Resistor Power Guide
Size resistors by power dissipation — package ratings, derating, and parallel dissipation.
Reference
Power formulas
- P = V² / R
- From voltage across the resistor
- P = I² · R
- From current through it
- P = V · I
- From both
- Derate to
- ≤ 50% of rated — especially in sealed enclosures
Typical ratings
| Package | Rated power |
|---|---|
| 0402 SMD | 1/16 W (62 mW) |
| 0603 SMD | 1/10 W (100 mW) |
| 0805 SMD | 1/8 W (125 mW) |
| 1206 SMD | 1/4 W (250 mW) |
| 2010 SMD | 3/4 W |
| 2512 SMD | 1 W |
| Through-hole 1/4 W | 1/4 W |
| Through-hole 1/2 W | 1/2 W |
| Through-hole 1 W | 1 W |
| Axial wirewound 5 W | 5 W |
| Axial wirewound 10 W | 10 W |
| TO-220 power | 25–50 W (heatsinked) |
Parallel dissipation
- Equal resistors in parallel
- Total P = N × single-resistor P
- Splitting high power
- Four 1 Ω 1 W in parallel = 0.25 Ω / 4 W capability
- Voltage derating
- High-voltage resistors can arc at rated voltage — check max working voltage
Notes
- Pulse handling differs from continuous: some resistors tolerate brief pulses many times their continuous rating.
- Thin-film handles sudden surges worse than carbon composition — pick accordingly.
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