This is a type
of resistor is made from a sintered semiconductor material which has a resistance that
changes with temperature. There are two types of
thermistor:
(i) negative temperature coefficient (NTC) – the resistance of the thermistor
falls with increasing temperature
(ii) positive temperature coefficient (PTC) - the
resistance of the thermistor rises with increasing temperature
The NTC is the type
most often used in schools so I will just consider that type in detail.
The increasing
temperature produces more free electrons and so the resistance falls. These electrons are
able to 'jump' from the valence band to the conduction band. This increase in conduction
electrons masks the effect due to the increase thermal motion of the atoms and
electrons.
At low temperature its resistance is large (thousands of ohms), at high
temperature its resistance is small (tens of ohms). Very little current will flow through it when
it is cold. This means that its resistance increases as the temperature falls.
A graph
for the thermistor of resistance against its temperature is shown in Figure 1.
You can
investigate this property practically by doing the experiment described in the 14-16
Experiment guides called Thermistor resistance.
The two graphs below show the variation of current with voltage for the two types of thermistor (NTC and PTC).
