We can make a wirewound resistor by winding resistive wire into a coil around an insulating core. We can also make an inductor by using a more conductive wire and winding it around a ferrite core. Both are essentially a coil of wire around a core, so what makes wirewound resistors behave more like resistors, and less like inductors?
There are several key distinguishing features between wirewound resistors and wirewound inductors, including differences in wire material and core material. But even with resistive wire and a non-conducting core, a wirewound resistor can still have a significant parasitic inductance which prevents use in high frequency applications, such as RF circuits.
Any reactance (inductance or capacitance) in the signal path will distort the signal, potentially beyond recognition. A pulse or high frequency signal is negatively affected by the inductance and capacitance of its path in different ways. Parasitic inductance will increase the rise time, that is, it will prevent the current from rising quickly. Parasitic capacitance, while less of an issue for wirewound resistors than inductance, will cause the signal to overshoot beyond the required level and cause additional unwanted distortion.
Precision resistors for high frequency signal paths therefore need to minimise their inherent inductance to improve their pulse load capability. One approach is to coil the wire using a bifilar winding, which folds the wire double before wrapping it around the core. This also helps minimise induction because the current in adjacent wires is in opposite directions, so the resulting electric fields are cancelled out. However, this method has a large parasitic capacitance since the voltage in adjacent wires is significantly different.
A better technique is the Ayrton-Perry Windings, in which two wires in parallel are wrapped in opposite directions around a core, one on top of the other, with a layer of insulation in between. The inductance is minimised because the wires are wound in opposite directions – currents in the inner wires are flowing the opposite way to currents in the outer wires, so the resulting electric fields cancel each other out. And the parasitic capacitance is minimised because the wires are in parallel, so voltages in adjacent turns in the inner and outer wires have the same voltage.
While Ayrton-Perry wirewound resistors are slightly more complex to manufacture, they can produce extremely favourable characteristics. This technique is the key to Riedon’s fast rise resistor range. Typical parasitic inductance for this range is less than 1μh for a 500Ω resistor, parasitic capacitance is less than 0.8pF for a 1MΩ resistor, and a typical fast rise time resistor in Riedon’s range enables a rise time of 20nsec or less.