Input โ enter any two
U
Voltage across resistor
I
Current through resistor
R
Resistance
Circuit diagram
Power formulas
| Known | Formula | Example |
|---|---|---|
| U, I | P = U ร I | 5V ร 0.02A = 0.1W |
| U, R | P = Uยฒ / R | 5ยฒ / 250 ฮฉ = 0.1W |
| I, R | P = Iยฒ ร R | 0.02ยฒ ร 250 ฮฉ = 0.1W |
| P, R | I = โ(P/R) ยท U = โ(PรR) | derive I or U |
Wattage ratings & tips
Standard ratings
- 1/8W (0.125W) โ SMD 0805/1206
- 1/4W (0.25W) โ most common through-hole
- 1/2W (0.5W) โ slightly larger axial
- 1W โ wirewound or metal oxide
- 2W, 5W, 10W โ power resistors, need heatsinking above ~2W
Safety rules
- Always use a resistor rated for at least 2ร the calculated power
- A 0.25W resistor at 0.25W will run hot โ use 0.5W minimum
- Above ~0.5W the resistor will get noticeably warm to the touch
- Above ~2W use a resistor with heatsink tabs or mount to PCB copper pour
- Derating to 50% of rated power is good practice for reliable long-term operation
Why resistors heat up
A resistor converts electrical energy directly into heat. The higher the resistance or current, the more heat is produced. In circuits with high currents (motor drivers, power supplies, LED arrays), always double-check power dissipation.
Common gotchas
- Pull-up/down resistors (10 kฮฉ) dissipate very little โ usually fine with 1/4W
- Current-limiting resistors for LEDs: check P = Iยฒ ร R at max current
- Shunt resistors in power paths can dissipate several watts
- In parallel resistors, each resistor carries only part of the current