Johnson A421 Digital Temperature Controller

The Johnson digital controller is used to control the temperature of a refrigerator, or similar cooling or heating device. Enter your desired temperature, called set point, and how tightly you want your temperature maintained, called differential, and let the controller do the rest.

Ranco Vrs. Johnson 
This Johnson controller is very similar to our popular Ranco brand controller (FE610) in application and appearance. The advantage of the Ranco is being able to switch between cooling and and heating from the digital interface, while with the Johnson you have to take the cover off and physically remove a jumper. The advantage of the Johnson is the very attractive price.

For technical aficionados the Johnson does have a feature called Anti-Short Cycle Delay. Cycling is the rapid turning off and on of an electrical device. Over time cycling can harm your appliance. The Johnson controller allows you to set a time interval, in minutes, that keeps the controller from turning on right after it shuts off no matter what the temperature reading is. This can be very useful when you are using a 1 degree of differential setting for tight control but want your compressor or power supply to have a minimum time period of rest before it turns back on. Temperature Range of -40 to 212F.