Peak Power

Peak power is distinct from average and continuous power. In a circuit with PWM switching, capacitor discharge, motor regeneration or lightning surges, the resistor sees current spikes whose instantaneous V × I far exceeds the long-term mean. A pre-charge resistor sized for 200 W average can briefly see 2 kW at the moment the contactor closes. A snubber resistor may average 5 W but spike to 500 W on each switching event.

The upper limit on peak power is set by two failure mechanisms. At very short pulses (< 100 μs) the heat is adiabatically trapped in the resistive element — the limit is the material's maximum instantaneous temperature before degradation (typically 400 – 800 °C, depending on alloy or paste). At longer pulses heat begins to diffuse outward; the limit becomes thermal stress in the substrate-to-element interface, where mismatched expansion can crack metallisations or open thick-film terminations.

Datasheet pulse power curves explicitly graph these limits — read them rather than relying on rules of thumb. For wirewound parts the published curve usually shows 10× rated for 5 s, 50× for 0.1 s, 100× for 10 ms. For thick film the multiples are lower but still meaningful: 5 – 10× rated for 100 μs is common. When a circuit has very short repeated pulses, evaluate the per-event energy and the inter-event cooling time as well as the peak.