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Hey everyone! Ive been on this site for awhile, and find it simply amazing. And now I have a simple question for some electricians :) Im OBSESSED with light, literally OBSESSED. the only bad thing, is I know NOTHING in electronics, but ive started learning and am trying to assemble a very simple led strip light with some conductive tape, and 12v power supply and about 16 leds. (yes i know there is a bar tutorial, but i need some explaning on a few parts.) I want to just have 16 leds in a parallel or series, (prefer parallel) so it can be put on a very slim piece of wood/metal/sheet or wherever i place it. I tried using the ledcalculator, but it keeps telling me i would need a resistor for EACH led, and that would just defeat my set up being "small and simple" So is there anyway to just have ONE resistor in the VERY beginning, to limit the current of 12v down to 3.5 to supply to all the 16 leds? Kinda like in my drawing (that is bad) Because i want the assembly to be as small as possible, and would like not to use one for every LED And also, how many leds would i be able to drive in a parrallel with a 12v power supply? Thnx so much in advance!

## 29 Replies

LED master22 (author)2008-10-03

you know... when i did a valentines day thing with about 9 leds.. i had to use two nine volt batteries... its also kinda simple math..nine leds divided by 18 V =2v each so No resistors are needed. for your setup, i would recomend tryin it with out a resistor on all of them.and i would connect them in series or you WOULD have to have a resistor for each because it would be 12v to each LED.
hope it helps!!!

killerjackalope (author)2008-04-04

If you're obsessed with light you might like this it's my first in a hopeful series of instructables about utilizing light for effect, it's not specific to any kind of lighting though so it's more just whats going on. The first is just bout colours and a bit of nice lighting.

T3h_Muffinator (author)2007-04-08

I can solve this!

First, we need to know the current output of the battery. If you don't know then that's okay!

Next, we need to know how much current each LED needs. Usually it's about 10mA minimum, and 30mA maximum (we'll use 25mA = .025A). Please confirm this before using the resistor value I'm going to give you.

Now, we use Ohm's law. Since the LEDs are in parallel, we add the current and keep the voltage the same. That means you have 3.5V of potential across the leds, so you need an (12V-3.5V) 8.5V drop and 16*.025A of current, or .4A. Ohm's law is V=I*R, so R = V/I, which is 8.5V/.4A OR appx 22 ohms.

It's going to be hard to find a resistor that small, but there's no harm in going anywhere from 22 ohms to 100 ohms, they'll still light nicely.

The short version: you need a resistor somewhere between 22 and 100 ohms that will go between +V and the positive strip of LEDs.

killerjackalope (author)2008-04-04

Just imagine making one of these with cree's or luxeon's, I'm think liquid nitrogen coolant...

mu0p (author)2007-04-10

Thnx everyone for the replies!!

t3h, thnx a bunch for figuring that out, after reading all your replies I kinda understood it alot better, and thought of this:
http://s73.photobucket.com/albums/i226/mu0p/?action=view&current=led.jpg

Would that work out, not having to use a resistor at all, or maybe a very small one at the end?

And could power up to like 100 leds like that? Like you said t3h?

Because, im trying to power as many rgb slow fading leds with just the conductive tape a 12v power supply and maybe some wire.

And here is the LED stats for the led im using

Items Symbol Absolute maximum Rating Unit
Power Dissipation PD 100 mW
Forward Current(DC) IF 30 mA
Peak Forward Current* IFP 100 mA
Reverse Voltage VR 5 V
Operation Temperature Topr <> -40 <>~ <>85 ?
Storage Temperature Tstg <> <>-10 <>~ <>100 ?
Lead Soldering Temperature Tsol Max.260Â°C for 5 sec Max.

(3mm from the base of the epoxy bulb)
*Pulse width?0.1msec duty?1/10
? Typical Electrical & Optical Characteristics ( Ta = 25?)
Items Symbol Condition Min. Typ. Max. Unit
Forward Voltage VF IF=20mA 3.00 / 3.80 V
Reverse Current IR VR=5V ? ? 10 uA
Dominant Wavelength ?D IF=20mA / / / nm?
Luminous Intensity IV IF=20mA 4000 / 6000 mcd
50% Power Angle ?? IF=20mA ? Â±10 ? deg

again, thnx alot everyone!!

westfw (author)2007-04-10

Um. Maybe. Using a resistor per set of series LEDs gives you some leeway in case things don't behave quite as you expect. Wire up the LEDs and transformer in a way that JUST works if everything exactly meets some Vf spec and power supply spec, and it could fail either way if those numbers weren't quite right...

Are you expecting multiple "slow fade rgb" leds to change color in a synchronized fashion when connected like this? I don't have any experience at all with such LEDs, but I'd be VERY surprised if they stay synchronized at all . It'd be worth some experiments on a solderless breadboard before you start soldering things together!

mu0p (author)2007-04-10

no, they get out of sync very fast, and thats the effect i want, because when they all blend together, you get a VERY wide range of colors. thnx for your reply, i just want as much info before i do this, so i dont go blowing like 150 leds

T3h_Muffinator (author)2007-04-10

if you want to make sure you don't blow anything first, start by using resistors that are WAY too big, then go down from there. for 42 LEDs off a 9v, I started off with a 1k resistor and went down to 340 something.

mu0p (author)2007-04-10

awesome! thnx a alot again! ill be doing this starting next week, and ill document it and put it up, but by all means, im not the one that should be teaching others on how to assemble anything electrical... you guys are the pros!

T3h_Muffinator (author)2007-04-10

Don't call me a pro! I'm only 16!!

mu0p (author)2007-04-10

well im 20, and you know more then me, so 16 or 42 you still know your stuff. but, maybe you can answer me this question at home i have 8 leds with the same stats i posted before. 3.5v 20mA and they were in a parrallel and aer powered by 3 AAA batteries, and if i leave it on more then about an hour, they just start going out, how come the batteries last so short? i know the rgb leds would go through button batteries in like 1-2 hours, but are they killing off 3 AAA so quick?

westfw (author)2007-04-11

Without a current limiting resistor, you don't know how much current your LEDs are actually using, but you can assume the 8 in parallel with consume AT LEAST 160mA. 160mA is a pretty big number for AAA batteries, so their capacity will be less than at lower currents. Maybe 600mAH for good alkalines or NiMH, and less for "heavy duty." That means a max expected lifetime of about 4 hours. But the rated capacity for alkalines is usually down to a voltage of 0.9V, while your LEDs will probably quit when they hit 1.1V or so...

T3h_Muffinator (author)2007-04-08

Didn't see your second question for a sec! You could power.... A LOT of LEDs. I powered 20 off of 5V @ 100 mA, so you will probably be able to power at least... 200. You might be able to pull off 400, depending on the current output and how bright you want them.

westfw (author)2007-04-09

You can't tell from the suppy voltage; you have to know what current the supply is rated for. Using t3h's other calcs, and strictly parallel wiring, you need a 400mA supply to drive 16 LEDs (or a 1A supply would drive 40 LEDs.) Using series/parallel wiring, you could power at least 3 times as many LEDs... You could put resistors on the bottom of the board to prevent it from becoming bigger:

midstate33 (author)2008-04-03

Where can I buy the PC board you have pictured in this post for the 15 LED's....tnx....Fred

westfw (author)2008-04-03

Mine? You'd have to make it. If you're really interested, I can make the EAGLE cad file available, I think...

T3h_Muffinator (author)2007-04-09

What software do you use to draw up schematics, I'm having a hard time getting used to eagle.

westfw (author)2007-04-09

That was eagle. (Thus the several instructables I've written on using EAGLE.) It DOES take a while to get used to it, but I gather that's true for any PCB package; the trick is getting used to drawing under much stricter constraints than the average "art" drawing tool. I think. Maybe.

T3h_Muffinator (author)2007-04-09

M'kay, thanks!

imarzouka (author)2008-03-22

Check out this calculator
http://ledcalculator.net

FrenchCrawler (author)2007-04-16
Dystonia (author)2007-04-11

you will need at a minimun 680 ohms resistor at several watts -"-DO NOT -- parallel " the restiors
as this will cut the resistence in half
example --2-- 680 watTs in parallel
will be 320 ohms =( LED BURN OUT )
but 2 --680 in series =1360 oms
there is a formula to do watts but i dont know it

westfw (author)2007-04-11

BTW, if you're "obsessed" with lights, you may also want to check out CandlePower Forums (at least, if you don't mind hanging out with a bunch of cops...)

westfw (author)2007-04-09

t3h's calculations look good to me. Keep in mind that this will need to be about a 5W to 10W resistor (8.5*0.4, nominal) and your power supply may or may not be outputting exactly 12V...

T3h_Muffinator (author)2007-04-09

Weissensteinburg (author)2007-04-08

We need las vegas

dentedvw (author)2007-04-08

I actually saw this at my local radio store (not radio shack). It was about \$35 per eight inch strip, and the LED's were quite bright. You could just cut with scissors, add strips, add wires between, whatever, and it was on sticky back tape. The whole thing was very thin, as it was all surface mount bits.

Weissensteinburg (author)2007-04-08

I'm pretty sure he wants to make it himself though.

panic (author)2007-04-08

1,Put all LEDs in series,and use a step-up circuit.You can find such circuit from MAXIM or TI or others. 2,Put every 3 LEDs in series with one resistor.You will need 6 resistors instead of 16.