Induction cooking apparatus having pan safety control

Claim 2

An improved induction cooking unit power supply system including in combination inverter circuit means comprised by a controlled conductivity control device for converting unidirectional potential to a high frequency pulsating excitation potential, the frequency and period of which is determined by the conduction interval of the controlled conductivity control device, said inverter circuit means including as a component thereof an induction heating coil that is excited by the high frequency pulsating excitation potential, and pan safety control circuit means synchronously operable with said inverter circuit means for assuring safe operation of said inverter circuit means, said pan safety control circuit means comprising sampling means responsive to conduction through the controlled conductivity control device for deriving an indication of the conduction intervals of the control conductivity control device, conduction interval limit setting means responsive to conduction through the controlled conductivity control device for defining safe limits to the respective conduction intervals for the control conductivity control device, comparison means responsive to said sampling means and said conduction interval limit setting means for comparing the actual conduction intervals to the preset safe limits, and safety means responsive to the output from the comparison means and controlling operation of said inverter circuit means in response to the actual conduction of the controlled conductivity control device failing to conform to the preset safe limits.

Claim 3

An improved induction cooking unit power supply system according to claim 1 wherein the inverter circuit means comprises a chopper-inverter circuit means utilizing a silicon controlled rectifier having a feedback diode connected in inverse parallel circuit relationship therewith across a pair of power supply terminals and having a gating circuit for controlling the turn-on of the silicon controlled rectifier coupled to the gating electrode, said self-excited gating circuit including a built-in low voltage gating circuit power source that energizes the gating circuit and this is connected across and derives its power from the power supply teRminals connected across the silicon controlled rectifier device, and wherein said safety means comprises a fast responding, gate control conduction device connected across the gating circuit power source in parallel circuit relationship therewith for shunting the gating circuit power source in response to an output signal from the comparison means and thereby shut down the chopper-inverter.

Claim 4

An improved induction cooking unit power supply system according to claim 2 further including full wave rectifier means connected to and supply the chopper-inverter circuit means and adapted for excitation from a conventional residential source of alternating current electric energy with said pair of power supply terminals connecting the output from said full wave rectifier means across the load terminals of the silicon controlled rectifier and parallel inverse connected feedback diode, a filter inductor connected in at least one of said power supply terminals intermediate the output of the full wave rectifier means and the chopper-inverter circuit means, a filter capacitor connected across the output of the full wave rectifier means intermediate the full wave rectifier and the filter inductor, and said chopper-inverter circuit means further comprising at least a commutating inductor and series connected commutating capacitor with the series connected commutating inductor and commutating capacitor being connected across the power supply terminals in parallel circuit relationship across the silicon controlled rectifier and parallel inverse connected feedback diode.

Claim 5

An improved induction cooking unit power supply system according to claim 3 further including a radio frequency suppression inductor and series connected capacitor connected in parallel circuit relationship across the commutating capacitor, the radio frequency suppression inductor also comprising the induction heating coil.

Claim 6

An improved induction cooking unit power supply system according to claim 4 further comprising zero point switch means connected in at least one of said power supply terminals intermediate the output from the full wave rectifier and the filter inductor and filter capacitor for controlling energization of the filter capacitor and the chopper-inverter circuit means, and zero point switching control means connected to and controlling operation of said zero point switch means for assuring soft starting of the chopper-inverter circuit means upon initially being placed in operation and for further reducing radio frequency interference effects of the circuit.

Claim 7

An improved induction cooking unit power supply system according to claim 5 further including power control means for changing the level of output power derived from the chopper-inverter circuit means by varying the commutating frequency of the chopper-inverter circuit means and hence the conduction intervals of the silicon controlled rectifier, and means for auto-matically changing the limit setting of said conduction interval limit setting means simultaneously with each change in the conduction interval of the silicon controlled rectifier by said power control means.

Claim 8

An improved induction cooking unit power supply system according to claim 6 further including over-temperature sensing and control means for sensing an over-temperature condition in a pan or other object being heated by the induction cooking unit, and inhibit means for inhibiting further operation of said zero point switching control means in response to the output of an over-temperature condition alarm signal from said over-temperature sensing and control means, said inhibit means also being responsive to the output from said comparison means in said pan safety control circuit means for shutting off said zero point switch means in response to an alarm output signal from said comparison means, and common resetting switching means manually controlled by an operator of the induction cooking unit for resetting both said pan safety control and said over-temperature sensIng and control means after actuation thereof.

Claim 9

An improved induction cooking unit power supply system according to claim 7 further including pan temperature control means for directly sensing the temperature of a pan or other metal base cooking vessel being heated by the induction cooking unit and having its output coupled to and controlling operation of said inhibit means for controlling operation of the zero point switching means in response to the sensed pan temperature.

Claim 10

An improved induction cooking unit power supply system according to claim 5 further including inhibit means for inhibiting further operation of said zero point switching control means in response to the output from said comparison means in said pan safety control circuit means, said inhibit means serving to shut-off said chopper-inverter circuit means in response to an alarm output signal from said comparison means, and resetting switching means manually controlled by an operator of the induction cooking unit for resetting said pan safety control circuit means after actuation thereof.

Claim 11

An improved induction cooking unit power supply system according to claim 1 further including power control means for changing the level of output power derived from the chopper-inverter circuit by varying the commutating frequency of the chopper-inverter circuit means and hence the conduction intervals of the silicon controlled rectifier, and means for automatically changing the limit setting of said conduction interval limit setting means simultaneously with each change in the conduction interval of the silicon controlled rectifier by said power control means.

Claim 12

An improved induction cooking unit power supply system according to claim 1 wherein said sampling means comprises first and second differentiator circuit means responsive to the conduction of the controlled conductivity control device for developing first and second timing signal pulses representative of the start and the termination of the conduction interval, respectively, said conduction interval limit setting means comprises monostable multi-vibrator circuit means responsive to the first timing signal pulse for generating after a preset delay period a pedestal gating signal pulse of preset time duration definitive of the preset safe period of condution limits, said comparison means comprises coincidence circuit means responsive to the pedestal gating signal pulse and to said second timing signal pulse for determining whether said second timing signal pulse occurs within the safe limits defined by the delayed pedestal gating signal pulse, and said safety means is connected to and controlled by the output from said coincidence circuit means.

Claim 13

An improved induction cooking unit power supply system according to claim 11 wherein the inverter circuit means comprises a chopper-inverter circuit means utilizing a silicon controlled rectifier having a feedback diode connected in inverse parallel circuit relationship therewith across a pair of power supply terminals and having a gating electrode, a self-excited gating circuit for controlling the turn-on of the silicon controlled rectifier coupled to the gating electrode, said self-excited gating circuit including a built-in low voltage gating circuit power source that energizes the gating circuit and that is connected across and derives its power from the power supply terminals connected across the silicon controlled rectifier device, and wherein said safety means comprises a fast responding gate control conduction device connected across the gating circuit power source in parallel circuit relationship therewith for shunting the gating circuit power source in response to an output signal from the coincidence circuit means and thereby shut down the chopper-inverter.

Claim 14

An improved induction cooking unit power supply system according to claim 12 further including full wave rectifier means connected to and supplying the chopper-inverter circuit means and adapted for excitation from a conventional rEsidential source of alternating current electric energy with said pair of power supply terminals connecting the output from said full wave rectifier means across the load terminals of the silicon controlled rectifier and parallel inverse connected feedback diode, a filter inductor connected in at least one of said power supply terminals intermediate the output of the full wave rectifier means and the chopper-inverter circuit means, a filter capacitor connected across the output of the full wave rectifier means intermediate the full wave rectifier means and the filter inductor, and wherein said chopper-inverter circuit means further comprises at least a commutating inductor and series connected commutating capacitor with the series connected commutating inductor and commutating capacitor being connected across the power supply terminals in parallel circuit relationship with the silicon controlled rectifier and parallel inverse connected feedback diode and, a radio frequency suppression inductor and series connected capacitor connected in parallel circuit relationship across the commutating capacitor with the radio frequency suppression inductor also comprising the induction heating coil.

Claim 15

An improved induction cooking unit power supply system according to claim 13 further comprising zero point switch means connected in at least one of said power supply terminals intermediate the output from the full wave rectifier and the filter inductor and filter capacitor for controlling energization of the filter capacitor and the chopper-inverter circuit means, and zero point switching control means connected to and controlling operation of said zero point switch means for assuring soft starting of the chopper-inverter circuit means upon initially being placed in operation and for further reducing radio frequency interference effects of the circuit.

Claim 16

An improved induction cooking unit power supply system according to claim 14 further including power control means for changing the level of output power derived from the chopper-inverter circuit means by varying the commutating frequency of the chopper-inverter circuit means and hence the conduction intervals of the silicon controlled rectifier, and means for automatically changing the limit setting of said conduction interval limit setting means simultaneously with each change in the conduction interval of the silicon controlled rectifier by said power control means.

Claim 17

An improved induction cooking unit power supply system according to claim 15 further including over-temperature sensing and control means for sensing an over-temperature condition in a pan or other object being heated by the induction cooking unit, and inhibit means for inhibiting further operation of said zero point switching control in response to the output of an over-temperature condition alarm signal from said over-temperature sensing and control means, said inhibit means also being responsive to the output from said comparison means in said pan safety control circuit means for shutting-off said zero point switch means in response to an alarm output signal from said comparison means, and common resetting switching means manually controlled by an operator of the induction cooking unit from resetting both said pan safety control and said over-temperature sensing and control means after actuation thereof.

Claim 18

An improved induction cooking unit power supply system including in combination inverter circuit means connected across a pair of power supply terminals for converting a supply potential applied across the power supply terminals to a high frequency pulsating excitation potential the frequency and period of which is determined by the conduction intervals of a controlled conductivity control device that comprises a part of the inverter circuit means, said inverter circuit means including as a component thereof an induction heating coil that is excited by the high frequency pulsating excitation potential, and pan safety control circuit means synchronously oPerable with the inverter circuit means for assuring safe operation of the inverter circuit means, said pan safety control circuit means comprising a low voltage pan safety circuit power source that is connected across and derives its power from the pair of power supply terminals, monostable multi-vibrator circuit means energized by said low voltage pan safety circuit power source and having a presetable period of operation for deriving a pedestal limit setting signal pulse of preset pulse duration after a preset period of delay upon the monostable multi-vibrator circuit means being triggered from its stable to its unstable state of operation, first sampling means for deriving a first timing signal pulse representative of the start of each conduction interval of the controlled conductivity control device and for supplying the same to the monostable multi-vibrator circuit means to control operation thereof, second sampling means for deriving a second timing s1gnal pulse representative of the end of each conduction interval of the controlled conductivity control device, coincidence circuit means energized by said low voltage pan safety circuit power source, means for supplying said pedestal limit setting signal pulse and said second timing signal pulse to said coincidence circuit means for deriving an output control signal from said coincidence circuit means upon the second timing signal pulse failing to conform to the preset safe limits defined by the pedestal limit setting signal pulse, and safety circuit means responsive to the output from said coincidence circuit means for shutting down said inverter circuit means in response to an output control signal from the coincidence circuit means.

Claim 19

An improved induction cooking unit power supply system including in combination inverter circuit means connected across a pair of power supply terminals for converting a supply potential applied across the power supply terminals to a high frequency pulsating excitation potential the frequency and period of which is determined by the conduction intervals of a controlled conductivity control device that comprises a part of the inverter circuit means, said inverter circuit means including as a component thereof an induction heating coil that is excited by the high frequency pulsating excitation potential, and pan safety control circuit means synchronously oPerable with the inverter circuit means for assuring safe operation of the inverter circuit means, said pan safety control circuit means comprising a low voltage pan safety circuit power source that is connected across and derives its power from the pair of power supply terminals, monostable multi-vibrator circuit means energized by said low voltage pan safety circuit power source and having a presetable period of operation for deriving a pedestal limit setting signal pulse of preset pulse duration after a preset period of delay upon the monostable multi-vibrator circuit means being triggered from its stable to its unstable state of operation, first sampling means for deriving a first timing signal pulse representative of the start of each conduction interval of the controlled conductivity control device and for supplying the same to the monostable multi-vibrator circuit means to control operation thereof, second sampling means for deriving a second timing s1gnal pulse representative of the end of each conduction interval of the controlled conductivity control device, coincidence circuit means energized by said low voltage pan safety circuit power source, means for supplying said pedestal limit setting signal pulse and said second timing signal pulse to said coincidence circuit means for deriving an output control signal from said coincidence circuit means upon the second timing signal pulse failing to conform to the preset safe limits defined by the pedestal limit setting signal pulse, and safety circuit means responsive to the output from said coincidence circuit means for shutting down said inverter circuit means in response to an output control signal from the coincidence circuit means.

Claim 20

An improved induction cooking unit power supply according to claim 17 wherein said monostable multi-vibrator circuit means comprising a first NPN junction transistor connected in series circuit relationship with first voltage dividing resistor means across the low voltage pan safety circuit power source, a first PNP feedback transistor connected in series circuit relationship with a second set of voltage dividing resistors across the low voltage pan safety circuit power source and having the base thereof connected to an intermediate point on the first mentioned voltage dividing resistor means, a first programmable unijunction transistor having its anode gate connected to an intermediate point on said second set of voltage dividing resistors, a series connected first variable resistor and capacitor circuit connected in parallel circuit relationship with the second set of voltage dividing resistors and having the junction of the variable resistor and capacitor connected to the gate of the programmable unijunction transistor, said last mentioned series connected resistor and capacitor circuit being variable to control the turn-on time of said first programmable unijuntion transistor, and a load resistor connecting the cathode of the first programmable unijunction transistor to the negative terminal of the low voltage pan safety circuit power source, a second NPN junction transistor connected in series circuit relationship with a third set of voltage dividing resistors across the low voltage pan safety circuit power source, the load resistor of the first programmable unijunction transistor being connected through a current limiting resistor to the base of the second NPN junction transistor, a second PNP feedback transistor connected in series circuit relationship with a fourth set of voltage dividing resistors between the base of the second NPN junction transistor and the positive terminal of the low voltage pan safety circuit power source, the base of the second PNP feedback transistor being connected to an intermediate point on the third set of voltage dividing resistors, and a second programmable unijunction transistor device having its anode connected through a second variable resistor to the collector of the second PNP feedback transistor and to a timing capacitor Connected in series circuit relationship with the second variable resistor and the collector-emitter of the second PNP feedback transistor across the low voltage pan safety circuit power source, said second programmable unijunction transistor having its anode gate connected to an intermediate point on the fourth set of voltage dividing resistors and having its cathode connected directly to the negative terminal of the low voltage pan safety circuit power source, the delayed pedestal timing signal pulse output from the monostable multi-vibrator circuit means being derived from the collector of the second PNP feedback transistor after a delay period determined by the setting of the first variable resistor and with the duration of the delayed pedestal timing signal pulse being determined by the setting of the second variable resistor, the trigger input signal to the monostable multi-vibrator circuit means being coupled to the base of the first NPN junction transistor through a differentiator network supplied through a voltage dividing resistor with the potential appearing across the controlled conductivity control device.

Claim 21

An improved induction cooking unit power supply according to claim 18 wherein said coincidence circuit means comprises a set of two PNP transistors having their emitter-collector circuits connected in series circuit relationship with a load resistor across the low voltage pan safety circuit power source, the base of one of the PNP transistors being connected through a current limiting resistor to the output from the collector of the second PNP feedback transistor in the monostable multi-vibrator circuit means and the base of the remaining PNP transistor being connected through an isolating diode to the output of a differentiator network supplied through a voltage dividing resistor with the potential appearing across the controlled conductivity control device, the output from the coincidence circuit being derived from the load resistor connected in series circuit relationship with the two PNP transistors, and being applied to the gating electrode of a latching silicon controlled rectifier that comprises the safety circuit means and that is connected in parallel circuit relationship with the self-excited low voltage gating circuit power supply for the main power silicon controlled rectifier and feedback diode pair through an isolating diode.

Claim 22

An improved induction cooking unit power supply system according to claim 19 wherein the inverter circuit means comprises a chopper-inverter and the controlled conductivity control device comprises a silicon controlled rectifier having a feedback diode connected in inverse parallel circuit relationship therewith across the pair of power supply terminals, said silicon controlled rectifier having a gating electrode, a self-excited gating circuit coupled to the gating electrode for controlling the turn-on of the silicon controlled rectifier, said self-excited gating circuit including a built-in low voltage gating circuit power source that energizes the gating circuit and that is connected across and derives its power from the power supply terminals connected across the silicon controlled rectifier device, and wherein said safety means comprised by the latching silicon controlled rectifier is connected across the low voltage gating circuit power source in parallel circuit relationship therewith for shunting the low voltage gating circuit power source in parallel circuit relationship therewith for shunting the low voltage gating circuit power source in response to an output signal from the coincidence circuit and thereby shut down the chopper-inverter.

Claim 23

An improved induction cooking unit power supply system according to claim 20 further including full wave rectifier means connected to and supplying the chopper-inverter circuit means and adapted for excitation from a conventional residential source of alternating current electric energy with said pair of power supply terminals connecting the output froM said full wave rectifier across the silicon controlled rectifier and parallel inverse connected feedback diode, a filter inductor connected in at least one of said power supply terminals intermediate the output of the full wave rectifier means and the chopper-inverter circuit means, a filter capacitor connected across the output of the full wave rectifier means intermediate the full wave rectifier and the filter inductor, and wherein said chopper-inverter circuit means further comprises at least a commutating inductor and series connected commutating capacitor with the series connected commutating inductor and commutating capacitor being connected across the power supply terminals in parallel circuit relationship across the silicon controlled rectifer and parallel inverse connected feedback diode, and a radio frequency suppression inductor and series connected capacitor connected in parallel circuit relationship across the commutating capacitor with the radio frequency suppression inductor also comprising the induction heating coil.

Claim 24

An improved induction cooking unit power supply system according to claim 21 further including zero voltage switch means connected in at least one of said power supply terminals intermediate the output from the full wave rectifier means and the filter inductor and filter capacitor for controlling energization of the filter capacitor and the chopper-inverter circuit means, and zero point switching control means connected to and controlling operation of said zero point switching means for assuring soft starting of the chopper-inverter circuit means upon intially being placed in operation and for further reducing radio frequency interference effects of the circuit.

Claim 25

An improved induction cooking unit power supply system according to claim 22 further including power control means for changing the power level output derived from the circuit by varying the frequency of operation of the chopper-inverter circuit means through varying the conduction intervals of the silicon controlled rectifier, means for automatically changing the limit setting of said conduction interval limit setting means simultaneously with each change in the conduction interval of the silicon control rectifier by said power control means, over-temperature sensing and control means for sensing an over-temperature condition in a pan or other object being heated by the induction cooking unit, means for inhibiting further operation of said zero point switching control in response to the output of an over-temperature condition alarm signal from said over-temperature sensing means, and common resetting switching means manually controlled by an operator of the induction cooking unit for resetting both said pan safety control and said over-temperature sensing and control means after actuation thereof.

Claim 26

A pan safety control circuit for an induction cooking unit inverter circuit having a controlled conductivity control device for assuring safe operation of the inverter circuit, said pan safety control comprising sampling means responsive to conduction through the controlled conductivity control device for deriving an indication of the conduction intervals of the control conductivity control device, conduction interval limit setting means responsive to conduction through the controlled conductivity control device for defining safety limits to the respective conduction intervals for the control conductivity control device, comparison means responsive to said sampling means and said conduction interval limit setting means for comparing the actual conduction intervals to the preset safe limits, and safety means responsive to the output from the comparison means and controlling operation of the inverter circuit for shutting down operation of the inverter circuit in response to the actual conduction of the controlled conductivity control device failing to conform to the preset safe limits.

Claim 27

A pan safety control circuit for an induction cooking unit inverter circuit having a controlled conductivity control device for assuring safe operation of the inverter circuit, said pan safety control comprising sampling means responsive to conduction through the controlled conductivity control device for deriving an indication of the conduction intervals of the control conductivity control device, conduction interval limit setting means responsive to conduction through the controlled conductivity control device for defining safety limits to the respective conduction intervals for the control conductivity control device, comparison means responsive to said sampling means and said conduction interval limit setting means for comparing the actual conduction intervals to the preset safe limits, and safety means responsive to the output from the comparison means and controlling operation of the inverter circuit for shutting down operation of the inverter circuit in response to the actual conduction of the controlled conductivity control device failing to conform to the preset safe limits.

Claim 28

A pan safety control circuit for an induction cooking unit inverter circuit accordIng to claim 24 wherein the sampling means comprises first and second differentiator circuit responsive to the conduction of the controlled conductivity control device for developing first and second timing signal pulses representative of the start and the termination of the condution interval, respectively, said conduction interval limit setting means comprises monostable multi-vibrator circuit means responsive to the first timing signal pulse for generating after a preset delay period a timing pedestal gating signal pulse of preset time duration definitive of the preset safe period of conduction limits, said comparison means comprises coincidence circuit means responsive to the pedestal gating signal pulse and to said second timing signal pulse for determining whether said second timing signal pulse occurs within the safe limits defined by the delayed pedestal gating signal pulse, and said safety means is connected to and controlled by the output from said coincidence circuit means.

Claim 29

A pan safety control circuit for assuring safe operation of an induction cooking unit power inverter circuit having a silicon controlled rectifier supplied from a pair of power supply terminals, said pan safety control comprising a low voltage pan safety circuit power source that is connected across and derives its power from the pair of power supply terminals, monostable multi-vibrator circuit means energized by said low voltage pan safety circuit power source and having a presetable period of operation for deriving a pedestal limit setting signal pulse of preset pulse duration after a preset period of delay upon the monostable multi-vibrator circuit means being triggered from its stable to its unstable state of operation, first sampling means for deriving a first timing signal pulse representative of the start of each conduction interval of the silicon controlled rectifier and for supplying the same to the monostable multi-vibrator circuit means to control operation thereof, second sampling means for deriving a second timing signal pulse representative of the end of each conduction interval of the silicon controlled rectifier and feedback diode pair, coincidence circit means energized by said low voltage pan safety circuit power source, means for supplying said pedestal limit setting signal pulse and said second timing signal pulse to said coincidence circuit means for deriving an output control signal from said coincidence circuit means upon the second timing signal pulse failing to conform to the preset safe limits defined by the pedestal limit setting signal pulse, and safety circuit means responsive to the output said coincidence circuit means for shutting down said inverter circuit in response to an output control signal from the coincidence circuit means.

Claim 30

A pan safety control circuit for assuring safe operation of an induction cooking unit power inverter circuit having a silicon controlled rectifier supplied from a pair of power supply terminals, said pan safety control comprising a low voltage pan safety circuit power source that is connected across and derives its power from the pair of power supply terminals, monostable multi-vibrator circuit means energized by said low voltage pan safety circuit power source and having a presetable period of operation for deriving a pedestal limit setting signal pulse of preset pulse duration after a preset period of delay upon the monostable multi-vibrator circuit means being triggered from its stable to its unstable state of operation, first sampling means for deriving a first timing signal pulse representative of the start of each conduction interval of the silicon controlled rectifier and for supplying the same to the monostable multi-vibrator circuit means to control operation thereof, second sampling means for deriving a second timing signal pulse representative of the end of each conduction interval of the silicon controlled rectifier and feedback diode pair, coincidence circit means energized by said low voltage pan safety circuit power source, means for supplying said pedestal limit setting signal pulse and said second timing signal pulse to said coincidence circuit means for deriving an output control signal from said coincidence circuit means upon the second timing signal pulse failing to conform to the preset safe limits defined by the pedestal limit setting signal pulse, and safety circuit means responsive to the output said coincidence circuit means for shutting down said inverter circuit in response to an output control signal from the coincidence circuit means.

Claim 31

A pan safety control circuit for an induction cooking unit power inverter according to claim 26, wherein said monostable multi-vibrator circuit means comprising a first NPN junction transistor connected in series circuit relationship with first voltage dividing resistor means across the low voltage pan safety control circuit power source, a first PNP feedback transistor connected in series circuit relationship with a second set of voltage dividing resistors across the low voltage pan safety circuit power source and having the base thereof connected to an intermediate point on the first mentioned voltage dividing resistor means, a first programmable unijunction transistor having its anode gate connected to an intermediate point on said second set of voltage dividing resistors, a series connected first variable resistor and capacitor circuit connected in parallel circuit relationship with the second set of voltage dividing resistors and having the junction of the variable resistor and capacitor connected to the gate of the programmable unijunction transistor, said variable resistor and capacitor circuit being variable to control the turn-on time of said first programmable unijunction transistor, and a load resistor connecting The cathode of the first programmable unijunction transistor to the negative terminal of the low voltage pan safety control circuit power source, a second NPN junction transistor connected in series circuit relationship with a third set of voltage dividing resistors across the low voltage pan safety control circuit power source, the load resistor of the first programmable unijunction transistor being connected through a current limiting resistor to the base of the second NPN junction transistor, a second PNP feedback transistor connected in series circuit relationship with a fourth set of voltage dividing resistors between the base of the second NPN junction transistor and the positive terminal of the low voltage pan safety control circuit power source, the base of the second PNP feedback transistor being connected to an intermediate point on the third set of voltage dividing resistors, and a second programmable unijunction transistor device having its anode connected through a second variable resistor to the collector of the second PNP feedback transistor and to a timing capacitor connected in series circuit relationship with the second variable resistor and the collector-emitter circuit of the second PNP feedback transistor across the low voltage pan safety control circuit power source, said second programmable unijunction transistor having its anode gate connected to an intermediate point on the fourth set of voltage dividing resistors and having its cathode connected directly to the negative terminal of the low voltage pan safety control circuit power source, the delayed pedestal timing signal pulse output from the monostable multi-vibrator circuit means being derived from the collector of the second PNP feedback transistor after a delay period determined by the setting of the first variable resistor and with the duration of the delayed pedestal timing signal pulse being determined by the setting of the second variable resistor, the trigger input signal to the monostable multi-vibrator circuit means bieng coupled to the base of the first NPN junction transistor through a differentiator network supplied through a voltage dividing resistor with the potential appearing across the silicon controlled rectifier and feedback diode pair.

Claim 32

A pan safety control circuit for an induction cooking unit power inverter according to claim 27 wherein said coincidence circuit means comprises a set of two PNP transistors having their emitter-collector circuits connected in series circuit relationship with a load resistor across the low voltage pan safety control circuit power source, the base of one of the PNP transistors being connected through a current limiting resistor to the output from the collector of the second PNP feedback transistor in the monostable multi-vibrator circuit means and the base of the remaining PNP transistor being connected through an isolating diode to the output of a differentiator network supplied through a voltage dividing resistor with the potential appearing across the silicon controlled rectifier and feedback diode pair, the output from the coincidence circuit being derived from the load resistor connected in series circuit relationship with the two PNP transistors, and applied to the gating electrode of a latching silicon control rectifier that comprises the safety means and is connected in parallel circuit relationship with the self-excited low voltage gating circuit power supply for the main power silicon controlled rectifier and feedback diode pair through an isolating diode.

Claim 33

An improved induction cooking unit power supply comprising inverter circuit means including gate controlled power thyristor means coupled to and supplying an induction heating coil load with periodic energization currents, rectifier means for supplying a rectified undulating semi-filtered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit means for developing gating signal pulses for applIcation to the control gate of the gate controlled power thyristor means comprising a part of the inverter circuit means, selectively operable zero point switching means connected intermediate the rectifier means and the inverter circuit means for selectively supplying energizing potential from the rectifier means to the inverter circuit means, zero point switching control means for selectively turning-on said zero point switching means at or near the beginning of each cycle of the rectified undulating semi-filtered unidirectional excitation potential, and pan safety control means responsive to the conductivity characteristics of the metal base cookware being inductively heated for controlling operation of said zero point switching control means for assuring safe operation of said inverter circuit means.

Claim 34

An improved induction cooking unit power supply comprising inverter circuit means including gate controlled power thyristor means coupled to and supplying an induction heating coil load with periodic energization currents, rectifier means for supplying a rectified undulating semi-filtered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit means for developing gating signal pulses for applIcation to the control gate of the gate controlled power thyristor means comprising a part of the inverter circuit means, selectively operable zero point switching means connected intermediate the rectifier means and the inverter circuit means for selectively supplying energizing potential from the rectifier means to the inverter circuit means, zero point switching control means for selectively turning-on said zero point switching means at or near the beginning of each cycle of the rectified undulating semi-filtered unidirectional excitation potential, and pan safety control means responsive to the conductivity characteristics of the metal base cookware being inductively heated for controlling operation of said zero point switching control means for assuring safe operation of said inverter circuit means.

Claim 35

An improved inductive cooking unit power supply according to claim 29 further including start-up delay inhibit control means for inhibiting operation of said zero point switching control means, and control means for applying an on-off control signal to the inhibit control means for controlling operation of said inhibit control means.

Claim 36

An improved inductive cooking unit power supply according to claim 30 wherein said zero point switching means comprises a silicon control rectifier connected in one of the power supply leads from the rectifier means to the inverter circuit means, said zero point switching conrol means comprises voltage divider means connected across the output of the rectifier means intermediate the rectifier means and the zero point switching means and a low voltage pilot switching silicon control rectifier having a control gate connected to the voltage divider and havng its output connected to supply gating current to the zero point switching silicon control rectifier, and said start-up delay inhibit control means comprises an inhibit latching silicon control rectifier having its gate connected to its anode and connected across at least a portion of said voltage divider means for clamping off the supply of gating potential to the pilot switching silicon control rectifier, an inhibit control transistor for selectively shunting the control gate of the inhibit latching silicon control rectifier, a delay timing resistor capacitor charging circuit connected to control the inhibit control transistor, inhibiting transistor means responsive to on-off control signals connected to control the charging of the capacitor in the resistor-capacitor charging circuit, and discharge transistor means connected across the delay timing capacitor for discharging the delay timing cpacitor upon the circuit being turned-off following a period of operation.

Claim 37

An improved induction cooking unit power supply according to claim 31 wherein the gate controlled power thyristor means comprises at least two series connected SCR/feedback diode pairs connected to and conrolling excitation of the inverter circuit means and supplied by said rectifier means with the gate electrodes of the SCRs being supplied in parallel with the output from the gating signals output circuit means whereby the power supply is capable of operation at higher power levels.

Claim 38

An improved inductive cooking unit power supply according to claim 31 further including capacitor switching means operable during non-conducting intervals of the inverter circuit means to switch different values of commutating capacitance into effective circuit relationship in the inverter circuit means to thereby control the power generated by the circuit, and inhibit switching ganged to operate in conjunction with said capacitor switching means for applying inhibit control potentials to the inhibiting transistor means for maintaining the zero point switch silicon control rectifier turned-off while the capacitor switching means is being switched to change the power output of the inductive cooking unit.

Claim 39

An improved induction cooking unit power supply according to claim 29 wherein said gating circuit means inCludes means for deriving a low voltage unidirectional self-excitation potential from the high voltage rectifier unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, gate controlled semiconductor switching means connected intermediate said low voltage excitation potential and said timing circuit charging means for controlling application of said low voltage excitation potential to said timing circuit charging means, the control gate of said semiconductor switching means being coupled to and controlled by the voltage across the gate controlled power thyristor means for controlling supply of the low voltage excitation potential to the timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a second gate controlled semiconductor switching means having a relatively flat current vs voltage conducting characteristic and having its load terminals connected through suitable voltage dividing means across the output from the rectifier means, the control gate of said second constant current semiconductor switching means being connected to and supplied by the voltage responsive switching means, and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching means and coupled to the control gate of the gate controlled power thyristor means.

Claim 40

An improved inductive cooking unit power supply according to claim 29 wherein the inverter circuit means is a chopper-inverter comprised by a filter inductor having an inductance L2, at least two series connected bidirectional conducting gate control semiconductor thyristor switching devices connected in series circuit relationship with the filter inductor across the output from said rectifier means with the filter inductor interposed between the thyristor switching devices and the rectifier means, a commutating inductor having an inductance L1 and commutating capacitor having a capacitance C1 connected in series circuit relationship across the thyristor switching devices and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, means for deriving output power coupled to at least one of the commutating components, the gating circuit means being coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper-inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the vlue w2t2 equals substantially pi /2 radians or greater where w2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching devices following each conduction interval is maintained substantially independent of load.

Claim 41

An improved inductive cooking unit power supply according to claim 35 further including a smoothing inductor having an inductance L3 and a smoothing capacitor having a capacitance C3 that are connected in series circuit relationship across at least one of the commutating components and have impedance values such that the combined impedance of the commutating capacitor, the smoothing inductor and the smoothign capacitor is capacitive in nature and series resonates with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio Frequency interference emission effects, and the smoothing inductor is an inductive heating coil.

Claim 42

An improved inductive cooking unit power supply according to claim 35 wherein said safety control means for assuring safe operation of the inverter circuit means comprises sampling means responsive to conduction through the controlled conductivity silicon control rectifier and feedback diode pair control device for deriving an indication of the conduction intervals of the silicon control rectifier and feedback diode pair, conduction interval limit setting means responsive to conduction through silicon control rectifier and feedback diode pair for defining safety limits to the respective conduction intervals therefore, comparison means responsive to said sampling means and said conduction interval limit setting means for comparing the actual conduction intervals to the preset safe limits, and safety means responsive to the output from the comparison means and controlling operation of the inverter circuit means for shutting down operation of the inverter circuit means in response to the actual conduction of the silicon control rectifier and feedback diode pair failing to conform to the preset safe limits.