THERMISTOR

• Thermally sensitive resistor• Non-metallic resistors (semiconductor

materials)

Bead Type Thermistor (simplest in construction)Symbol for thermistor

Types of Thermistor (by construction)

Bead Type

Disc Type

Washer Type

Rod Type

Thermistor Probe (Used in Lab)

Temperature Vs Resistance

• Negative temperature coefficient• High precision, Limited range [−90 °C to

130 °C ]• Exponentially decaying characteristics

R0 Resistance of Thermistor at T0 K

R Resistance of Thermistor at T K

β Material constant of the material ( 3K to 5 M)

0

1 1β -

T T0R = R e

Thermistor

RTD

Thermocouple

Temperature

A Comparative Study

Comaprison of temperature of Thermistor, RTD, Thermocouple

Application in Transducer

• Temperature (resistance) changes to electrical signals

• Active element in wheat-stone bridge or potentiometer

RT

• Resistance of the Thermistor (from input voltage, output voltage and other resistance)

• For equal bridge

• Knowing the value of new resistance, temperature can be estimated from lookup table

Determination of Temperature

' (1 2 / )

(1 2 / )T T

V ER R

V E

−=+

Assignment

• Obtain the following expression assuming equal bridge

' (1 2 / )

(1 2 / )T T

V ER R

V E

−=+