Current Draw in LED Mod

rawls · Post by **rawls** » Tue May 24, 2005 9:59 am

I have been fooling around with modding a psone screen with leds. I have been trying to test the current draw when different numbers of leds are hooked up, but have been unable to get a good reading. Essentially my multimeter has too large of an internal resistance to let the leds light up when it is set at a 200ma or 2A max setting. At 10A, with two leds, I get a reading of between 20 and 30 amps being used (I am running just the leds off of 6V with 100 ohms resistance). The problem is I hook up 3 leds, I get essentially the same reading. I could just bypass some of my resistance leading to the leds, but that would circumvent trying to get an accurate reading (disregarding the fact that the internal resistance of the multimeter is probably already screwing up the reading somewhat).

Really what I am aiming for is if anyone knows the true cost of adding an additional led to the psone screen mod. The specs for my leds (see below) say they should draw about 30A, so for that amount, I don't see why I don't just plop in four (as opposed to the three that Ben uses).

On the other hand, the leds I am using are extremely bright. They are Nichia NSPWF50BS and two of them in the psone screen seems good. (BTW, I had to buy these in bulk, so I have a hundred and if anyone wants to buy some cheap (say for a $ each), I'll sell them some. Another good thing is they are 5mm but they are rectangular and small enought to fit in the screen.) I am thinking, just to cover my bases, that I should put four leds in the screen but switch two so that if I don't need so much backlight I can just shut them down without de-modding the screen.

Thoughts, anyone?

nos_slived · Post by **nos_slived** » Tue May 24, 2005 5:25 pm

You have them in series or parallel?

rawls · Post by **rawls** » Wed May 25, 2005 8:55 am

In parallel.

I figured out why the current draw is the same. Two leds in parallel will see the same number of volts, just half the current (each) as compared to a situation where only one led is hooked up. What fixes the current is the resistor.

This leads me to believe that I may want a bit more current running across my diodes so I may need to drop the resistor value. My only question is what "burns out" an led? Is it too high a current, or too high a voltage (or both). On the spec sheets for my leds I have the max ratings for these so I may fool around with my circuit to see if I can approach the max ratings (without getting too close).

One other thing. I don't have Ben's book, but from reading this site it seems that he uses 3 leds in his psone screen hack. Why not 4? Seems to be just as easy, and from what little I have seen so far it doesn't seem to affect the current draw too much.

nos_slived · Post by **nos_slived** » Wed May 25, 2005 5:13 pm

Too much voltage kills LEDs. The LEDs only draw the amount of current that they need, so unless you somehow force too much current in, it is fine.

I think he used 3 LEDs because the CCFL was L-shaped, so he had one for every corner where the CCFL was.