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Heatsink calculations Mounting a power resistor on a heatsink offers several design advantages over non heatsink mounted devices. Low inductance compared to power wire-wound resistors. As the parts are attached to a heatsink there is a reduced susceptibility to vibration. Good thermal management leading to reduced heating effects on neighbouring components PCB space saving. Working out the heatsink requirements for power resistors is relatively straightforward. Careful design and testing is needed to make sure the case temperature does not exceed its rating. The allowable power rating (Pd, Watts) may be determined as follows: Pd = (TRmax –TA) /(RθJC + RθCS + RθSA) Where: TRmax is the maximum temperature for the resistor. TA is the ambient or temperature reference RθJC is the thermal resistance-resistor element to resistor surface RθCS is the thermal resistance-ceramic element to heatsink surface (pad or thermal grease)* RθSA is the thermal resistance of heatsink to ambient. RθCS + RθSA = ((TRmax –TA) / Pd) – RθJC Example of selecting a heatsink for a power resistor Using a power resistor with a RθJC of 4.17°C/W at an ambient temperature of 25°C with a power dissipation of 8 Watts, and a maximum resistor temperature of 150°C the total allowable thermal resistance for the heat sink and the attachment interface is as follows: Maximum total Rθ of the heat sink and interface = ((150°C – 25°C) / 8W) – 4.17°C/W ((150°C – 25°C) / 8W) – 4.17°C/W = 15.63°C/W – 4.17°C/W = 11.46°C/W If 1°C per watt is allowed for the interface, then a heatsink of approximately 10.5°C/W is required. Choosing a power resistor for your heatsink If a heatsink is available and the designer needs to put the resistor on it, the calculation to determine the power in a power resistor is as follows: Pd = (TRmax –TA) / (RθJC + RθCS + RθSA) However the quick way of doing both calculations is to use the calculators on the Rhopoint Components website: Maximum resistor power for given heatsink Heatsink size calculator www.rhopointcomponents.com Techtorial 10 – 2MT10.1