Introduction: Choosing the Resistor to Use With LEDs

Picture of Choosing the Resistor to Use With LEDs
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.

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.

Step 3: The Hard Way (Math!)

Picture of 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/I or, 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 buy a 390 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 1/1000th of an amp.

Comments

fogartyk (author)2017-11-13

So I am helping a student who has built something with 4 different colored LEDs (that she wired in parallel with copper tape. We are learning a LOT as we work through trying to educate ourselves on this so thank you in advance for your assistance.

First, I've gathered that the LEDs should not be wired in parallel. This is what we know about the LEDs: Red, yellow are 1.8-2.2 v; Blue & Green are 2.8-3.2v. 20 mA all. So if I use the LED series/parallel array wizard, and I use these settings: 9V, 9.2 forward voltage (1.8 (r) +1.8 (y) + 2.8 (g) + 2.8 (b), 20 mA forward current and 4 LEDs. I get a message telling me that I need a higher voltage source to light up. That makes sense to me. However if I bump up the voltage input to 12 (which to my untrained brain seems high for 4 leds...) I get a message that says my forward voltage looks suspiciously high, but I do get a schematic.

I've already learned a lot in researching this, and will try the schematic. However I have a 5th grader who designed a small artifact which has 4 LEDs in different colors. I just want to help her get it lit with a battery. Can anyone advise us?

kgfogarty (author)fogartyk2017-11-15

nevermind about above! Misguided!!

Zeta2 (author)2017-08-04

It's probably worth mentioning that as a lot of people will be wanting to use this for Car LED bulb applications, that 12V would be wrong for the source Volts as when the car is running it is provided with power from the alternator that is typically 13.8V.

AndreaM235 (author)2016-09-15

ok so how come i bought a set of led fairy lights from the dollar store and there is no resistor in them, there are 10 white LED's wired in parallel with a battery box , in the box is just a switch and holder for 2 AA batteries ,no resistor anywhere

they are small LED's with a flat top , last year i cut apart a string of them for a halloween project and confirmed there was no resistor,i used an AC to DC power supply to power them instead of the battery box with an output of 3.3v DC from an old sony discman, not sure the mA current output

i also plugged the power supply into a plug in electronic lamp dimmer the slide kind with a triac inside and dimmed it a little ,some of the LED's i guess burnt out after a while after about a few weeks or so and got dim, not completely burnt out ,but dimmed ,i also have a halloween wreath pre-lit with similar LED's but yellow orangish,the LED's are clear but emit an orange yellow light,in the battery box for that there is a resistor and it also runs AA batteries,3 of them but i power it with the same power supply setup ,i wire it through the battery box so i use the included resistor it is wired in parallel with the other fairy lights

so how come the dollar store fairy lights have no resistor but the wreath does?

why do the fairy string from the dollar store not have resistors?

i bought some white LED's from ebay (clear but emit light with a bluish hue)

they say VF 3.2-3.4 IV 12000-14000 , not sure what the 2nd thing is

i plan on using the same power supply setup, i have some 270ohm 1/2 watt resistors

so will i be ok if i wire say 12 LED's in parallel ,and use one of these resistors

in series between the power leads and + terminal of the power supply?

the wreath i have has only one resistor in the battery box so i assume they are only using 1 resistor for the whole string of about 20 LED's, but i see that on another site it is recommended to use one resistor for each LED, but i if use one resistor

can i just multiply the value for each of the LED resistor values i get?

for example when i calculate what resistor value i need using a vf of 3.2 and a power supply voltage 3.3 i get recommended to use a 5 ohm resistor ,so if want to use 12 LED's wired in parallel then can i just use one 60 ohm resistor in series between all the positive led leads wired together and the + of the power supply?instead of wiring a 5 ohm resistor in series with each individual LED? and would a 270 ohm be overkill or should i look for a 60 ohm resistor or just use more led's? i mean will i get dimmer LED's if i use too large of a resistor value? is it just important to use a resistor that is big enough to not fry the LED's and then it is ok to over that by a little?

does the recommended resistor vale just mean at lest that value or higher it does not have to be exact does it if we are talking in terms of going over not under right?

hydranix (author)AndreaM2352017-05-24

Do not ever use LEDs in parallel. The cheap chinese LED lights you're referring to are unreliable and can fail if given enough current. Try to use the light with rechargeable versions of the batteries it requires (they can provide much more current than regular non-rechargeables). You'll find the LEDs eventually burn out.

Even if using LEDs all manufactured at the same time, they will not have a consistent resistance across them. Further, the resistance of an LED changes dramatically with thermal conditions. If one LED was warmer than the others it would have a lower resistance. It's would pass more current, and heat up even more...

This is why we use an series resistor with LEDs when current is not constant. It mitigates most of the unpredictable properties of LEDs.

Proper parallel LED circuits have a series resistor for each LED.

sigshane (author)AndreaM2352017-01-21

In parallel circuits, the total circuit current is divided among all of the branch circuits, proportionally by branch resistance. So for the 10 white LED branches in parallel, assuming they all have the same resistance, through each LED will flow 1/10th of the total circuit current. If the total current was 1 amp, then each branch will have ~0.1 amp flowing through it. That's why no resistor was required - branch current will be low enough to be safely conducted by the LED.

In series circuits, total current is felt across each load device in the circuit. The wreath circuit is most likely wired in series, if it contained a resistor. The resistor value would have been sufficient to reduce current flow to a value that could be safely conducted by the series device with the lowest allowable current value.

Again, with parallel circuits, the total current is divided, so a current-limiting resistor may not be necessary, depending on the number of branches and resistance of each (search the internet to find out how to calculate parallel resistance, and current flow - it is too "wordy" to post here).

Hope this helps,

Shane

Peterthinking (author)2017-05-19

https://www.instructables.com/id/LED-Resistor-Finder/ Or make a test rig.

apraj77 (author)2016-08-28

390 Ohms is wrong.

12-3.5=8.5

8.5/0.025= 340 Ohms

Grathio (author)apraj772016-08-28

The calculation is for resistors, not the actual Ohms. 340Ω resistors are essentially nonexistent, so it's rounded to the nearest value for common resistors: 390Ω.

AndreaM235 (author)Grathio2016-09-15

how exact does it really have to be anyways?

i mean is it just guidance for the minimum value?

as long as you use at least that value you are good and it is ok to go over right?i have 10 LED string that are not using any resistor in them

also what if you wire a bunch or LED's in parallel do you just multiply the vales by how many LED's you are using then wire all the anode leads together and all the cathode leads together then put the resistor in series between the anodes and the positve power supply lead?

dsandy3 made it! (author)2016-08-29

Thanks

WakeelA1 (author)2016-04-16

i'm confuse..I can't find anything..I mean if I have 1 white led and 6 volt battery so how i can find resistance value?Plz tel me easy method...

MahadiH4 (author)WakeelA12016-08-24

R= [V(s)-V(led)/[ I(led)]
Voltege source minus voltege of led..devided by current of led...

IanK48 (author)2016-05-07

Hi i could really do with some help the leds are 3mm at 20 at 2.0 eight of them i want them in a circle one piece of wiring joining the minus's the other joining the plus's with a res soldered to each led i dont know what type of curcuit this is and im not sure what resistor i need its a 5v input

nreed4 (author)2013-07-08

What if you want to use leds of different forward voltages?

foggy34 (author)nreed42014-01-23

just use the approperiate resistor for Each led.

KDS4444 (author)foggy342016-02-25

...and hook them up in parallel, not in series...

nreed4 (author)foggy342014-01-23

Thanks!

Martythebest (author)2011-01-12

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)Martythebest2011-01-13

Right. As long as there are enough amps you're good. Too many amps is not a problem in any way.

KDS4444 (author)Grathio2013-12-21

...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.

Akin Yildiz (author)KDS44442014-04-15

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..!

KDS4444 (author)Akin Yildiz2016-02-25

I am sorry I didn't notice this reply two years ago. Let me see if I can get this right: what happens when you attach such LEDs to a 5V 700mAh power supply is that the LEDs will attempt to draw all the power they can from the power supply-- that supply is only RATED at 700 mAh, and so the POWER SUPPLY will start to overheat! (to say nothing of the LEDs, which will also likely go kaput). The mAh rating of any power supply is the amount of power (current) that it can safely PROVIDE without damaging ITSELF, it is NOT a measure of how much "power" or "current" it "provides" (per se). If your circuit only draws 20mAh from a 700mAh power supply, that's great! If your circuit tries to draw 1000mAh from the same power supply, the supply will be damaged, not the circuit. Power LEDs don't limit their own current draw-- this must be managed by some kind of external circuit. Obviously, hooking a power LED up to a power supply that has a very high mAh rating will quickly destroy the LED, even if the voltage provided by that supply is within specs (unlike a regular LED, which will be fine no matter what the current supplied is-- so long as that current draw is lower than the rating of the power supply, the power supply will not be damaged either).

MACKattacksnipe (author)Grathio2011-07-28

so a 7.2 VDC 3000 mAh RC car battery all i need to worry about is that 7.2 VDC

Taha Shahid (author)2014-02-27

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

Joxman2k (author)Taha Shahid2014-04-11

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

KDS4444 (author)Joxman2k2016-02-25

Also consider this: the little bouncer may actually get run over if the current is too fast-- i.e., you may destroy your resistor if it either isn't rated to withstand that kind of fast current OR if it doesn't provide enough outright resistance to the flow of the current. A resistor of 100,000 ohms rated for a certain wattage will be okay in a place where a resistor with the same wattage rating but only 5 ohms of resistance will get burned up by the same amount of current. A small bouncer who is able to say "no" to most everyone will be able to handle the situation without burning up, as will a "big" bouncer who says "yes" to lots of people. A fast current through a small (low wattage) resistor with a low level of resistance is what makes the resistor get hot/ incinerate!

AshwillP (author)Joxman2k2014-09-17

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.

It is very easy budy, Follow the simple steps here ...
How to calculate the value of resistor for LED’s (with different types of LED’s circuits)
link: http://electricaltechnology.org/2013/08/how-to-ca...

or simply use this online calculator:
Required Value of Resistor for LED’s Circuit Calculator
link: http://electricaltechnology.org/2014/02/LED-resis...

franko.vill (author)Joxman2k2014-09-01

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

Joxman2k (author)franko.vill2014-09-04

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 (author)Taha Shahid2014-03-26

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.

thatguygq (author)Taha Shahid2014-03-20

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.

AustinS40 (author)2016-02-14

So if I have a 12 volt battery and 20mA 2 volt LEDs, can I wire 6 of them in series without conencting a resistor?

AndrewB210 (author)2016-01-30

i have brought two sets of led battery powered led sets from a discount store i have used them in project drilling 24 holes around a big mirror frame and placed drilled ping pong balls over the leds. my question please as they are neatly pre wired and all in parallel and my need to have them run off 12 volt. i have tried different resistor calculators to find the value needed to drive 24 leds in parallel ? but i find that its not a recommended array and consequently not given a result. i thought of using a 470 ohm 1/2 watt resistor. any advise would be most appreciated

Thank you

Elecrow (author)2015-08-28

This is really useful.
(Source Volts - LED Volts) / (Current / 1000) = Resistance

MichaelP59 (author)2015-08-27

if you need more information about <a href="http://911electronic.com/tunnel-diode-characteristic-symbol-definition/">tunnel diode</a>, you can click in this link. I check this site and there are some information about tunnel diode.

dhaneshg (author)2014-11-07

i want use Led bulb in real electric circuit for indication at 230 volt. how much resistance is require to protect led???? plz reply

jarfil (author)dhaneshg2015-05-23

Be aware that mains power is rated at "230V rms", which means peaks of 325V.

Supposing a 3.5V 25mA LED, you would need a 12860ohm 8.03W resistor.

As a general rule of thumb: DO NOT DO THIS.

dhaneshg (author)jarfil2015-05-24

sir, can i use any IC or bridge circuit type somthing to glow LED buld?

oronnadiv (author)dhaneshg2014-11-11

Lets say you use 3.5V 25mA LED.

(230v - 3.5v) / (0.025A) = 9060ohms.

230v is not a game. Do not do it yourself.

dadibom (author)oronnadiv2014-12-08

And the watts the resistor would have to be rated for would be at 230*0.025 i guess?

robertino.evl (author)oronnadiv2014-11-16

RIP dhaneshg

stoneschan (author)robertino.evl2014-11-26

LMFAO this had my rolling.

dhaneshg (author)oronnadiv2014-11-16

if i use small stepdown transformer and zener diode to control voltage for LED than??

Zack_Cantrell (author)2014-12-23

Okay, so I have an SMD LED... VF=11v-12v
@mA900
Supply voltage of 12v

This seems to be an ignorant question but I would like to know that if the voltage drop of my LED is 11v-12v then by Ohms law should I or should I not need a resistor.. At most maybe a 1ohm

dhaneshg (author)2014-11-11

but sir at the time of Festivals we are use LED bulb series to decorate that time normal volt is 230volt naa? i open a series light and found a Bride circuit, but couldn't understand.. so plz tell me how to build it?

CharlesD1 (author)2014-11-03

After using the "wizzard" I'm still not sure if everything is correct. From a 12vdc power supply, I plan to power a 15 led array wired in parallel. The numbers from the retailer are as follows: Power = 0.5w, Working Voltage = 3.2-3.5, Current = 150ma. The wizzard calculator red flagged one of the lines. I am looking to get the correct resistance value for this array. Any help would be great. Thanks guys.

kel.moto.3 (author)2014-08-20

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.3 (author)2014-08-07

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.

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