Step 2 - Calculate the needed Resistance using one of Ohm's Formula R = V / I Where : R = Resistance in Ohms V = Force in Volts I = Current in Amps So the needed Resistance is - Resistance in Ohms = 10 Volts / 0.02 Amps (Remember this is the same as 20 Milliamps) 500 = 10/0.02 So what we need is a 500 Ohm Resistor, that was pretty easy.