Re: Finding power - gnd shorts


"Paul Keinanen" <keinanen@xxxxxx> wrote in message
news:4k3f84dp3felbt38568cs7fqa5ugqtmr6i@xxxxxxxxxx
On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
<Paul_E.Bennett@xxxxxxxxxxxxx> wrote:

In reviewing some of the other replies I noticed that no one had yet (at
the top level of the tree) suggested using a signal injector and a trace
probe. By injecting a current driven signal onto the power rail and
ground you can prod around with the trace probe and determine where the
short is by the signal level dropping nearest the location.

What kind of signal would that be ?

Using a variable frequency RF-generator you should be able to
determine when the tracks and the short is resonated e.g. at 1/2 or
1/4 wavelength. After determining the PCB material and hence the
velocity factor, you should be able to determine the distance from the
feedpoint.

One other method would be to inject a sub-nanosecond pulse edge and
determine, how long it takes for the reflection from the short to
return back to the generator.

A different approach would be to inject a significant current into the
+12 V line and try to detect the magnetic field on the +12 V trace,
until it disappears into the ground plane. This should work even if
the +12 V trace is within a multilayer PCB. Some Hall sensor might
useful, if DC current is used to create the magnetic field, but if an
AC (mains /audio frequency) current is running in the trace, some
pick-up coil should be able to pick up the stray magnetic field.

Paul

In the distant past I have used a milli-ohm meter for this, but alas, those
don't seem to be in everyone's test cabinet. Also, that didn't always find
a big short.

Other than blowing the crap out of everything on the board with AC, a 12V
supply that won't current limit might work.



.

Relevant Pages

  • Re: Finding power - gnd shorts
    ... the top level of the tree) suggested using a signal injector and a trace ... ground you can prod around with the trace probe and determine where the ... A different approach would be to inject a significant current into the ... +12 V line and try to detect the magnetic field on the +12 V trace, ...
    (comp.arch.embedded)
  • Re: Finding power - gnd shorts
    ... the top level of the tree) suggested using a signal injector and a trace ... ground you can prod around with the trace probe and determine where the ... A different approach would be to inject a significant current into the ... +12 V line and try to detect the magnetic field on the +12 V trace, ...
    (comp.arch.embedded)
  • Re: Finding power - gnd shorts
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