# Driving LED's



## randomperson21 (Jan 14, 2007)

Heya all,

I'm having a bit of a problem w/ a school science project, hopefully this is the right place to come for help. 

The project essentially involves stripping the CCFL tubes out of an LCD (old 19" widescreen, to be exact), and replace it with an array of LED's. Thats all well and good, until I realize that I'm a complete moron when it comes to electronics (most of my experience comes from wiring up lightbulbs to batteries and going "yay!"). 

Now here's my problem. What is the best way to power a large array of LED's? These things run @ 3.85v @ 1500ma max, i might have anywhere between 20-60 LED's. Also, they are pretty voltage and current sensitive, at least thats what I have gathered from the datasheet (attached). The exact model i'm using is the LXK2-PW14-U00.

Also, since i'm a total electronics n00b, i'm most likely going to have a ton of moronic questions. Thanks to all!


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## Thermopylae_480 (Jan 14, 2007)

Hmm, well it certainly is going to be much more complicated than hooking batteries to a lightbulb.  I imagine you'll need resistors, maybe transformers and capacitors, circuit boards, you'll need a multimeter, you'll need to figure out what kind of current is being fed to the CCFL, and use components to alter it to run the LEDs.  You'll need to diffuse the emitted light in some manner otherwise you'll end up with dots all over behind the LCD.  I'm envisioning something that is going to be no easy task, especially if you don't have electronics experience.  Perhaps something that suits your acquired skills better, would be better for your science project, and this could be relegated to a fun and challenging side project as you build the necessary skills?  A very admirable and interesting endeavor, and I by no means wish to dampen your spirits, as I think it would be rather fun myself.

Also, in my experience, science projects usually are meant to propose, test, and collect data on a hypothesis, than present it.  What theory do you intend to propose, and test, with this project?


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## randomperson21 (Jan 14, 2007)

IDK if i'm reading your post right thermopylae, but I don't really want to power the LED's off the CCFL's connection/ the onboard LCD power supply. Its fine if theres an external PSU that is solely dedicated to powering the LED array.

I already have all the optics worked out and built (mostly reusing the current wedge in the display coupled w/ 2 diffusers and a BEF for repolarization). There's also a 10mm acrylic mixing block coupled onto the edges of the wedge, allowing for the LED's to mix (they need about 7mm or so, lambertian radiation pattern, +-30 degrees or something like that). The light should be fairly uniform going into the wedge, i could add diffusers if necessary. I hope to emulate the light output of the CCFL tubes so the optical characteristics of the wedge and optic filters will not have to be changed. 

I realize the LED's i chose require a ton of power (1.5A instead of the usual 20ma or so...) which is what worries me the most. If you get up to 50+ LED's, which might happen, it'd be pulling something greater than 300w, which i imagine would require a pretty beefy psu.

oh yeah, I just picked up a cheap digital multimeter from frys, and i'm learning how to use it. i can measure current and voltage... but thats pretty much it so far. heh.


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## randomperson21 (Jan 14, 2007)

This is interesting. It looks like he just wired up a whole bunch in parallel.

I'd like to do similar, but with brighter, more powerful (and therefore more power hungry) LED's.


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## PuMA (Jan 14, 2007)

the way we make leds work in school, is to put resistor before the led (400 ohm) and hook up the power on it.


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## Carcenomy (Jan 14, 2007)

And just select the appropriate resistor for the number of LEDs you're using, and the voltage.

So explain to me why you'd want to cut the brightness of the LCD in half by swapping from cathode backlighting to LED backlighting? This is all quite interesting... the last conversion I saw was removing all backlighting and mounting the panel over an old OHP... instant high-res projector.


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## randomperson21 (Jan 14, 2007)

I think thats the problem. How do I figure out what resistor to use/ voltage? Would the voltage just be a bit under the max for the LED? Also, according to the datasheet, you can run it anywhere between 700-1500ma. how would I adjust that?

Hopefully, the resulting LCD w/ the LED backlight will be within +-10% of the original brightness of the LCD. The LED's i'm using are extremely high brightness with a relatively large radiation pattern, they aren't the cheap-o "hi-brite" things you can pick up at jameco or something. I have to be within roughly 1874 fL of the backlight brightness.


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## Carcenomy (Jan 15, 2007)

This should help.


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## randomperson21 (Jan 15, 2007)

Carcenomy said:


> This should help.



WOW! amazing! thanks!

Sorry, a few more questions still.
The wizard says that the circuit will pull something like 10k ma from the source. That would mean that my power supply would have to provide at least 10A? Do I need to be greatly concerned about the mA the resistors or the diodes dissipate (something like 56A and 72A)?

Also, its saying that "the wizard thinks the power dissipated in your resistors is a concern", I can't figure out why. Any suggestions there?

Once again, thanks a ton.


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## Carcenomy (Jan 15, 2007)

Probably the fact you're running such big vicious LEDs. For a power supply I'd almost consider something like a car battery charger rated at around 10A providing 12V.

And it'll say it's a concern because of the current passing through - maybe the resistors will require cooling?


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## randomperson21 (Jan 15, 2007)

That makes sense. I'll have to sink the resistors also. That should work out OK, mostly because I already have to sink the LED's. I'll just kill two birds with one stone.

Just out of curiosity, I'm only really familiar with those little round resistor thingies. I wonder if they make any that are beefier and can handle a greater current. hmm.


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## ex_reven (Jan 15, 2007)

heres an interesting read that i was looking at a while back 
http://www.dansdata.com/caselight.htm

Im not sure if its up to your level in technicality, but its worth checking out.
and as you said you were hopeless with electronics, he shows you what to solder where, how and why to get things working. Hope it helps


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## randomperson21 (Jan 15, 2007)

thats a great article. It really clears up quite a few things for me, especially resistor choices, etc. thanks for the read.


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## ex_reven (Jan 15, 2007)

thanks for posting in the wrong thread


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## randomperson21 (Jan 15, 2007)

Sorry, folks, but another question: feasibility of running these LED's off a computer's PSU. 

I have a ye olde thermaltake power supply lying around that might do the job. it can do something like 30A on the 3.3v, 40A @5v, 18A at 12v. The max current this thing is going to draw with all 20 LED's on @ 1000ma is 20A, most likely running off the 5V connection. That would mean each diode would be hooked up to a resistor (something like (5-3.6)/1 = 1.4 watt, i think). I could run it off the 12v rail, but I'd have to create series chains hooked up to the parallel circuit. Lower current draw, but higher voltage requirements, right? 

Does a computer PSU have anything funky built into them that disallows you to hook up stuff like that, i.e. will it blow up in my face if i try? All the psu's at jameco are expensive as heck, so hopefully I could use what i already have.


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## randomperson21 (Jan 15, 2007)

ex_reven said:


> thanks for posting in the wrong thread



ha! yea. stupid firefox and its 50 tabs. 


ok ok don't blame the program, blame the user. *slaps himself


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## ex_reven (Jan 15, 2007)

have firefox open new windows rather than tabs 
it makes porn downloading easier too


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## randomperson21 (Jan 15, 2007)

ha! i could, but it really clutters up my taskbar. I like neat.........

ok then, back to the origional question:

will i blow up the psu or not?


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## ex_reven (Jan 15, 2007)

if its a spare who cares?

just dont be near it when it does


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## randomperson21 (Jan 15, 2007)

good point. i'd prefer not to blow it up if i have tho tho.


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## PuMA (Jan 15, 2007)

u suggest u dont use 12v line to power the leds,as it will kill them very fast. 3.50V is the normal voltage for leds. Thats why u put resistors before, so that they dont get too much voltage. Make sure u put the leds the right way(+ and -), as u will most likely kill ur psu if u dont.


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## randomperson21 (Jan 15, 2007)

so maybe run them in parallel off the 5v rail (the psu i have is rated @ 40a at 5v)?

heres the schematic i whipped up. does this work:


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## PuMA (Jan 16, 2007)

u should probly use bigger resistors, like 200-400 ohms


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## randomperson21 (Jan 16, 2007)

the LED's are pretty beefy tho. 3.72v @ 1A.


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## PuMA (Jan 19, 2007)

sorry, I re-calculated and those 1.5 ohms should do fine. And yes ur schematics looks good, I think they'll work


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## randomperson21 (Jan 19, 2007)

ok, so i just got the LED's in yesterday, and i was trying to make them work.

Heres the problem:

i hook up the 1.5ohm 5w resistor to the gnd terminal of a molex connection, and then hook up my multimeter to the +5v, and its still reading 5.22v. I think the psu might have some internal feedback loop or something thats monitoring the 5v line, notices the drop in voltage, and then compensates for it.... is this correct?
what do i need to do to make that work correctly then?

Also, I was just thinking about the resistors, and now i'm not so sure that they will work.
the LED's have an internal resistance of 1ohm... so
5v/2.5ohm= 2A of current. Firstly, that is too high, the most these LED's can take is 1.5A MAX.
then,
2.5-1=1.5== resistor value. 1.5*2A=3V. OK that is a good value. 

But there is still 2A in the circuit, and the LED can only do 1.5A. How would I go about fixing that?

Thanks!


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## W1zzard (Jan 19, 2007)

what voltage do you want to run the LEDs off ? its probably easiest to put them in series. what are the specs of your leds?


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## W1zzard (Jan 19, 2007)

good read: http://www.theledlight.com/ledcircuits.html


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## randomperson21 (Jan 20, 2007)

W1zzard said:


> what voltage do you want to run the LEDs off ? its probably easiest to put them in series. what are the specs of your leds?



I was hoping to run it off a computer PSU, meaning that I could get 5v, 3.3v, or 12v. I'd prefer the 5v rail, because it can supply 40A of power. 

The full datasheet for the LED's is attached to the full post, but here are the important specs:
3.5V typ @ 1A. LED can be driven anywhere between 350ma and 1.5A. I'd prefer to run them at 1A, because that strikes a good balance between luminosity and heat output.
Also, the LED has an internal resistance of 1ohm. 

I'll take a look at that article too. Thanks!


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## randomperson21 (Jan 20, 2007)

Would something like this work?


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## PuMA (Jan 20, 2007)

theres a regulator in PSU wich keeps the circuitry at 5V all the time. If u got the right resistors U will see that theres 1.3 Volts in the resistor and the rest 3.7 in the leds. Try hooking up 1 led at first and see what happens. Measure the voltage with meter from the resistor and from the led and see if they're what they're supposed to be.


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## randomperson21 (Jan 20, 2007)

Ah. I was just measuring the voltage with the resistor inline w/ the multimeter, no LED.

But tell me this:
If I have a 1.5ohm resistor and I want to run the LED in parallel, then
5v/2.5ohm= 2A -- this is too high for the K2, I want to run it at 1A max. 
Is this a problem?


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## PuMA (Jan 20, 2007)

Im not sure what u mean, but dont want to run them at a current higher, that they're supposed : they will fry very fast.Just use A bigger resistor, or u could put Two resistors in parallel to achieve the wanted resistance. the first schematics looked very good and its the kind we use in school when we want to use leds, Just use that 1. But remember not to put ALL the leds in at first: Just put 1 and see what happens.


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## randomperson21 (Jan 22, 2007)

BWAHAHAHAHAHAHAH!
IT WORKS!

and i think i lost my eyesight in one eye. hah!

and the feeling in one of my fingers........

New problem: these things are toasty as heck. Think PIII without a heatsink, thats the only thing i can really compare it to.

I tried mounting it on a Zalman VGA ramsink w/ some AS5, but even then, the LED AND the ramsink got hot enough to burn. 

I'm thinking a bigger heatsink, and hold it there with some arctic silver thermal adhesive stuff.

Any other suggestions?


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## PuMA (Jan 22, 2007)

try using bigger resistor, so u can burn them with smaller voltage, wich makes the leds go less bright and hot. Maybe put two of those 1.5 ohms in parallel before the led.


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## randomperson21 (Jan 22, 2007)

i was thinking that for awhile too. problem is, I need em as bright as i can get em, only about 6% of the light emitted by these things makes it thru the LCD panel. I'm trying to hit a +-10% range with the origional LCD brightness. 

I mounted one of the LED's on a passive hsf from an old blown x300, seems to work really well. the LED doesn't even get hot! thats good. But its still too large

All of these LED's are going to be mounted linearly down a strip. I was thinking of attaching a Thermaltake vga mem cooler to each (they're a bit bigger than the Zalman ones., and then enclose all the heatsink assemblies in a tube-like thing. Then, i'll put a fan in one of the ends and force air down the whole tube, exhausting air out the other end.

Hopefully that will alleviate the heat problems... plus, I think these things are designed to run at high temps, I just have to make sure it doesn't get TOO hot or melt any other components.


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## PuMA (Jan 22, 2007)

sounds cool dude, be sure to report back here, and show us some pics


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## W1zzard (Jan 22, 2007)

there's no way a single LED uses 1 ampere.. usually 20mA = 0.020 A


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## randomperson21 (Jan 22, 2007)

W1zzard said:


> there's no way a single LED uses 1 ampere.. usually 20mA = 0.020 A



These are super super high intensity LED's. They pull anywhere between 320-1500ma. i can post a pic later.


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## randomperson21 (Jan 30, 2007)

OH MY GOODNESS IT WORKS!


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## PuMA (Feb 2, 2007)

nice1 dude  

did u solve the heat problem???

more pics! I wanna see what those leds look like


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## randomperson21 (Feb 2, 2007)

hmm not really. hooked it up to a temp probe, maxed out at 75C after 15 min or so. I've ran it for 30 min straight, no noticeable problems yet. Might rig something up this weekend tho...

More pics!!!

Materials:





Assembling the array:





Duct Tape!





Side view of finished LED array





Testing w/ spectrophotometer:





Running LCD




... Note the pretty yellow line of stuck pixels. oops!
Having some uniformity issues near the LED emitters, note the cones. Can't really fix this without adding another row of LED's or a light mixer. Don't have room and time for either. 
Final brightness, according to my initial calculations, is about 30% dimmer than the origional display. Haven't calculated for spectral response yet though. The initial graphs look good though.


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## PuMA (Feb 2, 2007)

really cool dude  
can I ask u what are u studiyng to become???


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## randomperson21 (Feb 2, 2007)

i'm thinking of majoring in computer engineering or electrical engineering. not sure which one yet.


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## PuMA (Feb 3, 2007)

oh you're going to be engiineer then. Im studiyng to become an electrician, and so far it's been a fun ride. We study computers too


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