The user must choose according to the working conditions of the component (power, room temperature). Maximum working temperature must not exceed 125 °C. The dissipated power is simply calculated by the following ratio:
P: Expressed in W
ΔT: Difference between maximum working temperature and room temperature or fluid cooling temperature.
Rth (j - c): Thermal resistance value measured between resistive layer and outer side of the resistor. It is the thermal resistance of the component.
Rth (c - h): Thermal resistance value measured between outer side of the resistor and upper side of the heatsink. This is the thermal resistance of the interface (grease, thermal pad), and the quality of the fastening device.
Rth (h - a): Thermal resistance of the heatsink.
Example: RTH (c - a) for RCH 25 power rating 20 W at ambient temperature +50 °C Thermal resistance RTH (j - c): 2.5 °C/W Considering equation (1) we have:
ΔT = ≤ 125 °C - 50 °C ≤ 75 °C RTH (j - c) = 1.4 °C/W (Special Features) RTH (j - c) + RTH (c - h) + RTH (h - a) = = = 3.75 °C/W RTH (c - h) + RTH (h - a) ≤ 3.75 °C/W - 1.4 °C/W ≤ 2.35 °C/W with a thermal grease RTH (c - h) = 1 °C/W, we need a heatsink with RTH (h - a) = 1.35 °C/W
The applied voltage must always be lower than the maximum overload voltage as shown in the special features table. The values indicated on the graph below are applicable to resistors in air or mounted onto a heatsink.
