choosing-the-resistor-to-use-with-leds.pdf

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Choosing The Resistor To Use With LEDsby Grathioon August 10, 2009

Table of Contents

Author: Grathio ..............................................................................................................

License: Attribution Non-commercial Share Alike (by-nc-sa) .............................................................................

Intro: Choosing The Resistor To Use With LEDs ......................................................................................

step 1: The Web Way .........................................................................................................

step 2: The Retro Way .........................................................................................................

step 3: The Hard Way (Math!) ...................................................................................................

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t r : r a t i

Author: Grathio author's websiteCreative swashbuckler. Jack of all trades, master of a couple. If it involves computers, cooking, mechanical design, material science, electronics,photography or just about anything else I'm interested in it and learning more about how it works.

I live in downtown San Francisco, make things for a living and have my laboratory in a walk-in closet, so you're not going to see projects from me that requlots of big tools. (Table saw? I wish.)

License: Attribution Non-commercial Share Alike (by-nc-sa)

Intro: Choosing The Resistor To Use With LEDsThis question gets asked every day in Answers and the Forums: What resistor do I use with my LEDs? So I've put together several dif ferent ways to figure it out.

Lets get right to it:Each of the steps do the same thing. Step 1 is the simplest and we go downhill from there.

No mater what way you choose you must fi rst know these three things:

Supply voltageThis is how much power you're putting into the circuit. Batteries and wall warts will have the output voltage printed on them somewhere. If youusing multiple batteries*, add the voltage together.LED VoltageSometimes "Forward Voltage" but usually just abbreviated "V".LED CurrentSometimes "Forward Current". This is listed in milliamps or "mA".

Both of these last two can be found on the packaging for your LEDs or on your supplier's web site. If they list a range ("20-30mA") pick a value in the middle (25 in thiscase). Here are some typical values, but use your own values to be sure you don't burn out your LEDs!:

Red LED: 2V 15mAGreen LED: 2.1V 20mABlue LED: 3.2V 25mAWhile LED: 3.2V 25mA

Okay, lets get started!

*Batteries in series.

Introduction photo credits:

LED photo by Luisanto.

Resistor photo by oskay.

Image Notes1. Output Voltage: 12V DC. This is the number you use for your calculations,not the "input" above.
http://www.flickr.com/photos/oskay/437342078/http://www.flickr.com/photos/luisanto23/2825405359/http://en.wikipedia.org/wiki/Series_and_parallel_circuitshttp://creativecommons.org/licenses/by-nc-sa/2.5/http://grathio.com/http://member/Grathio/http://member/Grathio/



Image Notes1. Radio Shack is not a very economical place to get parts, but in most cities it'sthe only place in town to buy them off the shelf. If you can wait for delivery it'smuch cheaper to shop on the Internet.2. Forward current: 25 milliamps.3. Forward Voltage: 3.3-3.6 volts. (We'll just round it to 3.45)

step 1:The Web WayThe easiest way is to use one of the online calculators provided below.

Just click on one and enter the info from the previous step and you're set! You only need to go to one.

The LED Center(For single LEDs)

The LED Center(For arrays of LEDs)

LED Calculator.net(For single or arrays of LEDs)

LED Calculator.com(For single or arrays of LEDs)
http://www.led-calculator.com/http://ledcalculator.net/http://led.linear1.org/led.wizhttp://led.linear1.org/1led.wiz



step 2:The Retro WayGo to Evil Mad Scientist Labs web page at this linkand print and make your own slide rule-like calculator.

PDF, assembly and usage instructions are all on the page linked above.

It's pretty nifty and ends up being about business card size so you can keep one in that box with the rest of your LEDs.
http://www.evilmadscientist.com/article.php/ledcalc



step 3:The Hard Way (Math!)All the calculators in step 2 are just doing some simple math that you can do at home:

The formula to calculate resistance in a circuit is: R=V/Ior, more relevant to what we're doing:

(Source Volts - LED Volts) / (Current / 1000) = Resistance*

So if we have a 12v battery powering a 3.5V 25mA LED our formula becomes:

(12 - 3.5) / (25 / 1000) = 340ohms.

But wait! (you might say) When I use one of the other calculators I get 390 ohms!And indeed you do. That's because its hard to buy a 340 ohm resistor and easy to b390 ohm one. Just use the nearest one you can easily find.

To learn more about this magic formula read about Ohms Law.

*We're dividing the current by 1000 because our listing in in miliaps, or 1/1000thof an amp.

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