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Ohm’s law and power calculator
Any two of V, I, R and P give the other two. It also tells you which standard resistor to buy and how much power rating to give it, because the number on paper is never the number on the board.
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The formula
V = I · R P = V · I = I² · R = V² / R
When to use it
Sizing a pull-up, a current-sense shunt, a bleeder, a dummy load — anywhere a resistor has to carry real current. Clear the fields you do not know and fill in two; the calculation runs on those two.
Pitfalls
- Run resistors at half their rating or less. A resistor at its rated power sits far above ambient and drifts; the 2× margin here is the minimum, and surface-mount parts on a hot board deserve more.
- The power rating of a chip resistor also depends on the pads: a 0603 rated 0.1 W assumes the datasheet’s copper area.
- Voltage rating is a separate limit. A high-value resistor across a high voltage can exceed its rated voltage while dissipating almost nothing.
Related note
Why sleep current is higher than the datasheet saysNeed the whole board designed, not just one resistor?