Electronic load protection circuit

摘要

A load protecting circuit includes a triac connected in series with the load to provide an open circuit when a voltage drop or current surge is applied to the load. The triac is gated through a transistorized gating circuit which includes a zero-crossing detector so that the gate of the triac is turned on or off only during zero-crossing points of the AC line voltage. The gating circuit is responsive to a voltage drop detector comprising a photo-optical isolator which turns off as the voltage drop in the input line drops below a predetermined value and, thus, turns off the gating current to the triac. The current surge protector comprises a transistor of reverse polarity from the transistors contained in the gating circuit so that a gate current derived from the current surge and applied to the transistor turns off the activating current for the gating circuit. The device also includes a varistor for protection against spike voltage impulses and a radio frequency suppression line.