# Ohm's Law

The mother of all electronic formulas, the one to rule them all...

U = R * I

U is the potential difference between two nodes in a circuit measured in Volt. R is the resistance between two nodes in a circuit measured in Ohm. I is the current flowing between two nodes in a circuit measured in Ampere.

## Current Limiting Resistor

Is an application of Ohm's law in that you have a know voltage (U), ie potential difference, and want to limit the current (R) that flows between the two potentials, nodes. In the simplest case the sought resistance (R) is just supply voltage (U) divided by the desired current (I).

## Current limiting a LED

One of the most common applications of current limiting resistors is to limit the current through a led or led backlight. In the led case the U is not just the supply voltage but the difference between the supply voltage (Us) and the forward voltage drop (Uf) of the led and thus the formula becomes (Us-Uf)/I=R. Most leds have a forward voltage drop of somewhere between 1,4 and 4,0 volts and this and the recommended maximum current for a specific led, or led backlight, can always be found in the datasheet for the component. If you want the led to last you should settle for a lower current than the maximum, a degree of safety margin that allows for a bit higher supply voltage and a bit lower resistance in the resistor.

Example:

• Imax 60mA (from datasheet)
• Uf 3.8V (from datasheet)
• Us 5V
• Ms 120% (safety margin)

((Us-Uf)/Imax)*Ms = ((5-3.8)/0.06)*1.2 = (1.2/0.06)*1.2 = 24