Ohm's law

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Calculate voltage, current or resistance. Enter any two values and leave the one to find blank. It also shows the power dissipated.

V = I × R Any 2 → third Power included

Calculation

Leaves empty the value to calculate. Enter the other two.

Voltage V (volts)

Current I

R resistance

LED on Arduino 5V

Red LED: Vf=2V, If=20mA. Series resistor: R = (5−2)/0.02 = 150 Ω. Use LED calculator for E12.

Current in a resistor

12 V across 470 Ω: I = 12/470 = 25.5 mA. Power: P = 12×0.0255 = 306 mW → use a 0.5 W resistor.

Resistance unknown

With 9 V and 45 mA → R = 9/0.045 = 200 Ω. Nearest E12 value: 220 Ω.

What is Ohm's Law?

V = R × I. Voltage in volts, resistance in ohms, current in amperes. Also: I = V/R and R = V/I.

How do I find current through a 470Ω resistor at 12V?

I = V/R = 12/470 = 25.5 mA. Power: P = 12×0.0255 = 306 mW.

How do I calculate power dissipated by a resistor?

P = V × I = V²/R = I²×R. If P > 0.25W, use a 0.5W or higher rated resistor.