Hello, I have modded a 300 watt atx psu to give me 12 volts for about 30-ish feet of led light strips. The exact product is found here:

http://www.amazon.com/SUPERNIGHT-No...rds=5050+non+waterproof+300leds+rgb+led+strip

I of course also bought a couple more rolls.

The 5 amp psu that came with it, is not supposed to be enough to power 30+ feet. I knew this but went ahead and used it until it blew up. At this point, I realized I had to buy another psu. But I had some old computers laying around, so I decided to mod the psu from one of the computers. Pretty much I got rid of all of the wires, other than ground, 12v and the green wire to power it on. I connected ground to the green wire to power it on and then connected the 12v plus ground to the led control box. The problem is, the led lights are super bright close to the power supply, and get dim towards the end of the line. Also the colors kinda distort towards the end. I set my voltmeter to 10 amps and measured the draw of the leds. It gave me a reading of 2.9 amps.

What is the issue here? I'm still fairly new to electronics, everything I know is mainly from trial and error as well as learning some simple breadboard circuits.

Let me know what else you need.

 

Measure the voltage drop of some LEDs near the end and compare it to the voltage drop of some LEDs at the start.

 

It starts at about 9.3, and drops 2 volts to 7.3. This is when testing the blue rail, when the color is set to white. On white, it is of course powering 3 rails, red, green, and blue.

And actually, testing it when it is on a single color, i,e red, it yields the same results.

So what does this tell us?

 

You should have a 10 watt, 10 ohm resistor between a 5 volt (red) wire and ground (black). Atx power supplies need about 0.5 amp load on the 5 volt line to stabilize voltage. Measure, but your 12 volt line is likely running at 9 or 10 volts.

If that doesn't help (or if it is not a problem), then the wires on these cheap LEDs are too small for your 3 amp load and the wire has too much resistance for the LEDs far from the power supply.