Question about fridge temperature controls

I don't think you will find one available that could somehow control the fridge only. You plug the entire fridge/freezer into the controller instead of a wall outlet and the controller turns the whole thing on or off.

 

What I did is buy a fermheat wrap that wrap around my car body and connect to a digital temp controller. The ferm wrap heats the wort up to the desired temp and turns off when it's too hot. You can set the temp range at 1 degree. I currently tape the digital probe on the outside of the carboy and cover with bubble wrap, tape and a towel.

My wort stays within temp range while by fridge stays 38 degrees.

 

My fridge has a separate dial for the freezer which I keep set at max so that more of the coldness goes there somehow although I don't know how it works. Check to see if your fridge has some sort of separate setting for the freezer and then experiment to see if setting a dial like that will be enough to keep things frozen if the external thermostatic controller is set at a fermentation temp of 60 for the fridge. I keep my fridge at 50 for my 'cellared' beers, and the ice packs that I use for coolers are generally frozen, although not 'hard' frozen, so it must be around 25 degrees in my freezer. Maybe yours would be lower.

 

You're fermenting in your car???

 

Not really. A little background: Your fridge has only one source of cold air and it is all delivered to the freezer compartment . . . and it's always on maximum*. Your freezer and cold food compartment have dampers that meter the cold air from the freezer section (which can be as low as -20'ish) to the cold food side.

It's fairly easy to control the temp in either section, typically the cold food section is where you would ferment because it's larger. This leaves the freezer at the mercy of your ferm temp which is normally well above common fridge temps. By adjusting the dampers you can regulate the freezer side somewhat, but it's held hostage by the ferm temp. Also, if fermenting, the freezer side will vary depending on the outside air temp, temp differential, and your seals (it may run more or less depending on these variables).

*Some high end units have dual/variable-speed compressors but you're unlikely to encounter these.

 

It sounds like you may not be too concerned about the freezer temp. If that's the case, then simply put the temperature sensor in the 'fresh food' section and you're all set. As others have said, the freezer temp will be, at best, unpredictable. Set the internal controller for the refrigerator compartment to its maximum setting so the external controller controls the compressor. Set the freezer control to its minimum setting (open up the dampers) to reduce the compressor run time. Again, this assumes you don't need the freezer.
Another thing you could do is cut a pair of 4" holes between the freezer and fresh food compartments and install a computer case fan to move the air around. Then both compartments will be the same temp.