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Which relay?

Hi, I have a customer that has LED lighting in his house and when the mains power goes out and then comes back on the LED's have an auto on function. This is proving quite annoying to him when it happens at say 2.00am.  What we need to achieve is having the power to the circuit remain off once the mains power is restored.  I have looked at latching and non latching relays but cant quite figure how they will work correctly.  Any advice or tips that can be provided would be most appreciated.Thanks, Neil

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August 8, 2018 09:07

Hi BradLevy

Looks like you have cracked it! Thanks so much for you input. This one was doing my head in for a while, couldn't think of the solutuion but once you see it it is obvious.
Thanks again

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August 6, 2018 14:45

You can use a single pole relay instead of a double pole relay, but if you do, SW1 has to be capable of handling the full output load. With the double pole relay, SW1 only needs to handle the relay coil current.

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August 6, 2018 14:45

The attached schematic image shows a circuit to do what you want.
It uses a double pole single throw (or double pole double throw) relay.

Initially, power to the load is off.
Pushing SW1 provides power to the relay coil, turning on the relay and providing power to the load using one set of relay contacts, and providing power to the relay itself through the other set of contacts. So even though the pushbutton closure is momentary, the relay will stay on after the button is released.
When a power failure occurs, the relay turns off, opening both contacts.
This interrupts its own power input, so the relay (and output power) will stay off until SW1 is pushed again.

SW2 is an optional normally closed switch. Pressing SW2 interrupts power to the relay coil, causing the relay contacts to open, disconnecting power from the load and from the relay coil, so the output remains off once SW2 is released.
If you omit SW2, replace it with a wire.

-Brad

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