Abstract A control circuit for preventing the application of accelerating potential in excess of a predetermined maximum value to the cathode-ray picture tube of a television receiver. Positive polarity pulses amplitude-related to the applied accelerating potential are derived from the receiver horizontal deflection system and impressed across the emitter-base junction of a transistor. The base of this transistor is biased at a DC level such that the impressed pulses will exceed the breakdown potential of the junction when the accelerating potential exceeds its maximum value. When this occurs a control voltage is applied by the transistor to the control electrode of a silicon controlled rectifier (SCR) which has principal electrodes connected between the screen of the horizontal deflection system output tube and ground. This causes the SCR to conduct, substantially reducing the screen current applied to the output tube and hence the energy available for powering the receiver''s sweep-excited high voltage power supply, and preventing further generation of excessive accelerating potential. To prevent the receiver from being rendered inoperative by a transient overload condition, a capacitor couples pulses from the output circuit of the receiver vertical deflection system to the screen of the horizontal output tube to periodically reduce the quiescent current through the principal electrodes following an overvoltage condition to a value below the minimum holding current required to sustain conduction, thereby restoring the SCR to a nonconductive state and the high voltage power supply to an operative state. During a sustained overvoltage condition the control circuit causes the receiver to alternate between operative and nonoperative states, which ''''blinking'''' action on the receiver viewing screen effectively indicates a malfunction.
In a television receiver of the type including an image reproducer requiring an externally applied high voltage accelerating potential, and a high voltage power supply energized by the horizontal output stage of said receiver for supplying said potential, a control circuit for preventing said accelerating potential from exceeding a predetermined maximum value comprising: means including an over-voltage detector coupled to said horizontal output stage for producing a control voltage in response to said accelerating potential exceeding said predetermined maximum value; bistable control means including a silicon controlled rectifier having a first principal electrode coupled to said horizontal output stage, a second principal electrode coupled to a reference potential, and a control electrode coupled to said overvoltage detector, said silicon controlled rectifier being actuatable by said control voltage from a nonconductive state to a conductive state and remaining thereIn until restored, to disable said horizontal output stage when said bistable control means is in said conductive state, thereby interrupting said power supply; and means coupled to one of said principal electrodes for periodically restoring said silicon controlled rectifier to said nonconductive state by reducing the current through said principal electrodes below the minimum holding current required to maintain said silicon controlled rectifier in its conductive state.
Claim 2
Dependent Claim
205 words
In a television receiver of the type including an image reproducer requiring an externally applied high voltage accelerating potential, and a high voltage power supply energized by the horizontal output stage of said receiver for supplying said potential, a control circuit for preventing said accelerating potential from exceeding a predetermined maximum value comprising: means including an over-voltage detector coupled to said horizontal output stage for producing a control voltage in response to said accelerating potential exceeding said predetermined maximum value; bistable control means including a silicon controlled rectifier having a first principal electrode coupled to said horizontal output stage, a second principal electrode coupled to a reference potential, and a control electrode coupled to said overvoltage detector, said silicon controlled rectifier being actuatable by said control voltage from a nonconductive state to a conductive state and remaining thereIn until restored, to disable said horizontal output stage when said bistable control means is in said conductive state, thereby interrupting said power supply; and means coupled to one of said principal electrodes for periodically restoring said silicon controlled rectifier to said nonconductive state by reducing the current through said principal electrodes below the minimum holding current required to maintain said silicon controlled rectifier in its conductive state.
Claim 3
Dependent Claim
33 words
A control circuit as described in claim 1 wherein said restoring means comprises a source of periodic pulses and means for applying said pulses to said principal electrodes of said silicon controlled rectifier.
Claim 4
Dependent Claim
36 words
A control circuit as described in claim 2 wherein said source of pulses comprises the vertical deflection stage of the receiver, and wherein said means for applying said pulses to said principal electrodes comprises a capacitor.
Claim 5
Dependent Claim
55 words
A control circuit as described in claim 3 wherein said horizontal output stage includes an electron-discharge device having a cathode, a control grid, a screen grid and an anode, said silicon controlled rectifier is coupled between said screen grid and ground, and said capacitor is coupled between said vertical deflection stage and said screen grid.