Failed LED

Dead LED
Dead LED

Doesn’t look like much, does it? It’s an ordinary blue LED that I used for the upper colon dot in a clock. Worked fine for a few dozen power-on hours, then it turned off a bit after 6:00 pm one day. Back on an hour later, more or less, then off again by the next morning, back on again, off again.

Might be a software error, as each colon LED is a separate TLC5916 display driver output. Might be a soldering problem, as my board doesn’t have plated-through holes. Might be (shudder) a burned-out transistor inside the TLC5916.

When it’s off, VCC appears on both sides, within a few tens of millivolts.

Resoldered the joints, after which it worked for a while. When it’s on, voltage measurements look normal: about 3.5 V drop across the diode and 1.5 V across the driver transistor.

No obvious code problems, but, then, code problems are never obvious.

Finally the thing stopped working for a few hours. I unsoldered it and there’s no continuity: it failed open. Peering deeply inside with a microscope shows nothing unusual: the flying gold wires look OK, the bonds look flat, and the chip has no burn marks.

Just a bad LED, I suppose. It’s surplus, of course, but that doesn’t mean much these days; there’s a lot of surplus going around.

Soldered in a replacement from the same batch and it’s all good.

So far, anyway.

2 thoughts on “Failed LED

  1. When we’re testing our LED drivers we always end up using actual LED’s rather than some sort of load, because we have no idea how the LED’s are going to fail, and if we don’t design the drivers to handle everything, well, bad news. Sometimes they just open. Sometimes they fail short and stay that way (mostly this happens with the monster ones that can handle a couple dozen amps: they don’t have bond wires per se.) Sometimes they spend a while alternating opening and acting normally, sometimes they alternate open and short. No rhyme or reason, even (for the most part) within a brand, although they usually end up open after enough abuse.

    1. sometimes they alternate open and short

      Thanks for that data point. All I’ve ever seen are failed-open LEDs; now I know enough to be worried.

      the monster ones that can handle a couple dozen amps

      Zowie! That’s well beyond my simple needs… but it’d make a killer bike headlight if I could tote the battery.

      Speaking of bright, I had to crank the TLC5916 drivers in the clock I’m building down to a max of 5 mA: all those glowing blue segments lit up the entire house! No point in wasting all that power behind neutral-density filters.

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