LPD8806 - "injecting" power midstream

[MattM]

Hi, I have a project that uses a whole mess 'o LPD8806 LED strips -- probably 20-30 meters. I have the strips distributed in a pretty organized fashion from the controller & power supply, and the setup is working mostly pretty well. Since each strip has its own connection back to the controller & power supply (I guess that'd make it a "star" architecture), the brightness of one lane has no effect on the brightness of the others. I've got about 900W of power supplies, balanced in such a way that the maximum draw that would happen if the strips were run wide open will not cause any problems.

So far so good.

Here's the problem. A couple of the longer runs are dimmer than I'd like -- not gradually dimmer toward the end of the run, but just dimmer overall. Since this particular strip is about 25 feet of cable away from the power supply, and is a very long run (about 140 pixels; at 32leds/m, figure about 4 meters worth), I certainly understand why.

There are all kinds of mentions all over Adafruit, both in the tutorial and in these forums, about splitting off power and applying it again every meter. Questions:

1. Is there a schematic for how to do this? In particular, I would assume that the grounds need to be tied together in some manner, but I'm not clear exactly how. I haven't seen a diagram anywhere showing what leads need to be tied together in order to accomplish this. Since these strips are very expensive, I'm nervous about frying them trying to get them to be a little brighter than they currently are.

2. Can you use a separate power supply to apply power farther down the line? I've seen some tutorials on the web where the instructor applies one set of the power supply's leads directly to a strip, and then connects a second set of leads to some lamp wire, which are in turn used to power the second strip in the chain further down the line. In my case, I do not have the ability to run another wire from the power supply to where the strips live, but I DO have AC power available near where the strips live and could place a second power supply on the strip side of things in order to boost the power getting through these strings. But the whole idea of having two separate grounds and +5Vs concerns me because it seems like there's a really good opportunity for a huge short circuit (or a really long fried strip) if I don't tie these in correctly.

Thanks for the help!
Matt

[adafruit_support_bill]

You can apply power at any point. The 5v and ground conductors run the full length of the strip.

If you are powering from multiple supplies, tie all the grounds together but cut the 5v connections between the separately powered sections.

[MattM]

Hi Bill -- thanks for the reply. I haven't been working on this for a couple of months, so I apologize for the late follow up to my earlier post.

I have been experimenting with injecting additional 5V power into my LPD8806 strips, with unexpected results.

First of all, the "before" scenario... in my setup, as I mentioned, a couple of the particularly long stretches of pixels aren't lighting correctly. I have read elsewhere that when there isn't enough voltage being delivered to one of these strips, you'll see the color drop off towards the end of the run. In my case, that's not what's happening at all -- the entire run of about 20 feet is off. If I program RGB white (255R, 255G, 255B) for the whole strip, I get a very dim brownish red through the entire strip. I metered the voltage on the strip, and it's only reading 2.6V, so it's very clear what the problem is.

So as I asked below, I took the suggestion to try injecting some additional power down the strip. In your message below, you mention that if I'm powering from two different power supplies (rather than just feeding another line from the same power supply to the opposite end of the strip, for example), I should cut the +5V connection running between the two segments of the strip, such that the data, ground, and clock feeds are shared between the two segments, but the first segment gets +5V from one power supply, and the second segment gets +5V from the other.

When I read that, I assumed it had something to do with what would happen if you ran the power supplies in series -- rather than delivering 5V to the strip, the combined power supplies would be delivering 10V, which would destroy the strips.

Here's the thing, though...I tested the above scenario on a short, expendable piece of strip to see what would happen: I provided power, data and clock from the front end of the strip, and then added an ADDITIONAL +5V AND ground from a second power source. The strip immediately got a lot brighter, which was expected, although I wondered if the strip would wink out from being over-driven.

Surprise, though...when I metered the strip, though, it still showed +5V -- exactly what I'm shooting for. The strip seemed to be doing exactly what it was supposed -- the colors REALLY popped and it seemed to be responding to its color information exactly as I'd programmed.

The only thing I noticed was that my LED controller (a SANDevices E682) is reporting that there's pixel power present, even with the power supply feeding the controller cut off. So the 5V is leaking backwards to the controller, but it doesn't seem to care about that.

So, the question:

in your message below, you mention cutting the +5v connection between strips when powering from two different power supplies. From my testing, at least, it does *not* appear that having two power supplies are resulting in an over-volt condition. I do notice that power is leaking back into the controller, such that the LED controller is reporting the presence of +5V even when the main power supply is disconnected, but is that really a problem? What is the concern we're trying to avoid by cutting the +5V connection between strips?

I have tried running a second run from my main power supply down to the other end of the strip (which, presumably, doesn't require cutting any connections between the strips). It DOES work, but the strip only meters +3.4V and isn't as bright, obviously, as it would be at +5V. So I hope you see my dilemma -- I'm giving up a pretty significant amount of brightness by not using a secondary power supply. If I'm going for brightness (and I am), I'd like to do what's going to get the maximum bang for the buck, but I can't tell if there's another consideration that I'm missing.

Thanks for your help!
Matt

[adafruit_support_bill]

The reason you want to cut the 5v line between 2 sections that are powered by different supplies is that it is generally not a good idea to connect power supplies in parallel.

Due to natural variability, one may be trying to regulate the output at 5.01v while the other is trying to regulate at 4.99. In that case, the higher output one will be taking on more than its share of the load.

[MattM]

Thanks Bill.

To explore this a little further -- if one power supply is regulating at 5.01V, and the other is at 4.99V, I understand that that means one supply is taking on more of the load than the other.

But why does that matter?

As this branch of my overall system currently stands, I have a 600W rated power supply driving pixels that collectively draw 580 watts. The main power supply (the one feeding the LED controller at the front end of the system), in other words, is rated to handle the whole load by itself. The issue isn't that the one power supply can't deliver enough watts -- the problem stems from the length of the cabling feeding this run and the voltage drop being caused by the length of the pixel string itself.

If I add a second power supply, am I not simply taking some of the load off of the primary supply? Is the 5.01V vs. 4.99V putting some sort of stress on the power supply's regulator circuitry?

Is there another factor in play that makes it not a good idea to run these supplies in parallel?

Thanks for indulging me. I understand the "how" now, just trying to figure out the "why."

Matt

[adafruit_support_bill]

Your original supply was not able to power the whole strip. So what if that is the one that is trying to regulate to 5.01v? Your new 600W supply will be coasting along comfortably while the old one will still be running at max output trying to push that last 0.01v out there.

In this case, the resistance of the strand will tend to limit the effect of any mismatch. But many things that 'work' are still not recommended practice.

[MattM]

Ah, got it. So in the one-power-supply scenario, because of the voltage drop, the power supply sees 2.4V coming back to it and says "I'd better turn up the voltage" in an attempt to get the voltage back up to the regulated 5.01V. It doesn't get there but the needle is nevertheless pinned all the way to "max voltage" in order to try.

In a two-power-supply scenario, were they run in parallel, since the two supplies may not exactly agree on the regulated voltage they're striving for, one supply is still going to have the "needle pinned all the way to the max" all the time, even though the difference between the two supplies is only 0.02V?

[adafruit_support_bill]

That's the basic idea. Neither supply knows that the other one is out there. They just do whatever they can to keep the voltage at the level they think it should be.