Which resistor to use for blue led

[Thomas499]

I bought the arduino starter pack, and found that the blue led requires more resistors than the green led does or it blows fairly quickly. The starter pack didn't give any heads up about this. All it says on the Kit Contents paper is that the led's are 3mm which is misleading because it gives the impression that they are all the same.

I've been using multiple resistors for the blue led, but to simplify my breadboard i want to just use one. The problem is, I don't know which one to use or purchase. I went to Radio shack and asked the people there. They didn't know, and they tried a Google search but were unable to tell me which resistor is needed.

I'd like to get all these resistors in one trip.
Green led appears to work well with a 220k
Which resistor is needed for the red led? 220k is my guess but I want to make sure, these leds cost $2 a piece
Which resistor is needed for a yellow led?
Which resistor is needed for a blue led?
 
All these led's that I have are the ones that come in the standard Arduino starter packs. I also have a clear one but it doesn't have three prongs like the 5mm, tricolor RGB... It only has two prongs. I haven't messed with it yet, but it must have come in the other starter pack. I do have some plastic strips that I think were designed to wrap around the led so you can make it whatever color you want. If that requires a different resistor please let me know as well.

[DVDdoug]

....With 5V and 220K, none of these LEDs should be "blowing".

Is there a manufacture's part number?   If you are going to spend $2 for an LED, I'd buy from a supplier that either gives you the specs or the part number so you can look-up the datasheet.

Blue LEDs typically have a higher voltage drop, which means a lower resistor value.

If you assume 5 volts applied and 3V across the LED, that leaves 2V across the resistor.  

If you want 20mA, that's 2V/0.02A = 100 Ohms.

[Thomas499]

I bought two starter packs so I have plenty of led's. I'm not going to purchase any for $2. At most i'd probably pay 40 cents for one, but even that seems like a premium price. For some reason I only have 3 or 4 blue ones, where I have upwards of a dozen each of the green and red ones. I noticed the first blue led I used worked well for a while, but after a while it died. I also noticed that it was much much brighter than the green and red leds so I figured what likely happened is the same thing that will happen to a light bulb if you use one that is rated for less wattage than the device you are attaching it to. It is much brighter than the other bulb, but doesn't last very long.

Because I don't have many blue led's I figured to play it safe and use multiple resistors. I did notice with the amount of resistors that I used to get it to appear to be a similar brightness as the red, and green led's, if I hooked a red or green led to the same resistor set up, they were so dim that you could barley tell they were on at all.

So are you saying a 100 Ohm resistor should be enough that the 220k used in the examples is just to play it safe?

[MarkT]

You need to know 3 things - the rated current for the LED, I, the forward voltage, Vf, and your
supply voltage, Vs

R = (Vs - Vf) / I

[alnath]

220k  

with a LED Vf of 2V, it means 3/220000 = 0.000013 A = 0.01 mA
vith a LED Vf of 3V you get 2/220000 = 0.000009 A = 9µA

pretty sure you won't be able to see any light
but you probably meant 220 Ohm

Anyway, you could just use a 220 Ohm resistor , and use a multimeter to mesure the voltage across the LED, you'll then know the Vf for each LED, and you'll be able to chose the resistor which will allow a ... say 10mA current through the LED

[DVDdoug]

So are you saying a 100 Ohm resistor should be enough that the 220k used in the examples is just to play it safe?

Yes.   Higher resistance means less current (Ohm's Law).   Resistance means "resistance" to current flow.

Because I don't have many blue led's I figured to play it safe and use multiple resistors.

It depends on how you connect multiple resistors...

Resistors in series sum-up and the current will be reduced....  i.e. Two 100 Ohm resistors in series makes 200 Ohms. 

Resistors in parallel reduce the total resistance and increase current.    Two equal value resistors in parallel are half the resistance.    There is a formula for unequal resistors, but the total resistance will always be less than the lowest-value resistor.

I also noticed that it was much much brighter than the green and red leds

There are a couple of factors at work... 

LEDs vary a LOT in efficiency.   There are "super bright" LEDs that are much brighter with the same milliwatts.  The specs will usually show brightness as mcd at the rated current....  Just looking at my Jameco catalog I see LEDs from less than 10 mcd to over 5000 mcd.

Our eyes have more sensitivity at different colors/wavelengths.   (IIRC, our eyes are most sensitive to green).

Power (Watts or milliwatts) is calculated Voltage X Current.   So a blue LED at 3V and 20mA is consuming 50% more power than a red LED at 2V and 20mA.

[Thomas499]

LEDs vary a LOT in efficiency.   There are "super bright" LEDs that are much brighter with the same milliwatts.  The specs will usually show brightness as mcd at the rated current....  Just looking at my Jameco catalog I see LEDs from less than 10 mcd to over 5000 mcd.

Power is (Watts or milliwatts) is calculated Voltage X Current.   So a blue LED at 3V and 20mA is consuming 50% more power than a red LED at 2V and 20mA.

So do resistors limit power used? For example, if I wanted to be very power efficient could I purchase a high efficiency led 5000 mcd, and put in a large resistor to limit the power supplied, therefore I could get the same brightness as a low efficient 10mcd led with using only a fraction of the power?

[jackrae]

As has already been said, you need the data sheet for the LEDs you are using to ensure they are being operated within their parameters.  However, it's generally accepted that most LEDs will be quite happy passing 10mA through them. 

Note that LEDs are current based devices so specifying forward voltage to calculate series resistor is fairly iffy.

As Alnath suggested there is a test you can do to assess a reasonable value of resistor. 

Connect your current meter in series with your LED.  Say the nominal forward voltage (Vn) across the LED is 2v (Vn this is only a guess for the purpose of setting up the test)  Vs is the supply voltage.
Pick a series resistor (Rn) to give a nominal current (In) of 10mA 
where Rn = (Vs - Vn)/0.01  ie  Rn = (Vs-2)/0.01
Measure the value of In, say it shows as 12mA. 
By transformation of the formula 0.01Rn = Vs - Vn and hence 0.01Rn + Vn = Vs  and Vn = Vs - 0.01Rn
But since In is actually 0.012A then actual Vf = Vs - 0.012Rn (which is about 20% lower than our guessed 2v)
Hence Rs the actual series resistor required to give 10mA is Rs = (Vs - Vf)/0.01

Whilst the above is not strictly true because LEDs are non-linear devices, it does get you into a reasonable range of resistor value.   

Having said all that, it's unlikely that using a 270ohm resistor will cause run-of-the-mill LED failures when working on a 5v supply.

If you want to work within optimum parameters then it is essential that you get hold of manufacturers data sheets for the LEDs being used.

[DrAzzy]

$2 per LED? Did you really mean a $, not a ¢? Basic LEDs (from half-assed ebay sellers) should be around $2 for 100 - ex: http://www.ebay.com/itm/261540613243 



I'm amazed if you burned out a blue LED while simply applying 5v to it through a 220 ohm resistor. That should give you probably around 10 mA (assuming ~3v voltage drop across the LED - blue LEDs generally seem to be pretty close to this) - most modern indicator LEDs are spec'ed for 20mA, if you can scare up a spec sheet at all.


@Thomas499 Within limits, you can get more efficiency by running LEDs at a lower current than they're spec'ed at - it's well known that LED efficiency decreases as current increases; this is speculated to be due to recombination of electrons and holes as the density of such charge carriers increases. This is why most high power LEDs consist of several LEDs connected in series or parallel. I don't know if you'll still reap efficiency benefits if you try to dim it to 1/100th of it's normal brightness. Also, be careful when comparing LED mcd ratings. mcd depends not only on brightness, but how widely or narrowly the light is focused (review definition of mcd, vs the lumen).

[Thomas499]

If you want to work within optimum parameters then it is essential that you get hold of manufacturers data sheets for the LEDs being used.

The Led's that I have came in an arduino starter kit. They didn't come with data sheets, I guess the makers of the kits figured newbs wouldn't know what to do with a data sheet anyway. Given that I myself am still trying to figure out how to read a datasheet

DrAzzy:
Well, there's a good reason Radio Shack will probably go bankrupt in the next year or so 1 led $2 Radio Shack Everything there is way overpriced. But it is nice to be able to walk around and take a look at what they have, don't expect the employees to know though. Now they did have a "Special" where you could buy a pack of 5 led's for $4.68 if I remember correctly. I take it you haven't been in a Radio Shack lately? To save you gas money they sell their cheap breadboards for $10 if that tells you anything.

[DrAzzy]

I'm not surprised there was no datasheet in an arduino starter kit (I haven't been able to scare up datasheets, part numbers, or any other documentation short of (often inaccurate) Vf and current specs. However, I'm disappointed that they put in LEDs that didn't work as expected with the resistors they included - that's really shoddy. You need to start people off with parts that work - then once they're comfortable with how things should work, then they can start buying dodgy parts from china and it won't be so bad. But it's not going to get people interested in electronics to sell them kits that don't work.


Re: the led actually burning out - are you sure you didn't slip up and power it without the resistor, even for a split second? If you put 5v on an indicator led, with no resistor, it will typically die *instantly* - you usually won't even see a flash like you do with incandescent bulbs.


Wow, I thought radioshack gave you more than 1 LED for $2...

[Thomas499]

are you sure you didn't slip up and power it without the resistor, even for a split second?

I did leave the module connected to my computer when I left town for a night not even thinking about it so the led was on for a solid 16 hours. I don't know if it's possible for it to overheat or what not. It is possible I used it in an experiment trying to figure out how to make a parallel circuit I don't remember which led's I used for that. As a beginner when you first get a kit you really don't know what you are doing and remembering all the crazy things you attempted trying to figure things out.... well it's definitely possible it was an error on my end. What is the life expectancy of a led by the way? Can you leave one on for a month straight and still expect it to work?

[larryd]

Should work for 25,000 - 100,000+ hours

http://en.wikipedia.org/wiki/Light-emitting_diode

[alnath]

The Led's that I have came in an arduino starter kit. They didn't come with data sheets, I guess the makers of the kits figured newbs wouldn't know what to do with a data sheet anyway. Given that I myself am still trying to figure out how to read a datasheet

this kit ?

if yes, then maybe you should read the description, and particularly this :

Click on the name of the component to download the datasheet of the part. This document will describe the design and functionality of the component.

if you click on this line :

3 LEDs (blue)

you get this page :
blue led
and you'll see that there is already an internal resistor, thus it needs to be powered at 12V (not 5V)

[Thomas499]

this kit ?

Yes, that was one of the kits. However, the datasheet didn't come in a paper version with the kit.

and you'll see that there is already an internal resistor, thus it needs to be powered at 12V (not 5V)

Tell me, the arduino can only supply 5V or less. Is there anything in the kit that would allow a beginner to power a led with 12V using an arduino? I know the blue led (with a 220 ohm resistor) is a solid 3 times brighter than the green or red led already. I wouldn't be surprised if it was really 5 times brighter. I can't imagine how bright it would be if it was really meant for 12V and no external resistors.

My guess though is it would probably through in the towel and die.

--------------edit-----------------
That sounded cocky. I apologize. But if the reason that particular blue led isn't working is because it requires 12V, then how am I supposed to power it?