Heat Sink Size & Power Resistor Rating Calculators

These calculators support thermal design for power resistors, helping you determine either the required heat sink performance or the maximum power that can be safely dissipated in a given setup.

Use the tools below to either select a suitable heat sink or verify an existing configuration..

Heat Sink Size Calculation for Power Resistors

Calculate the required heat sink thermal resistance for a power resistor based on operating conditions and thermal limits.



This calculator uses maximum resistor temperature, ambient temperature, power dissipation, and junction-to-case thermal resistance to estimate the heat sink performance required for reliable operation. A typical case-to-sink thermal resistance of 1°C/W is assumed.

Example Calculation

Given a power resistor with a junction-to-case thermal resistance of 4.17°C/W, operating at an ambient temperature of 25°C, dissipating 8W of power, and with a maximum resistor temperature of 150°C:

Given:

TRmax = 150°C
TA = 25°C
Pd = 8W
RθJC = 4.17°C/W
RθCS = 1°C/W

Calculation:

RθSA = ((150 - 25) / 8) - 4.17 - 1
RθSA = 15.625 - 4.17 - 1
RθSA = 10.455°C/W

Therefore, a heat sink with a thermal resistance of approximately 10.46°C/W is required.

Maximum Rated Resistor Power for a Given Heat Sink

Calculate the maximum power a resistor can safely dissipate when used with a specified heat sink.



This calculator determines the allowable power based on thermal resistance values and operating temperatures, helping to confirm whether a heat sink is suitable for a given application.

Example Calculation

Given a heat sink with a thermal resistance of 10.46°C/W, a junction-to-case thermal resistance of 4.17°C/W, a case-to-sink thermal resistance of 1°C/W, a maximum resistor temperature of 150°C, and an ambient temperature of 25°C:

Given:

TRmax = 150°C
TA = 25°C
RθJC = 4.17°C/W
RθCS = 1°C/W

RθSA = 10.46°C/W

Calculation:

Pd,max = (150 - 25) / (4.17 + 1 + 10.46)
Pd,max = 125 / 15.63
Pd,max = 8.00W

Therefore, the maximum rated power the heat sink can support is approximately 8.00W.

The results provided by this calculator are intended as a guide only. While care has been taken to ensure accuracy, Charcroft Electronics does not accept responsibility for any errors or omissions. All results should be independently verified before being used in design, specification or application.