Driving LED's

[randomperson21]

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!

 

[W1zzard]

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?

 

[W1zzard]

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

 

[randomperson21]

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!

 

[randomperson21]

Would something like this work?

 

[PuMA]

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.

 

[randomperson21]

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?

 

[PuMA]

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.

 

[randomperson21]

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?

 

[PuMA]

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.

 

[randomperson21]

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.

 

[PuMA]

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

 

[W1zzard]

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

 

[randomperson21]

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.

 

[randomperson21]

OH MY GOODNESS IT WORKS!

 

[PuMA]

nice1 dude

did u solve the heat problem???

more pics! I wanna see what those leds look like

 

[randomperson21]

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.

 

[PuMA]

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

 

[randomperson21]

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

 

[PuMA]

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