Electrical Engineering Calculator

Divide the voltage across the component by the current flowing through it. For example, a 12 V supply driving 0.5 A through a load means the resistance is 24 ohms. The formula is R = V / I, and it works for any DC circuit or AC circuit at a single frequency when using impedance instead of resistance.

Most standard LEDs have a forward voltage around 2 V and a recommended current of 20 mA. The resistor value is (5 - 2) / 0.020 = 150 ohms. A 220-ohm resistor is the most common choice because it provides a safety margin and the LED still appears bright at roughly 13.6 mA.

Two resistors in series split the input voltage proportionally. The output voltage equals Vin times R2 divided by (R1 + R2). A pair of equal 10 k-ohm resistors divides any input exactly in half - useful for reading a 5 V sensor with a 3.3 V microcontroller.

For a standard 20 A residential branch circuit in copper, AWG 12 is the minimum per the NEC. AWG 12 copper wire has a resistance of about 5.21 ohms per 1000 feet and can safely carry 20 A in typical installations. For runs longer than 50 feet, stepping up to AWG 10 reduces voltage drop below the recommended 3% limit.

Multiply the voltage across the resistor by the current through it (P = V times I), or equivalently P = I squared times R. A 100-ohm resistor carrying 50 mA dissipates 0.25 W, so a standard 1/4 W resistor is right at its limit - choosing a 1/2 W resistor gives a comfortable safety margin.