Troubleshooting 'shorted' power rails

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I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer.  Worked like a charm.

Temp. rise was about 1oC @ 20mW.  The IR thermometer? $14
well-spent.


Cheers,
James Arthur

Re: Troubleshooting 'shorted' power rails
On 5/19/19 5:22 PM, snipped-for-privacy@yahoo.com wrote:
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You can get a full thermal camera attachment for Android from the state  
store for $200, sometimes less like $15-20 off on sale from time to time:

<https://www.amazon.com/dp/B00NYWAHHM/ref=twister_B07QVJVVGD?_encoding=UTF8&psc=1>  


Re: Troubleshooting 'shorted' power rails
On Sunday, May 19, 2019 at 5:30:58 PM UTC-4, bitrex wrote:
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I actually posted that here some time ago. But I've gotten along fine
without one. Besides, I don't really need Google-candy tracking me just
so that I can do electronic stuff.

A 'smart' phone and all that baggage seems like vast overkill when a
$14 thermometer does 85% of the same thing, and without even spying on
me <gasp>.

Cheers,
James Arthur

Re: Troubleshooting 'shorted' power rails

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https://youtu.be/t5fICjcaJ3E?t15%m51s

Re: Troubleshooting 'shorted' power rails
On Sun, 19 May 2019 14:22:57 -0700 (PDT), snipped-for-privacy@yahoo.com
wrote:

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Thermal imagers are getting affordable and are great to have around.
Not only can they find shorts and bad parts, it's good to image a new
design and see if anything runs hot.

https://www.dropbox.com/s/mlaf2yce2z3xiv7/IR_0025.jpg?dl=0

https://www.dropbox.com/s/r4q2z8vnonwh4mc/IR_Board.jpg?dl=0

Our test people use a thermal imager too, to spot unusual stuff.

One eternal problem is electronics is "where is the current going?"

Sometimes you can measure microvolt drops in traces and planes to
answer that question. It would be great if we could see mag fields!



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Troubleshooting 'shorted' power rails
John Larkin wrote:
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But the cheap ones have low res.  What do you think is minimum useful  
res?




Re: Troubleshooting 'shorted' power rails
On Sun, 19 May 2019 20:04:58 -0400, "Tom Del Rosso"

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Don't know, but what is important is that it can focus really close to
the parts. Some imagers are fixed-focussed at infinity, useless for
electronics.




--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Troubleshooting 'shorted' power rails
On 5/19/2019 5:04 PM, Tom Del Rosso wrote:
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Seek Thermal is quite adequate for the task.

In many cases, the thermal profile is low resolution.
A heat source anywhere near anything else will warm that
and 'fuzz out' any possibility of extremely high resolution.

If you get one, make SURE you get one with variable focus.
The very early ones were fixed focus and can't get close
enough.  The newer ones have the SAME PART NUMBER
and variable focus.  Make sure the lens rotates.

Re: Troubleshooting 'shorted' power rails
On Sunday, May 19, 2019 at 8:05:07 PM UTC-4, Tom Del Rosso wrote:
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One pixel worked decently well for me. <g>

YMMV.

Cheers,
James Arthur

Re: Troubleshooting 'shorted' power rails
On Sunday, 19 May 2019 22:48:13 UTC+1, John Larkin  wrote:
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There is mag field viewing film. And you can make your own viewer by dropping iron filings in a shampoo bottle. Crude but I've seen it demoed and work.


NT

Re: Troubleshooting 'shorted' power rails
On Monday, 20 May 2019 05:55:59 UTC+1, tabby wrote:
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oops, baby oil not shampoo


NT

Re: Troubleshooting 'shorted' power rails
On 5/19/2019 9:55 PM, snipped-for-privacy@gmail.com wrote:
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HP made an AC current probe/tracer.
In combination with the pulser, you can lay it on a trace and determine
which way the current goes with the tracer.
Easy on outer layers.   Not so easy on inner layers or with multiple
ground planes in the way.


Thermal imager is easier...and a good excuse to buy a new toy.

Re: Troubleshooting 'shorted' power rails
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I had used a Tektronix DC current probe at one point in time to follow
current.

Greg

Re: Troubleshooting 'shorted' power rails
On 5/20/19 12:55 AM, snipped-for-privacy@gmail.com wrote:
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Many smartphones, even cheap ones, come with three-axis Hall effect  
sensors built in nowatimes. With a free app you can always have a tool  
for sensing e.g. hidden wires in walls or whether an outlet somewhere is  
getting power

Re: Troubleshooting 'shorted' power rails
On 5/20/19 4:08 PM, bitrex wrote:
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In fact it would probably be possible to write an app that lets you  
sweep the phone thru an volume of space and generate a 3D graphical  
representation of the field strength magnitude in that space. Hmm, maybe  
there's a market for that.

Re: Troubleshooting 'shorted' power rails
On Sunday, May 19, 2019 at 2:48:13 PM UTC-7, John Larkin wrote:

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It can be done.    The old Kyread solution (spray=on to floppies or
mag tape, it shows the tracks) is one way

<https://www.magneticdeveloper.com

and ferrofluid is another.    The sheet gizmos (actually a lot of microspheres with loose
grains inside) are fun to play with, but only  old 8" floppies have coarse
enough tracks to show up on those.

Re: Troubleshooting 'shorted' power rails
whit3rd wrote:
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I'd have to be really desperate before I'd apply any kind
of goo to my circuit board to find a short. I've always
homed in on the short by following the voltage gradient
with a sensitive voltmeter. It's easy if the current flows
in discrete tracks, but I've also found hard shorts between
full power planes on big boards full of ECL logic that way.

Jeroen Belleman

Re: Troubleshooting 'shorted' power rails
On Monday, May 20, 2019 at 8:00:37 AM UTC-4, Jeroen Belleman wrote:
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Right that's what I do, (measure DC mV with ~20 mA of current)  
My power supply tends to look dendritic and I don't know how well the DC
trick works with power planes.  

George H.  

Re: Troubleshooting 'shorted' power rails
On Mon, 20 May 2019 05:46:14 -0700 (PDT), George Herold

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A real short can stand an amp or more of current. A 1oz copper sheet
is about 500 uohms per square so you get lots of signal. The polarity
and voltage pattern can lead you to a short.

If you order 1 oz copper, you'll rarely get 1 oz, so there's even more
voltage!

We have all our PCBs bare-board tested, so we don't see PCB internal
shorts any more.  

Lately I put a 0.1 ohm resistor in series with every voltage
regulator, so I can snoop supply currents. That has lots of uses.

The mag fields would be milligauss, way too low for iron filing sorts
of tricks.  


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Troubleshooting 'shorted' power rails
On Sunday, May 19, 2019 at 5:48:13 PM UTC-4, John Larkin wrote:
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I've got a pal who'd loan me his FLIR, but it was a lot faster
this time just to whip out the 1-pixel wonder.

TDR might be good for finding shorted planes.  But boards and parts
are generally so good these days, I've never had that happen.

Cheers,
James Arthur

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