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Re: Finding power - gnd shorts
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LOL, try entering such a discussion on the radsafe mailing list.
Or just look at the archives over the last say 10 years and be
overwhelmed... :-).
There are two camps: one says radon can cause lung cancer,
the other says it prevents it actually... (hormesis being assumed
to be the cause for that).
These are the extremes, of course, and even those propagating them
like the rest state that it is really hard to gauge the effect of
radon
on lung cancer incidents as smoking outweighs it by far and defacto
masks the result of any study done (two major ones I know of). Or if
it does not mask them it makes them a lot less convincing (I am
not that deep into that).

Didi

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Re: Finding power - gnd shorts
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I have never heard that radon problems required the presence of sandy
soils.  The area I am in has few sandy soils and is part of a wide
area that can have radon problems.  It *is* very individual.  Two
houses next to each other, one can have a significant level while the
other has very little.  A lot has to do with the mirco-geography
(specifics of the soil and strata under the house) and the
construction.

BTW, anyone who doubts that radon can cause health problems is not
being very bright.  Radon is radioactive and is clearly a health
threat.  I suppose someone could challenge the levels at which the
health risk becomes significant, but then that always happens doesn't
it?  I bet those are the same people who believe in homeopathy and
poltergeist.  Oh, I shouldn't have said that.  Now the thread will
turn to the metaphysical and it will be my fault!

Rick


Re: Finding power - gnd shorts
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When I moved into this condo 2 years ago I worried over having
electric heat.  However the unit turns out to be very well
insulated, and it doesn't require summer air conditioning.  Even
here, in an expensive electricity area, my averaged electric bill
is $76 per month (US dollars).  That's year round.  Ambient
temperatures range from about 90 to -15 (Fahrenheit, which is about
35 to -30 C).  Somebody else shovels, mows, fixes, etc.  And the
heating price hasn't risen this year!!!  

I have no incandescent bulbs in the unit, which helps.  All compact
fluoroscent.  When the computer is idle it shuts down the display,
which also helps.  Someday I may get an LCD display.

The only real worry is long-term power failures in the winter.  The
first year one lasted about 3 days.

--
 [mail]: Chuck F (cbfalconer at maineline dot net)
 [page]: <http://cbfalconer.home.att.net
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Re: Finding power - gnd shorts
Un bel giorno rickman digiṭ:

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Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
his job. :)

--
emboliaschizoide.splinder.com

Re: Finding power - gnd shorts
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A car battery? And then you end up with one hellacious hole where the
short was, having turned a non-working PCB into a definitely dead PCB.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Finding power - gnd shorts
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Why does everyone think that the short will be the part that blows?
It could just as well be any point in the current path.  Even if there
are power planes (there are) the current has to get into the plane and
that is the point that could blow instead of the short.

Rick

Re: Finding power - gnd shorts
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You are right, that is why handling this type of problem is a TWO step
process.
First, use mV voltage gradient probing to locate the short,
and get an idea of the milli-ohms involved, then you
MOVE the current injection points, very close to the short,
to make sure that the short IS what will blow, when you ramp
the current  higher.

This assumes you do have a 'blowable' short, and not a missing
annular ring or larger area short....

-jg


Re: Finding power - gnd shorts
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I wouldn't assume that.  At a surplus store
I once saw a barrel full of very expensive
populated multilayer boards.  The boards all had
very nasty burn marks where someone had tried
the trick and failed.

Re: Finding power - gnd shorts
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I did ask how many you had.  No the short may not go first.  It is just
something to try. I once had 60 boards with defective(shorted)
decoupling caps. I burned them out, no board damage.
PCB shorts can be tiny (Yours is not) or the FR4 smokes.  If a trace
burnsit may tell you where to look on the other boards.

Re: Finding power - gnd shorts
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That's why the blunt application of a car battery isn't the ticket. You
have to pipe in the current via connections which may or may not have
thermal reliefs. The more the better. Power planes must feature such
connection areas. Then gauge what this connection can safely take and
crank things up. If the short doesn't blow before reaching the pain
threshold it probably ain't going to work.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Finding power - gnd shorts
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Hi
 I believe the car battery was to be used with a lamp in series. This
is
less than 5 amps.
 Still, I recommend the method I suggested. It is non-destructive and
doesn't
require feeding high current through the short.
 I've used it for years and it has always worked. I even used it once
to find
a short on the other side of a 10K resistor than the power line. This
was
on a board with 100's of 10K resistors wired similarly.
 Since it doesn't require feeding current through the short, the
resistance
of the short relative to the trace is not important as is the method
of looking
for the drop on the trace when feeding current through the short.
Dwight

Re: Finding power - gnd shorts

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The subject is finding the short, not fixing the short. ;)


--
Grant Edwards                   grante             Yow! Did I do an INCORRECT
                                  at               THING??
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Re: Finding power - gnd shorts
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Yep, and after the smoke has wafted off and the fire engines have left
the scene one can with certainty say "There must have been a short" :-)

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Finding power - gnd shorts

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Correct, but you just need more current!! :)

That indicates 5mOhms (which IS a low value)
- we have to throw a LOT of deliberate copper at a design to get 5mOhm
paths!

(you DID check bare boards were OK ?)

Hit that with 10A, and you now have 500mW, 20A is 2W
but you get the idea....

-jg



Re: Finding power - gnd shorts


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But tested to prove that they conform to the supplied CAD data.
Some CAD systems will pass ERCs but if there are any manually split planes
problems can be in the Gerbers.
<memories of 0.9mm drills and bits of green wire and tack.>

Geo

Re: Finding power - gnd shorts
A couple of people have mentioned the heating effect but if you only have a
maximum of 10mV and 1A that is only 10mW.
Even with a 10A limited supply, 100mW is not much dissipated over a couple of
copper planes?


Geo

Re: Finding power - gnd shorts
In comp.arch.embedded,
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A short is ohmic, so with a current of 10A, the voltage increases to 100mV,
power goes up to 1 whole Watt (P = I^2 * R). Enough to see at least some
heating effects. But as mentioned, with my method of freezing the board a
little, some luck is involved as well, it does not always work.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Re: Finding power - gnd shorts
On Tue, 22 Jul 2008 09:30:33 -0700 (PDT), the renowned rickman

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Do you have a blank board to check? I hope so.  Can you see the
clearance where vias etc. pass through the power planes by holding the
board up to a strong light?  

As others said you can follow the gradient. If the gradient does not
change then you've found a direction where the plane is not carrying
current. But with two internal planes this won't be all that easy.

Hate to say it, but it kinda sounds like it might be internal to the
board from what you're saying. Like maybe they made two gnd planes or
something like that.

Just a couple of times I've taken almost every single component out of
a board to find the problem with 100% certainty (failure analysis,
just to ensure we didn't get more like that). Whiskery shorts on
boards (sometimes intermittent so it could pass e-test) were the
hardest to find, but you could see them under a microscope. OTOH,
sometimes it's huge and right in front of your face.



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
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Re: Finding power - gnd shorts
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When you say you found various shorts, were they design flaws (which
would be repeated on each board) or manufacturing flaws (which would
be individual)?  This is one board out of a run of 110.  I have found
one other board with a short on the 3 volt rail, but this one is on
the 12 volt rail.  So there is no design flaw.

I am currently suspecting a decoupling cap at this point.  The
gradient is small and points toward one end of my 4.5 x 0.85" board.
The 12 volt plane is only on about half the board and power has been
applied to the end near the middle of the board.  The gradient points
to the opposite end.  When I get some more time to work on this, I
will test with power on the "opposite" end and test toward the power
connector.  If I see the same gradient, I will start removing
components.  If the gradient slopes the other way, I will suspect
multiple shorts which are likely an internal board problem (which I
expect is unlikely).

BTW, thanks for all of the suggestions everyone!  Some of the ideas
were obvious to me, others at least made me think a bit.  I appreciate
the different viewpoints.

I may try the freeze spray thing.  I don't have a supply that will put
out 10 A, but I have an AC transformer that puts out 1 VAC at some
huge current.  That with a diode might actually do the job, who
knows?

Rick

Re: Finding power - gnd shorts
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use car battery? ;)

-Lasse

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