Abstract A resistor that is safe under steady power is not automatically safe during startup, surge, discharge, or repetitive pulses. Pulse selection must satisfy peak power, pulse energy, average dissipation, and maximum pulse voltage at the same time. This article connects waveform conversion, data sheet curves, and repetitive-pulse checks into one workflow.
1 Rated Power Does Not Define Pulse Survival
Continuous power ratings assume a specified thermal path and ambient condition. During a short pulse, heat cannot spread through the substrate and PCB quickly enough, so local film temperature and electric field can reach a limit first. Treat a single event and a repetitive pulse train as separate cases.
The source identifies three independent constraints. Single-pulse peak power must remain below the duration-dependent curve, average power in a train must not exceed the allowed P70 dissipation, and high-value resistors must remain below the pulse-voltage limit. Margin in one dimension does not cancel a violation in another.
2 Convert Real Waveforms to Equal Energy Rectangles
Pulse curves are usually specified for rectangular power. For a triangular, exponential, or arbitrary waveform, integrate power to obtain energy W. Create an equivalent rectangle with the same peak power, so teq equals W divided by Ppeak, and compare that pair with the part-specific curve.
Vishay gives an example with a 100 volt peak applied to 1 kilohm, producing 10 watts peak. A 12 millisecond triangular voltage pulse converts to a 4 millisecond equivalent rectangle. The MMA 0204 example curve permits about 7 milliseconds at 10 watts, so that particular case passes. The result must not be generalized to another series.
3 Very Short Pulses Are Technology Limited
Below a few hundred microseconds, much of the energy remains in the resistive element. Film thickness, geometry, substrate, and terminations dominate before package-level cooling becomes effective. Carbon film, metal glaze, or wirewound construction can be advantageous for severe energy pulses, while precision thin film balances pulse capability with stability, low temperature coefficient, and low noise.
Always use the curve and acceptance criterion for the exact resistor family. Some limits are based on a specified resistance shift after endurance testing, not on immediate open-circuit failure.
4 Repetitive Pulses Require an Average Power Check
For a periodic waveform, average power is the energy in one period divided by T. A positive rectangular pulse reduces to Pavg equals Ppeak times ti divided by T. This result must remain below the temperature-derated continuous power.
Average power alone is insufficient. The repetitive pulse curve must also accept the peak and duration, because equal averages can be produced by very different peak stresses and local temperature excursions.
5 High Resistance Brings a Voltage Limit
Above a critical resistance, allowable load can be set by the maximum element voltage rather than thermal dissipation. A short high-voltage pulse creates strong electric field in the resistive film and can cause flashover or structural damage. Check peak working voltage, the pulse-voltage curve, and package creepage together.
Qualification should reproduce waveform shape, tolerance, temperature, and repetition count, followed by resistance-shift measurement. Safety-related and high-reliability designs need manufacturer-specific data and endurance evidence, not a one-time no-burn demonstration.
Design Review Matrix
| Check | Core Calculation | Common Omission |
| Single pulse | Peak power versus duration curve | Checking average power only |
| Nonrectangular energy | W is the integral of power | Using total pulse width directly |
| Pulse train | Pavg equals W divided by T | Ignoring the repetitive curve |
| Pulse voltage | Compare with part voltage limit | Selecting high values by watts alone |