Derating Curve

Every resistor datasheet includes a derating curve, and every responsible design must respect it. The curve recognises a simple physical fact: the resistor's element has a maximum hot-spot temperature it can survive long-term (typically 350 – 450 °C for wirewound, 155 – 200 °C for film, 275 °C for cement). The rated power is what produces that hot-spot temperature at the reference ambient — often 25 °C, 40 °C or 70 °C. Run the resistor at higher ambient and the allowed dissipation must drop proportionally so the hot-spot stays the same.

The typical derating curve is flat at 100 % up to the reference temperature, then slopes linearly to 0 % at the maximum operating temperature. A 100 W resistor rated at 70 °C with 200 °C maximum derates by 100 % / (200 − 70) = 0.77 % per °C — at 100 °C ambient, only 100 × (200 − 100) / (200 − 70) = 77 W is permissible; at 150 °C, 38 W; at 200 °C, zero. Many designs ignore this and run hot for marginal savings, then fail in field service when summer ambient pushes the resistor past its limit.

Derating curves apply to terminal temperature (for SMD chips, measured at the solder pad) or ambient (for chassis-mount). The thermal path matters: aluminum-housed resistors have a separate curve for free-air vs heatsink-mounted. Pulse loads have their own derating curve at low duty cycles. Always read the actual datasheet curve — generic 50 % derating rules of thumb are starting points, not substitutes.