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What is a good mechanical relay to use to power on an off a wall airconditioning unit? Answered

cross-posted from here: https://www.instructables.com/community/Relay-to-control-window-air-unit-first-attempt-f/

I'm working on a thermostat project to control window AC units (which I will create an instructable for when it works). The need is to switch on and off an AC unit. The unit runs on 240 VAC (split phase) and the technical documentation says to put it on a circuit with a 20 AMP circuit breaker. I will be controlling the relay with 5 volts from an Arduino (although I suppose I can use an additional small relay to control the larger relay with a higher control voltage, if necessary... the less moving parts the better.)

I already tried with a 20 AMP Solid State Relay (SSR) from Omron, but it failed. The problem is that, due to the voltage drop in an SSR, they get really hot. The hotter they get, the less current they are able to switch. The other problem is that SSRs fail with the circuit closed or partially closed. This is not ideal. You wouldn't want to come back to your place after a vacation and find your AC had been running constantly the whole time...

I am thinking I need to use an electromechanical relay, but there are so many options on Mouser that I'm feeling a bit overwhelmed! Can someone please help point me in the right direction or suggest a good relay to use?


The rating of a circuit breaker, which, god willing, shouldn't ever trip isn't the sort of rating a "relay" needs for an A/C.

You should be looking for devices called "CONTACTORS", which are BIG relays, designed to switch heavy loads. You won't find one with a 5V coil ! You WILL find one with a 24V DC coil. Contactors are actually usually rated for the horsepower or kW rating of the motor you're switching, so here, you need one that can handle 4.8 kW

Good brands are "Siemens", Square-D, Klockner Moeller. Cheap, reasonable brand are "Chint"


Thanks Steve! I did not know about contactors. Really appreciate the help.

You WILL need a VERY beefy protection diode on the coil of the contactor.