Typical keezer conversion instructions shown here on HBT and elsewhere indicate that an external temperature controller is required. However, this is not true. The frugal and patient DIYer can spend that money elsewhere.
The vast majority of chest freezers used for keezer conversions utilize a simple internal electromechanical thermostat. These thermostats are preset by the factory to operate with a typical range between -20F and 20F depending on the position of the user accessible dial. Keezer temperatures are in the 40F range, depending on taste. It is the inability to readily set the internal thermostat to keezer appropriate temperatures that drives the need for an external thermostat. Here is an example of a typical internal thermostat:
What is not widely known is how these electromechanical thermostats work, and that these internal freezer thermostats have a coarse temperature adjustment screw which can be used to widely adjust the temperature setting of the thermostat, even above freezing to keezer temps. The coarse adjust screw on the unit pictured above is in the 5 o'clock position relative to the fine adjust dial shaft. They are usually recessed within the unit and may be covered with tape. Some units have them on the side, but I have yet to find one without a coarse adjustment screw.
Electromechanical thermostats operate on the principle of the ideal gas law, PV=NkT, where P=Pressure, V=Volume, N=Number of gas particles, k=Boltzmannís constant, and T=Temperature in Kelvin. The idea is that by sealing a quantity of gas in an enclosed vessel the variables V, N, and k are held constant resulting in the pressure of the gas being directly proportional to its temperature.
Freezer thermostats use a long hollow tube as the enclosed vessel that is stuffed inside the freezer wall, with a short portion extending into the compressor compartment where one end is connected to a pressure sensor. The pressure sensor works by movement of a diaphragm set against a spring. The important part is that to overcome manufacturing and material process variables and to allow use of the same design for different applications the spring tension is designed to be widely adjustable by manipulation of the coarse set screw.
From the ideal gas law we know that at higher temperatures equal higher pressure against the diaphragm. Thus, to adjust the thermostat to trigger at a higher temperature the opposing spring force must be increased. Increased spring tension can be felt as increased resistance when turning the coarse adjustment screw. Below is a simplified diagram detailing the principle.
Some trial and error can be required to get the setting correct, and this is where patience comes in as it can take several hours for the temperature inside the freezer to come to steady state. Quick coarse temperature measurements can be made with a dry thermometer, but since hysteresis can result in several degrees of error final measurements should be made with a thermometer with some thermal mass for final adjustment (i.e. a floating thermometer in a glass of water).
: Since the thermostat switches line voltage to the compressor there is live electrical current inside the thermostat at all times when the freezer is powered. REMOVE POWER FROM THE FREEZER BEFORE ADJUSTING THE INTERNAL COARSE SET SCREW.