LED's and Resistors Guide

[Whisp]

Led and Resistor Walkthru


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I wrote this little bit of information to help out. I by no means am an electrical engineer but I have tested this and am not responsible for any damage done to anything you take from this mod walkthru. I also realize there is many other calculators out there but didn't seem to find a walkthru here so posted my old one that I wrote up a long time ago so that people could figure it out on their own.

There is 2 power lines on the computer power supply on the standard molex style cables. That is the 5 volt line which is the red wire and the 12 volt wire which is the yellow.

First off I am going to show a simple equation. The equation to figure out what type of resistor is needed the equation is (power line - led volatge) / led amps. When you buy your led make sure you know the volatage required for it and the amps of it. Some leds might be 10mA while some might be 20mA or can vary.

Anyways now that we have the equation lets apply it.

So lets say we want to put 1 led on a 5 volt line and the led is 3.6 volts and has a milliamp of 20 then this is what that would look like. Also you have to convert the milliamps rating for the equation so devide the milliamps by 1000. For example a 20 Milliamp rating divided by 1000 = 0.02 Amps .

(5 volt[power] - 3.6 volt [led]) / 0.02 amps [ 20 mA]
= 70

So now we know you need a resistor that is 70 ohms

If this was a 12 volt line it would then be:

(12 volt[power] - 3.6 volt [led]) / 0.02 amps [ 20 mA]
= 420

So on the 12 volt rail you need a 420 ohm resistor.

Anyways that is the basics.

If you want to add more then 1 led then you times the volts of the LED by the number of LED's and use that number in your equation.

--------------------

This would be for putting them in series.

Example:

two 3.6 leds with a mA of 20 would be 7.2 volts and you would still be at 20mA so the equation looks like this.

(12volt[power] - 7.2 volt [led's]) / 0.02 amps [20 mA]
= 240

--------------------

This would be the equation for parallel.

Example:

three 3.6 leds with a mA of 20 on a 5 volt rail.

In parallel you multiply the mA by the number of Led's you want to use so since we want 3 leds it would be a mA of 60.

(5 volt[power] - 3.6[led]) / 0.06 amps [60 mA]
= 23

--------------------

The end results for the resistors won't be exact on with what you can buy so you need to find the closest one. As I mentioned I am not an eletrical engeneer so some of that might be a bit off but I have used these calculations many times in my mods and have not had an issue or anything blow up yet.

[Frakk]

nice tutorial. now all you n00bs should figure out how to do the magic for the ubermods :D

[inspiredbyasphalt]

nice, this is the perfect guide.. thanks for that whisp, im no rocket scientist and that guide put it down in plain english! ta

[inspiredbyasphalt]

ok, one question... say i mess up and dont do it right, can i screw anything (other than the LED) by doing this?

[Xato]

No, unless you have something else in the circuit.
If you pour too much power into the LED, it will simply blow and close the circuit... And if by chance you manage to short out your power supply... modern PSU's (for about 5-10 years now) have short circuit protection... the other day, i short circuited my PSU while testing a tornado fan (damn those things have some torque)... the PSU was fine after i unplugged it and plugged it back in

[Whisp]

Might burn out the resistor on the setup of it but never seen any led go bad unless a lot of juice is put thru it.

Also depends on the power supply but most likely the power supply will just short and turn off. Thats not guranteed though.

I also think that just a burned out resistor and a led if it did would not cause the system to fail.

For example sometimes the leds on the front of your computers burn out and are still plugged in but the system still runs.

LED's are probabaly one of the easier electrical mods to start with.

As long as you have the proper resistor somewhere along the circuit of the item you are making since it dont matter if it comes at the end or at the beginning so sort of fail proof.

Just make sure you are attaching to the right voltage line with the appropriate resistors using the calculations and you should be good to go.

[Xbrid]

a tornado fan (damn those things have some torque)...
they are f**king loud as well. I replaced mine with Red led quads. much quieter. :)

[Xato]

Lol, they sure can pass a **** load of air over your Heatsink though. Never use without a fan control though, or the noise will become unbearable.

[inspiredbyasphalt]

ahhh good! that leaves a little room for 'trial and error' ;)

[humanentropy]

hello, im new

ive got a question.. please answer, i will be greatful

when you say parallel, do you mean to place the resistor before all the LEDs or for each LED to get a resistor. I am betting on the first.. im unsure.. its been awhile..

oh, and what are the advantages/disadvantages of parallel and series?

thank you so much.

[Nagoshi]

Seems like you never learned the electrical basics. Short explanation :

Series : The most basic circuit.
http://www.danieltrimpey.com/learn/electronics/images/series.GIF
If you try to make a circuit like that running on for example 6volts AC with 3 10v light bulbs, you will notice that the light bulbs will not glow very much. And if you remove a light (or if it burns) the circuit is cutted, closed, no more current is passing.

Parallel : ''Safer'' circuit, a bit more complicated but it is logical.
http://www.danieltrimpey.com/learn/electronics/images/parallel.GIF
Here, the power is equal everywhere, meaning that if the circuit is running 5V the current will be of 5V everywhere. You will probly have to use one resistor for each LEDs if you are not using the same leds in the circuit, or one big resistor for the lot; but don'T trust me for resistors, I dont know how they works, and how to use them. So as you have read in the first post formulas ^^^^ each LED will be using its own voltage and amps settings. Basically, 3 6V bulbs made for 10V will light at 6V full efficiency (like if you would only be using one 6V bulb on series). But this circuit can be a bit more dangerous, if you happen to make a shortcircuit error (like removing a light and just tie the wires together), if you ahve nothing on the circuit that will eat a bit of power then itll shorten.

I sucks for explanations, but its about that for series and parallel. Just tell me if I said something wrong, but its hard to tell everything in english (im originally french, and still not perfecly bilingual).

Hastly said :
Series uses less power, but the lights wont show up as brighly as if they were linked in parallel. Parallel allows your leds to bright as they would normally, and the circuit won't close if one LED closes.

[humanentropy]

uh.. i know.. thanks.. nice diagrams.

This would be the equation for parallel.

Example:

three 3.6 leds with a mA of 20 on a 5 volt rail.

In parallel you multiply the mA by the number of Led's you want to use so since we want 3 leds it would be a mA of 60.

(5 volt[power] - 3.6[led]) / 0.06 amps [60 mA]
= 23

do i put a 23ohm resistor 3 times in (once for each bulb) or 1 time, resisting for all 3?

[DaveW]

Once should be fine on paper; though in real life, there may be other factors. Watch for xmastree catching this thread, he seems to know his stuff, corrected me plenty of times. (although he's not the only one who knows his stuff, don't take that the wrong way...)

-Dave

[humanentropy]

ah, cool. i am thinking about making a UV flashlight since they cost so much to buy.. and stick some in my case.

[xmastree]

Watch for xmastree catching this thread, he seems to know his stuff,How could I refuse? 8)

Sorry I'm late, been at a Rotary convention all weekend.


And if you remove a light (or if it burns) the circuit is cutted, closed, no more current is passing.
The circuit will be open. Open circuit passes no current.


Parallel :
Here, the power is equal everywhere, meaning that if the circuit is running 5V the current will be of 5V everywhere.I know what you're trying to say, but that's a little misleading. The voltage across each bulb will be the same as the battery voltage. In the series circuit, assuming identical bulbs, they will share the battery voltage equally. In each case, the power will be equal.

As for 3x3.6V LEDs from a 5V source, better to give them individual resistors. firstly, because they might not be exactly the same, and the one with the lowest forward voltage will draw more current than the others, and secondly because if one dies, and stops conducting, it will force a higher current through the remaining ones.


oh, and what are the advantages/disadvantages of parallel and series?LEDS in series is more efficient since the load on the power supply is the same as for one. However, the subsequent lower value for the series resistor makes it more sensitive to voltage fluctuations.

Why? Well the voltage across the resistor will be lower, so a shift of, say 0.5V is a higher proportion of the resistor's voltage, so it will have a greater effect on the current.

Example... nominal 12V, 3.6V LED, 10mA required.

For one LED, the resistor would be 840 ohms, with 8,4V across it @ 10mA
Three LEDs, the resistor would be 120 ohms, with 1.2V across it @10 mA

At 11.5V, the one LED will have 9.4mA, the three will have 5.8mA
At 12.5V, the one LED will have 10.5mA, the three will have 14.2mA

+/- 0.5V has a much greater effect on 1.2V than it does on 8.4V, so the higher the voltage across the resistor, the better the regulation.

Getting complicated? :rolleyes:

For three 3.6V LEDs in a computer, I would give them each a resistor and run them off the 12V rail.

[humanentropy]

For one LED, the resistor would be 840 ohms, with 8,4V across it @ 10mA
Three LEDs, the resistor would be 120 ohms, with 1.2V across it @10 mA

so if i were in parallel, should i give each of the three LEDs a resistor, i should put put an 840 ohm resistor for each LED, right?

if i were in parallel, should i use one resistor for all three LEDs, i should use a 120ohm resistor before splitting off, right?

[xmastree]

so if i were in parallel, should i give each of the three LEDs a resistor, i should put put an 840 ohm resistor for each LED, right?
Yes'

if i were in parallel, should i use one resistor for all three LEDs, i should use a 120ohm resistor before splitting off, right?No, the 120 example is if they are in series. If you put them directly in parallel and use one resistor (not a good idea) it would need to be 280ohms.

[humanentropy]

ah, i meant series for the second, but now i understand!

thanks a bundle. now to look for good led websites..

[xmastree]

now to look for good led websites..

http://ledmuseum.home.att.net/

[humanentropy]

http://ledmuseum.home.att.net/
http://www.hebeiltd.com.cn/?p=leds.5mm i cant find the current rating.

http://www.123leds.com/ i might use that

[xmastree]

http://www.hebeiltd.com.cn/?p=leds.5mm i cant find the current rating.Top of the list, Iv@20mA, so they're all measured at 20mA.

[humanentropy]

ah, cool thx!

what do they do about samples? im confused. you have to pay for them? then how are they samples?

[xmastree]

According to http://www.hebeiltd.com.cn/?p=samples

"This new samples policy was necessary to reduce the huge amount spent every year on samples. Thanks for understanding."

So basically, they no longer give them away.

[XcOM]

[QUOTE=Nagoshi]......But this circuit can be a bit more dangerous, if you happen to make a shortcircuit error (like removing a light and just tie the wires together), if you ahve nothing on the circuit that will eat a bit of power then itll shorten.
....[QUOTE]

I done this, well atleast i heat shrinked it, and got a super bright LED that matched my power supply, but hay, for an LED, the worst that can happen is you put too mush beans through it, and you fry it,

[Nagoshi]

YEa.. but I was giving the explanations for the circuit, not on the led.. try the same thing on 120v or 240v, good luck if you're still alive after it explodes in your face..

Altho Im pretty sure the circuit is by some way ''protected'' on the motherboard or in the power-supply.

[Liquid_Scope_99]

thanks even i understand that it that doing something lol

[Nameless]

Okay, so I've got:

2x Red LED's @ 2.6VDC/28mA
2x Yellow LED's @ 3.0VDC/20mA

How do I wire up these guys, what would be better Parallel or Series, and do I need to account for the resistance and Volt droppage of the Molex/Wire in this or do we just ignore that?

Thanks!

Oh, and if anybody could explain the equation I need to use I'd appreciate it. I haven't taken electronics since I did a semester at ITT 14 years ago and I'm pretty rusty.

While I've been lurking around for weeks now, I just registered and this is my first post so, "HI!" :)

Edit: After thinking about this, since two of my LEDs require different current I think I need to run my Red LED's in Series and my Yellow LED's in Series, and then run the two different colors in Parallel to the 12V power source, since I don't think 5V or 7V can push this. Is that right?

[calumc]

Welcome to the forums, for getting your resistances just put your figures into this calculator: http://led.linear1.org/1led.wiz
It would be better to wire them in paralell (although you wont notice any diference in just two) and whatever the calculator gives you for one led you can use that to power the whole circuit as 2 LEDS wont draw much

[Nameless]

Welcome to the forums, for getting your resistances just put your figures into this calculator: http://led.linear1.org/1led.wiz
It would be better to wire them in paralell (although you wont notice any diference in just two) and whatever the calculator gives you for one led you can use that to power the whole circuit as 2 LEDS wont draw much

Well I get that part, I'm just trying to figure out of the it's going to matter that the Voltage and mA's for each of the two different LEDs is going to be an issue. I assume it's going to make the calculations a little more complicated since I can't find a calculator that tells me the diagram for wiring LEDs with different voltage draws and current requirements.

[calumc]

I wouldnt worry at all about that, it would come into play only if you had a large amount of leds. If I was doing it I'd just wire them all up to a 47ohm resistor on the 5v line

[Nameless]

I wouldnt worry at all about that, it would come into play only if you had a large amount of leds. If I was doing it I'd just wire them all up to a 47ohm resistor on the 5v line

Would that work on a 5v line? 3.0+3.0+2.6+2.6=11.2V

Now I realize that just wouldn't work in series, and that it probably would if I just hooked them up in parallel, but then I'm sticking resistors on each LED.

I just want to make sure I'm doing my math right mainly, like I said, because I'm way out of practice. :)

[calumc]

A quick diagram for you:
http://xs320.xs.to/xs320/07404/diag.jpg (http://xs.to)

[Nameless]

A quick diagram for you:
http://xs320.xs.to/xs320/07404/diag.jpg (http://xs.to)

BAH! I can't see it from work, damn Army network, so I'll check it out when I get home. Thanks!

*scribble scribble* Yea... Gotta make sure I stop and get my resistor on the way home.

[xmastree]

Edit: After thinking about this, since two of my LEDs require different current I think I need to run my Red LED's in Series and my Yellow LED's in Series, and then run the two different colors in Parallel to the 12V power source, since I don't think 5V or 7V can push this. Is that right?

That's what I would do.

As for the equation, for the red ones:

2x Red LED's @ 2.6VDC/28mA

So the total volt drop across the LED's is 5.2V, leaving 6.8V (12 - 5.2) across the resistor.
R=V/I so 6.8/.028 = 242.

So 240 ohms is the nearest one there.


2x Yellow LED's @ 3.0VDC/20mA

6V/.02A = 300 ohms

Don't try calumc's suggestion. Sorry, but putting LED's of different voltages in parallel will never work. The red ones will clamp the voltage to 2.6V, so the yellow ones won't conduct at all.

[SgtM]

BAH! I can't see it from work, damn Army network, so I'll check it out when I get home. Thanks!

*scribble scribble* Yea... Gotta make sure I stop and get my resistor on the way home.

Quick word of advice for you.. don't buy Radio Shack stuff. Nothing but crap from them. Check out www.lsdiodes.com. Great stuff.

[Nameless]

That's what I would do.

As for the equation, for the red ones:

2x Red LED's @ 2.6VDC/28mA

So the total volt drop across the LED's is 5.2V, leaving 6.8V (12 - 5.2) across the resistor.
R=V/I so 6.8/.028 = 242.

So 240 ohms is the nearest one there.


2x Yellow LED's @ 3.0VDC/20mA

6V/.02A = 300 ohms

Don't try calumc's suggestion. Sorry, but putting LED's of different voltages in parallel will never work. The red ones will clamp the voltage to 2.6V, so the yellow ones won't conduct at all.

Much thanks! That's what I was thinking, but it never hurts to ask just to be sure.

[Nameless]

Quick word of advice for you.. don't buy Radio Shack stuff. Nothing but crap from them. Check out www.lsdiodes.com. Great stuff.

Nice site. Any reason Radio Shack LEDs are crap in particular? I already got the ones I'm using in this project from there, but I'm sure I can use the site you posted in the future because they seem pretty sharp.

I was just wondering if you had some experience you'd like to relay about Radio Shack LEDs or stuff in general, as it sounds like you've had a bad experience or two.

[SgtM]

Just more expensive for the same thing. $4 for 1 LED vs .45 for 1 LED and freebies. I'll order from lsdiodes any day of the week.

[rendermandan]

Just more expensive for the same thing. $4 for 1 LED vs .45 for 1 LED and freebies. I'll order from lsdiodes any day of the week.

Ditto! I have an order in with them right now!

[xmastree]

Radio Shack stuff. Nothing but crap from them.

Any reason Radio Shack LEDs are crap in particular?

Just more expensive for the same thing.

So not really crap then, just over priced?

[Kill_Switch]

Nope crap.

I really encourage buying from resonable retailers.

www.lsdiodes.com is great.

even ebay, I've gotta 100, 470 ohm and all work great thus far.

[Nameless]

Thanks for all the chatter and replies everybody. :) Just wanted to say that before the thread got too sidetracked.

[ekko]

Nope crap.

I really encourage buying from resonable retailers.

www.lsdiodes.com is great.

even ebay, I've gotta 100, 470 ohm and all work great thus far.

you cant order from them anymore..

[Oneslowz28]

Yea LS Diodes has been out of biz for a while. I was actually one of their last customers. I found the best place to get LEDs cheap now is Ebay. I recently purchased 100 Super bright green 5mm and 200 super bright green SMD size 603 leds for less than $14 shipped.

[billygoat333]

ledshoppe.com is a pretty good site as well to get LEDs.