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Quick question RE: reverse polarity protection

Discussion in 'General Electronics Discussion' started by rogers, Apr 3, 2011.

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  1. rogers


    Apr 3, 2011
    Hi all,

    An LCD monitor I own has a 4pin DIN 24v power input (external power brick).

    Long story short, I plugged the 4 pin plug in the wrong way around for a second until I realised it wouldn't fit. However, a small spark came from the plug and one of the pins got a bit burned.

    The monitor works fine, but I would like to know more about what happened.

    Why does it still work (I'd have expected blown caps, damaged IC's and so on)?
    Why did one of the power pins on the plug burn?
    Even though it works fine, will any damage have been done to the monitor?

    I would greatly appreciate any answers, I don't know much about electronics but would love to learn more.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There are many reasons, some or all of which may apply:

    1) The power supply has an overcurrent shutdown which killed the power as soon as it detected an overload.

    2) The input to the monitor has diodes to short the power supply pins to each if reverse voltage is applied.

    3) The pins of the connector are arranged so that if you put the plug in the wrong way that one or more power supply rails are shorted to ground.

    4) The regulator section of the LCD monitor has been designed to handle relatively large voltage overloads

    5) the regulator section of the LCS monitor has crowbar protection to prevent damage if excessive voltage is applied.

    6) You are extremely lucky

    7) There is damage, but you haven't noticed it yet.

    The burn marl on the pin is due to a small amount of vaporised and oxidised metal that was blasted from part of the connector when excessive current caused that spark you saw. That metal would have partially redeposited itself as a (typically) dark mark.

    See point 7. There may be some damage. Depending on how the monitor is designed, the reverse polarity protection (if indeed that was where the current went) may not be rated to be hit more than a couple of times (indeed it may rely on parts surviving gross overloads, and eventually failing sacrificially). If other parts have been damaged, they may be out of spec now, or may age faster and eventually fail prematurely.

    In general though, I would refer to point (6) above. It doesn't always end this well, it is entirely possible to destroy a large number of hard to replace parts.

    You should be thinking good thoughts about the designer of this piece of equipment because it is quite likely that he (or she) foresaw your error.
  3. rogers


    Apr 3, 2011
    Thanks for your reply.

    If there were no protection methods in place then the monitor would not be working now?

    I guess I am just looking for piece of mind that it won't break in a few months or so!
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If there were no protection then you would most likely have seen and smelt some magic smoke, and there is a very good chance that it would no longer have worked.

    I can't promise that it will continue to work for the next several months, but if it is working now, I would expect that the lifetime of this unit would not be significantly different than any other.

    If you were sending it into space, or if it were part of some life support equipment, I might give you a different answer. I would only give you a different answer on the basis that in these cases you *really* want to be sure.
    Last edited: Apr 3, 2011
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