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

 

... 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.

 

Presumably you'll make one or two of the resistors variable to give an adjustable temperature and optimum hysteresis?

 

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.

 

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.

 

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.

 

I see no timer. Compressors have a duty cycle you need to respect.

It has a 2C hysteresis to avoid quick cycling the compressor.

 

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..

 

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..

 

The capilliary follows a route around the fridge to sample the temperature and average it across multiple locations rather than at a specific point.

A string of silicon diodes in series could be used as an averaging sensor.

 

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.

 

I use one of these on my chest freezer.. works great..

https://www.ebay.co.uk/itm/272381918505?hash=item3f6b3a3d29:g:0xoAAOSwYihgknH~

 

I use one of these on my chest freezer.. works great..

https://www.ebay.co.uk/itm/203345414451?hash=item2f5854c933:g:RhwAAOSwd4lgbBCP

How do you tell it whether it is controlling a heater or a cooler?

 

There is a setting in the menu for heat or cold and for hysteresis.

Just noticed that i posted the wrong item, I've edited now..

 

There is a setting in the menu for heat or cold and for hysteresis.

Ah, I see. It seems my fridge has a problem at the moment anyway. It struggeles to cool much so i guess I have to fix/replace that first.

 

There isn't much refrigerant inside the thermostat phial so it always condenses into a liquid where it's clamped onto the evaporator, the pressure drops accordingly and the contacts open. It's nothing to do with average temperatures.