Resistor Power Derating

Formula

Description

Resistors are rated for maximum power dissipation at a specific ambient temperature (typically 70°C). Above this temperature, the allowable power decreases linearly to zero at the maximum component temperature (typically 155°C for thick film). This derating ensures the internal temperature never exceeds the maximum rated value. In enclosed or high-temperature environments, the effective power rating can be significantly less than the nominal value. Always check the derating curve in the datasheet for the actual operating conditions.

Variables

• Pmax_derated — Maximum allowable power at operating temperature (W)
• Prated — Rated power at reference temperature (W)
• Tmax — Maximum component temperature (°C)
• Tambient — Actual ambient temperature (°C)
• Trated — Temperature at which full power is rated (°C)

Practical Notes

For a 0.25W resistor rated at 70°C with Tmax of 155°C operating at 100°C: Pmax = 0.25 × (155-100)/(155-70) = 0.162W. Good practice is to derate to 50% of nominal even at room temperature for improved reliability and reduced resistance drift over time.