Freezer thermostat - how does sensor work?

Question

My chest freezer just died. The thermostat dial seems to move a relay up/down and if I push the small stem that sticks out the end (Ranco type K50) the compressor cycles on. I guess I need a new thermostat. The dial won't move the level the extra 1-2 mm needed to activate the internal relay.

How does the sensing coil actually work? On air pressure? If I were to cut it open, I think it's hollow inside, similar to a furnace thermocouple. I presume the sensor somehow moves one end of the relay but I'd like to understand how the thing works since I'm curious.

Meat Plow · Accepted Answer

There is a gas/liquid inside the bulb at the end of the sensor tube thatcauses mechanical action on the other end. Don't know what the gas/liquidis but it may have properties similar to freon.

RichK · Answer

the coils is made of bi-metallic strip.  One metal expands more, than theother, which causes bending.  Most simple overtemp devices have somebi-metallic strip.The bi-metal may be a conducting member itself, or push on something, ortilt a mercury switch (home thermostat)RichK

Pat Coghlan · Answer

I referred to it as a coil, but only because when you buy one the (hollow?) tube is coiled up.  Okay, rolled up.You have to unroll it and insert it up a metal tube at the back of the freezer near the bottom.It looks like this:

Pat Coghlan · Answer

In this case, there's no bulb, but it does appear to have a small cap on the end to seal it off.Meat Plow wrote:

Meat Plow · Answer

Must be enough to not need the bulb which is reasonable considering thetarget temperature of your freezer being -10 to -20F.

Freezer thermostat - how does sensor work?

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