Duty Cycle

Duty cycle D = t_on / T, where t_on is the time per period that the resistor carries current and T is the full cycle. For a brake resistor that dissipates 50 kW for 4 s every 60 s, D = 4 / 60 ≈ 6.7 % and the average power is P_avg = 50 kW × 0.067 = 3.3 kW — the figure the resistor body must actually shed continuously.

Duty cycle interacts with two timescales. (a) Long duty cycles (≥ seconds) drive the bulk heatsink temperature — derating curves and continuous power rating dominate. (b) Short duty cycles (μs to ms) heat the resistance element faster than heat can escape the body, so the part is governed by pulse-energy rating, not average power. Between these regimes lies a transition determined by the part's thermal time constant: aluminium-housed 100 W parts ≈ 30–60 s, big tubular wirewound > 5 minutes, SMD 0805 chips ~10 ms.

To specify correctly engineers calculate both numbers: P_peak × duty cycle ≤ continuous power rating AND single-pulse energy ≤ pulse rating. Datasheets often publish a pulse-duty derating chart showing how much above the steady-state rating a part can absorb for given on-time and duty-cycle combinations. This is the make-or-break selection criterion for braking resistors, snubbers, defibrillator dump resistors and any other intermittent load.