Wirewound Construction

Wirewound construction is the oldest and most stable resistor topology. The resistance value is set by the wire's resistivity, diameter and turns count; tolerances of ±0.005 % are achievable by trimming. Because the wire is metallic and bulk, wirewound parts deliver excellent TCR (down to ±5 ppm/K with Manganin), very low excess noise, high pulse-energy handling and stable performance from −65 °C to +275 °C.

The main drawback is parasitic inductance: a coil of wire is, fundamentally, an inductor. A typical 100 Ω, 10 W wirewound has 1 – 10 μH of self-inductance, raising its impedance at high frequencies. Two winding techniques mitigate this: bifilar winding (current flows down and back through paired wires, cancelling flux) and Ayrton-Perry winding (counter-wound layers further reducing inductance to nanohenries).

Wirewound parts dominate three application classes: precision (instrumentation, voltage references, calibration standards), power (braking, pre-charge, discharge, load banks from 1 W to 100 kW) and harsh environment (mil-aero, traction). Aluminum-housed and ceramic-cement packages provide mechanical robustness and heatsink integration. The body becomes very hot in service — temperatures of 300 °C+ on the housing are normal at full rated power.