Author Topic: Short Finding on a PCB  (Read 1668 times)

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Online paul@yahrprobert.com

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Re: Short Finding on a PCB
« Reply #25 on: September 23, 2021, 08:15:20 pm »
For years I've been using the following method:
  1) Connect a signal generator to your short and pump a hundred ma or so into it with a sinewave about 500 Hz.
  2) Make a small pickup coil with fine wire and a hundred turns or so.  When I get back home in October I can take a picture of my coil, it has a ferrite core about 5 mm OD with a slot cut in it, and glued onto the end of a plastic rod.  Another source of good coils are old relays.  Just crack them apart and take out the coil and core.
  3) Make an audio amplifier with a lot of gain, maybe 5000 or so.  When I get home I'll dig up the circuit diagram.  3 transistors and misc parts.
  4) Then connect some headphones and start tracing.  Its amazing how you can narrow down where the current is flowing.  Also good for finding ground loops.
 
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Offline Shock

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Re: Short Finding on a PCB
« Reply #26 on: September 24, 2021, 03:55:22 am »
These are probably adequate for measuring milliohms but not very practical for finding shorts on a PCB, because what you want is the audio feedback telling you if you are near/at the short without taking your eyes off the PCB on which you are working, which is the main advantage of the Shorty/Shorty-with-display. The way I see it, the display in the Shorty-with-display is for confirmation.

I use a camera with 14x zoom that outputs to an video display so can freely look at instruments even if under magnification.

The tone function is almost entirely superfluous in my opinion. Products like the leak seeker use range windows to allow tone resolution, but it's not as simple to operate. The shorty w/display itself is unnecessary if you have other equipment, but the advantage is it's easier setup than the voltage drop, current and signal injection methods.

I'll take resolution over audio queues and led lights (other designs) any day. :D
Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 

Offline Shock

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Re: Short Finding on a PCB
« Reply #27 on: September 24, 2021, 04:10:23 am »
For years I've been using the following method:

Yes the inductive method has it's advantages, can also be used with a scope like near field probes to find dead integrated circuits.
Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 

Offline edigi

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Re: Short Finding on a PCB
« Reply #28 on: September 24, 2021, 07:29:37 am »
The method to find a short on a PCB can vary a bit depending on the actual case. If using that much power that the generated heat shows up in thermal camera is not an option then the other widely used method is a current limiting PSU + a good resolution voltmeter (accuracy is not a concern here). If a 6.5 digit benchtop DMM is not available another alternative is a handheld multimeter with high resolution mode (like Brymen BM869s).
By checking the voltages at various places of the PCB the short can be usually figured out within reasonable time (still slower than with thermal camera).

It's a kind of 4W resistance measurement but when measuring very low resistance it's always better to use a controllable current and voltage measurement (so DMM is in voltage measurement mode).

Finding shorts in RF stuff follows an entirely different principle, that is based on the time delay it takes to sense that there is a short (TDR). If a fast scope and fast edge signal generator is available that can be used directly but if not then via a signal sweep and inverse FFT (the VNA/AA way) to get back to time domain the distance to short can be also determined (FDR). These methods require knowing the propagation velocity though.

So there are really a plethora of options depending on the case...
 
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Offline Shock

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Re: Short Finding on a PCB
« Reply #29 on: September 24, 2021, 09:00:35 am »
High Q testers (aka ring/lopt/fbt testers) are useful for finding inductor/transformer leakage and shorts. Inductors typically look like a short to DC, so the tester applies a quick pulse and determines how good the decaying oscillations are, essentially how high the Q (quality) is.

The model I have is an old Dick Smith kit shown in the middle.




Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 


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