LED string only partially lighting - advice?

[scolley]

Hey folks. I've got a DIY LED fixture that has been working for quite a while, and I decided to swap out some LEDs for different colors. The fixture has two series wired strings of 12 XR-Es, for 24 total. Each series is driven by a Meanwell ELN-60-48D, with the pots trimmed to run at 750 ma. I swapped out some LEDs on one string. It works fine.

Then I swapped out most of the LEDS (swapping XR-Es for XR-Es) on the other string. When I finished soldering them in, I plugged in the Meanwell, and everything lit up. So then I unplugged the meanwell, and screwed the newly soldered string onto my heatsink, plugged the Meanwell back in, and now only the 1st 5 light up. What gives?

For a test I swapped out the two Meanwells, and no change. Only 5 light up. So that's not it.

The only clue I have is that when I plugged in the Meanwell to test the string after soldering, the string lit (just a tiny, tiny bit) before I even ramped up the voltage on the dimming pair. And it still does that, except that it only effects the first 5 on the string now. The last 7 remain stone cold dark.

Any ideas what causes this? Or how to fix it?

Thanks in advance.

[Skeptic_07]

sounds like the LED that isn't lighting up is has a short on its star somewhere causing the current to bypass the LED

[scolley]

Originally Posted by Skeptic_07

sounds like the LED that isn't lighting up is has a short on its star somewhere causing the current to bypass the LED

You are RIGHT. Or REAL close anyway. It was the last LED that WAS lighting. Thank you!

While I was waiting (hoping!) for replies, I went back and started continuity testing on my start to the heat sink (which is - BTW - grounded). And on the last star that would light, the negative pole was shorted to the heat sink. I've got NO idea how. But I thanked my lucky stars I order at least one extra of everything.

I swapped the bad star mount LED out (solder). Worked great. Plus... no more tiny, tiny light before powering the dimming pair. Then I mounted it to the heat sink (where things went bad last time), and they STILL worked great! All of them.

Thanks for the quick and accurate help Skeptic_07. I was clueless. And had no idea that such a short could happen on star mount LEDs.

Thanks for the help!

[Skeptic_07]

hey no problem! i had the same thing happen to me a couple times.. glad i could help

[scolley]

Yea, I figured it must have happened to someone before. Thanks!

And it's always good to get such things documented for the community - search-able later. To that end...

The LED that was bad was only LEDs I've ever ordered from LED Supply. I usually use RapidLED. And their stars come loose. But my order from LED Supply came with all the LEDs attached on what I assume to be the fabrication assembly. So they had to be snapped off, one by one. And in the process of snapping them off, with one I heard - and to some degree felt - a slight "crackling" type sound as I did so. I'll wager anything that was the one that was bad. Only had 7 so I'd bet dollars to doughnuts that was the one that gave me trouble.

I'd be real curious how often that happens. And if that means there's a real downside to ordering from vendors that do not ship loose LEDs.

That said, they all had their type printed on the bottom. With the LEDs from RapidLED, they arrive completely unlabeled, except for the package they arrive in. Once you take them out of that package... good luck keeping up with what is what.

[scolley]

In a brief follow-up, not only did I have 1 out of my 7 CREE NW XR-E's from LED Supply go bad, I had one of them lose it's lens. It popped right off.

Wow. I've never had that happen before.

This is my first purchase from LED Supply. And it's also my first purchase of LEDs mounted on LuxDrive Indus Star's. Maybe that's the problem. But 2 out of 7 LEDs being arguably defective is NOT something that's going to attract my repeat business.

Thought people would want to know.

[RiffRaff]

I just did an LED strand and it was the same way with some not lighting up when I tested the series. But they work individually. Did you have to replace the whole star? Or could I just fix the ones that won't light up? Also they were really dim when I series tested them but I haven't checked the settings on the drivers yet.

[scolley]

The last 7 of a series of 12 would not light. The first 5 lit. Last 7 not. But it was the last of the 5 that lit - next to the 7 that wouldn't - that had the short. Once I replaced that "working" star, the whole string was fine.

[RiffRaff]

I have two in the middle of a series on one side and the last one in the series on the other side. So my problem and yours are probably different huh?

[scolley]

Well, it certainly presents differently. Is your heat sink grounded by any chance. My was. And that's what allowed me to diagnose the problem. I just took a continuity tester and touched one probe to the positive terminal on a heat sink, and touched the other to the heat sink. If it did not beep (indicating a short), I repeated that but with the negative terminal and the heat sink. Then repeated all that for every star.

Eventually that let to the problem. If your heat sink is grounded, you might try the same. And if it's not grounded, then temporarily ground it and try.

Good luck.