Electronic – Using resistors at their rated power


A standard 1% metal film plated-through hole (PTH) resistor is usually rated at 250 mW. Under which conditions can it handle that power? Are special mounting precautions needed, or will mounting flat on a PCB, with 0.5 mm traces do?

Best Answer

A resistor will reach a thermal equilibrium when the dissipated energy equals the energy drained to the environment. To drain heat to the environment the resistor's temperature has to be higher than the environment's; the higher the temperature difference the more heat will flow. So the resistor can dissipate more power at low environment temperatures. Rated power may be specified at 25°C and derate at higher temperatures. These thick film chip resistors derate only from 70°C, as the following graph shows:

enter image description here

So a 100mW 0603 can still dissipate these 100mW at 70°C, but shouldn't dissipate more than 50mW at 100°C environment temperature.

The datasheet doesn't give suggestions towards copper layout (land patterns and trace widths) which will influence conducted heat. (Convection will be low for SMDs, and radiation almost zero; the temperature is too low for that.) It may be tempting to have lots of copper connecting to a pad, but make sure that this doesn't cause trouble in soldering.