Why doesn’t a wirewound resistor act like an inductor?

October 13, 2016 // By EDN Europe
Phil Ebbert, Riedon’s VP of engineering
On the surface, wirewound resistors and inductors appear to be constructed very similarly – by winding a conductor around a core. So why does the wirewound resistor not act electrically in the same way as an inductor?

There are several mechanical differences in the construction of each component that differentiate them. For instance, when fabricating the wirewound resistor, an insulator is used for the core, whereas the inductor normally features a ferrite core. The inductor also uses a wire that is more conductive in its construction. But even though there are differences in the construction of the two devices, the wirewound resistor may still exhibit some inductance. Since the effects of that inductance is linked to the frequency of the circuit, the higher the frequency, the more effect that parasitic inductance will have on the signal. This can become problematic at very high frequencies, for example in RF circuits.  

Inductance or capacitance effects in the signal path will lead to distortion of the signal, sometimes changing it so much as to make it unrecognisable. High frequency signals are affected in different ways by these reactances. Inductance affects the signal by slowing down the current rise time, while capacitance causes the pulse to overshoot the desired level, which causes more distortion. Capacitance is generally less of a worry than inductance for wirewound resistors.

These reactance effects mean that precision resistors intended for high frequency circuits should be constructed in a way that will minimise any inherent inductance. A method that is often used to minimise the inductance, while improving the pulse load capacity, is to use different methods of winding the wire. For example, bifilar winding doubles the wire before winding it round the core. Using bifilar winding forces the current in the adjacent wires to flow in opposing directions, cancelling the electrical fields out and minimising inductance. Unfortunately, bifilar winding means that there is usually a large difference in the voltage in adjacent wires, leading to parasitic capacitance.

Another winding technique to reduce inductance is known as Ayrton-Perry windings. This technique wraps one wire around the core, and then adds a layer of insulation on top, before winding a second wire on top of the insulation in the opposite direction to the first. As the two wires are wound in opposite directions, the current in each wire is also opposed, cancelling the generated electric fields and minimising inductance. However, as the wires in this technique are wound in parallel to each other, the voltage in adjacent wires remains the same level, and therefore capacitance is also minimised.

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Different resistor winding techniques

There is a slight downside in this technique in that Ayrton-Perry wirewound resistors are more complex to fabricate than other types of wirewound resistors, but this is offset by their electrical properties. Riedon uses Ayrton-Perry windings in the company’s fast rise resistor series. This series offers minimal inductance (under 1μh for a 500Ω resistor), capacitance (under 0.8pF for a 1MΩ resistor) and has a normal rise time of under 20nsec.

Riedon has produced a detailed guide for different types of resistors and their characteristics. Download the resistors e-book here.

 

Phil Ebbert

Vice president of engineering

Phil Ebbert, vice president of engineering, is in charge of resistor development at Riedon Inc. He is also responsible for