Background: I have an undersink 3-canister filter in my kitchen and wanted to monitor the amount of water going through the filter. I purchased a Digiten DFC15 water flow controller and a Digiten DV12V normally open solenoid.

Problem: I intended for the controller to supply 12 V to the solenoid when it reaches the set limit. Instead I found that it was supplying power (closing the solenoid) in run mode and stopping power when it reaches its limit. This is the exact opposite behavior to what I need.

Troubleshooting: I contacted the manufacturer and they said that it only works with a normally closed solenoid which I can't use because it would be on 24/7 and generate a lot of heat. They did not place this information in the documentation.

Idea: What I want to do is flip the behavior of the controller so that when it stops providing 12 V to the wires, my solenoid closes. I think if I wire a relay to the 12 VDC wires (positive and ground) and have an external 12 V power source I can achieve this, but it's unclear to me what kind and spec relay I need.

The solenoid is 12 VDC and 4.8 W, so 0.4 A. If there's a better way, please share that too. The power brick for the controller supplies 12 VDC, so with a splitter pigtail, I can draw my 12 VDC to power the solenoid when the relay closes. The other thing I'm not sure about is whether the relay solution will generate a lot of heat as well, since that's what I'm trying to avoid.

  • \$\begingroup\$ Either use a normally closed solenoid or use the Normally closed contacts. \$\endgroup\$
    – Solar Mike
    Jan 31 at 9:44
  • \$\begingroup\$ There are only one set of contacts on the DFC15, so no normally closed contacts. If I use a normally closed solenoid, it will be held open 24/7 for 3-6 months before the meter records it's limit. My understanding is that this will generate a lot of heat and eventually burn out the solenoid. Is that incorrect? \$\endgroup\$ Jan 31 at 9:49
  • \$\begingroup\$ A Normally Closed Solenoid will have the contacts held closed by spring pressure, not current flow. When energised the contacts are held open against spring tension by the current. \$\endgroup\$
    – Solar Mike
    Jan 31 at 9:50
  • \$\begingroup\$ @SolarMike and they will be held energised 99% of the time, wasting power. \$\endgroup\$
    – user253751
    Jan 31 at 10:19
  • \$\begingroup\$ you could possibly use a relay with a much higher coil resistance so it wastes less power, to reverse the signal. (or a MOSFET with infinite "coil resistance" but the circuitry for that is not as simple) \$\endgroup\$
    – user253751
    Jan 31 at 10:20

1 Answer 1


You basically answered this yourself, a relay is a sensible solution, with the solenoid between +12V and ground, in series with the contacts. The relay coil current will always be there, but it's far lower than the solenoid current. The heat from the relay is likely negligible.

Place a flyback diode across the solenoid and another across the relay coil (important!), like some regular 1N4007 for example.


simulate this circuit – Schematic created using CircuitLab

A regular 12V coil relay rated would work. They are usually multipole with several NO/NC, although you only need the NC. No point in under-dimensioning the current rating, 5A to 8A is pretty standard. Though you could also look for low coil current ones and let that be the deciding factor. Socket-mount is usually handy and contact material likely doesn't matter here.

Another alternative would be a MOSFET. It would draw less current still, but then you'd have to solder some components and maybe build a small PCB.

  • \$\begingroup\$ Any reason this 12v relay module wouldn't work? I've used them before for different projects. It's 12VDC powered, has a NC output and looks like it's rated up to 10A at 30VDC on the output side. The only thing not clear to me is whether I would set the jumper as the high trigger or low trigger. \$\endgroup\$ Jan 31 at 17:07
  • \$\begingroup\$ @objecttothis There's no telling if it is suitable without a schematic or at least reverse engineering. There has to be a flyback diode across the relay coil or otherwise you risk damaging your controller. It looks as if maybe that board has an optocoupler and that would work too. \$\endgroup\$
    – Lundin
    Feb 1 at 7:28
  • \$\begingroup\$ Thanks, they advertise it as having an octocoupler. \$\endgroup\$ Feb 5 at 10:43

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