# Heat Sink Size and Power Resistor Rating Calculators

## Heat Sink Size Calculation for Power Resistors

To optimise the performance of power resistors, calculating the correct heat sink size is essential. This calculation, pivotal for thermal management, involves determining the total allowable thermal resistance, with a typical case-to-sink resistance of 1°C/W. Proper heat sink sizing ensures efficient heat dissipation, preventing overheating and enhancing the resistor’s longevity and reliability. Inadequate heat sinks can lead to component failure, affecting system stability. By calculating the ideal heat sink size, designers and engineers can ensure optimal functioning of electronic circuits, making this a critical step in electronic design and maintenance, especially in high-power applications where thermal management is a key concern.

Formula: RθSA = ((TRmax - TA) / Pd) - RθJC - RθCS

°C
°C
W
°C/W
1.00 °C/W
0.00 °C/W

### Example Calculation

Given a power resistor with a thermal resistance of 4.17°C/W, used at an ambient temperature of 25°C, dissipating 8 Watts of power, and with a maximum resistor temperature of 150°C, the required heat sink thermal resistance can be calculated.

Given:
TRmax = 150°C
TA = 25°C
Pd = 8W
RθJC = 4.17°C/W
RθCS (assumed) = 1°C/W

Calculation:
RθSA = ((150°C – 25°C) / 8W) – 4.17°C/W – 1°C/W
RθSA = (125°C / 8W) – 4.17°C/W – 1°C/W
RθSA = 15.63°C/W – 4.17°C/W – 1°C/W
RθSA = 10.46°C/W

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

## Maximum Rated Resistor Power for a Given Heat Sink

If the size of a heat sink is already established, one can calculate the maximum rated power that it can handle for a power resistor. This calculation is essential for ensuring that the heat sink is capable of efficiently dissipating the heat generated by the resistor. The formula considers several factors, including the heat sink’s thermal resistance, ambient temperature, and the maximum allowable temperature rise of the resistor. By determining the maximum power rating, engineers and designers can ensure that the power resistor operates within safe thermal limits, preventing overheating, which could lead to component failure or reduced efficiency. This calculation is a critical aspect of thermal management in electronic design, particularly in high-power applications where maintaining temperature stability is crucial for system reliability.

Formula: Pd_max = (TRmax - TA) / (RθJC + RθCS + RθSA)

°C
°C
°C/W
°C/W
°C/W
0.00 W

### Example Calculation

For a heat sink with a thermal resistance of 10.46°C/W, assuming the thermal resistance from the resistor to the case is 4.17°C/W, and the interface resistance is 1°C/W, with a maximum resistor temperature of 150°C and an ambient temperature of 25°C, the maximum power dissipation can be calculated.

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°C – 25°C) / (4.17°C/W + 1°C/W + 10.46°C/W)
Pd_max = 125°C / 15.63°C/W
Pd_max ≈ 8.00W

Therefore, the maximum power that the heat sink can handle for a power resistor is approximately 8.00 Watts.

### Popular manufacturers

The table below offers some heat sink mountable resistor options we have available as well as some of their specifications:

SERIESTHERMAL RESISTANCERESISTANCE RANGETOLERANCETCRPOWER RATINGPACKAGE
MP915 Series
8.33°C/W0.02Ω to 1kΩ±1.0%
(from 0.05Ω)
-20 to +80 ppm/°C
(from 0.5Ω)
15WTO-126 style
MP916 Series
7.81°C/W0.01Ω to 0.019Ω±5.0%0 to +500 ppm/°C16WTO-126 style
MP925 Series
5.00°C/W5kΩ to 100kΩ±1.0%-20 to +80 ppm/°C25WTO-220 style
MP930 Series
4.17°C/W0.02Ω to 4.99kΩ±1.0%
(from 0.05Ω)
-20 to +80 ppm/°C (from 0.5Ω)30WTO-220 style
MP9100 Series
1.50°C/W0.05Ω to 100Ω±1.0%-20 to +80 ppm/°C (from 0.5Ω)100WTO-247 style
Isabellenhütte
PBH Series
4°C/W0.002Ω to 100Ω±0.5%
(from 0.01Ω)
<50 ppm/°C10WTO-247 style
Isabellenhütte
PBV Series
3°C/W0.0005Ω to 1Ω±0.5% (from 0.0015Ω)<30 ppm/°C (from 0.01Ω)10WNon-standard
Isabellenhütte
A-H2 Series
3°C/W0.001Ω to 100Ω±0.1% (from 0.001Ω)<30 ppm/°C10WNon-standard

Users are hereby informed that Rhopoint accepts no responsibility for any inaccuracies or issues that may arise from the use of our online calculators. All data and results should be independently verified prior to their application in any projects or calculations.