Ohm’s Law

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

I = V / R

I is the current in Amps.
V is the voltage in Volts.
R is the resistance in Ohms

With this formula and two values you can determine an unknown value.

I use this a lot when I’m designing my homemade circuits. Usually to find out what resistance values to use on certain legs.

For example:

I have a tiny LED bulb that is rated at 2mA. I have a 24VDC power supply which is outputting 5A. What size resistor do I need to protect the LED leg of the circuit.

We can easily determine this value because of Ohm’s Law.

0.002A = 24VDC / Unknown Resitance

Solving the equation we get:

Resistance = 24VDC / 0.002A
Resistance = 12,000 Ohms or 12 KOhm

This tells me I should use a resistor at least 12K.