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- Choose which two values you know.
- Enter them in base units (volts, amps, ohms, watts). Use decimals for milliamps, e.g. 0.02 A = 20 mA.
- Read the other two values.
Formel
Ohm's law relates voltage, current and resistance in a resistive circuit; combined with the power law it gives twelve formulas.
V = I × R
P = V × I = I² × R = V² / R
I = V / R = √(P / R)
R = V / I = V² / P
Example
- 12 V across a 220 Ω resistor.
- I = 12 / 220 = 54.5 mA; P = 12² / 220 = 0.655 W.
- So a standard ¼ W resistor would overheat; use a 1 W part.
Häufige Fragen
Does Ohm's law apply to AC?
For purely resistive loads, yes, with RMS values. For capacitors and inductors, use impedance instead of resistance.
How do I choose a resistor wattage?
Calculate the power it dissipates and pick a rating at least double that for reliability.
How do I size an LED resistor?
R = (supply voltage − LED forward voltage) / LED current. For 5 V, a 2 V LED and 20 mA: R = 3 / 0.02 = 150 Ω.