You have some circuit board with a short circuit in it somewhere and just to make life difficult a'la Murphy's Law, that short circuit involves a foil that carries current to a whole slew of destinations.
A method for finding exactly where such a short circuit is located is illustrated by the following real life example.
In this case, there were seven destinations for current emerging from a pair of paralleled 150 ohm resistors, one to a type '4424 IC and six more which were passing through a linear voltage regulator to a whole bunch of other ICs.
Only the foil feeding the '4424 showed an unambiguously measurable voltage drop, in this case, 1.4 mV. All of the other possible paths going elsewhere showed very much lower, negligibly small, voltage drops. Thus, the approximately 70 mA passing through the paralleled pair of 150 ohm resistors had to be mainly flowing toward that '4424.
Sure enough, a short circuit from one of the '4424 output pins to ground was found to be the culprit causing all the trouble.
I run quite often into similar problems and therefore developed one instrument, which inject very low level AC current into the trace and with a "magnetic receiver"the current could be traced to the point of the short
Posted by: Alexander Pummer | March 10, 2011 at 10:52 PM
Ahhh, brings back memories of boards covered in 7400 and 74LS logic. I used this method a lot as a technician, and a friend had a gadget from hp, that would sense the presence of strong current traveling through a pin by the magnetic field.
Mike Fortner in Plano
Posted by: Michael Fortner | May 22, 2011 at 04:10 PM
Outstanding piece of work you have done.
This type of posts is rarely found.
This site has proved its metals in the way of giving extra ordinary information
Posted by: PCB Manufacture | November 11, 2011 at 10:02 AM
Ahhh... yes. This is a great example for dealing with short circuits. One can detect where a short circuit started through this diagram.
Posted by: Estheban Schnaeder | December 06, 2011 at 10:19 AM