Hello-
Many years spent in the surface mount assembly design and test "school of
hard knocks" taught me the best methods to find defective components pulling
excess current from power rails in complex equipment.
First, Harvey is 100% correct. Feed the shorted rail with a current limited
+5V power supply. Set the current limit as high as your DUT's PCB traces and
interconnect wiring will safely handle without melting open. For 5V logic,
it is usually safe to feed 2A to the buss. Ensure that the leads from your
power supply can handle this current.
Second, the fun part... There are two ways to find the defect... thermal and
voltage.
Thermal: After the constant current source has been connected, wait 15
minutes. No harm will be done due to the inherent power limiting from the
bench supply. If the defect is not a hard mechanical short on the buss, such
as from a solder bridge, the defective component will become warmer than
anything else. Use your finger to locate the warm part. The back of your
index finger is very sensitive to slight warming relative to ambient. As an
example, 1V developed across a small component like a capacitor or dip chip
equates to 2W at a 2A feed. You will easily find this!
If you have access to a thermal imaging camera (like a FLIR Systems unit),
save your fingers. Even at very low excitation currents, any modern thermal
imager will show that heated component like a searchlight beacon. It must be
seen to be believed. A few years ago I bought one of these and it has paid
for itself 10 times over in saved labor costs and needless component
changes.
Of course, not everybody will want to go the thermal route, so here is
detail on a good way to find that bad part with any bench DVM: You MUST use
the DVM across each suspect part, meaning connect the DVM leads DIRECTLY
across the part's power pins. DO NOT connect the DVM minus lead to a common
ground somewhere... your voltage results will be meaningless. Remember, a
defective part will pull all voltages down, but you are seeking the
component with the LEAST voltage across it, meaning that you are at or near
the defective node. Once you get the hang of it, it goes quite quickly.
Good Luck!
Howard
