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Latching relay OFF on power lost

Discussion in 'General Electronics Discussion' started by kickit2, Jan 2, 2012.

  1. kickit2


    Nov 20, 2011
    So I am controlling a dual coil latching relay via a MCU. If works well and good so long as there is power to the MCU. Problem is if the MCU where to lose power for some reason, I want the relay to be triggered off for safety being that there is no longer any control.

    I was thinking that I could possibly charge a cap while the circuit is live but then isolate the second side of the coil from it with a jfet that is wired to the main rail. When power is lost, the jfet would lose saturation and power would be able to reach the coil and trip it off. Assuming I choose a large enough cap, is my theory good here, or is there a better way to do this that I am not thinking of?

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Place a normal (non latching) relay in series with the latching relay and have it powered by the MCU supply. When that fails, the circuit will open.

    Failing that, simply use a non-latching relay.

    Or for your FET idea, use a P channel mosfet with the gate held high by the supply voltage. When this falls to zero it could switch on the device and discharge your capacitor into the relay. You would have to ensure that the part of the circuit powered by the capacitor doesn't cause problems to other non-powered parts of the circuit.
  3. KMoffett


    Jan 21, 2009
    Can you post a schematic of what you currently have? Might help in finding options.

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