12v relay turns on when power fails SOLVED

I'm trying to find a relay that turns on a device (12v) when the power fails, and then turns off when the power comes back on. I found this video. Pretty much what I want to do. Is there a device out there that anyone knows about for sale that I don't have to make? I don't have the tools or skills, but I'm happy to buy a device and can go from there wiring it, etc. Thank you for your help.

Here's the video: https://www.youtube.com/watch?v=z62wzYubQ_c

by ironclads
April 15, 2021

If you want to build your own, you basically need one PMOS (Mosfet with P canal) one diode and one large resistor (100k or 1MEG).

by vanderghast
April 16, 2021

That only works for DC though.

by vanderghast
April 16, 2021

You don't need the pull down resistor if the main DC source, on failure, provides a path to ground (ideal source). With that pull down resistor, your are losing some energy in pure lost (heat) by this resistor. But if the failure is like a broken wire, letting the grid of the PMOS dangling (no completed loop), some path to the ground, for the grid of M1, has to be provided.

by vanderghast
April 16, 2021

Thank you! I'll look into it. Would you, or someone you know you can refer me to, be able to build this for me? Otherwise I'll check around to try and find someone. Of course, willing to pay. Thank you again.

by ironclads
April 19, 2021

Only 3 components are required: The PMOS, I suggest an IRF9Z24N, cost around 1.00$ . The diode, !N4001, around 0.15. And a resistor (100k or 1 MEG), around 0.20$. If there is an open electronic shop, open, near your place, that may worth the trip, else, you probably have to pay a high price just for the delivery. And don't forget the wires, neither the connectors to the 12 volt sources.

You can use a "dead bug" montage (stand alone in free air) or standard breadboard. For a final product, the easiest one is probably with a Manhattan Islands montage.

Please take note that the metal plate on the back on the transistor is internally linked to the middle pin (in this casing) which will be linked to the +12V of the back up DC source. Don't short that plate with the ground (the negative terminal of the DC sources or of the circuit you intend to drive).

Given the implied cost of the implied material and the "simplicity" of the circuit, why not trying to do it by yourself? The final packaging can even be a blob of a non conducting resine, which you can mold to fit your aesthetical tastes :-) You may develop a taste to build your own circuits, at low voltage, after that.

by vanderghast
April 19, 2021

If forgot to draw the connection to the circuit to be supplied with the 12 volt DC. You take it from the rightmost pin of the IRF9Z24N.

by vanderghast
April 19, 2021

1 Answer

Answer by ironclads

I searched around and found this. Funny thing, I actually had one in my components drawer. It's exactly what I needed.


Thank you for your help and time. I'm glad there is a place like this to ask questions and find answers.

ACCEPTED +1 vote
by ironclads
April 20, 2021

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