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Finding an open on pcb

Discussion in 'Electronic Basics' started by Uriah, Oct 23, 2006.

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  1. Uriah

    Uriah Guest

    I have a meter that tracks down short circuits on circuit boards but I
    was wondering if there was something that can trace a open circuit
    down. What I am trying to do is follow traces on a circuit board and
    see where they lead. The go from top to bottom and run all over the
    board. The short circuit device lets you follow the trace on the
    board. You don't have to actually touch the trace just move the tip
    around over the top of the board. It injects a AC signal and that is
    what you are following. But with an open a AC signal won't show. I
    think I am missing some simple and obvious here. But if not, is there
    a way to do this.
    Thanks
    Russ
     
  2. Chris

    Chris Guest

    Hi, Russ. Just use your DMM with audible diode/continuity checker.
    Where there's a short, you'll hear the beep. When you can't hear the
    short, it's open. The diode check doesn't apply enough voltage to the
    circuit to damage anything while it's unpowered.

    Simple and easy.

    Good luck finding the open!
    Chris
     
  3. Uriah

    Uriah Guest

    Thanks but I think I didn't state my problem clearly enough.

    What I need to do is follow node or pin on a chip around a large pcb.
    and find out where it ends and if there is an open. The problem is
    that
    the traces are so small and they go from top to bottom it takes a long
    time to track down one trace. I need to track down a hundred of these.
    The short locator has a AC probe that pulses when you are on the
    trace and when you go the wrong way it shows you that you are off
    track. Is there anything like that for an open?

    Thanks
    Russ
     
  4. But you're asking for two things.

    A short is easy, because you know something should not go to ground
    unless it's a ground because you have one common point, ground, and
    then you just have to put the other lead on the trace you are checking
    for a short.

    But if a trace is broken, there is now way anything can know that
    it's a broken trace unless it knows the circuit and can note
    the absence of a connection.

    Either you look at the schematic to check that (and if you have
    a schematic then you don't have to follow traces, just make
    sure there is continuity between points A and B like in
    the schematic.

    Or you trace out the circuit and hope that what you think is no
    connection is actually no connection and not an open circuit.

    YOu can streamline all this.

    If the issue is not tracing the circuit but troubleshooting, then
    at least you can use other clues to figure out what area is not
    working properly, which narrows down your search. Unless the board
    has seen a lot of rough handling, chances are good it's not a bad
    broken trace. (Note that shorts to ground can come from shorted
    components.)

    You can look at the board, and can impose certain guesses, and
    then figure out if the guesses are correct. Like if you see
    an IC that is common but which is not a common gate, the manufacturer's
    datasheet will likely give a sample circuit to use it in, and if
    you start with that schematic and trace the board, it's often
    easier than tracing blind. "This pin should go to that pin, and
    there should be a capacitor on that other pin" is a lot easier to
    verify than just tracing the circuit from start.

    Michael
     
  5. Jamie

    Jamie Guest

    i simply use a Low ohm meter., its great for finding board
    shorts and opens.
     
  6. Uriah

    Uriah Guest

    Thank you for all of the help. I really appreciate it. Just so I can
    be clear about this let me explain again. I am working on a mixed mode
    complex circuit board that has no paper and a lot of custom parts. I
    need a quick easy way to follow the traces on the board. The go from
    one side to the other and there can be up to fifty lines all grouped
    together. I can use my meter or a logic probe but it takes to much
    time. Is there a tool that you can use to sweep over the traces to
    follow a particular node without breaking into the trace. They make
    things like this for tracing AC wiring but I have to be careful on how
    big a signal I can inject. Lets say I start at a pin of a chip. I
    would like to quickly follow this trace and see where it goes. Using
    something like a probe that has a beep or light that gets stronger the
    closer you are to the correct path and weaker when you go the wrong
    way. Has anyone seen something that will do that?

    Thanks again
    Russ
     
  7. Chris

    Chris Guest

    Hi, Russ. Sorry, but the device you're looking for doesn't exist.

    An ohmmeter with an audible diode/continuity check works well if you're
    the board manufacturer. Just use the CAD software to do a node
    printout, and this will tell you (if it's not a power/GND node) most of
    what you need to know. Just go from point to point with the artwork in
    hand (or on screen) and find it.

    If this is a home brew etched board, do the reverse. Get your node
    list and confirm continuity on all the nodes. If a vendor sold you a
    board like this, dump the vendor.

    In nearly all cases, repair work (if the board was shipped and was
    working) assumes there is no broken trace, unless the repair tech did
    it while removing the board and trying to fix it. In which case, he's
    got a problem.

    Cheers
    Chris
     
  8. jasen

    jasen Guest

    capacitors on the circuitboard will pass an AC signal much better than the
    lacquer coating does, so the signal will migrate to other traces.

    If you want to be sure you have the right trace you're probably going to
    need to use DC, And that means you need a direct connection,

    use a tailor's pin or something else with a sharp point as a probe for and a
    low voltage continuity tester.

    I integreatd a led into the probe of one I made... this made it easiy to
    watch both the display and where I was pointing it,


    Bye.
    Jasen
     
  9. Rich Grise

    Rich Grise Guest

    Well, you could invest a few $K, or 10's of $K in a bed-of-nails tester,
    but you're still setting yourself an impossible task - you can't check
    for an open trace unless you already know where the trace is supposed
    to go to.

    Good Luck!
    Rich
     
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