Questions about Johnson Temperature Controller for my keezer

Currently I am using a Johnson A419 on my 7 cubic foot chest freezer to make it into a keezer. However, I want to use it as a ferm chamber some days. My concern is that I have read that too much frequent cycling will harm the freezer and currently my differential is set at 5 degrees. That seems like a bit of a big swing for a fermentation to go through as the keezer sometimes drops to 6 or even 7 degrees under my set point. What do I do? Do I set it to cut out instead of cut in? Do I change the differential?

The A419 has an anti-short-cycle function that should ensure you don't cook your compressor. It can be set as long as 12 minutes. You probably should look into that.

As for the large differential and even larger apparent temperature swings, the latter is likely due to where the temperature probe is located, and the former was set to cope with the latter (maybe?)

Keezers (and fridges to a lesser extent) will see the interior air temperature drop below any set point, due to the nature of the cooling mechanism: the compressor shuts off at the set point, but the expansion coil is still filled with working fluid changing to vapor and thus extracting more thermal energy until equilibrium is reached. If your temperature probe is hanging in free air it will "see" all of that. Otoh, if your probe is attached to a large thermal mass inside the keezer/fridge, the swing it sees will be significantly narrower.

I run two fridges and a keezer with external controllers, typically with only a degree or two of differential, by coupling the controllers as tightly as possible to what I'm actually trying to control: the beer.

For my fermenting fridge, I stick the probe against the carboy, slap a 1" thick chunk of foam insulation over it, and crank them down tight with a velcro strap. For my carb/cold holding fridge and my keezer, I do the same thing against full kegs. The result is infrequent cycling and brews held within a degree of where I want them to be...

Cheers!

I have actually engaged mine to the full 12 minutes already. Can you link me to the type of insulation you typically use? As of now I just had the probe in a water bottle full of water but I may use a gallon milk jug to provide even more thermal mass. Should I be worried about setting my differential as low as one degree away from set point though if I am not sure how well insulated I can keep the probe? If it is too insulated how will it keep the temperature correctly since it doesn't see any change in temperature?

It's just closed-cell foam that you'd dig out of a shipping carton, but pretty much anything that won't become a sponge will work fine.

How small you can set the differential is dependent on how effectively coupled the probe is to whatever you're trying to control, and how well isolated . You'll have to experiment to see how your keezer performs once you've re-positioned the temperature probe...

Cheers!

Do I fully encapsulate the probe inside the foam? Will it still register temperature changes accurately?

You lay the temperature probe flat against the side of the vessel (somewhere near the vertical middle) and then lay the insulation over the probe, then strap everything tight so the probe makes good contact with the vessel wall. I have some velcro straps that work great for this, whether around a carboy or a keg.

In my experience, thus installed the probe will read within a degree of the LC thermometer strips I have on all my carboys, and is pretty well isolated from the interior temperatures of my fermentation fridges or my keezer...

Cheers!