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I'm basically looking for a reverse relay: I want to switch around 5V with wall outlet power (230V 50Hz in my country). When wall power goes out, the low-voltage circuit should open. I don't have space to use a phone charger / wall wart, and I'd like to avoid anything involving just a diode + resistor which would heat up and waste energy.

Do these kind of relays exist? I previously thought this kind of thing could be accomplished easily but now that I'm looking, I can't find anything.

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    \$\begingroup\$ USB wall warts get pretty darn small! The important thing is they are listed by credible third party testing labs, so they are safe. Something you build won't be. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Jul 13, 2021 at 23:47
  • \$\begingroup\$ What are you actually trying to do? What is the purpose for this circuit? Why not just use a 5V adapter? It will naturally go off when the power goes off... \$\endgroup\$
    – J...
    Commented Jul 14, 2021 at 12:59
  • \$\begingroup\$ Your constraints are also puzzling - if you don't have room for a USB adapter then a 230VAC relay isn't going to be much smaller, if at all. More details here would get you a much better answer. \$\endgroup\$
    – J...
    Commented Jul 14, 2021 at 13:52

1 Answer 1

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I'm basically looking for a reverse relay.

No. You're looking for a relay. You just need one with a 230 V AC coil.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The relay contact will open when mains power is lost.

Don't omit the fuse.

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  • \$\begingroup\$ You might have problems with low-load conditions here, though. Ideally, find a relay with gold or rhodium contacts, not tungsten. \$\endgroup\$
    – Hearth
    Commented Jul 14, 2021 at 3:45
  • \$\begingroup\$ Fuse is nice, but PE and overvoltage protection aren't optional either... \$\endgroup\$
    – Dmitry Grigoryev
    Commented Jul 14, 2021 at 7:11
  • \$\begingroup\$ @DmitryGrigoryev PE isn't needed if you build it to class II, and overvoltage protection for a simple relay would be excessive if the fuse is chosen properly (slow blow type, low value) to protect everything from the accompanying overcurrent \$\endgroup\$
    – Chris H
    Commented Jul 14, 2021 at 13:06

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