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What is a Thermistor?

A thermistor is a component whose
resistance changes when its temperature changes.
A thermistor is a special type of resistor whose
resistance decreases as its temperature increases.

Plot of Resistance against Temperature for a Thermistor

A plot of resistance against temperature for
a thermistor is shown above. The plot is unusual
because it shows that a bigger current will flow
at a higher temperature. For most components, the
resistance increases as the temperature increases
and a smaller current flows at a higher temperature.

A plot of current against voltage is shown below.
The curve shows that it does not obey ohm's law.

Plot of Current against Voltage for a Thermistor

What is a
Thermistor used for?

A thermistor can be used in a circuit to detect a
temperature change. For example, a thermistor may
be used to keep a freezer at the right temperature.
A circuit with a thermistor in it can switch the freezer on
if the temperature gets too high and switch the freezer
off again when the correct low temperature is reached.

A thermistor can also be used to trigger a fire alarm.

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