GB986901A - Improvements in or relating to circuits for silicon controlled rectifiers - Google Patents

Abstract

986,901. Automatic voltage control. PYE Ltd. April 9, 1963 [April 19, 1962], No.15166/62. Heading G3X. [Also in Division H3] A circuit for firing a silicon controlled rectifier at an instant in each half-cycle of an alternating-current supply voltage which varies in accordance with the peak-voltage value of the immediately preceding half-cycle in such a sense as to render the output of the SCR less dependent upon variations in the supply voltage comprises a full-wave rectifier and storage capacitor, the voltage across which decays slightly between successive peaks of the fullwave rectified voltage,a further capacitor which is charged from said storage capacitor during the intervals when the storage capacitor is itself not receiving charge from the rectifier circuit so that at the beginning of each half-cycle the further capacitor holds a voltage proportional to the peak voltage of the previous half-cycle,the charging of said further capacitor being interrupted as soon as the storage capacitor commences being re-charged from the full-wave rectifier so that said further capacitor commences discharging through a discharge circuit therefore, and a bi-stable circuit which when the voltage across said further capacitor has fallen to a predetermined value is triggered to a conductive condition in which it supplies an output pulse to initiate firing of the SCR and provide a charging path whereby said further capacitor is again recharged to the voltage of the storage capacitor. In the circuit illustrated, when the capacitor D3 is cut off so that charge is not being supplied to storage capacitor C1 from the full-wave rectifier, the potential at the emitter of transistor TR1 is greater in a negative sense than that at the upper end of resistor R1 so that capacitor C2, previously discharged when diode D3 was conductive,is charged through the baseemitter path of transistor TR1 so rendering the transistor conductive. This maintains capacitor C3 charged to the voltage of capacitor C1 and also ensures that bi-stable transistor pair TR2, TR3 are non-conductive. When, as the peak of the following half-cycle approaches,diode D3 is rendered conductive,the voltage between emitter of TR1 and the upper end of resistor R1 reverses so switching transistor TR1 off and discharging capacitor C2 through resistor R6. The charging circuit for capacitor C3 being no longer completed this capacitor discharges through resistor R5 until its voltage falls to such a low value that it is smaller negatively than that at the junction of resistor R3 and Zener diode D5; diode D4 then conducts and draws current from the base of transistor TR3 thus triggering the bi-stable pair to the condition in which both are conducting. Capacitor C3 thereupon recharges to the voltage of capacitor C1 which is still being charged through diode D3. When D3 cuts off once again transistor TR1 is rendered conductive so diverting current from transistors TR2,TR3 to render them non-conductive as previously described and the cycle repeats. A control voltage may be applied at X to replace that developed across Zener diode D5. The bi-stable transistor pair TR2, TR3 may be replaced by a four-layer solid bi-stable device.