Choosing the Resistor to Use With LEDs

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About: Creative swashbuckler. Writer for MAKE Magazine, presenter of inventions on TV, radio, magazines and newspapers. Professional problem solver. Annoyingly curious. Hacker of all things from computers to clot...

Intro: 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

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

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!)

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.

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79 Discussions

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Belc15

Question 5 months ago

So, I want to add 30 LEDs. The LEDs have to be placed in 3 rows of 10 and I want to use us less resistors as possible. How do I do this?

(2V 15 mA Led)

(15V power voltage)

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fogartyk

10 months ago

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?

Screen Shot 2017-11-13 at 12.55.46 PM.png
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Zeta2

1 year ago

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.

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AndreaM235

2 years ago

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?

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hydranixAndreaM235

Reply 1 year ago

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.

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sigshaneAndreaM235

Reply 1 year ago

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

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Peterthinking

1 year ago

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

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apraj77

2 years ago

390 Ohms is wrong.

12-3.5=8.5

8.5/0.025= 340 Ohms

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Grathioapraj77

Reply 2 years ago

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.

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AndreaM235Grathio

Reply 2 years ago

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?

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WakeelA1

2 years ago

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...

1 reply
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MahadiH4WakeelA1

Reply 2 years ago

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

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IanK48

2 years ago

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

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KDS4444foggy34

Reply 2 years ago

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

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Martythebest

7 years ago on Introduction

if i get it right...amps are enabling the charge to flow...so it doesnt matter how many amps are created by power supply?

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GrathioMartythebest

Reply 7 years ago on Introduction

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