Controlled switch circuit for resistive and inductive loads - has parallel connected semiconductor switch and electromagnetic relay switching in sequence across load, and zero-crossing detector

摘要

The alternating current switch uses two switching elements in parallel across the load and either, or both, may be on or off. One element is a semiconductor switch (6), such as a triac or a thyristor, and the other (7) is an electromagnetic relay. A zero-crossing detector (4) fed by mains-derived alternating current (a) creates a train of pulses (6) which are ANDed with a control switching pulse (c) in a logic gate (9). The output (d) from the gate is fed to control and driver circuits (10,11) which drive the semi-conductor and electromagnetic switches. The semiconductor switch is fired in synchronisation with a zero-point in the alternating current cycle, thus achieving a change-over at a point of low circuit current. After the intrinsic switch-on delay, the relay is switched on in parallel. Since the current is already established, radio-frequency arcing interference at contact is eliminated. The control circuit (10) now switches the semiconductor switch off to avoid heating of this element. The semiconductor switch is brought on again just before the relay is switched off, and then off again to open the load circuit.