Calculate LED current limiting resistor

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...

Attachment 41332

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.

<!-- BEGIN /var/www/html/homemadetools/protected/modules/zeus/views/tool/postUpdate.php -->
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:

<div id="blocks"> <div class="block b1 pngfix"> <div class="bimg"> <div> <a href="https://www.homemadetools.net/homemade-led-limiting-resistor-calculation"> <img src="/uploads/252531/homemade-led-limiting-resistor-calculation.jpeg"/> </a> </div> </div> <div class="head pngfix"></div> <div class="left pngfix"></div> <div class="right pngfix"></div> <div class="blockover b1 pngfix"> <div class="title"> <a href="https://www.homemadetools.net/homemade-led-limiting-resistor-calculation">LED Limiting Resistor Calculation</a> <span> by <a href="https://www.homemadetools.net/builder/mklotz">mklotz</a></span> </div> <div class="tags">tags: <a href='https://www.homemadetools.net/tag/led'>LED</a> </div> </div> </div> </div>
<!-- END /var/www/html/homemadetools/protected/modules/zeus/views/tool/postUpdate.php -->

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.

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

Quote:

---

Originally Posted by 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

---

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.

[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

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

Excellent. Thank you this is useful.