Pulse Power

A resistor's continuous power rating is set by its steady-state thermal balance — heat generated equals heat lost to ambient. For brief pulses much shorter than the thermal time constant, the element heats adiabatically: incoming energy is stored in the body's heat capacity rather than escaping. This lets a resistor swallow 10 – 100× its continuous rating for a single millisecond pulse without exceeding the element's safe maximum temperature.

Manufacturers characterise this with two related curves. The pulse energy curve (joules vs pulse width) shows the single-shot energy capacity from microseconds to seconds; an aluminum-housed 100 W RH series part can absorb 2 kJ in 100 ms but only 100 W steady. The pulse power curve (watts vs pulse width at fixed duty cycle) shows the allowed peak power; at 10 % duty cycle a 100 W part might allow 1 kW peak. Both curves taper to the continuous rating at long pulses.

Pulse capability is essential for snubber, pre-charge, surge-suppression and braking applications where the resistor sees brief high-energy events between long quiet periods. Wirewound parts excel — the bulk metal element has large thermal mass and high allowed temperature. Thick-film chips have limited pulse capability set by the thin resistive layer's small heat capacity. Always check both energy and peak power against the worst-case event envelope, not just the average.