I have a very simple control system for two homebrew pumps using a 12VDC control circuit and two (2) solid state relays (https://www.sparkfun.com/products/13015). Specs on the pumps can be found here, they are simple 115VAC motors.

Each relay controls a single outlet to which a pump is plugged in.

When the relay is off, the pump is off. When the relay is on and the pump is plugged in with the relay on, the pump will start. When the relay is turned on while the pump is plugged in, the pump will not start. Unplugging it and plugging it back in will start the pump.

ASCII circuit diagram:

+12VDC          +       ~         
---- SW1 -----+           +----------------------
              |   RELAY   |                       HOUSE MAINS (120VAC)
--------------+           +--------- PUMP -------
+0VDC           -       ~

Am I using the wrong relay, or do I need to add a starter capacitor of some form to the pump side of the circuit?

Note: when I replace the pump with a 120V floor lamp, the lamp is controlled as expected, without having to unplug it and plug it back in after the relay is on.

  • \$\begingroup\$ The pump motor does like the leakage current? Hard to tell. Is there a problem with using a mechanical relay instead? I'll understand if you say you hate the clicking :) \$\endgroup\$
    – Alexxx
    Sep 10 '15 at 14:25
  • \$\begingroup\$ I do hate the clicking, but not so much as to make it a deal-breaker. If I need to go with a mechanical relay, I will, I used the SSRs because they work quite well in a similar control circuit switching on/off a chest freezer. \$\endgroup\$
    – John
    Sep 10 '15 at 14:30
  • \$\begingroup\$ Hard to tell, but my initial guess is that something with zero cross detector isn't working as it should. I would test the equipment at some other place, at your friends house. It could be that some interference is causing that voltage on mains is never 0V. \$\endgroup\$ Sep 10 '15 at 14:34
  • 1
    \$\begingroup\$ @MarkoBuršič - can you justify "that voltage on mains is never 0V"? \$\endgroup\$
    – Andy aka
    Sep 10 '15 at 14:36
  • \$\begingroup\$ Do you have your control signal connected up correctly with the right polarity? \$\endgroup\$
    – Andy aka
    Sep 10 '15 at 14:37

For inductive loads such motors a Random Turn On SSR with snubber should be used. Zero crossing SSR can be used when you have resistive loads such bulbs, heaters, etc.


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.