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Power Tips

Posted: Sat Aug 23, 2008 7:45 pm
by Cheater
Ok from my observations USB can power a single 8x8 board, but no more. ;)

At the moment I have two and a half boards wired up.
Ran out of LEDs so when I get more I'll complete my 16x16 square.

I've observed running off USB power that the voltage varies between 2.5v and 3v with the default text ticker running.
With more leds on, some of the chips will start failing and flicker badly.
Only some though, apparently ones at the end of the chain.
Amazingly the Arduino handles the power fluctuations easily.

Oh and dont plug a 12v wall wart in to the BBB with that many LEDs connected.
It turns red hot in about half a second and the thermal protection makes the entire display blink. :lol:

Now I just need to make a power supply capable of 5 amps.....

Re: Power Tips

Posted: Sat Aug 23, 2008 11:33 pm
by paul
Yes that's the tradeoff with the non-multiplexed displays. You get awesome optical power out, but you need an awesome power supply. Here's the math - with 1k Rext resistors the LED's are supposed to draw 20 ma each so, when you light them all up you've got 64 x 20 mA = 1.2 A per panel. Look around for a good switching power supply.

Another strategy could be to just limit how many LED's are on at one time - I don't particularly like this though - because if you make a mistake in the software and light up too many LED's (almost certain for me) then you're going to shut down the power supply, as you've found out.

Check this post in the same forum topic - there are some links to possible power supplies. Several looked like good bargains, from my memory.


Re: Power Tips

Posted: Fri Sep 26, 2008 9:01 pm
by Cheater
Ok the only way I've found to cheaply power my grid of four boards is to leech juice from my computer.

Apparently it doesnt mind the 5v rail dropping to 4v occasionally. :lol:
Need to tie in another computer's psu as well to make it more stable.

Now to figure out the daisy chaining system.... :?

Re: Power Tips

Posted: Sat Sep 27, 2008 9:15 pm
by paul
Try getting a surplus computer supply - you can find them supplying 20A at 5V for 10 or $15. Try the trash too, or a municipal computer recyling center. At least you know one part that will be recycled from the machines you recycle in reverse. A lot of computer recycling is completely dubious with etrash ending up being illegally or quasi-legally exported to Africa or Asia for completely environmentally hideous reclaiming of materials.

Many switching supplies need to be padded down (a constant current draw) to go on, so you might want to buy a 5 ohm, 10 watt power resistor to slap across the line of voltage output of the tap with the highest current listed (many have spec on the side). The resistor will get hot though so figure that into your design.

The desktop switching supplies keep getting cheaper though, so check the new prices before you decide on spending money on surplus.