Temperature coefficient of resistor

Temperature coefficient of resistor:-

The resistance thermometers (RTD) and thermistors employ the principle 
of change in electrical resistance with change in temperature. 

If the temperature changes, the resistance also changes due to changes 
in both length and sensitivity. 

Therefore materials used for resistance thermometers have temperature -
coefficient of resistivity much larger than the coefficient of thermal expansion. 
Thus, the temperature coefficient of resistance 'a' is given by

Where,

 Change in temperature, °C

ΔT = Change in temperature, °C

Δρ/ρ0 = Fractional change in resistivity

ΔR/R0 = Fractional change in resistance.

ρ0 = Resistivity at 0 °C

R0 = Resistance at 0 °C.

The change in resistance with temperature can be given by the following relationship.

R=R0(1+α1T+α2T2+-------------------+αnTn+--------)

Platinum, nickel, copper and tungsten are the commonly used resistance materials. 

These metals provide a definite resistance value at each temperature within its range.
 Curves indicate that the resistance of platinum and copper increases almost linearly with increasing temperature,

while the characteristic of nickel is nonlinear. Tungsten has relatively high resistivity,
 but its use is limited for high temperature applications.

Gold and Silver are rarely used owing to their extremely low resistivity. 
 Electrolytic copper which has highly consistent temperature coefficient of resistance i.e.,greater(higher) than platinum. 

Due to their low relativity, their application is limited to low range industrial purposes. Phosphor bronze alloys are found suitable for low temperature measurements. 
Generally Nickel is chosen for resistance temperature measurements
 which has high temperature coefficient, less expensive than platinum and 
good reliability. 

According to their intended application the resistance material is selected.