# Relay for 120v input on amps, use with 12v trigger



## patchesj

Looking for a relay to turn my amps on (no internal relay) with a 12v trigger. I'm begining to think I'll need to use 2 relays, a 12v coil low amp to then trigger a larger 120v coil and the mains power. Would be nice to get this done with a single relay. Anyone built something like this?


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## NegativeEntropy

First, this involves mains voltage wiring. If you are not confident in your ability to do this correctly (read: safely) do not proceed with a DIY approach. I would go so far as to say that if you have not done mains wiring before, do not do this yourself. 

Just buy something like this: http://www.amazon.com/XANTECH-AC1-Controlled-AC-Outlet/dp/B000NU0T62 Note that the specs on that page do not indicate how much DC current it draws. It would be best to find out first (see the section on coil amperage below as it's the same concept).

If you want DIY and can do it safely:

Is the 12V trigger latching (i.e. always on once triggered until turned off) or momentary? If momentary, this gets more complex and is beyond the scope of this reply. Sorry to get your hopes up:neener: 

The below assumes it's a latching 12V trigger.

You should be able to do this with 1 relay. Think of the relay as a switch for a 120 volt light, but instead of your finger flipping it, it's the 12 volt trigger. You just need the right specifications for the coil side (the 12 V trigger will energize the coil, which flips the switch) and the contact side (the actual 120 V switch).

Spend some time learning to read relay specifications. I'm rusty at it, but here's my take on what you'll need.

Coil Side:
1) You need a 12V or higher DC coil voltage and the turn on voltage to be less than whatever your 12V trigger puts out, say 9V.
2) The 12V side of the relay will draw a certain amount of current. This must be less than the current your trigger can output. e.g. if the trigger is labeled "100 mA max" the relay must draw less than this. It will be labeled something like "rated current" for the relay (for the coil side, not the contact side).
3) The relay must be non-latching (or it will stay on once your receiver is off).

Contact side:
1) Must be rated for the nominal of your mains voltage or higher. If you're in the US, that's 120VAC or more. e.g. 240VAC would be fine.
2) Get one rated for 15 or 20A. This will be enough and that way it'll be safe if something else is plugged into it. *15A assumes your amps draw less than ~1800 watts total* (again, assuming US power of 120VAC).

Wiring:
1) I'd get a relay socket and a compatible socket mounted relay.
2) Mount it and and a 120VAC receptacle all in a NEMA enclosure (I'd use a DIN rail, but I have a few feet of that laying around from previous projects). Wire it with a 3 prong cord that supports 15A or more. *Make sure to wire the ground properly to the receptacle and the enclosure if it's metal.* Wire in the 12V trigger to the coil side of the relay..
3) Check it with a circuit tester or DMM before hooking up your load(s).

Even though I'm a major geek and comfortable with electronics, after reading all of that (which just hits the highlights) I think I'd just buy an off the shelf product 

Good luck.


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