Delivering 640W to 2048 RGB LEDs on 5V

[Trevelyan]

Hi,

Despite my Computer Systems Engineering degree, I haven't played with electronics for over a decade, and I'd like to get back into it.

I have a project idea, and I think I've got all the design leg work done. However before I commit money to this project, I would like a second opinion. My assumption is that I'm completely out of touch and no longer know what I am doing, so please take a look.

I believe the digital parts of the project to mostly be a none issue. My biggest problem is powering 2048 LEDs that require that I deliver 640W over 5V, so a 128A current draw.

I've written up what I intend to do (and how I came upon the Idea) here: https://sites.google.com/site/trevelyansmisc/lightwall

In short, and in terms of power supply: I intend to have 512 LEDs, each drawing max. 60mA in parallel. Thus a total of 32A @ 5V. I will then connect 4 of these these in series. Thus 32A @ 20V achieving my 640W supply for 2048 LEDs. Thus avoiding DC-DC converters and yet more heat.

I plan to deal with connecting the logic parts of the circuit to the floating grounds using some form of Digital Isolator IC.

I need to ensure that each of the floating 5V power rails stay at 5V. Remember each LED has its own PWM and so current draw will fluctuate very rapidly. After some searching I landed on the idea of using a Zener diode to provide the base current to a power transistors (http://sound.westhost.com/appnotes/an007.htm). I need to look at possible transistors. I think I will probably pepper more then one across each rail to keep current and heat (in any one area) down.

Some capacitors would probably also help.

Is this a reasonable solution? or is there a better way?
Should I arrange the circuit differently somehow? e.g. dividing through 40v (8x 256 LED strips) to keep the current down even further? Can I get 40V from a ATX PSU or will I have to use some other supply?

Please help.

[k-ww]

I think that running tri-color leds that have a common catode or common anode in the fashion you propose will be more trouble than it's worth - it will be very difficult [ I think imposible] to keep each section of your circuit at 5V in the manner you propose.

If the LEDs had seperate anode and cathode connections for each color, you could easily connect ones of each color in series [3,4 or whatever can be driven by the voltage/current rating of the driver chips.

If you have to use common anode or cathode LEDs, I would use multiple 5V supplies stacked in series for the circuit you envision, or better yet, have all the circuits have a common ground and split 5V supplies - that would eliminate the need for the voltage isolator circuit[s].

I also doubt that you can modify an ATX supply in the way you propose, [for cheapness, the filter caps will be rated at very close to the output voltage[s] the ATX normally generates.]

[Trevelyan]

Thanks for the feedback!

The LEDs are on prefabricated flexible PCB strips and are controlled by LPD8806 chips that you feed a serial data line and clock. (They are shift registers with PWMs so you can chain them)

They are mass produced so cheaper then DYI, all I can change is the power-in, clock-in and data-in connection to the strip.
The chip gives 7bit 'brightness' control per colour, so ~3bytes per LED. See here: http://adafruit.com/products/306

I could connect the ground of various strip segments together and then have multiple PSU providing the 5V, but I am worried about the footprint that will leave in my living room, especially if I build multiples of these to cover the wall. In all cases I will need to get the Girlfriends approval for living room installation....

As to PSU; there are few guides, even youtubes videos, on altering ATX PSUs for higher voltages. Yes, in all likelihood I will have to change the end stage capacitors for ones with a higher voltage rating, probably still easier then building my own 600W PSU or cheaper then buying a special purpose PSU.

I updated my site to try and be a bit more clear and waffle a lot less: https://sites.google.com/site/trevelyansmisc/lightwall
All the moydering about the hows and whys are now in a new Journal section.

[k-ww]

1) Look at the SI8441 and associated parts - a lot cheaper than the ADUM isolators.

2) Your 5V shunt regulators when stacked, should be set to more than 5V - if they are not, and the power supply goes above 30V [6 times 5V], the regulators will load down the power supply. Consider what happens if they were a perfect 30V zener, and the supply voltage goes above 30V.
I suspect that if you run the setup the way you envision that it will always try to pull the 32A, regardless of the current your LED wall will pull. I hope you will have good heatsinks on them. I still think that multiple 5V supplies with a common ground is a better way to go. [running a power supply at maximum load shortens it's life also.]

[dkedr]

I'd say you should use your PSU as is and just step down the power for the strips from 12V. There are loads of cheap DC-DC converters on ebay capable of 3A. The advantage of this solution is you get 12V going down longer wires at a lower current so you will have smaller resistive losses, also you won't need to modify your PSU and you won't have to try and stack those LED strips in series, which in my opinion would lead to disaster. Also your PSU will have a 5V output which you can also use to power the LED strips, but I'd recommend using it to power those that are closer to the PSU to reduce the resistive losses in the cables.

Ebay also has a lot of step up regulators floating around, these are more expensive than the step down, but still fairly cheap, and using them will allow you to utilize the 3.3V rail of the PSU as well.

The added advantage of using multiple switching power supplies is that if one of them dies, you only have 1 to replace.

[st2000]

From your web page:

I want to have dynamic wall colours and lighting, according to what is going on in the room.

You realize ( or maybe I'm the fist to tell ) most commercial light panels are specially made laminated plastics which only have LED around the edges. You have seen some. You just may have not noticed.

I have those Adafruit LEDs. And they are bright enough to light up such a panel. The only problem is that the strip is too wide for the types of panels normally used. Such an arrangement will drastically cut down on the number of LEDs used. And, perhaps, solve your power problem serendipitously.

Also note, the current draw from those Adafruit LED strips depends on how many and how bright the LEDs are programmed to be lit. Light 1, 2 or 10 of one color per meter and you can get away with battery operation. Light all of them full on for a "white" color effect and you better be prepared to handle the current.

What ever you do, you will probably be better off distributing your regulators. A heavy current draw over a small resistance can impact low voltages significantly. That is, trying to protect your self from a 1 volt drop at 12 volts is simple. After all, it's less than a 10% fluctuation. But at 5 volts, that 1 volt drop would be a 20% fluctuation! This is one of the reasons most design with at least a 12 volt relay instead of a 5 volt relay.

So, if you use, say, a 12 volt supply, you could use 5 volt buck-converters for, say, every meter of LEDs. A DC-DC converter may be a buck. But some are buck & boost. You don't need or want to pay for the boost part if you only need to reduce the voltage. And, yes, you can go w/an analog regulator (i.e. 7805). But keep in mind each one will waste way way more energy then a buck power supply. (i.e. You'll end up having to buy a bigger power supply just for the 7805s let alone the LEDs.)

-good luck

[Willaim]

640 watts at 5v is 128 amps not even including losses
thats outrageous!!

the two best suggestions i saw were 1. using less LEDs and 2 using a higher voltage

higher than 12 volts too..
640w/12v=53.3 Amps still way too high
5.3A @120 volts
perhaps you could hook them up in groups of series and parallel christmas light esq??