Every resistor turns some electrical power into heat, and every package size has a limit to how much heat it can shed. On laptop boards, most resistors carry tiny signal currents and run cold. A few — in power paths, LED drivers, current sensing and bleed circuits — work much harder. This calculator tells you how hard a resistor is working and whether a given package is up to it.
How to use it
- Enter the resistance.
- Choose whether you know the voltage across the resistor or the current through it, and enter that value.
- Choose the package size.
- Read the power, the typical rating of that package and the percentage of the rating in use.
The formulas
- P = V² ÷ R when you know the voltage across the resistor.
- P = I² × R when you know the current through it.
The voltage must be the voltage across the resistor — the difference between its two ends — not the voltage of the rail it connects to.
Typical ratings
The ratings used here are the usual values for general-purpose thick-film chip resistors: 0201 about 1/20 W, 0402 1/16 W, 0603 1/10 W, 0805 1/8 W, 1206 1/4 W, 1210 1/2 W, 2010 3/4 W and 2512 about 1 W. Individual series vary — high-power and current-sense parts can be rated much higher in the same size — so check the datasheet for critical positions. Package dimensions are covered by SMD Package Size Converter (Imperial ↔ Metric).
Keep a margin: designers usually aim for half the rating or less, because the rating applies at a specific ambient temperature and a laptop's inside is warm.
Worked example: a burnt resistor
A 47 Ω resistor in an 0603 package sits in a supply path, and you measure 2 V across it in operation. The power is 2² ÷ 47 ≈ 85 mW — about 85 % of an 0603's typical rating. That explains the discoloured body: it runs hot all the time. If the schematic shows the same value in an 0805 or 1206 package, someone has fitted the wrong size during an earlier repair.
Where it matters on a laptop board
- Choosing a replacement for a burnt resistor.
- Checking whether a modification or a jumper resistor will overheat.
- Understanding why a resistor near a short turned black.
Resistor basics are in resistor, and dissipation in general under power dissipation.
FAQ
Can I replace an 0402 resistor with an 0603 one?
Electrically yes, if the value and tolerance match; mechanically it may not fit the pads. The larger part handles more power.
Why is my resistor hot when the calculation says it is fine?
Heat from nearby parts adds to its own. A resistor next to a hot MOSFET or inductor runs warmer than the calculation suggests.
Do current-sense resistors follow these ratings?
Usually not. Sense resistors are special low-ohm parts with their own, often higher, ratings. Use the part number from the schematic.