Temperature control system for proportional electric heating

Claim 2

A temperature control system for proportional control of at least one electrically energized heating element, comprising: a gate-triggerable semiconductor current switching device adapted for being gated to energize said heating element from a source of a.c. voltage; a thermistor for sensing the temperature in a zone to be heated by said heating element, said thermisor having a resistance which varies as a predetermined function of the temperature in said zone; means interconnected with said thermistor for providing a sense voltage which varies as a function of the resistance of said thermistor; means for providing a periodically varying voltage; comparator means for comparing the instantaneous magnitudes of said sense voltage and periodically varying voltage and adapted to provide a signal for causing gating of said switching device when there is a predetermined relationship between the magnitudes of said sense voltage and periodically varying voltage; fail-safe means interconnected with said comparator means and said thermistor for preventing said signal for causing gating from being provided if said thermistor is either open-circuited or short-circuited, whereby energization of said heating element is proportionally controlled in response to the temperature sensed in said zone through normal operation of said thermistor.

Claim 3

A temperature control system as set forth in claim 1 wherein said fail-safe means comprises a transistoR interconnected with said comparator means and with said thermistor, said transistor being normally conductive for permitting normal operation of said comparator means and becoming nonconductive for preventing normal operation of said comparator means in response to an open circuit condition of said thermistor.

Claim 4

A temperature control system as set forth in calim 1 further comprising driver circuit means for supplying a gating signal for said switching device which is out of phase with respect to said a.c. voltage.

Claim 5

A temperature control system as set forth in calim 1 further comprising driver circuit means for supplying a gating signal for said switching device which is out of phase with respect to said a.c. voltage.

Claim 6

A temperature control system as set forth in claim 1 further comprising means for detecting zero-crossings of said a.c. voltage and for permitting said gating signal to be initially supplied to said switching device substantially only at a zero-crossing of said a.c. voltage.

Claim 7

A temperature control system for proportional control of at least one electrically energized heating element, comprising: a gate-triggerable semiconductor current switching device adapted for being gated to energize said heating element from a source of a.c. voltage; driver circuit means for supplying a gating signal for said switching device which is out of phase with respect to said a.c. voltage; zero-crossing circuit means for detecting zero-crossings of said a.c. voltage; first transistor switching means interconnected with said driver circuit means and said zero-crossing circuit means and adapted for permitting said gating signal to be initially supplied to said switching device substantially only at a zero-crossing of said a.c. voltage; second transistor switching means interconnected with said driver circuit means and adapted for permitting said gating signal to be supplied to said switching device in response to a heat demand signal; a thermistor for sensing the temperature in a zone to be heated by said heating element, said thermistor having a resistance which varies as a predetermined function of the temperature in said zone; means interconnected with said thermistor for providing a sense voltage which varies as an inverse function of the resistance of said thermistor; means for providing a periodically varying voltage; comparator means for comparing the instantaneous magnitudes of said sense voltage and said periodically varying voltage and adapted to provide said heat demand signal when the magnitude of said sense voltage is instantaneously greater than the magnitude of said periodically varying voltage; fail-safe means interconnected with said comparator means and said thermistor for preventing said heat demand signal from being provided if said thermistor is either open-circuited or short-circuited, whereby energization of said heating element is proportionally controlled in response to the temperature sensed in said zone through normal operation of said thermistor.

Claim 8

A temperature control system for proportional control of at least one electrically energized heating element, comprising: a gate-triggerable semiconductor current switching device adapted for being gated to energize said heating element from a source of a.c. voltage; driver circuit means for supplying a gating signal for said switching device which is out of phase with respect to said a.c. voltage; zero-crossing circuit means for detecting zero-crossings of said a.c. voltage; first transistor switching means interconnected with said driver circuit means and said zero-crossing circuit means and adapted for permitting said gating signal to be initially supplied to said switching device substantially only at a zero-crossing of said a.c. voltage; second transistor switching means interconnected with said driver circuit means and adapted for permitting said gating signal to be supplied to said switching device in response to a heat demand signal; a thermistor for sensing the temperature in a zone to be heated by said heating element, said thermistor having a resistance which varies as a predetermined function of the temperature in said zone; means interconnected with said thermistor for providing a sense voltage which varies as an inverse function of the resistance of said thermistor; means for providing a periodically varying voltage; comparator means for comparing the instantaneous magnitudes of said sense voltage and said periodically varying voltage and adapted to provide said heat demand signal when the magnitude of said sense voltage is instantaneously greater than the magnitude of said periodically varying voltage; fail-safe means interconnected with said comparator means and said thermistor for preventing said heat demand signal from being provided if said thermistor is either open-circuited or short-circuited, whereby energization of said heating element is proportionally controlled in response to the temperature sensed in said zone through normal operation of said thermistor.

Claim 9

A temperature control system as set forth in claim 5 wherein said switching device comprises a bidirectional thyristor.

Claim 10

A temperature control system as set forth in claim 5 wherein said comparator means comprises a pair of transistors interconnected as a differential comparator, said sense voltage being supplied to the base electrode of one of said transistor, said periodically varying voltage being supplied to the base electrode of one of the other of said transistors.

Claim 11

A temperature control system as set forth in claim 7 wherein the emitter electrodes of said transistors are connected in a common circuit, said fail-safe means comprising a further transistor having its collector-emitter circuit in series with said common circuit and its base electrode connected to said thermistor, said further transistor being normally conductive to permit normal operation of said differential comparator and becoming nonconductive to prevent normal operation of said differential comparator in response to an open circuit condition of said thermistor.

Claim 12

A temperature control system as set forth in claim 7 further comprising a selectively variable iMpedance element connected in a circuit with said thermistor for preselecting values of said sense voltage corresponding to respective values of the resistance of said thermistor, thereby to permit preselection of a temperature to be maintained in said zone by energization of said heating element.

Claim 13

A temperature control system as set forth in claim 5 wherein said periodically varying voltage is of a sawtooth waveform of a period substantially greater than the period of said a.c. voltage.