| 摘要 |
A circuit is disclosed for initially supplying full-wave rectified alternating current (A.C.) to a solenoid and, after a predetermined time, supplying automatically half-wave rectified A.C. to the solenoid to reduce the heating thereof. Two controlled rectifiers (SCR's) are connected with two diodes in a bridge configuration to supply, when both controlled rectifiers are conducting, a full-wave rectified A.C. to the solenoid. The first SCR is triggereed into conduction during each half cycle by a fixed firing signal. Pulses supplied by the collapsing magnetic field of the solenoid when the first SCR turns off in each cycle are used to charge a capacitor through a resistance connected across the gate and cathode of the second SCR thereby initially triggering the second SCR alternately with the first SCR into conduction in each cycle. However, as the capacitor charge builds up incrementally to maximum, the successive current pulses become smaller until they are insufficient to continue the triggering of the second SCR, which then turns off leaving only the first SCR conducting to supply half-wave rectified A.C. to the solenoid which is sufficient to hold its armature in but insufficient to cause abnormal heating. A diode gate controls the discharge path for the capacitor. This diode is reverse-biased by a circuit which clamps its anode to the most negative excursions of the applied A.C. voltage and prevents discharge of the capacitor until the A.C. voltage is removed, whereupon the capacitor is discharged and is thus reset for the next charging operation initiated when the A.C. is again restored. The first SCR may be replaced by a diode if low-level shut-off of the system is not desired.
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