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Thread: 24 pin chip question & additional notes from troubleshoo

  1. #1
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    Default 24 pin chip question & additional notes from troubleshoo

    I thought this was interesting. While troubleshooting my issue that is now marked 'RESOLVED' I found that each time I started sending data out (the clock Signal running) without any have any pixels light, a column was lighting. I was working on this column before noticing this so I assumed I shorted something out.

    I started by unplugging the cathode for that column and surrounding columns. It still continued to light. I check everything around it for a short, didn't see to be a the case. I am puzzled as to how with only the anode plugged into the control board it was able to light up the column. Not all the leds in the column were lit.

    I began to fix this (with the cathode still unplugged) un soldering LEDs until the row stopped lighting when power and single was on. In the end I replaced 5 LEDs in the column and had no further problems.

    My guess is the LEDs had an internal short. I read something about the MBI / Allergro chips having a feature to identify shorts but am not sure how that works. I am still puzzled how it was able to light with only the anode hooked up.

    ms
    Matt Stemple

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    If the cathode of a given LED was not connected or touching anything and the led was lighting you are a pioneer and I want to shake your hand. :D

    Are you sure you didn't have the cathode feed disconnected and the led was still connected to the others? If so there is a way this can happen. All diodes leak... I will not go into techincals and theory here but suffice it to say that with a number of leds still attached to the the cathode of the led in question you can have enough leakage and the voltage drop you will have to create a voltage differental across the led to cause it to glow. Is this what you had? Electricians in house wiring use to call this backfeed. If the cathode leg was sticking out into clean air and doing this man your on to something or you need to move away from your home NOW.

  3. #3
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    Was the LED cathode totally disconnected, or was it attached to the cathodes of the other LEDs in the same column? In second case it is possible that a shorted (or reversed) LED would cause other columns to light up.

    --

    Phil

  4. #4
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    It was traveling backwards and the LEDs in question are dead now, so the LEDs failed in the circuit while I was troubleshooting something else most likely.

    When I said unplugged the cathode, I meant unplugged the cathode bus column from the board, not unplugged the LEDs from each other. All cathodes for the column remained hooked together while their anodes were hooked to the driver board. Also they were only emitting 1/2 as much light during this issue. So based on the leaking description that sounds like the case.

    As I removed the suspected LEDs from the cathode bus column (the ones not lighting) the problem was resolved. Each of the removed LEDs were hosed.

    There was no magic thin air energy transfer going on in thin air here... With the cathodes all hooked together there was a good length of bus wire to conduct in. This makes more sense now.

    ms
    Matt Stemple

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