GB982539A - Improvements relating to d.c. supply filter circuit arrangements - Google Patents

Abstract

982,539. Smoothing D.C. WESTING- HOUSE BRAKE & SIGNAL CO. Ltd. Jan. 24, 1964 [Feb. 20, 1963], No. 6782/63. Heading H2F. A direct current smoothing circuit comprising an inductor L and capacitor C includes controllable rectifier means shunting the inductor and rendered conductive by a sensing circuit on the occasion of a transient surge at the output terminals. As shown, a thyristor SOR1 shunts the inductor L and has a resistor R3 and Zener diode ZD1 in its gate-cathode circuit. The sensing circuit comprises a diode D2, resistor R1 and a parallel RC circuit R2, C1 in series across the output terminals, the junction of R1 and C1 being connected to the gate through R3. In operation, when the current I falls rapidly due to changing load conditions the current through L cannot change rapidly and current is diverted to the capacitor C causing an increase in output voltage. The capacitor C1 which has also charged to the normal peak voltage tends to follow the increase in voltage on overshoot; however, by virtue of the time constant or the circuit R1, C1, the rise of voltage on C1 is delayed. As a consequence a forward bias is applied to gate cathode circuit, firing the thyristor before the overshoot has risen by more than a few volts. A circulating current flows through L and SCR1, and the excess voltage is limited to the forward voltage drop across SCR1 which may be only a few volts. The Zener diode ZD1 and resistor R3 protect the thyristor on switching on. In modifications D2 and R1 may be omitted since a similar path exists through the gate-cathode circuit and R3; an additional diode may be connected in series with ZD1; a transistor amplifier may be used in the sensing circuit; and the thyristor may be replaced by a transistor and diode in series.