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 :
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.
(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).
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)].
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
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