How A Thermostat Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How A Thermostat Works

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The schematic of a simple rod thermostat is shown above. The thermostat comprises two elements – Brass and Invar - that expand at different rates. The brass tube expands a lot as it gets hotter but invar expands very little.  When the liquid whose temperature we want to control is cool the brass does not expand so the switch is closed and the electric heater heats the water.  When the water reaches the set temperature, the brass tube has expanded enough to pull the Invar rod away from the switch.  This opens the switch and breaks the circuit.

The electric heater will stay disconnected until the brass tube contracts enough to close the switch again.  Normally the thermostat and heater are together in one unit.  The temperature at which the switch is opened can be adjusted by changing the tension of the spring which closes the switch.  Thermostats are not very accurate ( ± 30 degree C) but they are long lasting and cheap.