Intrinsically-safe timing circuit - has zener diode shunting series circuit of thermal timing relay and switching relay

摘要

The intrinsically-safe timing circuit consists of a thermal timing relay in series with a switching relay. The series combination of these two relays is shunted by a single zener diode. The resulting combination is connected to the output of a 4-diode rectifier bridge with a ballast resistor at its input. The zener diode reduces the voltage across the relays and contributes to making the circuit independent of voltage even if the zener diode is open circuit.