Overload protection circuit for solid state switch
First Claim
1. In a control device for controlling the application of power from an alternating current power source to a load having a normal impedance, said control switch connected in series with the alternating current power source and the load so that alternating current is supplied to the load when the switch is in a conductive state, means for inhibiting the initiation of conduction by the solid state switch when the impedance of the load is so substantially below its normal value that current of a magnitude potentially destructive to the solid state switch would pass through the switch if it were in its conductive state, comprising:
- a shunt impedance connected in shunt with the solid state switch and in series with the load and the alternating current power source, said shunt impedance forming an internal power supply for the control device deriving power from the alternating current power source from the voltage across the solid state switch when the solid state switch is nonconductive;
first means for inhibiting the application of triggering signals to the solid state switch following each zero-crossing of the alternating current power source until a predetermined voltage is built up across said shunt impedance to provide an interval of electric power when the solid state switch is nominally conducting; and
second means for inhibiting the application of triggering signals to the solid state switch for a predetermined time when the rate of rise of the voltage across said shunt impedance exceeds the rate which would exist if the impedance of the load was said normal value.
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Accused Products
Abstract
A control device for energization of a load employs a bidirection triggerable solid state switching device connected in series with an alternating current power source and the load. In order to prevent the passage of excessive, potentially destructive currents through the switch if the load is shorted out, the microprocessor which controls the energization of the switch senses the time required for the voltage across the switch to attain a predetermined value. If the load is shorted the potential difference across the switch increases and attains the predetermined value a shorter time after zero-crossing of the alternating current. When this occurs more than a predetermined number of consecutive times, the generation of triggering pulses for the solid state switch is inhibited for a predetermined time. In the preferred embodiment the control device is a thermostat for controlling a temperature modifying load such as a furnace or air conditioner.
46 Citations
17 Claims
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1. In a control device for controlling the application of power from an alternating current power source to a load having a normal impedance, said control switch connected in series with the alternating current power source and the load so that alternating current is supplied to the load when the switch is in a conductive state, means for inhibiting the initiation of conduction by the solid state switch when the impedance of the load is so substantially below its normal value that current of a magnitude potentially destructive to the solid state switch would pass through the switch if it were in its conductive state, comprising:
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a shunt impedance connected in shunt with the solid state switch and in series with the load and the alternating current power source, said shunt impedance forming an internal power supply for the control device deriving power from the alternating current power source from the voltage across the solid state switch when the solid state switch is nonconductive; first means for inhibiting the application of triggering signals to the solid state switch following each zero-crossing of the alternating current power source until a predetermined voltage is built up across said shunt impedance to provide an interval of electric power when the solid state switch is nominally conducting; and second means for inhibiting the application of triggering signals to the solid state switch for a predetermined time when the rate of rise of the voltage across said shunt impedance exceeds the rate which would exist if the impedance of the load was said normal value. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A control device for controlling the application of power from an alternating current power source to a load by control of conduction between a pair of terminals connected in series with the alternating current power source and the load, said control device comprising:
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a triggerable bidirectional solid state switch connected across the pair of terminals for conduction between the pair of terminals when triggered; a control device power supply connected across the pair of terminals in shunt with said solid state switch for deriving power for operation of the control device from the alternating current power source from the voltage across said solid state switch when said solid state switch is nonconductive; a control circuit connected to said solid state switch for selective generation of a triggering signal to trigger said solid state switch; a trigger delay means connected to said solid state switch and said control circuit for delaying application of said triggering signal to said solid state switch following each zero-crossing of the alternating current power source until the voltage across said solid state switch reaches a predetermined voltage in order to provide an interval of electric power to said control circuit power supply when said solid state switch is nominally conductive; a voltage rise time measuring means connected to said control device power supply for determining if the rate of rise of voltage across said control device power supply exceeds a predetermined rate related to the nominal impedance of the load; and an inhibiting means connected to said control device and said voltage rise time measuring means for inhibiting application of said triggering signal to said solid state switch for a predetermined length of time longer than one cycle of the alternating current power source if the rate of rise of voltage across said control device power supply exceeds said predetermined rate. - View Dependent Claims (13, 14, 15)
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16. A control device for controlling the application of power from an alternating current power source to a load by control of conduction between a pair of terminals connected in series with the alternating current power source and the load, said control device comprising:
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a triggerable bidirectional solid state switch connected across the pair of terminals for conduction between the pair of terminals when triggered; a control device power supply connected across the pair of terminals in shunt with said solid state switch for deriving power for operation of the control device from the alternating current power source from the voltage across said solid state switch when said solid state switch is nonconductive; a trigger delay means connected to said solid state switch for delaying application of a triggering signal to said solid state switch following each zero-crossing of the alternating current power source until the voltage across said solid state switch reaches a predetermined voltage in order to provide an interval of electric power to said control circuit power supply when said solid state switch is nominally conductive; and a microprocessor means connected to said solid state switch, said control device power supply and said trigger delay means, said microprocessor means programmed for selectively generating a triggering signal for application to said trigger delay means to trigger said solid state switch for application of power from the alternating current power source to the load in accordance with a predetermined control function, determining if the rate of rise of voltage across said control device power supply exceeds a predetermined rate related to the nominal impedance of the load, and inhibiting generation of said triggering signal for a predetermined length of time longer than one cycle of the alternating current power source if the rate of rise of voltage across said control device power supply exceeds said predetermined rate for more than a predetermined number of consecutive times. - View Dependent Claims (17)
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Specification