How to Choose a Resistor's Power Rating (Watts)?
A resistor's wattage is chosen so it can safely carry the power it will dissipate as heat. A wrong choice ends with the resistor overheating and its value drifting, or burning out.
How Is the Power Calculated?
The power a resistor dissipates is found with three formulas: P = V×I, P = I²×R or P = V²/R. This power turns entirely into heat.
Worked Example
If 0.1 A flows through a resistor and its value is 100 Ω: P = I²×R = 0.1² × 100 = 1 W. In this case a 1/4 W resistor burns; for a safe choice at least 2 W is needed.
Derating (Leaving a Margin)
| Calculated power | Recommended resistor |
|---|---|
| 0.1 W | 0.25 W |
| 0.5 W | 1 W |
| 1 W | 2 W (or above) |
General rule: pick a resistor rated at at least 2× the calculated power. In a high-temperature environment leave a bigger margin.
Temperature Derating Curve
A resistor's datasheet includes a "derating curve": above a certain temperature (e.g. 70 °C) the power it can carry starts to drop. So in hot enclosures you don't rely on the full nominal wattage; leave extra margin.
Common Mistakes
- Looking only at the ohm value: the right resistance but insufficient wattage = a burnt resistor.
- Leaving no margin: a resistor running right at the limit heats up and its life shortens.
- Forgetting the ambient temperature: in a hot environment the resistor's power capacity drops.
Frequently Asked Questions
How many watts should the resistor be?+
What's used where high wattage is needed?+
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