The name Thermistor is derived from Thermally Sensitive Resistors. These are two types namely NTC and PTC. Thermistors have negative temperature coefficient of Resistance.
Working:– When temperature increases, the resistance of Thermistor decreases and vice versa. Thermistors sre semiconductors made from a specific mixture of pure oxides such as:-
- IRON etc.
Types of Thermistors.
The thermistor is placed in one leg of a wheatstone bridge circuit. At balance condition, when there is no change in temperature, the galvanometer G indicates zero. As the temperature increases or decreases, the resistance of the Thermistor also increases of decreases due to which the wheatstone bridge circuit becomes unbalanced. The deflection of the galvanometer G can be calibrated on the temperature scale,
R1 RX = R2 R3
RX =R2 R3/R1
RX = Resistance of Thermistor.
Characteristics:– The Thermistor is the following characteristics R=ae b/t
a & b are constant. R=Resistance of Thermistors at Absolute temperature (T).
Thermistor Testing With Digital Multimeter
Select DMM Selector switch to Resistance mode (Auto Range) DMM Means Digital MultiMeter.
- Connect DMM + ve probe to one end of Thermistor
- Connect DMM – ve probe to another end of Thermistor
- See the DMM Display reading in normal temperature.
- Switch on the soldering Iron (20W) at near the thermistor.
VERIFICATION: The DMM reading shows when increases temperature by the heater, the value of the Resistance of the thermistor is decreases. It means that thermistor is NTC type (Negative Temperature coefficient)
- Otherwise increase its Resistance that thermistor is PTC (Positive Temperature coefficient)
- For accuracy measurement need a Wheatstone Bridge Circuit.