Resistor inductance

 
Resistor inductance

What is resistor inductance? Inductance is an electrical property of conductors by which an electrical current passing through the conductor induces an electromotive force in the conductor itself (self-inductance) and other conductors nearby. Since resistors are made of conductive materials, they, too, exhibit inductance as an unwanted, parasitic effect. This effect is especially noticeable if the resistor is made out of wire formed into a coil shape. Depending on the application, resistor inductance might be easily disregarded, especially in DC circuits. However, parasitic resistor inductance can be a significant factor in high-frequency AC applications. The reason for this is that the impedance of a resistor rises with the applied voltage frequency due to the increase in its reactance. Inductors and resistors Electrical loads can be divided into two types: real (or resistive) loads and reactive loads. Real loads are used to convert electrical power into heat. An ideal resistor is a purely resistive load, which means that all the electrical power applied to the resistor is dissipated as heat. On the other hand, reactive loads convert electrical power into a magnetic or electric field and temporarily store it before returning it to the rest of the circuit. Reactive loads can be inductive or capacitive. Inductive load store energy in the form of a magnetic field, while capacitive loads store energy in the form of an electric field. The main difference between ideal resistors and ideal inductors is therefore that resistors dissipate electrical power as heat, while inductors turn electrical power into a magnetic field. Ideal resistors have zero reactance and as a result zero inductance. Unfortunately, electrical devices are not ideal in practice and even the simplest resistors have a slight parasitic inductive reactance. Parasitic inductance Resistors are used when a purely resistive load is required, so inductance is often [… read more]

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]