Non-Inductive Winding

A standard helical wirewound resistor behaves as an inductor in series with the desired resistance — typical 100 Ω, 50 W parts measure 5 – 20 μH, presenting an impedance of several ohms above the resistance value at 100 kHz. In snubbers, dummy loads for RF amplifiers, oscillator-damping networks and current-sense applications in switch-mode converters, this is unacceptable.

Non-inductive winding solves the problem geometrically. The simplest method, bifilar, folds the wire back on itself so current in adjacent turns flows in opposite directions and the magnetic fields cancel. Ayrton-Perry winding takes this further with two layers of counter-wound thin wire, reaching residual inductance below 100 nH for 100 Ω parts. Reverse-layer methods alternate winding direction every layer to balance flux.

Non-inductive parts are essential in dynamic-braking resistors for VFDs (where the chopped DC waveform contains significant 4 – 16 kHz energy), in pulse-discharge resistors for capacitor banks (where a low τ requires low L as well as low R), in damping resistors for IGBT snubbers, and in precision shunts for switching power supplies. The trade-off is roughly 2× the wire length per ohm and higher cost, but no other construction matches wirewound's pulse-energy capability at low inductance.