I've been trying to switch a solenoid valve using a relay controlled by a signal from a microcontroller. The circuit works fine when connected to a normal load like a light bulb, but some random behavior (of the digital circuitry) was observed when connecting the circuit to the (highly inductive) solenoid valve. The problem was reduced to some degree by adding a snubber circuit between the terminals to the load and increasing the decoupling capacitor for the circuit to 1000uF. Although the circuits works now (with the occasional randomness, about 1 in 100 false trigger of the relay), there is one more persistent problem. The relay contacts close with a very large noise (almost like a crack of a whip). The contacts open with no problem. The relay also switches normally with a lightbulb load. Does anyone have any idea why this is and how I can fix it?

The circuit I used is shown below. Relay Circuit

The valve used is a 230V Danfoss solenoid valve.

  • \$\begingroup\$ It's probably unrelated, but do you have a pull-down in addition to R9? \$\endgroup\$ Jun 11, 2016 at 3:44
  • \$\begingroup\$ This is an excellent illustration why we use Solid-State Relays here in the 21st century. \$\endgroup\$ Jun 11, 2016 at 4:43
  • \$\begingroup\$ Where is the 1000 uF capacitor? In the snubber circuit? \$\endgroup\$
    – Transistor
    Jun 11, 2016 at 7:07
  • \$\begingroup\$ Use a optic isolator on MC output. Why don't use a triac with MOC series isolator. Relay not fast and solenoid got a lot EMF(Your relay will not work long time !). \$\endgroup\$
    – dsgdfg
    Jun 11, 2016 at 7:26

1 Answer 1


First -- your snubber network should be across the contacts, AND C5 needs to be changed for a 250VAC X2 capacitor to keep it from being toasted by surges and other junk on the mains.

Beyond that, if you can't stand the noise, replace K1 with a suitably rated SSR but keep the snubber network in place.


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