I have a 220 V, 65 W fridge that I use for camping and a 12 V/220 V, 1000W inverter.

I need a circuit diagram that will switch on my 12 V/220 V inverter when my 220 V fridge thermostat switches on. The same inverter will then power the fridge until the thermostat switches off again.

So when I plug in the fridge to the inverter's 220 V output, the thermostat will be on because the fridge is warm, but the fridge won't run because the inverter is off. The inverter must then sense that the thermostat is on and then the inverter must switch on as well. The reason I want to do this is because I dont want the inverter running while the fridge is off because the inverter pulls 5.5 A from the battery while running with no load and this uses up power from my battery.

The fridge itself only draws about 5 A from the battery (apart from the current spike when it starts). I only want the inverter on when the fridge needs it. I would also need a 30 seconds delay if possible from when the inverter switches on until the fridge gets power, to give the inverter time to start up properly.

I am not a elecronics boffin and would like a circuit that is as simple as possible, one that I can build myself.

  • 4
    \$\begingroup\$ I would suggest that you have a look at your inverter. I have a similar set-up, and drawing 5amps (60Watts) when off-load suggests a highly inefficient (or even faulty) inverter. Mine draws 0.7 amps when off load. \$\endgroup\$ May 4, 2020 at 9:48
  • \$\begingroup\$ It's possible to rewire the inverter and put a thermostat in parallel with the on/off switch - although re-wiring your inverter will probably void its warranty and bring a chance that you'll break it. \$\endgroup\$
    – user253751
    Jul 24, 2023 at 9:35
  • \$\begingroup\$ @ConanTheGerbil whether faulty or a junk/unsuitable design, ~60W of waste heat in idle would be enough to condemn it. But even 0.7A idle will deplete the battery pretty quickly - my 40W campervan fridge runs <25% of the time so averages in the high single W. That's a 12V compressor so gets round a lot of the hassle \$\endgroup\$
    – Chris H
    Jul 24, 2023 at 12:26

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. Control circuit for inverter switching.

This does not have a time delay. If you really need that then get a mains time-delay relay and add it between the inverter and the fridge. I would have thought that a 1 s delay would be more than enough.

With the added constraint that the fridge must be unharmed so that it can be run on mains:

  • The additional 12 V T'STAT will need to be mounted inside the fridge.
  • Set T'STAT to the desired temperature and set the fridge thermostat to minimum. That way the fridge will be controlled by T'STAT rather than the fridge stat and it will ensure that you won't be running the inverter while the fridge's 'stat is open.
  • When running on mains set the fridge's 'stat to normal position.
  • \$\begingroup\$ Rather than rewire the fridge, it would be simpler to add another thermostat, and set it to the desired temp, using it to control the relay. You can find these for US$12 or less, e.g. amazon.com/… . This keeps 220 VAC mains separate from 12 VDC. \$\endgroup\$ May 4, 2020 at 1:15
  • \$\begingroup\$ Thank you, remember the thermostat is part of an 220v ac circuit not 12 v. I am sorry, I was not very clear in may question, when i am at a camping site where 220v ac is available the fridge will not be supplied by the inverter, but directly from an 220ac supply. \$\endgroup\$
    – Redge
    May 4, 2020 at 8:39
  • \$\begingroup\$ I apologize, I made my comment before I read your explanation, I only saw the diagram. \$\endgroup\$
    – Redge
    May 4, 2020 at 8:41
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    May 4, 2020 at 8:54
  • \$\begingroup\$ I did not mention in the beginning that the inverter is a UPS.It has a DEXON 1000W pure sine wave inverter. Could the fact that it is a UPS be the reason why it draws current when not powering any device? If so would it be difficult to make it only an inverter and not a UPS? \$\endgroup\$
    – Redge
    May 14, 2020 at 9:32

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