Bifilar Winding

Bifilar winding takes advantage of a simple physics observation: a current loop generates an opposing magnetic field to one of equal magnitude flowing in the opposite direction. By winding two adjacent conductors and connecting them so current flows out through one and back through the other, the net flux linkage drops by 90 – 99 % compared with a single-helix coil of the same length and turn count.

The construction is straightforward. The two parallel wires are wound onto the ceramic former together; the start of one wire and the end of the other are tied together at the far end (the “U-bend”), and the two free ends become the resistor's terminals. Total resistance is the sum of both wires' DC resistance. Typical residual inductance is 0.5 – 2 μH for a 1 kΩ, 50 W bifilar part, compared with 50 – 200 μH for a conventional winding of the same body size.

Bifilar parts trade off a bit of capacitance — the two wires lie close together at the same potential difference, creating a few picofarads of distributed capacitance. For most power and pulse applications this is benign, but very-high-frequency designs may prefer Ayrton-Perry winding, which balances both inductance and capacitance better. Bifilar wirewound is the standard choice for non-inductive cement-encased and aluminum-housed resistors below 10 MHz.