About DC circuit - a digital input to the PLC

Discussion in 'General Electronics Chat' started by Anatoly_Sml, May 14, 2015.

  1. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    Hello,
    Recently I had a weird issue - 5A fuse burned a few times for 24VDC circuit protection. It 's a digital input to the PLC, that normally current there about 0.5A .
    When searching, the only trouble I found was a very oxidized terminal at some point , but no shortened lines, ground faults etc.
    Do you think it may create any "rush current" at DC circuit to burn fuse ?
    Thank you !
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    sounds like moisture was present at sometime in the past clean the corrosion and look for signs of overheating on the board. normal oxidizing of copper and tin doesnt result in furry corrosion build up.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Oxidation = increase resistance and even could be conductive causing a short.
    Typically the time curve for a fuse will allow it to sustain "inrush" without blowing.. But I wouldn't suspect inrush on a digital input..
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Normally it is hard to over-current a PLC P.S. when it is used purely for inputs as they are current limited through an Opto etc.
    Are you using it also for outputs by any chance?
    Is it only blowing on the activation of one particular input maybe?
    Max.
     
  5. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    Well, the fuse burned every time when one particular input was activated .
    I agree there wasn't condition for the "in rush " current, but after all, after I just "refreshed" the termination, the problem gone ... Unless it's just coincidence .
     
  6. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    Well, we don't have any moisture there ; I understand resistance could be increased, but really don't see conditions for the short !
     
  7. MaxHeadRoom

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    It could have tracked to ground possibly?
    Max.
     
  8. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    We
    ll, I really don't see condition for that; terminals are dry, clean , wires were oxidized ( fairly black , it's actually very common for the plant by a some reason ).
     
  9. MaxHeadRoom

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    If there is a cause of oxidization present, there is no knowing how much got inside the unit possibly?
    Max.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Then you must examine everything in that circuit and whatever output is doing also assuming its on the same supply.
    Are you willing to post the entire circuit diagram? if not only so much help we can offer.
    But seeing that you can recreate it 100% of the time with 1 specific input you need to examine everything thats happening/connecting when that input is triggered.
    Even disconnecting parts of the circuit to see if you can narrow it down even further to a specific sub-circuit.
     
  11. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    Well, I did check pretty much everything related , and all finding is the badly oxidized wires by the terminals.
    I believe it may created some kind of sparking there, and the question is, could it create a spike that burned a fuse ?

    There is nothing the circuit diagram may help : fused line ( 2 wires #14 ) from relay output card ( just dry contact ) to the DC sink input card provides interlock between 2 pcs of equipment.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So it sounds as though you Are using the supply for the outputs, this means that there can be outboard loads unpredictable or beyond your control and it can be a fault on the load itself.
    What PLC is it? Many just provide a 24vdc supply for the inputs alone.
    Also It almost sounds as though you have an output tied to an input, which would not make sense?
    Max.
     
  13. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    " Also It almost sounds as though you have an output tied to an input, which would not make sense? "

    In this case, I have input that provides interlock with another machine. Digital signal for the input provided by the dry contact from the output card of the 2nd machine , pretty straight forward .
    There is Allen-Bradley MicroLogix, DC input slots have independent power supply.

    After I stripped the ends of the wires, no problem so far.
    Would you believe the bad contact can create over-current in DC circuit ?
     
  14. MaxHeadRoom

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    I misunderstood, I took it the input and output were on the same PLC unit.
    I cannot really see how a corroded contact can cause O.C. unless there was leakage over to a ground or common?.
    Max.
     
  15. Anatoly_Sml

    Thread Starter New Member

    May 14, 2015
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    I understand .
    Thank you very much !
    Anatoly S.
     
  16. RichardO

    Well-Known Member

    May 4, 2013
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    Just some random thoughts. Be warned that I have no real-world experience with these kind of failures...

    Have any changes been made to the wiring recently? Could a wire be frayed or shorted in a cable? Could a wire have become shorted to a (grounded) chassis or case?
     
  17. Anatoly_Sml

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    May 14, 2015
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  18. MaxHeadRoom

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    Unless there is an internal fault, you cannot normally 'over current' a PLC input, the input when ON activated is connected directly across the supply and the current limited internally by the input circuit which normally consists of an opto isolator.
    Max.
     
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