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Simple Temperature Switch - Fan on at 80°F

mberding

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Sep 20, 2019
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So I've done some Googling and Amazon searching and so forth, but I must be searching for the wrong thing.

I'm looking for an extremely simple thermostatic switch that I can use with a 80mm computer fan. I want it to turn on (close the circuit) when it's around 80°F or more, and turn off when it cools back down. I'm looking to add some forced ventilation into a compartment where I have an inverter, battery charger, etc. I don't need it to be adjustable temperature, and I don't want any parasitic draw, if that's at all possible.

Can someone point me in the right direction please?

Thanks
 
You're looking for magic. Good luck.

Seems like it!

My old RV had a mechanical thermostat that took zero power. When the temperature dropped to the pre-set value, it would close the circuit and the furnace would come on. I feel like having it the other way around (when the temperature rises instead of drops) shouldn't be that difficult. Apparently it is though!
 
You could use a mechanical thermostat with a NC relay. The relay would be closed and driving the fan by default.

As the temp lowers to the set point, the thermostat switch will close, which would OPEN the relay shutting off the fan. Unfortunately, the relay coil would have a 1-2W power drain keeping the fan off.

You could learn a new skill and design an arduino based fan control to simply turn the fan on or off as needed. The arduino would pull maybe .03A

You might try to find an old mercury-switch-based thermostat and see if you can mount it upside-down or in an orientation where the mercury switch would work as you desire.
 
Depending on the amps and voltage a normal thermostat like would be used for a household a c unit
 
These come in different temp ranges.


Ah ha! That's the key. "Normally Open Thermostat Switch" is the key phrase I wasn't thinking of and led me to my solution.

A quick search yielded the product below, which is perfect for my situation. It uses zero current, it's tiny, and super-simple to wire up.

Thank you everyone!

 
27°F is a pretty substantial difference. Pretty sure the NO thermostat I linked above will work and is adjustable... darn near magic.
 
I've used an old style Honeywell thermostat before, they use a bimetal coil and mercury switch so they use zero power for opeartion, and you can adjust the on/off temp with a twist of the dial. The mercury switch will handle a computer fan just fine.
Honeywell thermostat.jpg
 
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This is exactly what you are looking for, unfortunately it is set for 115 degrees. I wouldn't mind buying one set for say 90 degrees.

 
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