Resistor Power Derating

Pmax = Prated × (Tmax−Tamb)/(Tmax−Trated)

Quick answer

This calculator solves Pmax_derated = Prated × (Tmax − Tambient) / (Tmax − Trated) for Derated Power. With Rated Power 22 W, Max Temperature 50 °C, Ambient Temperature 110 °C, Rated Temperature 235 °C, the result is 7.135 W.

Calculator

Result
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Formula

Pmax_derated = Prated × (Tmax − Tambient) / (Tmax − Trated)

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.

Symbols and units

Symbol Meaning Default unit Accepted units
Pmax Derated Power W µW, mW, W, kW
Prated Rated Power W µW, mW, W, kW
Tmax Max Temperature °C °C
Tamb Ambient Temperature °C °C
Trated Rated Temperature °C °C

Step by step

  1. 1. What you know: Rated Power 22 W, Max Temperature 50 °C, Ambient Temperature 110 °C, Rated Temperature 235 °C
  2. 2. The relation: Pmax_derated = Prated × (Tmax − Tambient) / (Tmax − Trated)
  3. 3. Substitute: Prated = 22 W, Tmax = 50 °C, Tamb = 110 °C, Trated = 235 °C
  4. 4. Result: Derated Power = 7.135 W

Worked examples

Minimum range

Given: Rated Power 22 mW, Max Temperature 2.5 °C, Ambient Temperature 5.5 °C, Rated Temperature 11.8 °C

Result: Derated Power = 7.097 mW

Low range

Given: Rated Power 0.22 W, Max Temperature 25 °C, Ambient Temperature 55 °C, Rated Temperature 118 °C

Result: Derated Power = 70.97 mW

Typical

Given: Rated Power 22 W, Max Temperature 50 °C, Ambient Temperature 110 °C, Rated Temperature 235 °C

Result: Derated Power = 7.135 W

High range

Given: Rated Power 2.2 kW, Max Temperature 85 °C, Ambient Temperature 187 °C, Rated Temperature 400 °C

Result: Derated Power = 0.7124 kW

Maximum range

Given: Rated Power 22 kW, Max Temperature 850 °C, Ambient Temperature 1870 °C, Rated Temperature 4000 °C

Result: Derated Power = 7.124 kW

How Derated Power changes with Rated Power

Rated Power (input) Derated Power (result)
4.84 W 1.57 W
12.1 W 3.924 W
24.2 W 7.849 W
36.3 W 11.77 W
48.4 W 15.7 W
72.6 W 23.55 W
96.8 W 31.39 W
0.1936 kW 62.79 W
0.3388 kW 109.9 W
0.484 kW 157 W

The same result in other units

Unit Value
µW 7135000
mW 7135
W 7.135
kW 0.007135

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Frequently asked questions

How do you calculate Resistor Power Derating?

Use Pmax_derated = Prated × (Tmax − Tambient) / (Tmax − Trated). With Rated Power 22 W, Max Temperature 50 °C, Ambient Temperature 110 °C, Rated Temperature 235 °C, Derated Power works out to 7.135 W.

What is the formula for Resistor Power Derating?

The formula is Pmax_derated = Prated × (Tmax − Tambient) / (Tmax − Trated). Resistors are rated for maximum power dissipation at a specific ambient temperature (typically 70°C).

What units does Resistor Power Derating use?

Derated Power is shown in µW, mW, W, kW. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

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