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. enter image description here

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?

  • 3
    \$\begingroup\$ 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... \$\endgroup\$
    – PlasmaHH
    Feb 25 '15 at 23:15
  • \$\begingroup\$ 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. \$\endgroup\$ Feb 25 '15 at 23:25
  • \$\begingroup\$ @BrunoEsteves What thickness of copper is your PCB going to have? \$\endgroup\$ Feb 25 '15 at 23:31
  • \$\begingroup\$ @Nick Alexeev I'm not sure yet \$\endgroup\$ Feb 25 '15 at 23:34
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ 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

Datasheet here

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

Larger version ofdiagram here

enter image description here


I just found this earlier SA Q&A

What transistor or combination of transistors should I use to make a fully controllable AC switch?

Diagram below from Andy's answer matches what I described above.
Isolated gate drive need not actually be truly isolated - ask.

enter image description here

  • \$\begingroup\$ Russel, this is great, and exactly what I was looking for. \$\endgroup\$ Feb 26 '15 at 9:43
  • \$\begingroup\$ Regarding the PSMN0R9-30YLD you suggested, can I use the arduino to control the gate? Or should I use an isolated gate? \$\endgroup\$ Feb 26 '15 at 10:13
  • \$\begingroup\$ @BrunoEsteves Look at the circuit and establish how it works. You will see that when the FETs are off the gate-source is isolated from Vin or Vout so you cannot "just connect" an Arduino to it - you have to get voltage to this floating node. The symbol shown for the opto coup;er above has a battery in it ! :-) - that means it is photo voltaic - the light creates voltage in the floating node. BUT you can instead have a floating cap which charges from the max and min voltage points via high value resistors. OR use a photo voltaic coupler. Or a floating small battery (Heath Robinson). Or ... \$\endgroup\$
    – Russell McMahon
    Feb 26 '15 at 10:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.