I'm trying to run a backup pump that will be actuated by a no flow condition from the primary pump. Everything is 12V DC. The flow switch can only handle 0.4A at 12V DC. The issue I'm running into is finding a solenoid that can be actuated with less than 0.4A that is affordable and reliable. I've only found a traditional mechanical solenoid which is actuated by 0.7A.

I've been told that the flow switch can activate a relay which in turn activates a solenoid. How would I design a circuit like this?

  • \$\begingroup\$ Sadly, the first part looks like a 'shopping question' - see help center for advice on writing good questions. You might want to reword it a bit. How might you specify 'affordable' and 'reliable'? Do you know about solid-state relays, or are you saying they are no good either? \$\endgroup\$
    – gbulmer
    Commented Sep 22, 2014 at 18:43
  • \$\begingroup\$ A solenoid and a relay are both effectively electromagnets. The technique using transistors to drive a relay can also be used to drive a solenoid. Therefore, you shouldn't need both a relay and a solenoid. \$\endgroup\$
    – JYelton
    Commented Sep 22, 2014 at 18:48
  • \$\begingroup\$ @jyelton then explain every car system ever. Obviously they are both electromagnets, but when one requires significantly more power than the other, and the trigger can only power the weaker one... Op understands this but wants to check. \$\endgroup\$
    – Passerby
    Commented Sep 22, 2014 at 19:03
  • \$\begingroup\$ @Passerby Not saying you never combine relay and solenoid, but for 0.4 to 0.7A, I don't see why it's needed here. \$\endgroup\$
    – JYelton
    Commented Sep 22, 2014 at 19:08
  • 2
    \$\begingroup\$ What does the solenoid actually do with respect to the two pumps? Normally, you just connect redundant pumps in parallel, with check valves on their output sides. Why is a solenoid needed at all? \$\endgroup\$
    – Dave Tweed
    Commented Sep 22, 2014 at 19:50

1 Answer 1


All you would need is a 12V coil relay rated for sufficient contact current (for example, 10A relays are common and inexpensive). It might also be advisable to connect a diode (eg. 1N4004) across the relay coil (and another across the solenoid coil) to reduce sparking in the flow switch (and relay contacts).

Panasonic JS-1A relays and similar are one choice for the relay.


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