# How switch 20A AC between 2 wires

I'm trying to translate a circuit I made from breadboard to PCB and need some help finding out what components I should be looking for.

Basically, so far I have a very simple circuit running 12V AC, at about 10-20A using a relay.

I'm controlling this relay with an arduino, so if I give 0v the circuit flows towards circuit A, and if I give 5v, the relay switches the flow to circuit B.

My issue is that the relay is too big for my application, I need something smaller, that I can fit in to a smaller chip.

From my research FET's is what I should be using, but are there other easier components I can use? With FET's, I will need to use an optocoupler to drive them from the arduino correct? Are there simpler ways?

• What is "too big" and what is your max size requirement? anything where 20A goes through cant be tiny, and 20A micro relays arent that big... Feb 25 '15 at 23:15
• I'm looking for something the size of a small FET. Like this aosmd.com/pdfs/datasheet/AO4407.pdf. That's about 5mm by 5mm. Even if I need to use two of them, that's about the size I'm looking for. Feb 25 '15 at 23:25
• @BrunoEsteves What thickness of copper is your PCB going to have? Feb 25 '15 at 23:31
• @Nick Alexeev I'm not sure yet Feb 25 '15 at 23:34
• I think trying to use a tiny FET or two will be counterproductive because of the space needed for insulation and cooling. In any event, as this is AC, you should be looking at some kind of opto triac/diac arrangement. Feb 26 '15 at 1:03

SOME FETS around that size are usable.
You will need two in series with opposed polarity for AC.
Join Gates.
Join Sources.
Two Drains are an AC switch terminals.
Drive gate to source with floating voltage (details if needed).

BUT with say 5 milliOhm FETS your dissipation is
I^2 x R = 20 x 20 x 5/1000 = 2 Watts per FET.

With 1 milliOhm FETS that's 0.4 Watts/ FET. 1 mO FET is "special but doable".
Careful design and heatsinking needed.
What are your size constraints and why?

Slightly larger. Just.
5mm x 6mm x 1mm.
30V 100A+.
Under 1 milliOhm so eg say ~ 0.5W at 20A.
In stock Digikey \$1.98/1 http://www.digikey.com/product-detail/en/PSMN0R9-30YLDX/568-11554-1-ND/4965575

Heatsink well enough and they will work well.
Need not be "large" but must be adequate.

Larger version ofdiagram here