Replacement fridge thermostat circuit?

Thread Starter

AlbertHall

Joined Jun 4, 2014
11,309
I want to replace the thermostat on my fridge as the temperature control is not good and varies with the weather. I came across this design which I like as it does not require calibration. I always tend to be suspicious of WWW designs so what do you guys think of this one?

http://danyk.cz/tstat_en.html

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drc_567

Joined Dec 29, 2008
1,110
... Just a thought ...
There is usually a removable air intake filter of some sort, located at the lower fridge front panel. After a time, the filter can become clogged with dust and restrict air flow to the condenser coil. It might be a good idea to check it before any modifications.
 

Alec_t

Joined Sep 17, 2013
12,065
Presumably you'll make one or two of the resistors variable to give an adjustable temperature and optimum hysteresis?
 

Thread Starter

AlbertHall

Joined Jun 4, 2014
11,309
Presumably you'll make one or two of the resistors variable to give an adjustable temperature and optimum hysteresis?
According to the article that shouldn't be necessary to keep the tempaerature between 2C and 8C. We shall see.
 

Alec_t

Joined Sep 17, 2013
12,065
I haven't found an authoritative limit, but isn't 8C a tad high for normal food storage?
The mechanical stats in fridges often have a 'boost' setting for quick chilling of large food items. Would that be a useful addition? Presumably it justs cranks the set temperature down somewhat.
I'd be interested to hear how your circuit performs in practice, as I suspect my freezer stat is on its way out and I'm considering an electronic replacement.
 
Last edited:

Thread Starter

AlbertHall

Joined Jun 4, 2014
11,309
I haven't found an authoritative limit, but isn't 8C a tad high for normal food storage?
The mechanical stats in fridges often have a 'boost' setting for quick chilling of large food items. Would that be a useful addition? Presumably it justs cranks the set temperature down somewhat.
I am particularly concerned with keeping my meds cool. Their upper limit is 8C (brief excursions above this are OK), and they absolutely musn't freeze. The syringes are in a cardboard box so they will respond slowly to temperature changes.
 

Irving

Joined Jan 30, 2016
1,497
Oddly enough I was looking at exactly the same idea. I have a small fridge that the thermostat has failed on and I can't get a replacement for love or money as its no longer made. The tricky bit is putting the sensor in the right location in the fridge. The old electromechanical thermostat has a (typically 18" - 30") capilliary filled with a liquid that expands and contracts with temperature to adjust the switch point (its not a bi-metallic switch like a room stat - somehing new I learnt!). The capilliary follows a route around the fridge to sample the temperature and average it across multiple locations rather than at a specific point. For meds a single measuring point might be acceptable, but for food its probably not good enough, and you'd need a few sampling locations. I had planned to use five DS18b20 one-wire probes epoxied to the inner wall of the fridge and parallelled together on the one-wire power/data circuit with a ESP32 on the outside with an OLED screen to display internal temperature, and an SSR to control the compressor/circulation motor and wifi/bluetooth reporting of temperature, activity, etc..
 

prem85

Joined Mar 29, 2021
1
There are nice 3 digit relay modules on ebay that you can set on/off differential and temperature [C degrees] for less than $9. I have used them - delay usually can be made ok by setting hysteresis high enough - or rely on thermal mass... temperature sense is 2 wire thermistor..
 

du00000001

Joined Nov 10, 2020
62
Want to measure some average temperature?
A solid copper wire (1 sq. mm or more) does this trick: attach its end to the temperature sensor of your choice (a good, thermally conductive connection is the key) - that's it.
Concerned about corrosion? Tin the wire or varnish it.
 
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