Resistor power fault analysis
If something shorts, a resistor can end up with the whole battery across it or the full fuse current through it. Enter both, pick the resistor or type its power rating, and see how small or large it must be to live through the fault.
- Current case
- —
- Voltage case
- —
Every standard package at these values
| Package | Rating | Current case | Voltage case |
|---|---|---|---|
| SMD thick film | |||
| 0201 | — | — | — |
| 0402 | — | — | — |
| 0603 | — | — | — |
| 0805 | — | — | — |
| 1206 | — | — | — |
| 1210 | — | — | — |
| 2010 | — | — | — |
| 2512 | — | — | — |
| Through-hole | |||
| 1/8 W | — | — | — |
| 1/4 W | — | — | — |
| 1/2 W | — | — | — |
| 1 W | — | — | — |
| 2 W | — | — | — |
| 3 W | — | — | — |
| 5 W | — | — | — |
| 10 W | — | — | — |
Typical standard-part ratings and max working voltages at 70 °C ambient or below. Click a row to analyse that package, and check the datasheet of the exact part before you rely on a number.
The formula
Current case: P = I²·R → R ≤ P / (I²·m) Voltage case: P = V²/R → R ≥ V²·m / P
When to use it
Fault analysis on anything fed from a battery or a fused rail: a pull-up that could end up across the full supply, a sense or series resistor that could carry the fuse current, a test point that might get shorted to ground. The voltage case limits how low R can go; the current case limits how high. The rating is the horizontal line on the plot, and the vertical lines where the two curves cross it are the limits. Pick a package for its standard rating, or type the power for a high-power part.
Pitfalls
- Use the worst-case voltage, not the nominal one: a Li-ion cell is 4.2 V when full and a car rail sits near 14.4 V with the engine running, before any load-dump spike.
- A fuse carries its rated current forever and well above it for seconds. Use the fuse rating as the current, and remember a short that clears fast may still overheat a small part first.
- The package ratings are for standard thick-film parts. High-power and anti-surge lines put 1/4 W or 1/2 W in an 0805, so type the datasheet power when you use one; the package’s max working voltage still applies.
- Power ratings are at or below 70 °C ambient. Most parts derate to zero by 155 °C, so a hot enclosure needs a bigger package or a larger margin.
Related note
Reverse-polarity protection for a DC power inputWant the whole board checked for what happens when something shorts? We run the fault cases on every rail before it ships.