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I am talking about specifically this thermostat used in cooling systems enter image description here

My query to engineers & technicians is that: Can we check the working of above thermostat with only continuity tester and without connecting it to Main Power supply?

In other words, can this thermostat toggle from On to Off state without needing any external power?

When I did experiment on my side, I found that it remains in On state (continuity exists between terminals of internal relay) whether I put it in hot or cold environment. I guess, that without actually installing it in operational-condition we can’t check it with a continuity tester.

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    \$\begingroup\$ How about a data sheet for the part? A picture tells a thousand words but pictures can lie. \$\endgroup\$ – Andy aka Apr 8 at 15:43
  • \$\begingroup\$ Without more information on the specific part, the only accurate answer is "Yes, fridge thermostats are either active or passive." ... \$\endgroup\$ – brhans Apr 8 at 15:49
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    \$\begingroup\$ The thermostat in that image almost certainly contains a real metal to metal switch driven by the expansion of the working fluid in the tube. You should be able to hear it click when you alter the set point by turning the shaft. if it clicks, all the mechanical parts are probably OK. If it clicks and there's no change in continuity, then the switch is probably welded closed \$\endgroup\$ – Neil_UK Apr 8 at 15:59
  • \$\begingroup\$ Are you testing an old one, or a spanking new one from a reliable source? Because @Neil_UK's comment is pretty much mine; he just got there first. \$\endgroup\$ – TimWescott Apr 8 at 16:25
  • \$\begingroup\$ @Andyaka that’s a good idea. But I did not find any data sheet of it. \$\endgroup\$ – kaka Apr 8 at 16:34
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These things are strictly mechanical with the expansion of a gas in the capillary operating a snap-action switch. There is no electronics within and no potentiometer.

You can test the switch operation by cranking the knob until it clicks on and off (and measuring the contacts) but to test the actual functionality you need to change the temperature of capillary. It's possible the capillary has been damaged and the gas has leaked out, but that's unusual without some visible damage.

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  • \$\begingroup\$ So now my question is: If I decrease the temperature of a capillary (lets say by putting the thermostat in freezer) would its terminals be shown OPEN by continuity meter then? \$\endgroup\$ – kaka Apr 8 at 17:06
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    \$\begingroup\$ The thermostat contacts close to turn the compressor on (to reduce the temperature). So if you set it so that it's just barely closed (continuity) then it should open when you put it in the freezer for a while. \$\endgroup\$ – Spehro Pefhany Apr 8 at 17:08

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