LPD8806 Strip Power Requirements?

[mhenstell]

Hello,
I am about to embark on a rather large project involving these strips, with about 120 meters. At 2A max per meter, thats 240A at 5V. Not undoable, but expensive. But I'm totally stumped here: I have a 5m length of this strip already, which pulls about 1.5A at full brightness. Obviously it would be nice if I needed a much smaller amount of power, but I don't understand how this strip could only be pulling 300ma/meter at full brightness, despite being rated for about 10W/meter. It's blindingly bright. Any ideas? Thanks!

[pburgess]

The resistance of the copper layer of the flex strip limits the current to a degree...if you were to add power taps every meter, you'd see the current and brightness both increase.

1.5A does seem low though. I've seen some strips that 'interlace' the even/odd LEDs very quickly, which might explain the lower-than-expected current...is the reel you have from Adafruit or another source?

[mhenstell]

This is the 5m Adafruit strip. We just did the math and it does indeed come out to about 1.6A for the whole 5 meter length, which doesn't seem to add up to what the strips are rated for (~8 watts vs 50). Is this inconsistant with your measurements?

[pburgess]

Yeah, I'm reading about 1.4A for a 1-meter strip, and 2.7A for 5m. What's your power supply and measurement setup look like? Are you doing any shenanigans like powering the strip from both ends?

[mhenstell]

How odd. I am powering the strip at one end from a regulated 5V 4A power supply. Putting a multimeter between in line with the positive output gives me about 1.6A at full for the whole 5m length. Also tried running from a digital bench supply to be sure voltage was a constant 5V, and even with amps cranked it was still pulling less than 2A.

Without taking strip resistance into account, the math based on the schematic on the adafruit tutorial says that this strip can only pull 1.6A.

I will try and confirm again this evening in case I was doing something screwy late at night, but even so, I'm curious why the strips are rated so far above what they actually pull.