I'm using this circuit to replace the thermostat in a small mini fridge. In the fridge, I took the white and black wires, attached them to the NO and COM ends of the relay, and have the Arduino open the relay based on temperature through pin 7.

I should mention that there is a green ground wire that I have not grounded within the fridge.

My Schematic

Should I ground this wire to the side of the fridge? Should it be attached to the NC port of the relay?

Inside Of Fridge

Eventually, the relay no longer opens. I can tell by the sound and by the fact that thr fridge never makes noise, as it does when I replace the relay. The red LED on the relay still shines, indicating that it should be open. I have replaced the relay twice and both stopped working at some point, and today I was able to fix it again with a new relay.

The schematic below shows how I have everything wired to the Arduino, and there is also a schematic on the back of the fridge that I have added

Fridge Schematic.

Note that the thermostat is no longer there, and was previously attached to the black and white wire, and the green one was grounded off to the side. I have added a picture of another mini fridge I have, which has the same setup with a blue wire instead of a white wire.

Other Fridge

I (clearly) don't have a great base of knowledge on circuitry, so perhaps I need to add a component somewhere? Would the ground wire play into why the relays quit opening?

  • \$\begingroup\$ are you running the relay at 3.3v power? that might not be enough as it shows 9v. If that's true, you need to feed the module 12v and hope the 5v signal is still enough. Underpowering it could make it fail early as the switching is slower than expected and thus the arcing is more. it might also be under-rated for the inrush current of the fridge; try going up to a bigger relay or use an SSR with zero-cross detection built in. \$\endgroup\$
    – dandavis
    Aug 2 at 20:50
  • \$\begingroup\$ Fridges are highly inductive loads. Unless you have a snubber circuit with your relay you will fry the contacts in the relay. \$\endgroup\$
    – Majenko
    Aug 2 at 22:26

1 Answer 1


Priorities First:
Ground the fridge,

OK, I am an electric inspector by actual trade, but you should newer power unearthed appliance unless its specifically designed to do so (too much to explain).

all the real electric majour injury/death videos I have ever seen were fridge related ;-)

Now secondary point. The relays you buy of ebay (amazon) are designed for one purpouse: To be made cheaply. your average fridge size relay is usually 3-10 times the size of your tiny relay and the way compressor on a fridge works is by eating loot of power for short periods, so the switching of such a powerfull load as fridge compresor takes more then SONGLE or relay PHAT (made with relays even cheaper then SONGLE). More power efficient fridges don't have much more efficient compressor, but instead keep the temperature inside better. For short periods (few years) you should manage with good quality relay something like Shrack or Omron rated atleast 20Amp (from profesional stores like RS components, mouser, CPC (Element14)). Some higher end fridges even use two part switching with first relay having a resistor bypas. If its a comercial fridge then its common to have small relay on a control board switching a powerfull replacable DIN relay.

Lastly compressor does not like starting so you don't want it to be going on for one degree at a time. Make sure your arduino have sufficient range for switching (FOr example to target -2 degrees, you would set arduino to activate compressor when it goes above -2 and leave it running until at-least -5. Many thermostats have builtin in-rush limiting to protect compresor from too much current on start. industrial fridges even control what position the compresor stops at... Dont forget the resetable oveheatstat.

Warning: both Shrack and especialy Omron are heavily faked and I have not actualy sucesfully boght real one of them on amazon yet, so make sure you get them from professional suppliers, who get them from TE or Omron directly as the fakes often get to chainstores too...

Stay safe

  • \$\begingroup\$ Just ordered one of these. Hoping for delivery Thursday, so I’ll keep you posted. Thank you!! People like you are lifesavers for answering stuff like this (quite literally considering I didn’t ground it haha)! \$\endgroup\$
    – Andrew Hansen
    Aug 2 at 23:38
  • \$\begingroup\$ in all honesty people doing amateur arduino mods to domestic appliances who forget to ground things make me VERY nervous ... \$\endgroup\$
    – danmcb
    Aug 3 at 9:12
  • \$\begingroup\$ If you are based in UK, you will be suprised how many "Professionally designed" devices are usafe here. The UK largest DIY store chain called BnQ has its main device brand "Cokie Lewis" one more dangerous then another. you cant blame people who do their own stuff for not knowing how to do, if people who should know don't bother. its good that this person made mistake and learned from it. \$\endgroup\$
    – Tomas
    Aug 5 at 1:04

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