THERMISTORS

• It  is  a bulk semiconductor device having  negative temperature  coefficient  ofresistivity i.e., the resistance decreasing exponentially with increasing temperature.

• Thermistors are frequently used as temperature sensors in the range of 100to 300°C. It   is made   of metal oxide aluminum, nickel, copper, zinc, manganese, cobalt etc.,

• The resistivity of thermistor varying between 0.1 to lo9. These sensors aremade in the shape of bead, disc, rod and pellets etc.

• Beads may be glass coated or enclosed in evacuated or gas filled envelops forprotection   against   corrosion.   Washer   shaped   thermistors   can   be   bolted together for series or parallel connection.

• The  device  resistance  decreased  when temperature  raises current through  athermistor  causes  power  dissipation  which  raises  the  device  temperature. Thus the device resistance in dependent upon ambient temperature and self biasing.

• When  very small  currents  flow through  the thermistor  will  have no  effect sothe increases to a peak value at which the heating effect of the current begins    to significantly change the thermistor resistance.

• Further  increase in  current  causes  a progressive reduction  in  resistance  andconsequently a reduction in voltage across the device.

Uses



1. In electronic circuits used as temperature compensating device.

2. As a temperature sensor in electronic thermometers.

3. As a sensing element in microwave power measuring equipment.

4. As thermal delay

5. In control devices actuated by charges in temperature.

Advantages

1. Higher temperature coefficent

2. Smaller in size

3. Low thermal capacity.

4. High speed of response

5. Improved accuracy of measurement.