Thermostat Working Principle

Thermostat is a device which is used to maintain a desired temperature in a system like refrigerator, air-conditioner,iron and in a number of devices.

Thermostat works on the principle of thermal expansion of solid materials.

Bimetallic strips Thermostat

A traditional thermostat has two pieces of different metals bolted together to form what’s called a bimetallic strip (or bimetal strip). The strip works as a bridge in an electrical circuit connected to your heating system. Normally the “bridge is down”, the strip carries electricity through the circuit, and the heating is on. When the strip gets hot, one of the metals expands more than the other so the whole strip bends very slightly. Eventually, it bends so much that it breaks open the circuit. The “bridge is up”, the electricity instantly switches off, the heating cuts out, and the room starts to cool.

But then what happens? As the room cools, the strip cools too and bends back to its original shape. Sooner or later, it snaps back into the circuit and makes the electricity flow again, so the heating switches back on. By adjusting the temperature dial, you change the temperature at which the circuit switches on and off. Because it takes some time for the metal strip to expand and contract, the heating isn’t constantly switching on and off every few seconds, which would be pointless (and quite irritating); depending on how well-insulated your home is, and how cold it is outside, it might take an hour or more for the thermostat to switch back on once it’s switched off.

bimetallic thermostat Principle

How a bimetallic thermostat switches on and off

  1. An outer dial enables you to set the temperature at which the thermostat switches on and off.
  2. The dial is connected through a circuit to the temperature sensor (a bimetal strip, shown here colored red and blue), which switches an electrical circuit on and off by bending more or less.
  3. The bimetal (“two metal”) strip is made of two separate metal strips fastened together: a piece of brass (blue) bolted to a piece of iron (red).
  4. Iron expands less than brass as it gets hotter, so the bimetal strip curves inward as the temperature rises.
  5. The bimetal strip forms part of an electrical circuit (gray path). When the strip is cool, it’s straight, so it acts as a bridge through which electricity can flow. The circuit is on and so is the heating. When the strip is hotter, it bends and breaks the circuit, so no electricity can flow. Now the circuit is off.
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6 thoughts on “Thermostat Working Principle”

  1. nice post ……
    I have one doubt…
    how the material properties are getting changed based on the input temperature in dial switch in thermostat working principle…..

    if i set a temp 30 deg C …. the bimetal bends at 30 deg C

    if i set temp 100 deg C ….the bimetal bends at 100 deg C

    how this happens ???

    • Changing thermostat value actually changes the position of wire 2 relative to bimettalic strip.
      if you increase value for example in above case, it will move the touching end of wire 2 with bimetallic strip furthur to right causing the touching end stay for long time with bimettalic strip.


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