I have a micro controller based irrigation controller which should control solenoid valves.

The solenoid is 24V AC type, with 25 Ohm resistance. I am considering to use some triac based SSR to switch the 24V AC circuit. The input (emitter) is fine, just needs a limiting resistor to keep the current around 30mA.

I want to have the circuit as simple as possible, but reliable as well. I think if I use a zero switching triac based SSR then there will be no need to worry about any voltage spike from the inductive solenoid, right? The output can be just the 24V AC source connected to the solenoid through the SSR output. And of course the SSR should be capable switching current around 1A.

Thank you for your advice.

  • \$\begingroup\$ It not clear what exactly you want. For example "The input (emitter) is fine,.." what do yo mean? Almost any small SSR you can buy on the market is capable of your needs, and no it shall not be zero-cross type, rather random fire. \$\endgroup\$ Commented Aug 27, 2020 at 12:47
  • \$\begingroup\$ Even if the SSR is zero-start capable, there could still be inductive kickback (on the AC side) when de-energized. If a 120vAC or 240vAC SSR is used, it would likely be fine. If it fails after awhile though, you'll want to add an RC snubber across the SSR's AC terminals to quench the kickback. \$\endgroup\$
    – rdtsc
    Commented Aug 27, 2020 at 13:14
  • \$\begingroup\$ Hm... zero switching means zero start only? My understanding was it should switch off at (near) zero as well - actually that is how triacs work - but it was long time ago studied some electronics at school.... And that zero switch off means no kickback \$\endgroup\$ Commented Aug 27, 2020 at 22:57

1 Answer 1


Triac is not really an ideal solution for 24V/1A. It would be a relatively large loss. What I can suggest is a 2A MOSFET optocoupler. A few mA (<3) is enough to turn it on and the on resistance is low. The symmetrical TVS provides good protection against switch-off transients.


  • \$\begingroup\$ When using a MOSFET optocoupler, could it be simpler should I use DC power to control the valve ? The solenoid is happy with DC as well. \$\endgroup\$ Commented Aug 28, 2020 at 7:12
  • \$\begingroup\$ Yes, DC control would be simpler. Since galvanic isolation is not required, a (logic level) FET and a protection diode would suffice. \$\endgroup\$
    – csabahu
    Commented Aug 28, 2020 at 7:37

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