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Topic starter
2020-09-20 10:31 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!
Mike
2020-09-20 1:12 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.
DroneBot Workshop reacted
2020-11-11 2:48 pm
salvage the relays off of the board. 99% of the time, A/C relays are DC relays with diodes in them to change the A/C to D/C. It all sounds feesible, just tedious tho. Between the code for the thermo samplers, LED displays, setting up the hysteresis boundaries and timing so you wont have the fan and comp kicking in at the same time and pulling enuf amps to weld everything together 😉 I guess you could retain all the fancy functions of econ and fan speeds etc... sounds like a good challenge.
2020-11-11 3:50 pm
And promises a future video - which if it is available I haven't been able to find.
It's right here (made it after your original post):
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.
Yes, I agree. You'd need a way of emulating that logic otherwise it might cause your very expensive AC to become a very expensive weight holder, which is what I use my not-so-expensive AC for!
😎
Bill
"Never trust a computer you can’t throw out a window." — Steve Wozniak
2020-11-11 4:29 pm
@dronebot-workshop (and others)
DIY electrical equipment design for this type of thing is a full blown engineering project and it's probably bigger than you think.
And if you cause a fire any insurance company involved will accuse you of using uncertified equipment and refuse to pay up. Or if you sell the house containing this equipment you may be exposed to all kinds of liability if anything goes wrong with it. At this scale I don't think it's worth the risk.
As a long time home owner I am sure I could do a better job of household appliance design than what is on the market but I hesitate going beyond the experimental stage because of this.
DroneBot Workshop reacted
2020-11-11 6:14 pm
I agree completely. The insurance aspect alone is enough to keep me away from this sort of project, you're also dealing with some pretty high currents and a high-pressure compressor.
Getting the original replacement controller board as @triform suggested is the best route to go.
😎
Bill
"Never trust a computer you can’t throw out a window." — Steve Wozniak