Best practive Relay Triggring -Looking to understand

Thread Starter


Joined Aug 24, 2014
Dear Members,

I am into automation, Is it better to design the relay circuit for LOW - Trigger or High to Trigger?

Bhimsen Bukkebag


Joined Jan 27, 2019
It is best to choose the fail safe mode. If the safe position is OFF then high is fail safe, if the safe position is ON then low is fail safe.
You can also look at "safety relays".

Something I discovered later was a concept called "monitored contacts". It's used in fire alarm systems. I probably should have used the concept when designing a safety system, but I, sort of, did. Cables (fire alarm cable) within the room were 24 VAC contact closures. Cables that left the room used 3 wires. One was the closure and one was power to an LED. You could loose one wire and not know. If a cable were cut, the panel would know. The side effect was that the panel knew when the building fire alarm was reset. The trip from the Fire Alarm Panel would have a labeled indicator labeled FAP and there would be a small LED lit by that light. While the FAP was tripped, the LED would be off. When not tripped it would be ON.
I should have used 2 conductor monitored contacts where there would be a resistor across the contact.

Example feed the system with a resistor and a power supply V, with the same value resistor across the contact, you can get 0, 0.5*V and V back depending on the state. Fixed typo - made it clearer.
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