first (3) and second (4) inputs of said safety circuit, said heater switch being a triac having a control input, connected to an output of an integrated circuit (IC) unit, for duty cycle time division control of said triac through said control input of said triac in a duty cycle range varying from at most 25% duty to 100% in response to heat settings and in response to an output of said safety circuit connected to a control input of said integrated circuit unit, said integrated circuit unit being supplied with d.c. power at a reference voltage by d.c. power supply means connected to said source of alternating electric current, said integrated circuit unit also having manual control means for controlling said heater, said safety circuit comprising:

a first resistive voltage divider (R1, R2) having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said second heater feed conductor (2) which end is connected to said heater switch (T1) and the end of said first safety link return conductor which is remote from its connection to said first heater feed conductor;

a second resistive voltage divider having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said first heater feed conductor (1) which end is connected to said fuse (5) and the end of said second safety link conductor (4) which end is remote from its connection to said second heater feed conductor (2), the greater of said resistances of each of said first and second voltage dividers being connected respectively to said first safety link return conductor and to said first heater feed conductor,

the ratio of said greater to said smaller resistance being greater for said second voltage divider than for said first voltage divider;

a first rectifier diode (D1) having a first electrode connected to said tap of said first voltage divider and a second electrode connected to a first network comprising a first capacitor (Cl) shunted by a resistor (R5) leading to ground potential, said first network having a first predetermined time constant;

a second rectifier diode (D2) having a first electrode connected to said tap of said second voltage divider and a second electrode connected to a second network comprising a second capacitor (C2) shunted by another resistor (R6) leading to ground potential, said second network having a second time constant;

said connection of said first rectifier (D1) to said first capacitor (C1) also being connected, through a third diode (D3), poled oppositely to said first diode (D1) in a series connection therewith interposed between said first diode and said control input of said integrated circuit unit;

a semi-conductor inverting amplifier stage having a first main path electrode connected to said reference voltage, having a control electrode connected, through an input resistor (R8) to said second diode (D2) where said second diode is connected to said second capacitor (C2), for blocking conduction between main path electrodes of said amplifier stage by a signal rectified by said second diode (D2) and having a second main path electrode connected both to a load resistor (RIO) leading to ground potential and, through a fourth diode (D4), to said control input (9) of said integrated circuit (IC) unit (14), the polarity of said fourth diode (D4) being opposite to that of said third diode (D3) when said third and fourth diodes are considered as being in series through their common connection and being the same as the polarity of said third diode when said third and fourth diodes are considered as respectively belonging to parallel paths to ground through respective resistances (R5, R10);

wherein said first and second networks serve to make a time that passes after initial power up of said heater before said integrated circuit unit can respond to a fault substantially equal to a time necessary for both the safety circuit and the integrated circuit (IC) unit to react to a fault.

2. The safety circuit of claim 1 wherein a predetermined minimum value of said duty cycle is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

3. The safety circuit of claim 1 wherein said first network and said second network each have a time constant not less than 0.04 sec. and not greater than 0.06 sec.

4. The safety circuit of claim 2 wherein said first network and said second network each have a time constant not less than 0.04 sec. and not greater than 0.06 sec.

5. The safety circuit of claim 1, wherein said first voltage divider has a ratio between 8% and 12% said second voltage divider has a ratio between 0.3% and 0.7% and wherein the overall resistance of said second voltage divider is at least 50% greater than the overall resistance of said first voltage divider.

6. The safety circuit of claim 2, wherein said first voltage divider has a ratio between 8% and 12%, said second voltage divider has a ratio between 0.3% and 0.7% and wherein the overall resistance of said second voltage divider is at least 50% greater than the overall resistance of said first voltage divider.

7. A safety-assuring control device for an electric alternating-current heater, said heater having a heating element provided by a web of PTC electroresistive material extending between first and second heater feed conductors, said first heater feed conductor (1) being connected to a protecting fuse (5) and connectable therethrough to an ungrounded pole of a source of alternating electric current and said second heater feed conductor (2) being connected to an electrically controlled heater switch (T1) and connectable therethrough to a grounded pole of said source of alternating electric current, said first and second heater feed conductors, at respective ends remote from said fuse and from said heater switch, being respectively connected to first and second safety link return conductors which lead respectively towards first and second connections to said control device, said control device comprising an integrated circuit unit (14) having means for cyclically varying the on time of said heater switch in consecutive equal periods of a constant major fraction of a minute from an on time of a few second stepwise to a continuous on time, a safety circuit connected to said first and second safety link return conductors and to an input (9) of said integrated circuit unit (14) and a source of a direct current supplied at a steady potential more than 4 volts and less than 7 volts from ground potential, connected to an input of said integrated circuit unit (14) and to said safety circuit,

said electrically controlled heater switch is a triac having control connections means (12);

said integrated circuit unit (14) has a safety circuit input, an input connected with said source of direct current, as well as least one heat setting input (15, 16,

     17) connected to heat setting means, a grounding connection, an input (20) for voltage of said electric alternating current and an output connected to said control connection means (12) of said triac and is programmed for control of said means for cyclically varying the on time of said heater switch in response to said heat setting means and to said safety circuit and for otherwise interrupting or shutting off said heater switch in response to occasional input from said safety circuit,

a first resistive voltage divider (R1,R2) having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said second heater feed conductor (2) which end is connected to said heater switch (T1) and the end of said first safety link return conductor which is remote from its connection to said first heater feed conductor;

a second resistive voltage divider having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said first heater feed conductor (1) which end is connected to said fuse (5) and the end of said second safety link conductor (4) which end is remote from its connection to said second heater feed conductor (2), the greater of said resistances of each of said first and second voltage dividers being connected respectively to said first safety link return conductor and to said first heater feed conductor,

the ratio of said greater to said smaller resistance being greater for said second voltage divider than for said first voltage divider;

a first rectifier diode (D1) having a first electrode connected to said tap of said first voltage divider and a second electrode connected to a first network comprising a first capacitor (C1) shunted by a resistor (R5) leading to ground potential, said first network having a first predetermined time constant;

a second rectifier diode (D2) having a first electrode connected to said tap of said second voltage divider and a second electrode connected to a second network comprising a second capacitor (C2) shunted by another resistor (R6) leading to ground potential, said second network having a second time constant;

said connection of said first rectifier (D1) to said first capacitor (Cl) also being connected, through a third diode (D3), poled oppositely to said first diode (D1) in a series connection therewith interposed between said first diode and said control input of said integrated circuit (IC) unit;

a semi-conductor inverting amplifier stage having a first main path electrode connected to said reference voltage, having a control electrode connected, through an input resistor (R8) to said second diode (D2) where said second diode is connected to said second capacitor (C2), for blocking conduction between main path electrodes of said amplifier stage by a signal rectified by said second diode (D2) and having a second main path electrode connected both to a load resistor (R16) leading to ground potential and, through a fourth diode (D4), to said control input of said integrated circuit unit, the polarity of said fourth diode (D4) being opposite to that of said third diode (D3) when said third and fourth diodes are considered as being in series through their common connection and being the same as the polarity of said third diode when said third and fourth diodes are considered as respectively belonging to parallel paths to ground through respective resistances (R5,R16);

whereby at low voltage at said input (9) of said integrated circuit unit (14) causes a shutting off of said heater switch at least for a predetermined period and

wherein said first and second networks serve to make a time that passes after initial power-up of said heater before said integrated circuit unit can respond to a fault substantially equal to a time necessary for both the safety circuit and the integrated circuit unit to react to a fault.

8. The safety-assuring control device of claim 7 wherein a predetermined minimum non-zero value of said on time of said heater switch in consecutive equal periods is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

9. The safety-assuring control device of claim 7, wherein said integrated circuit unit (14) is programmed to provide a duty cycle period not exceeding one minute in which variable on-time has a minimum duration of at least one second and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

10. The safety-assuring control device of claim 8, wherein said integrated circuit unit (14) is programmed to provide a duty cycle period not exceeding one minute in which variable on-time has a minimum of at least one second duration and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

11. The safety-assuring control device of claim 7 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by again reading a signal at said safety circuit input (9) of said integrated circuit unit and if a fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

12. The safety-assuring control device of claim 8 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by again reading a signal at said safety circuit input (9) of said integrated circuit unit and if a fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

13. The safety-assuring control device of claim 7, wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

14. The safety-assuring control device of claim 8 wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

15. The safety-assuring control device of claim 7, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

16. The safety-assuring control device of claim 8, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

24. The safety-assuring control device of claim 8 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by again reading signals at safety circuit inputs of said integrated circuit unit and if the fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

17. A safety-assuring control device for an electric alternating-current heater, said heater having a heating element provided by a web of PTC electroresistive material extending between first and second heater feed conductors, said first heater feed conductor (1) being connected to a protecting fuse (5) and connectable therethrough to an ungrounded pole of a source of alternating electric current and said second heater feed conductor being connected to an electrically controlled heater switch (T1) and connectable therethrough to a grounded pole of said source of alternating electric current, said first and second heater feed conductors, at respective ends remote from said fuse and from said heater switch, being respectively connected to first and second safety link return conductors which lead respectively towards connections to said control device, said control device comprising an integrated circuit unit having a read-only memory, a program counter, an arithmetic logic unit and, a random access memory, and a data bus interconnecting at least said arithmetic logic unit, said program counter, said random access memory and a time counter, said read-only memory being connected to said program counter, said arithmetic logic unit and said random access memory, wherein:

said electrically controlled heater switch is a triac having control connections means (12);

said integrated circuit unit (27) has first and second safety circuit inputs (22,23) and an input connected with a source of direct current supplied at steady potential more than 4 volts and less than 7 volts from ground potential, as well as least one heat setting input (15,16,17), a grounding connection, an input (20) for voltage of said electric alternating current and an output connected to said control connection means (12) of said triac;

said second safety link return conductor, at its end adjacent to said control device, is clamped to ground to ground potential in a first polarity and to said steady d.c. potential in a second polarity, opposite to said first polarity, by respective diodes (D9,D10), is connected to a first current limiting resistor (12) leading to the connection of said fuse with said first heater feed conductor and, is connected to said second safety circuit input (23) of said integrated circuit unit (27); and

said first safety link return conductor, at its end adjacent to said control device, is connected, at least after an applied initial voltage drop exceeding 50 volts, to a second current limiting resistor (10) leading to a junction (25) which, in addition to being connected to said second current limiting resistor, is connected to a third current limiting resistor (11) which leads to ground potential, said junction being clamped to ground in said first polarity and to said steady d.c. potential in said second polarity by respective diodes (D6,D8) said junction being connected to said first safety circuit input (23) of said integrated circuit unit through a diode for selecting half cycles of alternating voltage corresponding to said alternating electric which are of a predetermined polarity;

said input (20) of said integrated circuit unit (27) for voltage of said electric alternating current being clamped by a diode (D5) to said steady d.c. potential and connected through a fourth current limiting resistor (R9) to said first heater feed conductor at or near its connection to said fuse (5), and

said integrated circuit unit being programmed by its read-only memory to enable said triac to conduct alternating current continuously or periodically so long as a.c. power frequency pulses going from ground potential to approximately said potential of said source of direct current are supplied to said first safety circuit input (22) of said integrated circuit unit while the potential at said second safety circuit input (23) of said integrated circuit unit remains within a predetermined voltage, less than one volt, from ground potential and to disable said triac for at least half a second when a.c. power frequency pulses going to approximately said potential of said source of direct current are supplied to said second safety circuit input (23) of said integrated circuit unit and likewise when the potential at said first safety circuit input (22) remains within said predetermined voltage, less than one volt, from ground potential.

19. The safety-assuring control device of claim 17, wherein a predetermined minimum value of said duty cycle is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

21. The safety-assuring control device of claim 17, wherein said integrated circuit unit (27) is programmed to provide a duty cycle not exceeding one minute in which variable on-time has a minimum duration of at least one second and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

23. The safety-assuring control device of claim 17 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by again reading signals at safety circuit inputs of said integrated circuit unit and if the fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

25. The safety-assuring control device of claim 17 wherein said integrated circuit unit includes an independently running counter connected for resetting said integrated circuit unit to a predetermined place in a program for controlling operations of said integrated circuit unit at regular intervals.

27. The safety-assuring control device of claim 17, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

29. The safety-assuring control device of claim 19, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

31. The safety-assuring control device of claim 17, wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

33. The safety-assuring control device of claim 17, wherein said connection of said first safety link return conductor, at its end adjacent to said control device is through a break-over device having a minimum breakdown turn-on voltage which is greater than 50 volts, said break-over device having a first electrode connected to said first safety link return conductor and a second electrode connected to said second current limiting resistor (10).

35. The safety-assuring control device of claim 33, wherein said break-over device comprises a bulb containing a gas capable of being ionized by an electrical field and wherein a leak resistor having less resistance than said second current-limiting resistor (10) is connected between said second electrode of said break-over device and said second safety link return conductor.

37. The safety-assuring control device of claim 35, wherein a predetermined minimum value of said duty cycle is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

39. The safety-assuring control device of claim 35, wherein said integrated circuit unit (27) is programmed to provide a duty cycle not exceeding one minute in which variable on-time has a minimum duration of at least one second and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

41. The safety-assuring control device of claim 35 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by reading again safety circuit input signal and if the fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

43. The safety-assuring control device of claim 35 wherein said integrated circuit unit includes an independently running counter connected for resetting said integrated circuit unit to a predetermined place in the program of said integrated circuit unit at regular intervals.

45. The safety-assuring control device of claim 35, wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

47. The safety-assuring control device of claim 17, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

49. The safety-assuring control device of claim 35, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 m and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

51. The safety-assuring control device of claim 37, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

53. The safety-assuring control device of claim 39, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

55. The safety-assuring control device of claim 41, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

57. The safety-assuring control device of claim 43, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

59. The safety-assuring control device of claim 45, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (27) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

18. A safety-assuring control device for an electric alternating-current heater, said heater having a heating element provided by a web of PTC electroresistive material extending between first and second heater feed conductors, said first heater feed conductor (1) being connected to a protecting fuse (5) and connectable therethrough to an ungrounded pole of a source of alternating electric current and said second heater feed conductor being connected to an electrically controlled heater switch (T1) and connectable therethrough to a grounded pole of said source of alternating electric current, said first and second heater feed conductors, at respective ends remote from said fuse and from said heater switch, being respectively connected to first and second safety link return conductors which lead respectively towards connections to said control device, said control device comprising an integrated circuit unit having a read-only memory, a program counter, an arithmetic logic unit and, a random access memory, and a data bus interconnecting at least said arithmetic logic unit, said program counter, said random access memory and a time counter, said read-only memory being connected to said program counter, said arithmetic logic unit and said random access memory, wherein:

said integrated circuit unit (34) has first and second safety circuit inputs (22,23) and an input connected with a source of direct current supplied at steady potential more than 4 volts and less than 7 volts form ground potential, as well as least one heat setting input (15,16,17), a grounding connection, an input (20) for voltage of said electric alternating current and an output connected to said control connection means (12) of said triac;

said second safety link return conductor, at its end adjacent to said control device, is connected to said second safety circuit input (23) of said integrated circuit unit (27) and is connected to the tap connection a resistive voltage divider (R13,R14) connected from ground potential to said d.c. potential so as to put said tap connection at no more than 1 volt from ground potential; and

said first safety link return conductor, at its end adjacent to said control device, is connected, at least after an applied initial voltage drop exceeding 50 volts, to a first current limiting resistor (10) leading to a junction (25) which, in addition to being connected to said first current limiting resistor, is connected to said first safety circuit input (23) of said integrated circuit unit and is connected to a second current limiting resistor (11) which leads to ground potential, said junction being clamped to ground potential in a first polarity and to said steady d.c. potential in a second polarity, opposite to said first polarity, by respective diodes (D6,D7);

said input (20) of said integrated circuit unit (27) for voltage of said electric alternating current being clamped by a diode (D5) to said steady d.c. potential and connected through a third current limiting resistor (R9) to said first heater feed conductor at or near its connection to said fuse (5), and

said integrated circuit unit being programmed by its read-only memory to enable said triac to conduct alternating current continuously or periodically so long as a.c. power frequency pulses going from ground potential to approximately said potential of said source of direct current are supplied to said first safety circuit input (22) of said integrated circuit unit while the potential at said second safety circuit input (23) of said integrated circuit unit remains within a predetermined voltage, less than one volt, from ground potential and to disable said triac for at least half a second when a.c. power frequency pulses going to approximately said potential of said source of direct current are supplied to said second safety circuit input (23) of said integrated circuit unit and likewise when the potential at said first safety circuit input (22) remains within said predetermined voltage, less than one volt, from ground potential.

20. The safety-assuring control device of claim 18, wherein a predetermined minimum value of said duty cycle is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

22. The safety-assuring control device of claim 18, wherein said integrated circuit unit (27) is programmed to provide a duty cycle not exceeding one minute in which variable on-time has a minimum duration of at least one second and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

26. The safety-assuring control device of claim 18 wherein said integrated circuit unit includes an independently running counter connected for resetting said integrated circuit unit to a predetermined place in a program for controlling operations of said integrated circuit unit at regular intervals.

28. The safety-assuring control device of claim 18, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

30. The safety-assuring control device of claim 20, wherein said heater switch comprises first and second triacs (T1,T2) each having a control input connected to an individual output of said integrated circuit unit, said first triac (T1) being connected to said second heater feed conductor and to said second triac (T2) and said second triac (T2) being connected to said grounded pole of said source of alternating electric current, and wherein said integrated circuit unit is programmed for switching on both of said triacs simultaneously through their respective control inputs, whereby if a triac fails to respond to a turn off condition at its control input, the turn off is nevertheless accomplished by the other triac.

32. The safety-assuring control device of claim 18, wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

34. The safety-assuring control device of claim 18, wherein said connection of said first safety link return conductor, at its end adjacent to said control device is through a break-over device having a minimum breakdown turn-on voltage which is greater than 50 volts, said break-over device having a first electrode connected to said first safety link return conductor and a second electrode connected to said first current limiting resistor (12).

36. The safety-assuring control device of claim 34, wherein said break-over device comprises a bulb containing a gas capable being ionized by an electrical field and wherein a leak resistor having less resistance than said first current-limiting resistor (12) is connected between said second electrode of said break-over device and said second safety link return conductor.

38. The safety-assuring control device of claim 36, wherein a predetermined minimum value of said duty cycle is made effective by said integrated circuit unit at said control input of said triac in initial startup to reduce the initial average current for a predetermined initial time interval so that said fuse (5) can be selected without regard to inrush current values that might otherwise occur and wherein said triac is fired in consecutive cycles of said alternating electric current of said alternating electric current source immediately after each null voltage transition of said consecutive alternating electric current cycles.

40. The safety-assuring control device of claim 36, wherein said integrated circuit unit (27) is programmed to provide a duty cycle not exceeding one minute in which variable on-time has a minimum duration of at least one second and the duty cycle varies from at most 10% to full on, and wherein said integrated circuit unit is able to detect a voltage designating a fault reported by said safety circuit in substantially less than 0.2 sec., and wherein said integrated circuit unit is programmed from an initial detection of said voltage to interrupt the pulses necessary for keeping the triac heater switch in its on condition for a major fraction of a second instead of turning the triac off entirely, and wherein, after at least one interruption for a major fraction of a second, the detection of another fault reported by said safety circuit results in turning off said triac heater switch indefinitely.

42. The safety-assuring control device of claim 36 wherein said integrated circuit unit is programmed to respond, after a triac turn off for a major fraction of a second in response to an initial fault detection, by reading again safety circuit input signal and if the fault is again detected, the turn on of the triac is again blocked and an LCD display is switched into a blinking mode.

44. The safety-assuring control device of claim 36 wherein said integrated circuit unit includes an independently running counter connected for resetting said integrated circuit unit to a predetermined place in the program of said integrated circuit unit at regular intervals.

46. The safety-assuring control device of claim 36, wherein said d.c. power supply means provides d.c. power at an electro-positive reference voltage with respect to ground potential.

48. The safety-assuring control device of claim 18, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

50. The safety-assuring control device of claim 36, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

52. The safety-assuring control device of claim 38, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

54. The safety-assuring control device of claim 40, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

56. The safety-assuring control device of claim 42, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

58. The safety-assuring control device of claim 44, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the count number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.

60. The safety-assuring control device of claim 46, wherein a second triac (T2), having a control input, is connected between the junction of said first heater feed conductor (1) and said fuse (5) and ground potential and has its control input connected to a second control output of said integrated circuit unit (34) and wherein said integrated circuit unit is programmed to count the number of turnoffs of turn-off commands for the triac (T1) of said electrically controlled heater switch and to compare the counter number with a predetermined number which is at least 3 and less than 17, and when said predetermined number of said turn-off commands is reached, said second triac is turned on for blowing said fuse (5) and thereby disabling the heater.