When the temperature is low the contacts are closed and the current passes through the thermostat. When the temperature is high, one metal expands more than the other and the bonded bimetal strip bends upward to open the contacts, preventing current from flowing.The thermostat is made of two thermally different metals bonded back to back .
There are two main types of bimetallic bands based mainly on their movement when subjected to temperature changes. There are “snap-action” types which produce an “ON / OFF” or “OFF / ON” type instantaneous action on the electrical contacts at a set temperature point, and slower “slow-acting” types which change. gradually position as the temperature changes.
Instant acting thermostats are commonly used in our homes to control the temperature set point of ovens, irons, hot water tanks. They can also be installed on the walls to control the home heating system.
Slow-acting thermostats usually consist of a bimetallic coil or spiral. Although very economical and available over a wide temperature range, standard snap action thermostats have one major drawback: they have a large hysteresis between opening and closing of electrical contacts.
The temperature oscillation range can therefore be quite high. Commercial bimetallic household thermostats have temperature adjustment screws that allow you to more precisely set the desired temperature set point and hysteresis level.