7 Segment Display meets Fire - How Bad Is It?

Discussion in 'General Electronics Chat' started by CoachKalk, Jul 22, 2012.

  1. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    139
    2
    Good evening everyone. I am hoping for the best, but I will need your help with the best course of action.

    I have been using the 5" Display circuit shown below for some time now. While working on replacing a different power supply (separate from this circuit), I quickly found out I screwed up.

    When I powered everything up, my display did not come on. Hmm, that's strange I think to myself. Just as I step around the corner to have a look see, I see a puff of my first "magic smoke" and to top it off, I even get a small fire to boot!

    The V+ wire for the 20V supply had come loose and was making contact with one of the 100 ohm resistors. Houston, we have a problem! I had some extra 150 ohm resistors handy, so I replaced the fried resistor.

    I was obviously concerned about other damage, but all I could do was power it up. When I powered up, everything looked great - 3 zeroes (I was expecting 1 segment to be a bit dimmer, but I couldn't really tell.

    But, after watching the displays count, I quickly found a problem with the 2 digit. Every time a 2 is displayed, the "c" segment also lights up (should just be a,b,g,e and d. All other digits display fine.

    I am not sure the best way to attack the problem. Is it possible the 4553 or 4543 got messed up as well? Or should I replace all of the 7-segment resistors?

    So, how bad is it?


    [​IMG]
     
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  2. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    Do you have an oscilloscope or only a DVM?

    What is Q4? Is this a resistor pack?

    My guess is that U3 4543 needs to be replaced.

    You should be able to test this by probing U3 pin-11 (c output) when the number 222 is being displayed (if all you have is a voltmeter for testing).
     
    Last edited: Jul 22, 2012
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I don't have a definitive answer to your problem, however I notice that the C segment is used in all numbers 0 through 9, except for 2. So, I speculate that something is triggering the C segment for all numbers, but because all except 2 need the C segment, that is the only time the error is evident. In other words, I would tend to believe that the C segment is permanently enabled and would look to find a reason why. Hope it helps.
     
  4. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    Darn. From the title, I was expecting to see a picture of flames and a panicked geezer frantically swatting them with his hastily yanked off t-shirt :D
     
  5. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    139
    2
    The Q4 is a ULN2804. I cropped my larger circuit which had all of the designations - sorry about that. I only have a DVM. I will give your suggested test a try tomorrow for sure.

    Thanks for your help.
     
  6. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    139
    2
    Tracecom - good catch about the c segment. I didn't realize that. Duh! I should have noted which segment resistor got fried - I will have to check tomorrow - but smart money is c.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    In that case, replace Q4 ULN2804 first.

    (You are using only 7 channels of 8 on the package. You can try switching "c" to the spare channel.)
     
    Last edited: Jul 23, 2012
  8. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    139
    2
    MrChips - Nice call on the extra channel. Switched segment c to the extra channel and I am back in business. Nothing like a little destructive test to confirm that 20V will kill a ULN2804 channel! Glad I could contribute ... :eek:!

    Thanks everyone. I was fortunate it wasn't worse.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Why use 20v? It is high, and will likely damage your LED displays with excess reverse voltage as the matrixing exposes the off LEDs to -20v.
     
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