Finding an open on pcb

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

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

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

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

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

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

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

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