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.

[DIYer]

Thanks mklotz! We've added your LED Limiting Resistor Calculation to our Electrical category,
as well as to your builder page: mklotz's Homemade Tools. Your receipt:

LED Limiting Resistor Calculation by mklotz

tags: LED

[mklotz]

The wattage, W, being dissipated in the resistor is given by...

W = I²R = 0.02² * 470 = 4E-4 * 470 = 0.188 watts

The common standard power ratings of resistors are 0.25W, 0.5W, 1W, 2W, 5W, and 25W so even the smallest size can be used in this application.

[Saaglem]

I've learned the colour coding in the following.
Bad Boys Rape Our Young Girls But Violet Gives Willingly.

Bad = Black - 1 Ohm = 1 Ohm
Boys = Brown - 10 Ohm = 10 Ohm
Rape = Red - 100 Ohm = 100 Ohm
Our = Orange - 1000 Ohm = 1K Ohm
Young = Yellow - 10000 Ohm = 10K Ohm
Girls = Green - 100000 Ohm = 100K Ohm
But = Blue - 1000000 Ohm = 1M Ohm
Violet = Violet - 10000000 Ohm = 10M Ohm
Give = Grey - 100000000 Ohm = 100M Ohm
Willingly = White - 1000000000 Ohm = 1G Ohm

Just saying

[mklotz]

I've learned the colour coding in the following.
Bad Boys Rape Our Young Girls But Violet Gives Willingly.

Bad = Black - 1 Ohm = 1 Ohm
Boys = Brown - 10 Ohm = 10 Ohm
Rape = Red - 100 Ohm = 100 Ohm
Our = Orange - 1000 Ohm = 1K Ohm
Young = Yellow - 10000 Ohm = 10K Ohm
Girls = Green - 100000 Ohm = 100K Ohm
But = Blue - 1000000 Ohm = 1M Ohm
Violet = Violet - 10000000 Ohm = 10M Ohm
Give = Grey - 100000000 Ohm = 100M Ohm
Willingly = White - 1000000000 Ohm = 1G Ohm

Just saying

Yes, that and its less PC variants have been a mnemonic for the code for a long time.

However, each color denotes a digit, not a resistance as you have indicated. It's the combination of three digits that denote the resistance.

[bob47907]

[QUOTE=mklotz;194134]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...


Marv,

A bit of poking around for infrared LEDs (which I occasionally use in isolation applications) yields Vb around 1.9v.


Bob

[Saaglem]

Your right.... my mistake. That ohm should not have been there!

[mr_modify1]

Excellent. Thank you this is useful.