Temperature Coefficient of Resistance

The temperature coefficient of resistance, or TCR, is one of the most important parameters that characterize a resistor performance. The TCR defines the change in resistance as a function of the ambient temperature. The common way to express the TCR is in either ppm/°C (or ppm/K), which stands for parts per million per degree Celsius (or Kelvin). 

Resistors are available with a TCR that is negative, positive, or stable over a certain temperature range. For pure metals, this coefficient is a positive number, meaning that resistance increases with increasing temperature. For the elements carbon, silicon, and germanium, this coefficient is a negative number, meaning that resistance decreases with increasing temperature. For some metal alloys, the temperature coefficient of resistance is very close to zero, meaning that the resistance hardly changes at all with variations in temperature (a good property if you want to build a precision resistor out of metal wire!).

Choosing the right resistor could prevent the need for temperature compensation. In some applications it is desired to have a large TCR, for example to measure temperature. Resistors for these applications are called thermistors, and can have a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC).