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I want to build irrigation system for my garden. Most solenoid valves for that purpose work at 24VAC.

I'm wondering if I could use triacs to drive them. They are cheap and small (in comparison with relays) but I have some concerns about overcurrent and short-circuit conditions (like coil interturn short circuits, cables damaged by rodents etc.) and voltage drop (relatively high at 24VAC).

I don't want to reinvent the wheel.

How this is done in commercial irrigation systems? <- edit: is this question opinion based?

Relays, triacs or maybe something else?

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    \$\begingroup\$ Not opinion based imo. \$\endgroup\$
    – Drew
    Commented Mar 31, 2022 at 1:57
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    \$\begingroup\$ I thought most solenoid valves use 12V or 24V DC, and for either DC or AC no one is using traics. Can you let us know the links to the applications you mentioned? \$\endgroup\$
    – tlfong01
    Commented Mar 31, 2022 at 2:04
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    \$\begingroup\$ The residential irrigation controllers I've had have used TRIACs to drive the valves and AC excitation. \$\endgroup\$ Commented Mar 31, 2022 at 2:04
  • \$\begingroup\$ I echo what Kevin says. A 16A triac is cheap and you’re probably not likely to have anywhere near 16A available from the supply, so it should be pretty robust. You can use polyfuses to limit the current. Relays can be sourced cheaply as well but make sure you put a varistor across the solenoid as relays don’t switch at zero crossing. \$\endgroup\$
    – Kartman
    Commented Mar 31, 2022 at 4:03
  • \$\begingroup\$ @tlfong01 Take a look at Rain Bird GB-R Sprinkler Valves for example. In systems like this everything is powered from 24V transformer for safety \$\endgroup\$
    – Kamil
    Commented Mar 31, 2022 at 8:20

2 Answers 2

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Most modern irrigation controllers use triacs to control the solenoids. However, relays work also well and have the possible advantage of galvanic isolation between the control and load sides of the circuit.

We use Aromat (NAIS / Panasonic) JS1 relays by the tens of thousands. We find they are more cost effective than the (beefy) triacs that our loads would require. Although the coils are DC, the contacts handle AC or DC with no problems.

If you do decide to use triacs, over-size them to accommodate the occasional short circuit that inevitably happens.

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You can use relays or you can use MOSFETs or FETs or BJTs which are inexpensive too and have good current handling capacity (assuming you will be using any microcontroller for triggering the MOSFET at desired timings or needs).

And don't forget the protection components like a freewheeling diode for back-EMF control and a capacitor for power supply.

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