Resistor properties

 

The function of resistors is to oppose the flow of electric current in a circuit. Therefore their primary parameter is the resistance value. The manufacturing tolerance must be adequately chosen for each specific application. The ultimate resistance value may deviate from the specification because of many reasons. One is the temperature coefficient of resistance, or TCR, which is often specified for precision applications. Stability defines the long term variations of the resistance. After a long duration of electric load, the resistance value will not return to its original value. Electric noise appears in every resistor, and is for low-noise amplifying applications of importance. For high frequency applications, the inductance and capacitance properties play a role. Next to the characteristics related to resistance value, the maximum power and voltage can be specified. The maximum power rating is mainly for power electronics important, while resistors in electronic circuit boards mostly never reach the maximum power rating. For high voltage circuits, the maximum rated voltage must be taken into account. The quality of a resistor in terms of durability and reliability is for some applications more important than for others. An overview of the most common resistor properties and characteristics to describe a resistor are detailed below. Low Temperature Coefficient of Resistance (TCR) The TCR is dependent on the resistive material and the resistor construction. The temperature dependence of electrical resistivity is determined by the material: Number of phonons Coefficient of expansion from the material Power rating The power rating indicates the maximum dissipation that the component is capable of. The rated dissipation is normally specified at room temperature and decreases at higher temperatures. This is called derating. Typically from 70°C derating is specified. Above this temperature, it can only utilize a reduced power level. This is illustrated by a derating curve. The [… read more]