# Encyclopedia Magnetica

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ayrton-perry_winding

# Ayrton-Perry winding

 Stan Zurek, Ayrton-Perry winding, Encyclopedia Magnetica, E-Magnetica.pl reviewed by Jeff Jones, 2021-04-04

Ayrton-Perry winding - type of winding used for making wire-wound non-inductive resistors or such that have very low equivalent series inductance (ESL). The wire is typically made of resistive alloys with low thermal coefficient like copper-nickel or nickel-chrome.1)

Ayrton-Perry winding

Resistors with an Ayrton-Perry widing are used in electronic applications, such as: audio2), precision current shunts and voltage dividers, attenuation and amplification (e.g. setting a gain of an amplifier), decade box resistances, etc.3)

There are several other technologies which provide “low inductance” resistors. For example, the conductive material used in thick-film resistors, however they are not as resilient to overload and high current pulses as wire-wound resistors.4)

This type of winding is named after William Edward Ayrton (English physicist and electrical engineer, 1847-1908) and John Perry (Irish mathematician and engineer, 1850-1920).

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Ayrton-Perry winding in a decade box resistance
Types of windings: 1 - ordinary helical, 2 - bifilar, 3 - on a flat former, 4 - Ayrton-Perry

## Self-inductance and self-capacitance

In an Ayrton-Perry winding, two wires are wound spirally around a former in opposing directions, and joined at each end to form a parallel electrical connection. Wire-wound resistors made in such a way exhibit low inductance, because the magnetic effects from each wire cancel each other, and as a result the amount of the instantaneous magnetic energy is greatly reduced.

Additionally, if the wires are made to cross at equal lengths then they will cross each other at the points of equal potential which further reduces the parasitic self-capacitance.5).6) The crossing wires should be still insulated from each other, for example by using enamel (see also: enamelled wire).

However, the wires are connected in parallel, so for the same length of resistive wire the resistance is lower, as compared to other types of wire-wound resistors. For instance, bifilar winding made with the same length of wire has four times greater resistance, but also a greater self-inductance. On the other hand, the total surface area of the wire is also increased, so such resistor can withstand greater currents without overheating.

If such winding is made from a non-magnetic wire (which is typical), it exhibits low inductance and reduces signal loss, so it is suitable for example for design of audio circuits.7)

## Further improvements

Windings in Dorst's patent8): Fig.1 - 1st spiral layer, Fig.2 - 2nd layer (connected in parallel to 1st layer), Fig.3 - 3rd layer, Fig.4 - 4th layer (in parallel with 3rd layer)

US Patent Office, Public domain

Ayrton-Perry winding can be made on a round or a flat former, and the latter can further reduce its equivalent series inductance (ESL)9)10)

However, at very high frequencies (MHz range), the inductance may be still too high and more elaborate winding methods may be employed. An example is described for example in US patent US242168811), in which four wires are used, wound in two opposing pairs. All wires are then connected in parallel.