4.2 A thermistor circuit device - temperature dependent resistor - how it works and uses

 Circuit 42 shows how you can investigate the resistance of a thermistor.

The voltmeter is wired in parallel with the thermistor, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

You must decide on the initial p.d. and see how the current varies.

You calculate the resistance of the thermistor from Ohm's Law equation: V = IR, so R = V/I

Somehow you need to vary the temperature of the thermistor resistor e.g. dipping it into a beaker of water of varying temperature, making sure the circuit is insulated from the water.

You can make measurements from 0 to 60^oC by using ice and then warm-hot water and try to get measurements for every 5 or 10^oC incremental rise in temperature.

You should find that the resistance falls with increase in temperature because a thermistor is a temperature dependent resistor.

The higher its temperature, the lower a thermistor's resistance (e.g. tens of ohms) and much higher at low temperatures (e.g. thousands of ohms).

High resistance in a cool environment and low resistance in a warm environment.

You can see this trend clearly in the resistance - temperature graph for a thermistor.

Thermistors can therefore respond to changes in temperature.

 

Uses of thermistors

Thermistors can act as temperature detectors and are used in thermostats, temperature sensors - cooling systems in car engines etc.

Circuit 32 shows in principle how to control a cooling fan in a room.

Or any other heating system in principle.

(real thermistor circuits are more complicated)

The fixed resistor and cooling fan are wired in parallel. This means they always have the same potential difference across them.

However, the thermistor is a variable resistor.

The p.d. of the power supply is shared out between the thermistor and the 'loop' consisting of the fixed resistor and fan wired in parallel.

The output component (fan) and the thermistor are wired in series.

(I've indicated this with blue arcs - not meant to be wires!)

The greater the component's resistance, the greater proportion of the p.d. it takes.

If the room gets hotter, the resistance of the thermistor decreases, so it takes a smaller shared of the p.d.

Therefore the p.d. across the fixed resistor and fan rises (V₁ increases, V₂ decreases).

The fixed resistor and cooling fan motor are wired in parallel, so have the same p.d. V₁ across them.

The greater the p.d. across the fan, the faster it goes as the power output can increase (P = IV).

If the room cools, the thermistor's resistance increases and the process reverses and the fan slows down or stops.

 

Thermistors are used as temperature detectors e.g. electronic thermostats in heating and cooling systems in the home or electric kettles (relatively low temperatures), or in high temperature situations like a car engine.

 

Footnote on the I-V graph for a thermistor  (graph (2) on the right)

The graph of current versus voltage for a thermistor is similar to that of a filament bulb.

Its a non-linear graph and the phrase non-linear component may be used.

When the current (A) is NOT proportional to the p.d (V) so the thermistor is described as a non-ohmic conductor (doesn't obey Ohm's Law, a non-ohmic resistor!).

The passage of current heats up the filament and the rise in temperature causes the resistance to increase.

As the current increases, more heat energy is released and the filament gets hotter and hotter, so further increase in temperature further increases the resistance.

This decreases the rate at which the current increases with increase in potential difference.