locating shorts Re: HP ScanJet ADF

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Mon Jun 3 12:04:13 2002

>From: Joe <rigdonj_at_cfl.rr.com>
>
>
>>Probably a shorted decoupling capacitor (if it's a 'dead short' or close
>>to one). I have been known to carefully cut power traces to find out
>>which section is shorted, and then home in on the problem.
>
> That's the way that I've traced shorts down. However since then I've bought a HP Current
probe, they're supposed to be able to trace a short down the correct path. I've never had a need
for it since I bought it so I dont know how well it works. Has anyone had any experience using
one?


Hi
 I've used a current probe a number of years ago and found
it worked OK for trace short, when they were accessable
on the surface. I've since found a method that works fine
for me, using standard bench top items. You need a 4 or 5
voltmeter that has at least a 200 uV range and a current
limiting power supply.
 What you do is to place about a 1 amp current from end to
end of the curcuit that is shorted to another curcuit.
Do not place the 1 amp through the short. This will generally
create a 20-100 uV drop across the curcuit. Now, clip one
lead of the meter on the other curcuit ( the one that this
curcuit is shorted to ). With the remaining lead, trace along
the curcuit with the current flowing through it. When you
get to the point where the short is, the voltage will be zero.
On either side, the voltage will be plus or minus, telling you
which way to go. Check the meter regularly for offset by
connecting both meter leads together. This will help you especially
when you get close. For curcuits with branches, you may need to
move where you put your end to end current to follow another
branch.
 This method also works for power planes. Say there is a bypass
cap that is shorted someplace. Place the current source on
opposite corners of the board and trace until you have a line
( usually curved ) across the board that measure zero ( I usually
tape a piece of string along this line ). Move the current source
to the other corners. Again, find the line. Where the lines
cross, you'll find the short.
 There are variations of this method that I've used to find
multiple shorts.
Dwight
Received on Mon Jun 03 2002 - 12:04:13 BST