Bi-Metal Thermostats

Bi-metallic (bi-met) temperature controls (thermostats) have been around for a very long time, but their simplicity, dependability, size and cost still make them a good choice for certain applications.

Also known as "thermoswitches", bi-metal thermostats come in two primary styles - a "disk" type, which looks more like a button, and a "cartridge" style. Both operate on the same basic principle of differential expansion. Disk type devices are used in many household appliances, such as clothes dryers or coffee pots, as temperature control or as hi-limits. Cartridge style thermostats are used in more industrial applications and OEM equipment.

Internal view
of differential
expansion type
thermostat.

A bi-metallic sensor consists of two metallic strips encased in a tube. The strips are usually welded to the top and bottom of the tube, and arched toward each other in the middle. The facing center of each strip has an electrical contact welded to it. The tube and the strips have different coefficients of expansion. When heated, they expand at different rates pulling apart or pushing together the electrical contacts.

These devices can be normally closed (opening on temperature rise) or normally open (closing on temperature rise). Disk type thermostats are not adjustable and have a fixed setpoint. Cartridge style thermostats usually have a setpoint adjustment screw.

Bi-metallic thermostats are designed to switch full voltage with a maximum current capability. Disk type thermostats are usually limited to 1 amp or below. Cartridge style thermostats carry ratings up to 12 amps at 240 volts.