I need to commute on/off a coil feeded with 12 Vac from a microcontroller output (5V).

I've found a cheap Solid State Relay but the datasheet and a forum says this doesn't work with less than 75 Volts ac.



Others models operate in a high dc input.

What can I do? (in a cheap way).


thanks to Jack Creasey. the circuit work perfect. adding a triac in the output I can drive the voltaje like this image. MO3021 + Triac

  • \$\begingroup\$ What is your current level 12 VAC? \$\endgroup\$ Commented Jul 18, 2017 at 21:35
  • \$\begingroup\$ it may vary according to various vendors. Lets supposse 400 mA max. \$\endgroup\$
    – Ando
    Commented Jul 18, 2017 at 21:56
  • \$\begingroup\$ @Ando You probably should do a search on Digikey looking for optoisolator triacs. The VO2223A, for example, might work. Dissipation in these tiny packages would be more my concern. But some of these devices have a pretty low voltage drop across them at the current you mention suggesting perhaps as low as 400-500 mW, which you may be able to get away with. Also read: st.com/content/ccc/resource/technical/document/application_note/… \$\endgroup\$
    – jonk
    Commented Jul 18, 2017 at 22:55
  • \$\begingroup\$ Here's a somewhat related question about SSR. The SSR described there is made with MOSFETs. Furthermore, it can be wired either for DC or AC. \$\endgroup\$ Commented Jul 19, 2017 at 1:04

1 Answer 1


Any SSR will have a significant forward (on) voltage drop of 2-3 V. Typically where two triacs (one opto-isolated and one power) are used there is a very significant turn on voltage required. While I have not seen one needing the 75 V specified for the Q3MB-202 I have seen plenty that don't turn on till about 20-25 V.

I'd suggest that you could use the MOC301x or MOC302x optically isolated triacs. These will turn on with only 3-4 V but at the current levels you want they will typically drop about 1.5 - 2 V RMS. If you actually want to get 12 V RMS on your load you may have to use a 15 V RMS transformer output.

You can parallel 2 * MOC30xx triacs and they will work well (you could expect less on loss). BUT ...you must have both the LEDs on all the time you want the AC on. You can parallel the outputs and you can either operate the LEDs in series or drive separately with their own limiting resistors.

Another possibility is to use a DC SSR. These typically have a FET output and you could use a low Vf Schottky diode bridge to switch the AC. This would have a slightly lower loss voltage probably around 1.5 V, though there is not a lot of gain.

  • \$\begingroup\$ I need 12 V Ac, but I can expect almost a 4 Volts loss. A single MOC will work. Do they need more components attached? \$\endgroup\$
    – Ando
    Commented Jul 19, 2017 at 14:41
  • \$\begingroup\$ No, on the triac side you need no more components. On the LED side you need current limiting of course. \$\endgroup\$ Commented Jul 19, 2017 at 16:58
  • \$\begingroup\$ thank you. I've simulate the circuit on proteus and found a provider with that chip. Its cheap. I've ordered the components today. \$\endgroup\$
    – Ando
    Commented Jul 19, 2017 at 20:37

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