Bad LEDs?

by Beez65 on Sat Apr 09, 2011 4:58 pm

I have assembled my 3x blue kit, and I have what appear to be several bad LEDs on two of the three boards.
First off, my soldering technique is pretty good as I've been doing it professionally and as a hobby for over 30 years. :D
Not that I've never made a mistake but I don't think that's it here. :oops: The leds in question are in "forwards" and had the long leg in the correct hole.

I put a DMM across one of the LEDS that does work and I get the same results as one that does not. About 2.9VDC when on. Any suggestions to debug? I'm thinking about pulling out one of the "bad" leds and replacing it with a known good one but I know there's always a chance to pull the plating out of the hole doing that. Suggestions of what to try first? :?:

As a suggestion for debugging, it might be helpful to have a "lamp test" mode, hold the button for shutdown and ALL-ON for one sec followed by ALL-OFF for one sec, repeat. Release the button and it shuts down.

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Re: Bad LEDs?

by Beez65 on Sat Apr 09, 2011 9:04 pm

More Information:
I just completed the avrTinyISP and that works fine. I have downloaded and uploaded images in all four banks on all three spoke pov boards. Also, they do the bootup LED dance. It's hard to believe that out of 120 LEDs I got about 5% failures?
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Re: Bad LEDs?

by Agent24 on Sun Apr 10, 2011 2:25 am

If you don't want to try desoldering first you could probably try to tack a known working LED on the backside of the board on the pads of an LED that doesn't work, see what happens.
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Re: Bad LEDs?

by adafruit_support_bill on Sun Apr 10, 2011 4:58 am

If you are reading 2.9v across the leads, tacking a known good one on the back as suggested would be an easy test.
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Re: Bad LEDs?

by Beez65 on Mon Apr 11, 2011 12:41 am

Well, as Alice would say, "curiouser and curiouser".

I took the board that has the most "bad" leds on it. Six, two on one side and four on the other.
I took a known good blue led from the baggie of leftovers, tested with a laboratory power supply.
I tacked it on to each bad led in turn. Four of six times not only did the tacked on led light, but the suspect one lit too! :shock:

Just to keep things more interesting, two out of the six bad leds did not light, but the tacked one did. :?

What the heck am I looking at here? Flakey solder joints that fix when I tack on a led and then fail again when I take it off? I tried reheating and adding a little more solder. Fail. Flakey outputs on the shift register chips or insufficient output loading? :?:

Thanks for any and all suggestions.
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Re: Bad LEDs?

by adafruit_support_bill on Mon Apr 11, 2011 4:53 am

Post some photos of the front & back of the board and we'll have a look.
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Re: Bad LEDs?

by Beez65 on Wed Apr 13, 2011 9:28 pm

Looks like I also neglected to mention that when I tacked on the LED, most of the time it fixed the on board LED. When I removed the tacked on LED, ALL of the time the on board LED failed again.

Here are some pix.

Image

Image

Image

Image
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Re: Bad LEDs?

by Agent24 on Wed Apr 13, 2011 9:42 pm

That really is odd.

Your previous explanation of "Flakey outputs on the shift register chips or insufficient output loading?" does seem to fit with what you observed, although not knowing much about it I can't say if that actually makes sense or not.

Have you tried replacing the ICs and seeing what happens then? - I suppose that may be rather hard though as they aren't socketed and are PTH....
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Re: Bad LEDs?

by adafruit_support_bill on Thu Apr 14, 2011 5:24 am

What are you using for batteries? 2.9v sounds marginal for a blue LED. I have seen some with forward voltages as low as 2.8v, but the ones in the store are rated at 3.2-3.8. I'm assuming they are shipping the same ones for the SpokePOV kits.
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Re: Bad LEDs?

by Beez65 on Thu Apr 14, 2011 4:59 pm

Batteries are rechargeable NiMH. New and freshly charged.
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Re: Bad LEDs?

by Agent24 on Thu Apr 14, 2011 5:04 pm

Maybe that's the problem, NiMH will be 1.2v rather than 1.5v for primary cells...
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Re: Bad LEDs?

by adafruit_support_bill on Thu Apr 14, 2011 5:09 pm

Can you try it with 3 alkalines?
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Re: Bad LEDs?

by Beez65 on Sat Apr 16, 2011 11:15 am

Fixed!
I decided to bite the bullet and remove and replace the LEDs, and each one I replaced, I tested it and now they are all good! :mrgreen:

Thanks, Limor, for including 10% spares because I needed eight of the nine! :wink:

I'm going to test the removed ones on the lab supply but about half of them died on the operating table, with one of the leads getting pulled out of the plastic case.

Thanks for the help!
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Re: Bad LEDs?

by thejukeboxer21 on Sun Jan 08, 2012 12:54 am

I seem to have had a similar problem with my Blue LED POV kit. I completed it, and then found that a number of LEDs didn't come on, maybe about 8 or 9 at first, but tried re-flowing some of the LED and related solder connections and found that a couple lit back up. In the end though, I found that 5 or 6 didn't light up. I did a few multimeter tests and found that the "bad" LEDs had different resistance measurements(back through the circuit) than the working ones. Anyway, I had two extra LEDs from my two kits and decided to take some out and put these in. Voila! They work! However, I still need a few more to replace some of the bad LEDs. Can you send some new ones my way? I need maybe another 4/5. Thanks!

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Re: Bad LEDs?

by adafruit on Mon Jan 09, 2012 4:53 pm

surely, email support@adafruit.com and ask for 10 extra 5mm Blue LEDs for yer 'POV
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