Calculate LED current limiting resistor

[mklotz]

LEDs are used often enough in home projects that it's worth a short tutorial on determining the proper resistor to be used with one.

The breakdown voltage of an LED (Vb) varies with color. This chart shows ranges of that voltage...



A typical supply current is I=20 ma. The required resistor can then be calculated from Ohm's law (R = V/I)

R = (Vs - Vb)/0.02

here Vs is the power supply voltage.

An example will help. Suppose we have a red LED we wish to power with a 12 volt battery. From the figure, the average Vb for a red LED is 1.9 volts.
Then:

R = (12-1.9) / 0.02 = 10.1 * 50 = 505 ohms

The standard series of resistors looks like..

1– 1.2 – 1.5 – 1.8 – 2.2 – 2.7 – 3.3 – 3.9 – 4.7 – 5.6 – 6.8 – 8.2 – 10

All of these values can be multiplied by powers of ten (1.2 – 12 – 120 - 1200, etc.).

So the closest we can come to 505 is a 470 ohm resistor.

The resistor color code is

0 black
1 brown
2 red
3 orange
4 yellow
5 green
6 blue
7 violet
8 gray
9 white

The third color band indicates the power of ten multiplier.

so a 470 ohm resistor will have yellow-violet-brown bands.

Most LEDs will have a flat area on the base to indicate the cathode (negative) terminal. Sometimes, the cathode lead is made shorter to indicate which it is.