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Driving LED's

Discussion in 'Cases, Modding & Electronics' started by randomperson21, Jan 14, 2007.

  1. randomperson21

    randomperson21 New Member

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    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!

    Attached Files:

  2. Thermopylae_480

    Thermopylae_480 New Member

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    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?
    Last edited: Jan 14, 2007
  3. randomperson21

    randomperson21 New Member

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    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.
    Last edited: Jan 14, 2007
  4. randomperson21

    randomperson21 New Member

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    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.
  5. PuMA

    PuMA New Member

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

    Carcenomy New Member

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    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.
  7. randomperson21

    randomperson21 New Member

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    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.
  8. Carcenomy

    Carcenomy New Member

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  9. randomperson21

    randomperson21 New Member

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    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.
  10. Carcenomy

    Carcenomy New Member

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    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?
  11. randomperson21

    randomperson21 New Member

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    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.
  12. ex_reven New Member

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    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
  13. randomperson21

    randomperson21 New Member

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

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    thanks for posting in the wrong thread :laugh: :p
    :roll:
  15. randomperson21

    randomperson21 New Member

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    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.
  16. randomperson21

    randomperson21 New Member

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    ha! yea. stupid firefox and its 50 tabs.


    ok ok don't blame the program, blame the user. *slaps himself
  17. ex_reven New Member

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    have firefox open new windows rather than tabs :D
    it makes porn downloading easier too :p
  18. randomperson21

    randomperson21 New Member

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    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?
  19. ex_reven New Member

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    if its a spare who cares?

    just dont be near it when it does :p
  20. randomperson21

    randomperson21 New Member

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

    PuMA New Member

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    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.
  22. randomperson21

    randomperson21 New Member

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    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:

    [​IMG]
  23. PuMA

    PuMA New Member

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    u should probly use bigger resistors, like 200-400 ohms
  24. randomperson21

    randomperson21 New Member

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

    PuMA New Member

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    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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