Tracking Down a Short Device or Technique

[Brad]

Hi,

I am trying to track down a short on a computer board. With power source
disconnected, there is almost 0 ohms reading between +5 volt "line" and
ground.

There was a device called "LeakSeeker" 82B that was designed
for tracking down a short. Has anyone used this or any other device, or do
you know a technique?

Thanks in advance, Brad

Before you type your password, credit card number, etc.,
be sure there is no active keystroke logger (spyware) in your PC.

 

[rush14]

Hi,

I am trying to track down a short on a computer board. With power source
disconnected, there is almost 0 ohms reading between +5 volt "line" and
ground.

There was a device called "LeakSeeker" 82B that was designed
for tracking down a short. Has anyone used this or any other device, or do
you know a technique?

Thanks in advance, Brad

Before you type your password, credit card number, etc.,
be sure there is no active keystroke logger (spyware) in your PC.

Power to ground shorts can be a bear to trouble shoot sometimes. I
assume this board worked at one time and if so I suspect it is a
defective device rather than a manufacturing defect such as a solder
bridge, reversed component, pwb fault, etc.

I would try the following:
1) Inspect the components for any sign of overheating. You might even
try sniffing around in different areas of the pwb for the smell of
overheating.
2) Briefly apply power to the board using a current regulated power
supply (key word is briefly) and feel around for overly warm parts or
signs of smoke. When I worked in industrial electronics one method we
used was placing a sheet of temperature sensitive liquid crystal on
the board to locate overheating devices. Try Edmund Scientific for a
source if they are still in business.
3) There is a gadget called a Toneohm made by Polar instruments that
has several troubleshooting modes for locating various type of short
circuits. I don't think you'd be willing to pay for one though.
4) If you find a suspect device disconnect its power lead and apply
power again to see if the short goes away.

One other thing to keep in mind is the measured resistance between
power and ground on some circuit boards is naturally low. Be sure you
really do have a power to ground short.

I'm sure there are other methods you could use but this might be
enough to help.
Good Luck
Rush

 

[G]

Hi,

I am trying to track down a short on a computer board. With power source
disconnected, there is almost 0 ohms reading between +5 volt "line" and
ground.

There was a device called "LeakSeeker" 82B that was designed
for tracking down a short. Has anyone used this or any other device, or do
you know a technique?

Thanks in advance, Brad

Before you type your password, credit card number, etc.,
be sure there is no active keystroke logger (spyware) in your PC.

I used to use a Tektronix Hall Effect scope probe for DC current.

Use an ESR meter or another meter capable of very fine low ohms
readings. I think I have heard of a device with a tone of some sort to
provide an audible reading.

greg

 

[N Cook]

I would progressively cut tracks.
I am aware, but not used, a 4 probe system in the manner of a traditional
Wheatstone bridge to monitor differential voltages and so currents in
tracks.

 

[JW]

What is this computer board? Please be specific.
If a motherboard I'd start by isolating the power FETs.

I use a Toneohm 850A. But even that can be tough with extremely low
resistance power planes.

I would progressively cut tracks.

Problem would be that most computer boards have internal power and ground
planes.

 

[**THE-RFI-EMI-GUY**]

Use a current limiting power supply to power the +5V rail, and using a
millivoltmeter, attach the + lead to the +5V rail and use the - lead to
probe all the +5 V nodes of the board looking for the voltage increase
as you get close to the short.

Hi,

I am trying to track down a short on a computer board. With power source
disconnected, there is almost 0 ohms reading between +5 volt "line" and
ground.

There was a device called "LeakSeeker" 82B that was designed
for tracking down a short. Has anyone used this or any other device, or do
you know a technique?

Thanks in advance, Brad

Before you type your password, credit card number, etc.,
be sure there is no active keystroke logger (spyware) in your PC.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

 

[[email protected]]

I use my Fluke 8050 for that. It has the resolution at low ohms to do
it. It also has an mho function but I have not found it to be
necessary.

A good ohmmeter that can resolve the resistance of the traces on a
circuit board is what you need for the most efficient solution.

JURB

 

[[email protected]]

I am trying to track down a short on a computer board. With power source
disconnected, there is almost 0 ohms reading between +5 volt "line" and
ground.

[...]
3) There is a gadget called a Toneohm made by Polar instruments that
has several troubleshooting modes for locating various type of short
circuits. I don't think you'd be willing to pay for one though.

I've used one recently - it's like (I imagine) playing a Theremin (but
with
contact probes). There's also an LCD display for the low ohm value.
As
I recall, it wasn't a slam dunk finding the short, but the indications
were
strong enough to identify the most likely suspects.

Depending on how close to zero ohms your reading is (and the
resolution
of your meter), you might be able to find the location of the short by
walking
the probes around the board (on decoupling caps and other 5V pins) to
see
whether the reading gets even closer to zero.

TM

TM

 

[Sam Goldwasser]

I used to use a Tektronix Hall Effect scope probe for DC current.

Use an ESR meter or another meter capable of very fine low ohms
readings. I think I have heard of a device with a tone of some sort to
provide an audible reading.

THe problem is that digital PCBs often have power and ground planes. The
sheet resistance of these is so low that finding the location of a short
is going to difficult based on resistance alone.

It does work on PCBs with separate power traces - like my Tek 485 when it
decides to pop a tantalum cap.

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[JeffM]

I am trying to track down a short on a computer board.

It would be good if you would keep the answer in the same thread as
the question.
news:[email protected]

 

[bob]

Hi I've used a number of techniques
A Milliohm will often do it but you need some practice in how to narrow down
your search I currently have a kleithly that works well
or in the past I did as someone suggested and pumped current through the
short and used thermal fax paper to detect any heating
Also can use a very sensitive voltmeter and effectively trace the current
Most of my experience was with blank boards from a manufacturer or a simple
shorted bypass cap
Bob