How do I cause a normally open solenoid to close when power is removed?

Question

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.

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.