Finding an open on pcb

[Uriah]

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

 

[Chris]

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

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

 

[Uriah]

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

 

[Michael Black]

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?

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

 

[Jamie]

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

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

 

[Uriah]

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

 

[Chris]

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

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

 

[jasen]

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?

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

 

[Rich Grise]

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?

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