Electrical Engineering ⇒ Topic : Thermistors ,Construction and its Applications

Thermistors. A thermistor is a heat sensitive device usually made of a semiconductor material whose resistance changes very rapidly with change of temperature. A thermistor has the following important properties :

• The resistance of a thermistor changes very rapidly with change of temperature.
• The temperature coefficient of a thermistor is very high.
• The temperature co-efficient of a thermistor can be both positive and negative

                                             FIG .(A)

                                          FIG .(B)

Construction. Thermistors are made from semiconductor oxides of iron, nickel and cobalt.They are generally in the form of beads, discs or rods (See Fig. A). A pair of platinum leads are attached at the two ends for electrical connections. The arrangement is enclosed in a very small glass bulb and sealed.

Fig. (B) shows the resistance/temperature characteristic of a typical thermistor with negative temperature coefficient. The resistance decreases progressively from 4000 Ω to 3 Ω as its temperature varies from 50°C to +150°C.

Applications

(A) A thermistor with negative temperature coefficient of resistance may be used to safeguard against current surges in a circuit where this could be harmful e.g. in a circuit where the heaters of the radio valves are in series See Fig. (c).

                                              FIG .(C)

A thermistor T is included in the circuit. When the supply voltage is switched on, the thermistor has a high resistance at first because it is cold. It thus limits the current to a moderate value.As it warms up, the thermistor resistance drops appreciably and an increased current then flows through the heaters.

(B) A thermistor with a negative temperature coefficient can be used to issue an alarm for excessive temperature of winding of motors, transformers and generators [See Fig. (D)].

                                                              FIG. (D)

When the temperature of windings is low, the thermistor is cool and its resistance is high.Therefore, only a small current flows through the thermistor and the relay coil. When the temperature of the windings is high, the thermistor is hot and its resistance is low. Therefore, a large current flows in the relay coil to close the contacts. This completes the circuit for the signal lamp or buzzer