this page is a collection of notes on how temperature control works in a kegerator, and in particular, how our kegbot does it.

one basic problem with home-made kegerators is that you often want the form-factor of a "chest" (ie, a rectangular cabinet with a top-loading door), rather than an upright unit. the problem is, it is hard to find refrigerators in this form factor, and far easier to find a freezer unit.

using a freezer in a kegerator system is fine; mechanically, the two appliances are nearly identical. however, beer is not meant to be frozen, and for optimum taste, should be kept within a few degrees of a temperature, typically just above freezing. so, we have a basic problem: how do we force the freezer to keep things a little warmer?

the traditional solution to this problem is to either replace or externally modulate the freezer's thermostat. in either method, the basic mechanical operation of the freezer is exploited: to make things cold, turn on the compressor. if it is cold enough, turn off the compressor. repeat infinitely.

replacing the thermostat involves physically swapping out parts of the freezer's hardware, and is not very well documented.

the more popular method is to buy and attach an external controller, such as this one. this kind of controller always comes with a new thermostat that you have to insert into the freezer -- another hole to drill.

both traditional methods have a drawback for us: there is no way to set or change the high and low thresholds from the kegbot. in the first method, the temperature is controlled by a mechanical knob on the back of the freezer. in the external method, there is usually a knob or buttons on the front of the controller. plus, neither of these methods give us any sort of measure as to what the temperature actually is -- we'd need a digital thermostat interface to our controller to learn that, and that's something we'll probably want anyway.

the solution which is now in use is this: observe the temperature from a digital thermometer. if the temperature climbs above a certain threshhold (T_H), activate the fridge via a relay. similarly, disable the relay when the temperature drops below another threshhold (T_L)

this solution is novel, because:

• hi and lo threshholds can be controlled in software, and changed on a whim (eg, if colder beer is desired that night)
• adjustable and known hysteresis
• adding a digital thermometer is less intrusive
• it gives us new, useful hardware: both the relay and the themometer can be put to other uses by the controller: we can shutdown the freezer (with the relay) if an emergency is detected, and we can log (from the temperature sensor) historical temperatures.

this method works and has been in use since 09/2003 with success! the fridge switching relay was upgraded (to a latching one) in 01/2004, and the temperature sensing is now performed by a DS1820 on a dedicated 1-wire bus.

1/2005: in retrospect, using a latching relay was very stupid. i don't know why i thought it would be good. it's a great relay, but the problem is, if the microcontroller loses power (but the fridge does not), you could have the relay stuck on, causing the fridge to become a freezer! this hasn't been a problem in a long, long time, but the new board layouts use non-latching relays (at a small cost in power dissipation.)