Protection circuit for switches liable to failure - has separate controls for normal conditions and failure

摘要

The protection circuit is used to prevent switches operating incorrectly as a result of failure or damage to the switch system and each switch or set of contacts is connected over a diode and resistor to two switching stages. The first stage assumes control of the switching operations during normal conditions and the second stage assumes control during failures. Each switching stage contains a transistor whose collector is coupled to a diode forming part of an AND-gate in yet another switching stage. This last switching stage has a transistor that operates an indicating relay.