Instructables
This question gets asked every day in Answers and the Forums: What resistor do I use with my LEDs? So I've put together several different 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 first know these three things:

  • Supply voltage This is how much power you're putting into the circuit. Batteries and wall warts will have the output voltage printed on them somewhere. If you're using multiple batteries*, add the voltage together.
  • LED Voltage Sometimes "Forward Voltage" but usually just abbreviated "V".
  • LED Current Sometimes "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 this case). Here are some typical values, but use your own values to be sure you don't burn out your LEDs!:

Red LED: 2V 15mA
Green LED: 2.1V 20mA
Blue LED: 3.2V 25mA
While LED: 3.2V 25mA

Okay, lets get started!

* Batteries in series.

Introduction photo credits:
LED photo by Luisanto.
Resistor photo by oskay.
 
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Step 1: The Web Way

Picture of The Web Way
The 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)

Step 2: The Retro Way

Picture of The Retro Way
Go to Evil Mad Scientist Labs web page at this link and 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.
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Taha Shahid6 months ago

Can anyone define the difference between voltage and current in a simple way?

Voltage is pressure. Current is flow.

Voltage is how excited the electrons want to move through a wire. Current is how many electrons are actually moving through a wire.

If a nightclub had a popular band playing that night, Voltage would be how excited the patrons would be to get in they would be close together at the door waiting. A Resistor would be the size of the bouncer at the door. If he is small and weak he would let everybody in as quick as they wanted. A big bouncer would let a single line in, one at a time. Current would be how fast the nightclub fills up.

Hope that helps :P

Not really true hey, but I really laughed at how awesome you described it. Remember current has to keep on flowing, so if you say current would be how fast the nightclub fills up, doesn't it mean that the circuit will be sort circuited? Or am I maybe confused. It is more that a resistor can only allow a certain amount of current to pass before burning, meaning that the club is the resistor and not the bouncer. Smaller club equals less current at a time flowing through, where a larger club would let more current flow through at the same time. This has all have to happen in a loop, when one goes in one goes at, or the club will be over capasity.

this is a great teaching method , hope some professor know this technik

Thanks franko :) Most schools just teach so you can pass tests, which is basically memorizing knowledge. Understanding comes from relating what is taught to something you already understand, by way of relationships between the parts. Understanding comes from asking questions. In order to ask questions you need to be curious. Wisdom and excellence comes from asking the right questions.

Using the above example:

Wire gauge would be how big the doors are. The larger the door the less you can fit on the front of the bar. 12 gauge wires are big, where as 22 gauge wires are small so you can fit more.

milliampere-hour (mAh) would be how long the line is.

The LED would be the band.

A capacitor would be the bouncer letting in say 10 customers at a time after 10 customers leave.

A diode would be the exit that locks after you leave.

Wattage would be the heat that is produced as customers go through the door.

... it's not a perfect senario but it explains the relationships well. you could also use a restaurant, football stadium, outdoor concert, even driving to work in the city.

:P

r0r0s Taha Shahid5 months ago

Voltage is a difference in electrical charge between two points and the de driving force for the current which is the resulting flow of charge. It is easier to understand by looking at analogues in either liquid flows or mechanics.

In a liquid flow through a pipe our Voltage analogue would be the pressure difference between the two ends of our pipe and the current is the same as the volume flow of liquid through the pipe. If you fill your pipe with sand or a wad of fabric you have increased the resistance and you will have to apply a higher pressure to get the same flow.

In mechanics a simple comparisson can be made with a block on an inclined board: the heigth difference between the two sides is our driving force (like voltage in electrics) and the resulting speed of the block sliding down the plane is similar again to current (though in electrics you will have many small blocks sliding down the same plane in a row). Our resistance in this case is the friction between the block and the board, if we reduce the friction by using a smoother surface like glass the block will slide down faster.

Voltage is a "potential" so it describes how much the electrons want to get from + (positive) to - (negative). At really high voltages, the electrons will even jump an air gap (create a spark) to get from one side to the other. Current is a measure of how many electrons are in motion when a circuit is powered and running. The more electrons that are in motion, the more physical work they can do (like starting a car or melting a fuse). An open switch has no current going through it, since the electrons are not able to move through it, but it might have a voltage across it. Too much voltage across a switch can let the electrons spark and jump through it anyway. If you close a switch that has a voltage across it (switch it from off to on), you let the electrons flow through it and suddenly, there is a current.

kel.moto.329 days ago

Actually ,i cut all the leds into singles for use with different projects,and discovered that these little leds are very adaptable ,i have been running a green single led on 2 double aa batteries non stop for 5 days straight and still as bright as first day ,i also made a solar powered orange led lantern with one 1.2 volt rechargable battery .the led doesn't require a resistor unless you are using them in strings or a/c , they are negative power capable ,and that part gets tricky .so before you use them on a project ,test led with 3 volts max d/c to see which way the current flows ,and mark the positive side . now you have endless supply of lights ,for projects .one thousand leds uses less power than a 40 watt light bulb.

they take a certain amount of power before they light up .so its just math after that and basically only when the are used as an a/c string.or higher voltage. if using as a power on light and want to use a resistor then a .025 m/a 6 volt would be perfect the 6 volt ac fuse is like i said in the power plug which comes apart. im working on a cordless phone rechargeable battery pack as another power source.cordless phones are also very inexpensive at thrift store.now you have lots of Christmas leds and a rechargeable phone power supply for maybe 10$ .plus the resistor u need may be inside the phone .free..and not have to search for it.

kel.moto.31 month ago

For you guys looking for resisters most are built in the led ,its actually a 5 or 6 volt fuse located in both plugs off a string of lights ,different total amounts of leds

of led strings give you different fuses and resisters .check a 10 led string compared to a 35 led string and use whichever fits your project also cutting off male end and using 2 AA batterys ......as well as a power supply with no more than 20 MA depending on total combined led voltage, 78 to 90 volts string is recomended.

Grathio (author)  kel.moto.31 month ago

This advice only applies to commercial strings or strips of LEDS. If you're using individual LED components, you must look at the specifications and see what the current and voltage draw are for the LED, and choose a proper matching resistor.

mshafin5 months ago

it says while instead of white

if i get it right...amps are enabling the charge to flow...so it doesnt matter how many amps are created by power supply?
Grathio (author)  Martythebest3 years ago
Right. As long as there are enough amps you're good. Too many amps is not a problem in any way.
KDS4444 Grathio9 months ago
...Unless you are operating a "power LED" in which case too many amps will blow your LED... which is why such LEDs require a driver, not just a simple resistor.

hello KDS4444, i have been looking for this answer on the internet for a while now. so what you are saying is that when using "high power LEDs" we actually have to watch out for the mA output of the power source, not just the output V? for example if i have 2 LEDs @ 2.5V and 350mA connected in series to a 5V 700mA, will it actually hurt them? it should be a 5V 350mA power source? also, if they were to match up perfectly, why do we still have to use a resistor? can anyone please answer these questions. love and peace..!

so a 7.2 VDC 3000 mAh RC car battery all i need to worry about is that 7.2 VDC
nreed41 year ago
What if you want to use leds of different forward voltages?
foggy34 nreed47 months ago

just use the approperiate resistor for Each led.

nreed4 foggy347 months ago

Thanks!

N3em3 years ago
Thank you so much its helps alot ,but i have qustion

how can i bulid led curcit like this
http://www.youtube.com/watch?v=E7Z6WqAP-SI
foggy34 N3em7 months ago

here, http://www.240sxforums.com/forums/resource-center/119623-s13-led-tail-light-conversion.html

merlin131 year ago
Great tutorial for a beginner like me, thanks a lot! I just have one question. Most of the calculators I've seen (and the one you mentioned) calculate the resistor using the LED forward current. So for a 3.3V supply, using an LED with 2.0V voltage and 20mA forward current, the resistor comes up to be 65ohm. Perfect!

But if I want to use this LED with a Raspberry Pi let's say, the max current on pins is 4mA. If I use this resistor, I risk damaging the Raspberry Pi pin, right? So instead I calculate the resistor using 4mA as the current which gives me 330ohm resistor. This is the right value I need to use, is this correct?
Thanks for all your help and again, thanks for the tutorial!
jcook321 year ago
Ok for the project I'm doing the LED calculator says i need a . I found LED's on eBay that come with resistor, but the resistors they have to pick from are 4.5V, 5V, 6V, 9V or 12V resistors can I use any of those? or do I still need to buy the 120 ohm resistors
http://www.ebay.com/itm/160878209257
Grathio (author)  jcook321 year ago
What they're saying in the listing is if you tell them what voltage you're running the LEDs, they'll calculate the correct resistor and send it along -- You don't need to calculate.

However I would verify that they send appropriate resistors once they arrive.
Using math is often the fastest way. Simple algebra. It usually takes me longer to find a datasheet or some other information than it does to find & solve for R in Ohm's Law. R= V/I boom that simple.
This is just an application of ohms law.
V=IR (Voltage = Current times Resistance)
if the supply voltage is 5 the forward voltage is 2.5 and the forward current is 25 mA or .025 amps.
R=V/I so R= (5-2.5)/.025 so R=2.5/.025 which is 100 ohms. It is simple to use and more effective than an online calculator. There is a good site that explains this:
http://cuttingedgescienceclub.blogspot.com/2012/01/ohms-law-practice-problems.html
scroll down to the section labeled LED problems or you can look at the earlier sections to help.
tutdude982 years ago
thanks its help me :D
Vick Jr4 years ago
So the supply current (like those listed on wall warts) makes absolutly no difference? It won't hurt to use a supply voltage with a very high current, as long as the resistor is calculated using the supply voltage, LED voltage, and LED current?
Grathio (author)  Vick Jr4 years ago
Yes, that's correct.  You can perhaps think of Amps as "available power".  If you're not using it, it doesn't cause any trouble.

You need to make sure you have enough amps to supply everything you're powering, so if you're powering t12 LED's that pull 30 milliamps each you need a power supply with at least (12x30mA) 0.36 Amps. More than that will not cause any problems.

It's the same with the (somewhat) common task of buying a replacement computer power supply.  You can use pretty much any power supply as long as the voltage is the same and it has at least as many amps.

It's like using rope to climb down a cliff.  If you run short of rope (amps) you're in big trouble.  So you might as well bring at least as much as you need.
Gaoh Grathio3 years ago
that's it!
just the answer for my problem.
the current of the power supply always confused me, cause i thought it was the source for LED to burnt.
thanks, this is helpful.
This post just cleared up everything that confuddlywizzled me for a long time. Thank you for using such easily interpreted analogies!
Saprobic4 years ago
I have a question about over use of resistors. How much flexibility in teh value do you have? Say I calculate a required resistance of 120 Ohms. If I have a resistor of 470 Ohms will it still work?
Grathio (author)  Saprobic4 years ago
 It really depends on the circuit and the LEDs you're using.  While it's better to use larger (more Ohm) resistor than a smaller one, going too high will keep your LED from lighting much if at all.

As a rule don't go under by more than 10% or over by 25%.  If you're looking at 120 Ohm,  that would be a range of 108-150 Ohm.

However if you have several 470 ohm resistors you can connect them in parallel [wikipedia link] to divide their resistance.  In this case 4 in parallel would give you about 118 Ohm which would be just right for you.
Great thank you so much for that. So this leads me to wonder if I run resistors in series do they add?
Grathio (author)  Saprobic4 years ago
Yes!  So if you run two 68 Ohm resistors in serise you'll get 136 Ohms, which is more or less in the range for your needs.
sinopro4 years ago
If I use 3 LED lamps in white, each is 3.2V and 25mAH, totoal is 3.2V 75mAH, and battery is 9V, not 12V, what the resistor should be?
Grathio (author)  sinopro4 years ago
It entirely depends how you wire the LEDs. Are they in series or parallel?

I recommend that you go to Step 1, click on one of the links there and discover the answer yourself.
phillyj5 years ago
What if you salvage LEDs. There's no sure way to know for sure their ratings, right?
Grathio (author)  phillyj5 years ago
I don't know of any way to be 100% sure but you can get close:

I've done it this way. Get:
  • Your LED
  • Your DC power supply that you plan to use.
  • A 10k potentiometer.
  • A multimeter
  • A prototyping breadboard (Not required but makes all of this a lot less messy.)
Connect pin 1 of the potentiometer (typically the leftmost pin) to +V on your power supply, connect pin 3 (the rightmost pin) to -V. Turn the potentiometer all the way to the left (off) and plug your LED between the middle pin on the pot and -V. (If your LED still has pins, the long pin goes to the pot, the short one to -V)

Slowly turn the potentiometer up. If you turn it most of the way and don't see a light try turning the LED around.

Turn the pot until you get a bright light without the LED warming up. Cut power to the circuit and use your multimeter to read the resistance between pins 1 and 2 on the pot. That value is roughly the resistor you need to use.

At least that's how I've done it. If someone else knows a better way, please post it.

This is probably a good subject for an Instructable...
Look man, this isn't hard. Unless you think math is hard, and based on your title for step 3, maybe you do. I've heard similar sentiments from Teen Talk Barbie(tm).
;-)

But I digress.

Step 1: Start with a resistor and DC voltage source you know won't kill the LED. For example a 12V battery or wall-wart, and a 10K resistor.

You know the current drawn by this arrangement will be less than 1.2 mA, no matter what Vled is. How do you know this? According to your formula, the current through your resistor is: I = (Vsupply - Vled)/R = Vsupply/R - Vled/R
Maybe you don't know Vled exactly, but you expect it to be positive, thus: I < Vsupply/R

Step 2: At this point you may be able to see a tiny amount of light comming out of the LED. If you do that means the LED is good, and biased in the forward direction.

Step 3: Using a mulimeter, measure the actual voltage drop across your 10K resistor. Suppose it happens to be 8.4 V
How much current is flowing? Because you used a power-of-10-sized resistor, the math is easy: 8.4V/10K = 0.84 mA

Step 4: Now you that you know the LED is working maybe want to get agressive, and swap in a smaller resistor to get more current (and more light output). Can you predict what the current will be with your new resistor, assuming the Vsupply and Vled remain the same? Yes you can, since I = (Vsupply - Vled)/R = Vconstant/R.

Pick a reistor that's half as large (5K), and you expect the current to double (1.68 mA)

Pick a resistor that's 10 times smaller (1K), and you expect the current to get 10 times bigger (8.4 mA)

Pick Rnew= (1/a)*Rstart, and expect the current to get a times bigger.

Etc.
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