power and ground continuity

[sudheervemana]

Hi all,

In my circuit board there is an regulator which gives
3.3v output and this was used by various ic's.when the board was
tested for continuity across the pins ,it was working fine .After a
few testings were done on the board there was a continuity across the
power 3.3v and Gnd.what could be the reason behind this,In my circuit
board all the power pins with 3.3v supply are placed with a by-pass
capacitors and the capacitor other ends are connected to ground.i
think that if any capacitor got fails (gets short circuit) ,then the
power 3.3v and GND are showing continuity.I want to know the shorted
by-pass capacitor on my board.how to find out this capacitor on my
circuit board ,otherwise there is a continuity across 3.3v and GND.

Is there any other method to find the shorted
components on the circuit board by using externally with the bare
board(that does not contains the components and contains only the foot
prints of the respected components ).Please give me your suggestions
to find the continuity across the power and GND in my board and also
clarify me what does an capasitor shows( i mean its state is either
the short or open) when it fails on the circuit board .
Thanks in advance,

With Regards,
Sudheervemana.

 

[Robert Baer]

Hi all,

In my circuit board there is an regulator which gives
3.3v output and this was used by various ic's.when the board was
tested for continuity across the pins ,it was working fine .After a
few testings were done on the board there was a continuity across the
power 3.3v and Gnd.what could be the reason behind this,In my circuit
board all the power pins with 3.3v supply are placed with a by-pass
capacitors and the capacitor other ends are connected to ground.i
think that if any capacitor got fails (gets short circuit) ,then the
power 3.3v and GND are showing continuity.I want to know the shorted
by-pass capacitor on my board.how to find out this capacitor on my
circuit board ,otherwise there is a continuity across 3.3v and GND.

Is there any other method to find the shorted
components on the circuit board by using externally with the bare
board(that does not contains the components and contains only the foot
prints of the respected components ).Please give me your suggestions
to find the continuity across the power and GND in my board and also
clarify me what does an capasitor shows( i mean its state is either
the short or open) when it fails on the circuit board .
Thanks in advance,

With Regards,
Sudheervemana.

First, i doubt that any of the capacitors are shorted; in fact, i
think the ergulator was damaged during testing.
But, to find the part that is shorted, connect a DC current source
between some ground point and some 3.3V supply point with the open
circuit voltage limited to 3.0V; - side to ground and + side to the 3.3V
side.
I would start with a current of 10mA for initial tests, and see if the
culprit could be found with a current no more than 100mA.
Then take a DVM on the lowest voltage scale, - lead to some arbitrary
ground point (can be the same pad of the current source - but *on* the
pad at a different point), and use the + DVM lead to probe around at
different 3.3V places, looking for the lowest voltage.
Once found, one can then "guild the lilly" by moving the - DVM lead,
looking for the lowest voltage.
If both leads are at the lowest voltage points, then you have found
the part.