The key is to purchase a temp controller for the fridge. If you search your favorite homebrew store for them you can find several different chocies. The most basic ones you plug the fridge in to and they have a temp probe that goes in the fridge (generally you want to attach it to your fermentation vessel). You set the temp you want it at and when the temp hits that level it shuts power to the fridge. When your fermentation vessel begins to warm up it kicks the power back on and cools things back down. Most of them are in 60 to 80 dollar range.