Switch AC motors on...
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Switch AC motors on and off with Arduino  


New Member
Joined: 2 months ago
Posts: 1
2020-09-20 11:31 am  

Hi All,

I have a large (and expensive) air conditioner which is now obsolete because the control board is no longer manufactured (or that's the story from the vendor).

The control board essentially switches three motors (two fans and a compressor) in a timed sequence and switches the compressor off when it gets too cold. 

The existing control board has a 7 amp fuse and the three motors are switched using AC 220 volt relays.

The DroneBot video "Arduino High-Current Interfacing – Transistors & MOSFETs" ends with the warning "But they cannot be used to control AC devices, so don’t even try." And promises a future video - which if it is available I haven't been able to find.

What I'm proposing to do is either have three 5 volt DC 10 amp relays directly attached to the Arduino or alternatively three 12 volt DC relays (so it is an independent power supply) switched using three mosfets. These relays will switch the 220 volt AC relays.

In view of the dire warning on the above referenced video am I about to do something daft?

Much appreciated!


Reputable Member
Joined: 1 year ago
Posts: 331
2020-09-20 2:12 pm  


I would try and source the control board outside the vendor. Unless it's some soviet made oddity from the cold war era, you should be able to find it 😉

The board probably has a bit of "logic" to run the compressor or fans in certain combinations based on conditions. It may have a cycle phase it uses to get the compressor to a settled or know state based on temperature or current readings from the compressor or motors.

Most times in the HVAC world, the switching is done by a relay and contactor combination. With that setup, you will control a 12 or 24 volt (normally 24vac) relay to switch the contactor on or off.  Having a uC controller in the mix will need some special considerations for it to be stable and safe. Past saying that, I will stop as this is not for the inexperienced with multi-phase high current AC.