I'm working on automating an irrigation system which has a large well pump that uses 24-32 V, directly from a solar array.

This was previously switched manually by a light switch, but after my uncle's death, I'm managing it remotely from four hours away, so I'm looking for a reliable solution.

I'm hoping to manage it remotely via a Raspberry Pi or Sonoff (ESP8266) controller. The pump will pull up to 24 A, but typically uses about 12 A. The power is nominally 24 V, but is unregulated and can peak at 32 V.

I've previously used a 60 A rated SSR, but this failed closed, likely due to being in an enclosure, and lacking thermal paste/padding between it and the heatsink. An emergency replacement also fried a Sonoff with three 10 A relays in parallel (specifically the PCB traces).

So I'm now weighing three options:

  1. Use a mechanical relay, rated for 40+ A, which might require a smaller relay to activate from the 3.3 V Pi GPIO. My concern is with a high-current inductive load burning it out due to break-arcing. Should I consider contactors instead of relays for high-current DC?
  2. Use another SSR, but keep it outside. This would need to handle 110°F (43°C) ambient conditions and not burn out like the previous one.
  3. Use a linear actuator to mechanically switch the good old light switch. This is a bit hacky, but could work reliably. The advantage of a switch over relays is that the pump is "on" for 8 hours at a time, typically switching on/off once a day.

Is there a such thing as a digitally controlled discrete switch that doesn't have the energy waste associated with a relay or SSR during high duty cycles?

The pump will run all day except on cloudy/rainy days, in which the battery bank needs full solar power, hence the long duty cycles, to which a switch seems better suited than a relay.

Any advice about the choice between relay/contactor, SSR, or actuated switching would be very helpful, including factors I haven't considered.

As a reward for your reading this, here are some pictures of my previous attempts, minus missing magic smoke!

Fried SSR

Fried Sonoff PCB Traces

  • \$\begingroup\$ Have you considered a hybrid relay? \$\endgroup\$
    – jonk
    Commented Jun 24, 2020 at 22:39
  • \$\begingroup\$ @jonk, no I haven't heard of this. I'll look it up! \$\endgroup\$
    – Excalibur
    Commented Jun 24, 2020 at 22:41
  • \$\begingroup\$ Also, another consideration about the light switch (#3), is that it's rated for 20A AC, and isn't a DC rated switch. I'm aware of the issues with arcing, but given how few times its toggled (1-3 times daily), perhaps this isn't a huge concern? \$\endgroup\$
    – Excalibur
    Commented Jun 24, 2020 at 22:43
  • 1
    \$\begingroup\$ Just an SSR + a regular relay. Momentarily activate the SSR, first. Then, with that engaged, activate the mechanical relay. Once that's engaged, the SSR won't matter as it is bypassed. But you can deactivate the SSR then, if you want. To turn off, activate the SSR again, turn off the mechanical relay, then turn off the SSR. This protects the relay contacts from arcing but also avoids the need for a big heat sink for the SSR, as well. It's nice. \$\endgroup\$
    – jonk
    Commented Jun 24, 2020 at 22:44
  • 1
    \$\begingroup\$ @Excalibur If I were doing this for DC, some of it would be a lot easier. I wouldn't need pairs of SCRs, for example. Also, please note that DC relay ratings aren't going to apply well to a hybrid situation. That is, if you actually make one. That's because they don't have to deal with the make/break cases that standard mechanical relays assume when writing their specs. In your hypothetical hybrid case, you'll never have more than a couple of volts (at most) across the contacts when they engage. So keep that in mind. \$\endgroup\$
    – jonk
    Commented Jun 26, 2020 at 0:08

1 Answer 1


Mechanical Contactor in my opinion will be cheaper, try getting motor starting grade 24vdc and then you can drive it with a WebRelay output (24V). I haven't used it for well pumps but assuming your motor is a DC pump with conventional features, you will also want to withstand inrush current being high, You did not specify motor features or duty so it might be hard to size contractor with out. WebRelay also has inputs so you can use them on the aux contacts to monitor. You can power webrelay form separate 9v source, 24V or even PoE.

  • \$\begingroup\$ Thanks for introducing me to WebRelay, it looks like a more industrial (more reliable!) option than sonoffs/pi etc. Finding motor-rated DC-switch contactors is surprisingly illusive. I'm curious what the advantage over an automotive relay (I bought a 120 amp marine relay per the conversation above). \$\endgroup\$
    – Excalibur
    Commented Jul 6, 2020 at 15:19
  • \$\begingroup\$ I agree, it will heavily rely on the application itself. And on your needs, expectations and budget. Industrial vs Automotive usually fall on a broad spectrum vs single purpose trade off. Which renders in having low total cost of ownership by economy of scale towards two different aspects. Industrial needs robustness and commonality/scalability and automotive favors specialization and cost/space reduction. I'm not familiar with marine product profile but sounds like a smart choice for both cost and performance aspects. \$\endgroup\$
    – Juan Ojeda
    Commented Jul 8, 2020 at 6:20

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