Choosing the Resistor to Use With LEDs

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Introduction: Choosing the Resistor to Use With LEDs

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 clothe…
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.

1 Person Made This Project!

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88 Comments

0
krishparvin11
krishparvin11

Question 9 months ago on Step 3

Need how many ohm and watt resistor for 5mm led

0
said lamy
said lamy

Tip 1 year ago

thanks for all

0
dhaneshg
dhaneshg

7 years ago on Introduction

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

0
treesap
treesap

Reply 1 year ago

This is a late answer, but will be helpful to others. I suspect the voltage referred to here is AC, while LEDs require DC. This would need a diode (ideally 4 of them, connected to make a full-bridge rectifier, to avoid flickering) in addition to the large resistor. (A diode is like a 1-way valve--it prevents electricity from flowing "backwards", essentially making AC into DC.)

0
jarfil
jarfil

Reply 6 years ago on Introduction

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.

0
dhaneshg
dhaneshg

Reply 6 years ago on Introduction

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

0
oronnadiv
oronnadiv

Reply 7 years ago on Introduction

Lets say you use 3.5V 25mA LED.

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

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

0
dadibom
dadibom

Reply 6 years ago

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

0
dhaneshg
dhaneshg

Reply 7 years ago on Introduction

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

0
shineaquacare
shineaquacare

Question 2 years ago on Introduction

24v DC to led red green yellow which resistance best life for led

0
treesap
treesap

Answer 1 year ago

That is literally the exact question this Instructable answers. Read it, and you will have your answer!

0
pdoc51
pdoc51

Question 1 year ago

Hi, I am trying to rig up a single LED for an aircraft, 24 volt system, breakout box. the amperage could be high, say 5-10 amps, maybe higher. It will be tested between 2 banana plug sockets. I need to have a LED light that will withstand higher current applications. And it needs to stay small, like for on top of a mini banana plug grounding plug. Any thoughts?

0
treesap
treesap

Answer 1 year ago

Obviously it's 10 years too late for this guy, but maybe this will help a future reader.

Amperage doesn't work like that. As you probably know, the ampere is a unit of current. As such, it only applies in the context of a particular device. When you say the system is 5-10 amps, this is referring to the maximum amount of current the source can supply. It doesn't mean that is how much current a device connected to it has to use. The device connected will draw only the amount of current that it needs. The only thing one must be sure of is that the power source is rated to supply more amps than the device connected to it.

In this case, the LED will only draw a handful of milliwatts! This is perfectly fine, since the power supply is capable of more current than that. You could use the LED, no matter the amperage capability of the source–whether the system is capable of providing 1 amp or 1 million amps!

There's no such thing as a "high-amperage" LED. They all draw next to no current at all! This is one of the fantastic things about them, and a major reason they're replacing incandescent and fluorescent lights all around us.

0
Predator_Acolyte
Predator_Acolyte

Question 2 years ago on Introduction

Hello! I need help very badly. If the LED module I am using has 3 leds, (2 reds and 1 white) do I add the total value of the volts and mA’s? Please help me

0
Belc15
Belc15

Question 3 years 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)

0
fogartyk
fogartyk

4 years 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
0
kgfogarty
kgfogarty

Reply 4 years ago

nevermind about above! Misguided!!