Finding power - gnd shorts
rickman
ground. I am having a hard time finding where the short is so it can
be fixed.
The bare boards were supposed to be tested, so I don't suspect the
board itself. I have visually inspected everything I can including
looking under the chips as much as I can see and found no sign of a
problem.
My bench supply current limits (foldback actually) and I am seeing
about an Amp into the 12 volt rail. Probing with a volt meter I can
see 10 mV at the point where I connect the power to the board. This
drops to about 1 mV on the other edge of the board. But I can't find
a particular point where the voltage says "here it is"!
Any ideas on how to find and fix this short?
Rick
larwe
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
Faced with the same problem I had no choice but to use a dremel tool
to do a binary search to isolate the problem to a particular half,
then quadrant, then octant, then ....
Of course a Real Man(sm) would just blow the short out with AC line
voltage.
dalai lamah
Increase the current until you see something heat up or melt. :)
lang...@fonz.dk
I've heard a story of some big expensive board that had similar
problems, I believe dust or something on the films for the pcb made
some tiny shorts.
some brave guy just took the biggest PSU he could find and puff they
all worked
-Lasse
cbarn...@aol.com
A small compass
Jim Stewart
If you were a really old fart like me, you'd
have a dusty old Toneohm on one of the back
shelves of the lab. Ask around.
Mark Borgerson
@z66g2000hsc.googlegroups.com>, gnu...@gmail.com says...
holes. I forgot to do that on a recent design and the inner layers
came right up to the plated-through mounting holes for one module.
I had to drill out the mounting holes and warn the end user to
mount the module with nylon screws until the next board revision.
Mark Borgerson
Jim Granville
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
>
> The bare boards were supposed to be tested, so I don't suspect the
> board itself.
but you have checked anyway, right ?
> I have visually inspected everything I can including
> looking under the chips as much as I can see and found no sign of a
> problem.
>
> My bench supply current limits (foldback actually) and I am seeing
> about an Amp into the 12 volt rail. Probing with a volt meter I can
> see 10 mV at the point where I connect the power to the board. This
> drops to about 1 mV on the other edge of the board. But I can't find
> a particular point where the voltage says "here it is"!
>
> Any ideas on how to find and fix this short?
Normally the 4 wire 'microvolt gradient' method is enough to
narrow down the location.
That, and a highlighted view of the PCB layers, showing just the two
offending nets, can give you a smaller set of 'candidiate locations' for
a short.
Find the lowest voltage diff point, and then vary the current 2:1
and get a milliohm value from the dV - that also gives a clue of what
can be the cause, and possible solutions.
Copper or solder whisker faults can be cleared with a fusing current,
applied in the right place :)
-jg
Stef
Ground is a plane as well? If so, most of the power will be dissipated
in/around the short.
Spray the board (or section by section) with cold spray until you get
a thin layer of ice (just a white haze). Then run a current big enough
to get some power in the short. If you're lucky, you will see a spot
defrosting more rapidly then the rest of the board. Try to spray
evenly, uneven spraying can defrost in different times too.
The fancy option would be a heat camera, but I suspect you don't
have one sitting on a shelf. ;-)
Maybe filming the defrosting board and playing it back slowly will
help?
--
Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)
Grant Edwards
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
Finding the short is easy:
1) Turn up the PS current limit until until the voltage jumps
and the current drops to 0.
2) Examine board for charred spot.
;)
> The bare boards were supposed to be tested, so I don't suspect
> the board itself. I have visually inspected everything I can
> including looking under the chips as much as I can see and
> found no sign of a problem.
>
> My bench supply current limits (foldback actually) and I am
> seeing about an Amp into the 12 volt rail. Probing with a
> volt meter I can see 10 mV at the point where I connect the
> power to the board. This drops to about 1 mV on the other
> edge of the board. But I can't find a particular point where
> the voltage says "here it is"!
>
> Any ideas on how to find and fix this short?
We used to use a current probe to find shorts on blank PC boards.
--
Grant Edwards grante Yow! In 1962, you could buy
at a pair of SHARKSKIN SLACKS,
visi.com with a "Continental Belt,"
for $10.99!!
d...@nadler.com
Rick, what ever you do, we want to see it on You Tube ;-)
Looking forward to that,
Best Regards, Dave
Joerg
First try gradient. Dump in constant current, switch the DVM to the
200uV range and probe at 1/2" or so distance. IOW the probes "walk"
behind each other in lockstep. Move the trailing one for greatest
gradient, then keep going until you see a steep drop or reversal. That
should lead you to the "sink". Of course this won't work well if there
is more than one short. I once had a whole series of tested (!) boards
that had four shorts each.
If this fails try to get a hold of a camera that is somewhat infrared
sensitive. Snap a pic in the dark, load onto PC and stretch the
histogram to wazoo. Sometimes that shows a distinct hot spot or possible
more than one. The temperature at the short is usually a lot higher than
elsewhere on the plane.
If you are near San Francisco maybe John Larkin lets you put it under
his FLIR camera. That ought to show it.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
CBFalconer
>
... snip ...
>
> The bare boards were supposed to be tested, so I don't suspect
> the board itself. I have visually inspected everything I can
> including looking under the chips as much as I can see and found
> no sign of a problem.
Have you taken an unstuffed board and tested that assumption?
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.
Paul Carpenter
6a398a...@z66g2000hsc.googlegroups.com>, gnu...@gmail.com says...
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
Is this multi-layer or 2 layer?
I have seen creases/folds on inner layers (8 layer board) cause this
type of problem.
> The bare boards were supposed to be tested, so I don't suspect the
> board itself.
If you still have any blank boards I would test them FIRST!
I have seen bare board testing that basically follows the netlist for
continuity, but does NOT check for track to track or track to plane
shorts.
> I have visually inspected everything I can including
> looking under the chips as much as I can see and found no sign of a
> problem.
First obvious thing is does ANYTHING get noticeably warm/hot.
Without knowing the circuit you could have a combination of
faults (reversed components, interplane short, wrong component on
a power amp can cause some combinational effects).
Let alone a drop of solder from a soldering iron bridging two power
rails I once did by accident.
> My bench supply current limits (foldback actually) and I am seeing
> about an Amp into the 12 volt rail. Probing with a volt meter I can
> see 10 mV at the point where I connect the power to the board. This
> drops to about 1 mV on the other edge of the board. But I can't find
> a particular point where the voltage says "here it is"!
When you measure 1mV have you just moved one probe. Moving both probes
gives to nearest and further away points on the two tracks to determine
voltage gradient. The gradient method and how tone ohms basically work
(the closer the two probes are to the short or one point, the lower
the impedance, giving usually a higher frequency o/p).
> Any ideas on how to find and fix this short?
See everybody elses comments as well.
--
Paul Carpenter | pa...@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/> PC Services
<http://www.pcserviceselectronics.co.uk/fonts/> Timing Diagram Font
<http://www.gnuh8.org.uk/> GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/> For those web sites you hate
JSprocket
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
>
I had this some years ago with my very first 4 layer PCB. Aaaargh! I
took a board and sawed it in half: the short should be in either the
left or the right half. It was in both. So I sawed the two halves in
half. The short was now in all four pieces....
Eventually I took a belt sander and removed the outer PCB layers, thus
revealing the fault: the VCC layer has VCC connections, while the ground
layer had both VCC and ground connections. Armed with this, and the
Gerbers, I went back to the manufacturer... who was forced to confess
that some underling had decided that the two layers were meant to be
merged. So I eventually got my clean PCBs, but it took several weeks all
told.
JS
Neil
current. 1 amp is a lot. Something should be getting warm. If it is
not the board, that leaves wrong / bad components, backwards parts, and
a schematic error.
Geo
>The bare boards were supposed to be tested, so I don't suspect the
>board itself.
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
Geo
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
Stef
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.
Jim Granville
Did they also claim they had been 'bare-board tested' ? ;)
They may well have passed, as many testers learn from a sample!
-jg
Didi
> ....
> Of course a Real Man(sm) would just blow the short out with AC line
> voltage.
Yahoo, that would be it! In 220V countries even better than in the
US... :-)
I have not resorted to that, but have been close - I used a brand-new
12V 7Ah NiMH battery I had just assembled in similar circumstances.
I had a tiny (visible) short on an inner layer and I did need that
board to
work at that moment... It worked and has been working for
many years since.
Didi
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/
Original message: http://groups.google.com/group/comp.arch.embedded/msg/06a618bf575439fa?dmode=source
Paul Keinanen
>larwe wrote:
>> ....
>> Of course a Real Man(sm) would just blow the short out with AC line
>> voltage.
>
>Yahoo, that would be it! In 220V countries even better than in the
>US... :-)
The problem is that in 230 V countries, the mains fuses are only 10-16
A, so if there is a stubborn short on the PCB, the 50-100 A short
circuit current may blow the mains fuse but leave the PCB short
unaffected :-(.
For stubborn shorts, it might be a better idea to use a welding
transformer.
Even if you do not try to evaporate the short, one idea might be to
let some AC current (a few amps) flow through the PCB and use a
miniature coil connected to the oscilloscope probe and try to locate
in which PCB track branches the AC current flows to the short. By
rotating the coil, you might even be able to determine if the current
is flowing in a track along the long or short axis of the PCB.
Paul
Spehro Pefhany
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"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Dombo
> On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:
>
>> larwe wrote:
>>> ....
>>> Of course a Real Man(sm) would just blow the short out with AC line
>>> voltage.
>> Yahoo, that would be it! In 220V countries even better than in the
>> US... :-)
>
> The problem is that in 230 V countries, the mains fuses are only 10-16
> A, so if there is a stubborn short on the PCB, the 50-100 A short
> circuit current may blow the mains fuse but leave the PCB short
> unaffected :-(.
The mains fuse doesn't immediately blow at 16A. I've seen copper
evaporate from the PCB with a short on the the mains side with the mains
fuse still left intact.
rickman
Yes, this sounds like an excellent idea! Maybe I can even evaporate
the entire power plane completely eliminating any possibility of a
short!
Rick
ian....@gmail.com
Some 20 years ago I had a 28 slot, 8 layer backplane with this sort of
problem - dust on the artworks during manufacture. I had to us a 10
amp power supply to vaporise all of the shorts. Most went at about an
amp but there were a few that took more than 5 Amps before they
opened. Fortunately no active components on that board.
Ian
Stef
For that purpose I can recommend one of our machines with 40,000uF caps
at 350 Volts, they're ideal for evaporating copper from boards!
rickman
> rickman wrote:
>
> ... snip ...
>
> > The bare boards were supposed to be tested, so I don't suspect
> > the board itself. I have visually inspected everything I can
> > including looking under the chips as much as I can see and found
> > no sign of a problem.
>
> Have you taken an unstuffed board and tested that assumption?
No, but I have over a hundred boards that are working just fine.
Actually, I have 103 working boards... unfortunately, I need to
deliver 104, no kidding! I have three other boards where I have
identified the problems and can be fixed. One of those has at least
two open vias. These boards are from the batch I got from Sunstone
where they had a 25% X-out rate due to plating problems in the vias.
I ordered 6 panels. They made 7 and I still didn't get enough
boards. They had to make a seventh panel with 16 working boards to
complete the order.
I was very concerned about vias opening in the field. So I am glad
that I have only found one board with open vias after assembly. I
just got 56 boards out of a four day bake which I will be retesting.
Rick
rickman
Many posts indicate that I should expect the something to be getting
warm from the 1 Amp of current. Yes, the power supply is getting
warm. The voltage on the board is only 5 mV max. That equates to 5
mW. I think it would take a very sensitive device to see the
temperature rise from 5 mW of power. Handing the board wearing gloves
would leave a larger thermal imprint than this.
But using a FLIR camera does sound like fun!
Rick
dke...@hotmail.com
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
>
> The bare boards were supposed to be tested, so I don't suspect the
> board itself. I have visually inspected everything I can including
> looking under the chips as much as I can see and found no sign of a
> problem.
>
> My bench supply current limits (foldback actually) and I am seeing
> about an Amp into the 12 volt rail. Probing with a volt meter I can
> see 10 mV at the point where I connect the power to the board. This
> drops to about 1 mV on the other edge of the board. But I can't find
> a particular point where the voltage says "here it is"!
>
> Any ideas on how to find and fix this short?
>
> Rick
Hi Rick
I've described this methode before. Most people seem to think
that a method of finding the short requires feeding current through
the short. My method uses a different idea.
Put your supply across the 12v line, from one end to the other,
in current limit. The line then has a small voltage drop across it.
Place a DVM lead on the ground. The location isn't important
since there is no current in the ground.
Use the 200mv scale. This works best with a 4 or 5 digit meter.
With the other lead, probe along the 12V line. When the meter
reads closest to zero, you are at the location of the short.
I have used this method many times when other methods
seem to fail.
The idea is to consider the 12 v line to be like a resistor with
a tap on it. The ground it the tap. With the second lead as
another tap, when both taps are at the same location along
the resistor, there is 0 volts.
Dwight
rickman
wrote:
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
Didi
> On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:
>
> >larwe wrote:
> >> ....
> >> Of course a Real Man(sm) would just blow the short out with AC line
> >> voltage.
> >
> >Yahoo, that would be it! In 220V countries even better than in the
> >US... :-)
>
> The problem is that in 230 V countries, the mains fuses are only 10-16
> A, so if there is a stubborn short on the PCB, the 50-100 A short
> circuit current may blow the mains fuse but leave the PCB short
> unaffected :-(.
Yes, that's a clear advantage of the 110V (one can expect twice the
current at 220 is needed to blow the fuses), but 220 is still more fun
for
this application - sparkles, smoke, danger etc. (I recently got 100n
at 3kV
discharged left to right hand fingertips... was making a new 3kV 60W
power
supply, would not have been a success without any of that :-).
For practical purposes a car battery should be the best source to
blow PCB areas.
Didi
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/
Original message: http://groups.google.com/group/comp.arch.embedded/msg/19187ceba73c1bf2?dmode=source
Mark Borgerson
@l42g2000hsc.googlegroups.com>, gnu...@gmail.com says...
> On Jul 22, 8:02 pm, CBFalconer <cbfalco...@yahoo.com> wrote:
> > rickman wrote:
> >
> > ... snip ...
> >
> > > The bare boards were supposed to be tested, so I don't suspect
> > > the board itself. I have visually inspected everything I can
> > > including looking under the chips as much as I can see and found
> > > no sign of a problem.
> >
> > Have you taken an unstuffed board and tested that assumption?
>
> No, but I have over a hundred boards that are working just fine.
> Actually, I have 103 working boards... unfortunately, I need to
> deliver 104, no kidding! I have three other boards where I have
> identified the problems and can be fixed. One of those has at least
> two open vias. These boards are from the batch I got from Sunstone
> where they had a 25% X-out rate due to plating problems in the vias.
> I ordered 6 panels. They made 7 and I still didn't get enough
> boards. They had to make a seventh panel with 16 working boards to
> complete the order.
What was the size of your vias? I've been using 8 mil trace and space
and 21 mil vias for many years without problems. However, some of
the larger fine-pitch stuff I use now will soon force finer traces
and spacing.
>
> I was very concerned about vias opening in the field. So I am glad
> that I have only found one board with open vias after assembly. I
> just got 56 boards out of a four day bake which I will be retesting.
>
> Rick
Mark Borgerson
Paul Keinanen
>Yes, that's a clear advantage of the 110V (one can expect twice the
>current at 220 is needed to blow the fuses), but 220 is still more fun
>for this application - sparkles, smoke, danger etc.
By the way, can you make a hot sausage in a 110 V country by just
sticking the neutral at one end of the sausage and live conductor into
the other end ?
Paul
rickman
wrote:
If I get to that point, would you like to provide a unit for
evaluation and I will post the results. Maybe this will generate a
lot of sales???
Rick
rickman
>
> What was the size of your vias? I've been using 8 mil trace and space
> and 21 mil vias for many years without problems. However, some of
> the larger fine-pitch stuff I use now will soon force finer traces
> and spacing.
I used 6/6 and 10 mil vias in 24 mil pads. Sunstone originally said
they could make 10 mil vias, but after I got the boards they said they
used a 13 mil drill since that was within their spec of +- 3 mil. As
long as they got the holes on the pads, I didn't care, but some of the
holes are right at the edge of the pads. I've never been able to tell
if this is a problem as long as the trace is not cut from the pad.
Rick
rickman
wrote:
Good idea! When I get some time I will try this. Thanks.
Rick
lang...@fonz.dk
use car battery? ;)
-Lasse
CBFalconer
> Didi <d...@tgi-sci.com> wrote:
>
>> Yes, that's a clear advantage of the 110V (one can expect twice
>> the current at 220 is needed to blow the fuses), but 220 is still
>> more fun for this application - sparkles, smoke, danger etc.
>
> By the way, can you make a hot sausage in a 110 V country by just
> sticking the neutral at one end of the sausage and live conductor
> into the other end ?
No, that operation here requires an adaptor and half-length
sausages. :-)
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.
Stef
rickman <gnu...@gmail.com> wrote:
> On Jul 23, 10:02 am, Stef <stef...@yahooI-N-V-A-L-I-D.com.invalid>
> wrote:
>> In comp.arch.embedded,
>> rickman <gnu...@gmail.com> wrote:
>> > On Jul 23, 8:45 am, Dombo <do...@disposable.invalid> wrote:
>>
>> >> The mains fuse doesn't immediately blow at 16A. I've seen copper
>> >> evaporate from the PCB with a short on the the mains side with the mains
>> >> fuse still left intact.
>>
>> > Yes, this sounds like an excellent idea! Maybe I can even evaporate
>> > the entire power plane completely eliminating any possibility of a
>> > short!
>>
>> For that purpose I can recommend one of our machines with 40,000uF caps
>> at 350 Volts, they're ideal for evaporating copper from boards!
>
> evaluation and I will post the results. Maybe this will generate a
> lot of sales???
No extra sales I'm affraid. It is not their intended use but was their
self-destruct mechanism. :-)
--
Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)
There are two kinds of pedestrians... the quick and the dead.
-- Lord Thomas Rober Dewar
Joerg
It might make sense to rent one, find the short, then take it home and
image your house from the outside. It'll find all the leaky areas where
heat or A/C losses occur. One guy found a really hard signal near a
crawl space vent. Turned out an A/C duct had fallen off.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Antonio Pasini
[snip]
>
> Good idea! When I get some time I will try this. Thanks.
>
Buy a bag of very "light" iron powder (I saw that on sale on USA); take
a thin glass panel out from a window, cover it with water and a drop of
soap, mix the iron powder in the water, and carefully put the whole
panel upon your board. Put many amps across the board.
Watch the magnetic flux lines around the current path.
Clean up the whole mess...
:-)
(I just looked a video of a guy building magnetic field "readers" with
bottles, water, and iron poweder... fun pictures!)
Seriously, when I was interested in knowing where the short happened, I
used some dedicated instruments; it was very quick and effective. I'm
not at work now and I don't recall the model.
Basically, a gradient search should tell you where the short is located.
But if you just need the board, I would simply blow it up ad many said.
rickman
> Paul Keinanen wrote:
> > Didi <d...@tgi-sci.com> wrote:
>
> >> Yes, that's a clear advantage of the 110V (one can expect twice
> >> the current at 220 is needed to blow the fuses), but 220 is still
> >> more fun for this application - sparkles, smoke, danger etc.
>
> > By the way, can you make a hot sausage in a 110 V country by just
> > sticking the neutral at one end of the sausage and live conductor
> > into the other end ?
>
> No, that operation here requires an adaptor and half-length
> sausages. :-)
I have a hot dog cooker around here somewhere from when I was a kid.
It has two rows of spikes that you impale the dogs between. When you
close the lid, it complete the circuit to the 110 VAC and the dogs
cook in 2 or 3 minutes. Pretty cool huh? But they get a slightly
metallic taste so I guess that's why you don't seem them in the stores
now.
Rick
Mark Borgerson
@x41g2000hsb.googlegroups.com>, gnu...@gmail.com says...
I suspect that, even with a lid interlock switch, it wouldn't meet
current UL standards. If a dog falls off the neutral end, you
might get a ground fault or a hot chassis.
Given the ubiquity of the microwave oven, it's probably easier
to put a few vent holes and nuke it for 30 seconds.
Mark Borgerson
Nils
You guys need a 100µF 500V Electrolytic cap for that! They are cheap
because they're standard filter-caps in guitar amplifiers.
I know for sure that they can melt copper-trace with ease. They also
have enough power to fry your DMM and weld the probes onto the circuit
board as well :-)
Been there, done that..
Nils <--- who builts tube guitar amplifiers every once in a while, just
because it is so much fun the see arcs and sparcs on a cicuit-board.
rickman
<removethis_antonio.pas...@alice.it> wrote:
> Il 23/07/2008 17.53, rickman ha scritto:
> [snip]
>
>
>
> > Good idea! When I get some time I will try this. Thanks.
>
> Buy a bag of very "light" iron powder (I saw that on sale on USA); take
> a thin glass panel out from a window, cover it with water and a drop of
> soap, mix the iron powder in the water, and carefully put the whole
> panel upon your board. Put many amps across the board.
> Watch the magnetic flux lines around the current path.
> Clean up the whole mess...
>
> :-)
>
> (I just looked a video of a guy building magnetic field "readers" with
> bottles, water, and iron poweder... fun pictures!)
I seem to recall a magnetic liquid with some very unusual properties.
I don't recall the exact form, but in all of the images I saw it was
intact and 3D. It would illustrate some very odd fields, one caused
it to look like a spiky soccer ball. Anyone know what it was?
Rick
rickman
wrote:
Let's see... the board cost $150 to make and a FLIR camera cost maybe
$500 to rent. My ducts are in the basement where I can see them all
and this 50 year old house is nothing but a draft with windows.
I don't think I'll rent the FLIR camera, but I sleep well at night
knowing I don't have to worry about Radon building up inside... :^)
On the other hand, I am getting very worried about making enough money
to be able to fill my heating oil tank this year. After all, I only
make an engineer's pay. It might not be long before I sell the
furnace as scrap to pay for electric space heaters!!!
Rick
dalai lamah
> Many posts indicate that I should expect the something to be getting
> warm from the 1 Amp of current. Yes, the power supply is getting
> warm. The voltage on the board is only 5 mV max. That equates to 5
> mW. I think it would take a very sensitive device to see the
> temperature rise from 5 mW of power.
Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
his job. :)
Paul E. Bennett
> I have a small module that is shorted between the +12 volt plane and
> ground. I am having a hard time finding where the short is so it can
> be fixed.
>
> The bare boards were supposed to be tested, so I don't suspect the
> board itself. I have visually inspected everything I can including
> looking under the chips as much as I can see and found no sign of a
> problem.
>
> My bench supply current limits (foldback actually) and I am seeing
> about an Amp into the 12 volt rail. Probing with a volt meter I can
> see 10 mV at the point where I connect the power to the board. This
> drops to about 1 mV on the other edge of the board. But I can't find
> a particular point where the voltage says "here it is"!
>
> Any ideas on how to find and fix this short?
>
> Rick
In reviewing some of the other replies I noticed that no one had yet (at
the top level of the tree) suggested using a signal injector and a trace
probe. By injecting a current driven signal onto the power rail and
ground you can prod around with the trace probe and determine where the
short is by the signal level dropping nearest the location.
Non-destructive methods are usually preferred.
--
********************************************************************
Paul E. Bennett...............<email://Paul_E....@topmail.co.uk>
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
dke...@hotmail.com
Hi
The main trick is to get enough drop along the trace. Use as much
current as
you can. For smaller traces, I've used 1 to 5 amps someplace.
This trick even works for power plane to power plane shorts but you
need
to do two different directions across the plane that you are driving
power
across. You form two lines of zero volts. Where they cross is the
location of
the short.
If the 12v line has branches, you may find the zero volt is down a
branch. Just
move the location of one of the current injection points to the end of
the branch
and search along the branch for the zero volt location.
Dwight
Nils
>
> I seem to recall a magnetic liquid with some very unusual properties.
> I don't recall the exact form, but in all of the images I saw it was
> intact and 3D. It would illustrate some very odd fields, one caused
> it to look like a spiky soccer ball. Anyone know what it was?
>
Hi Rick.
Must be the stuff marked under the name Ferro-Liquid.
http://www.youtube.com/watch?v=c_D29D9nyq0&feature=related
Fun stuff for sure..
Nils
Geo
<Paul_E....@topmail.co.uk> wrote:
>In reviewing some of the other replies I noticed that no one had yet (at
>the top level of the tree) suggested using a signal injector and a trace
>probe. By injecting a current driven signal onto the power rail and
>ground you can prod around with the trace probe and determine where the
>short is by the signal level dropping nearest the location.
>Non-destructive methods are usually preferred.
Hah! - no fun - AMPS...
Geo
JeffM
>[...]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?
A car battery has been mentioned multiple times.
That will give you beaucoup current--with no diode to fail.
Folks who have a low beam burn out on their dual-beam headlights
and save the old bulb will find that they have a 13.8V 65W load.
1 or 2 of those will limit what is still A LOT of current.
Paul Keinanen
<Paul_E....@topmail.co.uk> wrote:
>In reviewing some of the other replies I noticed that no one had yet (at
>the top level of the tree) suggested using a signal injector and a trace
>probe. By injecting a current driven signal onto the power rail and
>ground you can prod around with the trace probe and determine where the
>short is by the signal level dropping nearest the location.
What kind of signal would that be ?
Using a variable frequency RF-generator you should be able to
determine when the tracks and the short is resonated e.g. at 1/2 or
1/4 wavelength. After determining the PCB material and hence the
velocity factor, you should be able to determine the distance from the
feedpoint.
One other method would be to inject a sub-nanosecond pulse edge and
determine, how long it takes for the reflection from the short to
return back to the generator.
A different approach would be to inject a significant current into the
+12 V line and try to detect the magnetic field on the +12 V trace,
until it disappears into the ground plane. This should work even if
the +12 V trace is within a multilayer PCB. Some Hall sensor might
useful, if DC current is used to create the magnetic field, but if an
AC (mains /audio frequency) current is running in the trace, some
pick-up coil should be able to pick up the stray magnetic field.
Paul
Joerg
That's going to cost you in winter.
> I don't think I'll rent the FLIR camera, but I sleep well at night
> knowing I don't have to worry about Radon building up inside... :^)
>
Why is everyone concerned about Radon? Does it really occur a lot?
> On the other hand, I am getting very worried about making enough money
> to be able to fill my heating oil tank this year. After all, I only
> make an engineer's pay. It might not be long before I sell the
> furnace as scrap to pay for electric space heaters!!!
>
Careful, in our neck of the woods the utility then really socks it to
you. Get past 130% of a rather paltry baseline usage and it jumps up.
Get past 200% of baseline and electricity becomes hyper-inflationary.
Out here electricity is certainly not the future. So, we got wood
burners. I already know what next winter's heating bill will be: $1200,
for roughly six months or winter (global warming didn't happen out here).
Joerg
> Un bel giorno rickman digitň:
>
>> Many posts indicate that I should expect the something to be getting
>> warm from the 1 Amp of current. Yes, the power supply is getting
>> warm. The voltage on the board is only 5 mV max. That equates to 5
>> mW. I think it would take a very sensitive device to see the
>> temperature rise from 5 mW of power.
>
> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
> his job. :)
>
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.
rickman
wrote:
> dalai lamah wrote:
> > Un bel giorno rickman digitò:
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
rickman
> On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
>
As a matter of fact, the last day job I had the company had just
bought a $50,000 TDR. I am sure fixing a $150 board is exactly the
job they had planned for it. Even with that, I'm not sure it could
show me the short. I don't know for sure if it is in the boards or on
top of it. The TDR won't tell me that.
Rick
Jim Granville
> Many posts indicate that I should expect the something to be getting
> warm from the 1 Amp of current. Yes, the power supply is getting
> warm. The voltage on the board is only 5 mV max. That equates to 5
> mW. I think it would take a very sensitive device to see the
> temperature rise from 5 mW of power. Handing the board wearing gloves
> would leave a larger thermal imprint than this.
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
Jim Granville
It can be a problem if etchant is able to sneak into the plated hole,
you can guess what happens next!
-jg
Jim Granville
> 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?
Sounds ideal - Forget the diode, and find a variac :)
If the AC is under 500mV pk, it is unlikely to damage
anything and if your 5mV/1A number was right, 500mV
will be reached at ~100A - something should happen before
you wind up that far ;)
-jg
Jim Granville
>
> One other method would be to inject a sub-nanosecond pulse edge and
> determine, how long it takes for the reflection from the short to
> return back to the generator.
That's another good idea! - just a nudge (or two ;) up the technology
scale from the milli-volt voltage gradient probing method I described
earlier!
-jg
Jim Granville
>
> 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.
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
Eric Smith
> As a matter of fact, the last day job I had the company had just
> bought a $50,000 TDR. I am sure fixing a $150 board is exactly the
> job they had planned for it. Even with that, I'm not sure it could
> show me the short.
It wouldn't easily identify the physical location of the short, though
it might help narrow it down. In my case, a $150 used current tracer
(HP 547A) and a $150 used signal generator (HP 8654A) worked great when
I last had to solve this kind of problem. It led me right to a solder
bridge under a closed frame DIP socket. $300 in used tools can help a
lot more than a new $50K tool, if the $50K tool doesn't happen to be the
right one for the job. On the other hand, it would certainly be nice
to have a high-performance TDR at hand when I need one. I've always
had to make do with the signal generator and an oscilliscope.
I'd like to replace the 8654A with an Agilent 33220A, but I can't
really justify the expense. I haven't quite figured out why test
equipment resellers are charging more a used 33120A than Agilent
charges for a brand new 33220A.
Eric
Jim Stewart
> On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> dalai lamah wrote:
>>>> Many posts indicate that I should expect the something to be getting
>>>> warm from the 1 Amp of current. Yes, the power supply is getting
>>>> warm. The voltage on the board is only 5 mV max. That equates to 5
>>>> mW. I think it would take a very sensitive device to see the
>>>> temperature rise from 5 mW of power.
>>> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
>>> his job. :)
>> 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/
>>
>> "gmail" domain blocked because of excessive spam.
>> Use another domain or send PM.
>
> 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.
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.
CBFalconer
>
... snip ...
>
> On the other hand, I am getting very worried about making enough
> money to be able to fill my heating oil tank this year. After
> all, I only make an engineer's pay. It might not be long before
> I sell the furnace as scrap to pay for electric space heaters!!!
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>
Try the download section.
CBFalconer
Actually Variacs are transformers, and can supply lots of current.
If you can find an old filament winding transformer (About 6.3 V at
2A) its output is strictly limited by the number of maggots that
can run around in its core. The number will be considerably larger
than 2A, but strictly limited. Of course, such an overload will
destroy the transformer in a short time, but that period is long
compared to the desired testing interval. For less current, find a
smaller transformer.
Grant Edwards
>> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
>> his job. :)
>
> 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.
The subject is finding the short, not fixing the short. ;)
--
Grant Edwards grante Yow! Did I do an INCORRECT
at THING??
visi.com
Paul Keinanen
<notthis...@removethispacbell.net> wrote:
>rickman wrote:
>> Let's see... the board cost $150 to make and a FLIR camera cost maybe
>> $500 to rent. My ducts are in the basement where I can see them all
>> and this 50 year old house is nothing but a draft with windows.
>>
>
>That's going to cost you in winter.
>
>
>> I don't think I'll rent the FLIR camera, but I sleep well at night
>> knowing I don't have to worry about Radon building up inside... :^)
>>
>
>Why is everyone concerned about Radon? Does it really occur a lot?
Radon can be a problem in houses built on sand that has been recently
(e.g. during the Ice Age) split from some uranium rich stones, such as
granite.
Instead of ventilating the whole house, just make 2-3 vertical holes
under the house and install an air pumps sucking the air and radon out
of these wells and blow it outside the house. The slight
under-pressure in the wells will such the air from the surrounding
sand and thus prevents the radon from entering the house through the
floor.
Paul
Didi
> ....
> >> I don't think I'll rent the FLIR camera, but I sleep well at night
> >> knowing I don't have to worry about Radon building up inside... :^)
> >>
> >
> >Why is everyone concerned about Radon? Does it really occur a lot?
>
> Radon can be a problem in houses built on sand that has been recently
> (e.g. during the Ice Age) split from some uranium rich stones, such as
> granite.
>
> Instead of ventilating the whole house, just make 2-3 vertical holes
> under the house and install an air pumps sucking the air and radon out
> of these wells and blow it outside the house. The slight
> under-pressure in the wells will such the air from the surrounding
> sand and thus prevents the radon from entering the house through the
> floor.
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
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/
Original message: http://groups.google.com/group/comp.arch.embedded/msg/d64babe473048109?dmode=source
Neil
> On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> dalai lamah wrote:
>>>> Many posts indicate that I should expect the something to be getting
>>>> warm from the 1 Amp of current. Yes, the power supply is getting
>>>> warm. The voltage on the board is only 5 mV max. That equates to 5
>>>> mW. I think it would take a very sensitive device to see the
>>>> temperature rise from 5 mW of power.
>>> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
>>> his job. :)
>> 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/
>>
>> "gmail" domain blocked because of excessive spam.
>> Use another domain or send PM.
>
> 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
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.
Neil
> On Jul 22, 9:30 am, rickman <gnu...@gmail.com> wrote:
>> I have a small module that is shorted between the +12 volt plane and
>> ground. I am having a hard time finding where the short is so it can
>> be fixed.
>>
>> The bare boards were supposed to be tested, so I don't suspect the
>> board itself. I have visually inspected everything I can including
>> looking under the chips as much as I can see and found no sign of a
>> problem.
>>
>> My bench supply current limits (foldback actually) and I am seeing
>> about an Amp into the 12 volt rail. Probing with a volt meter I can
>> see 10 mV at the point where I connect the power to the board. This
>> drops to about 1 mV on the other edge of the board. But I can't find
>> a particular point where the voltage says "here it is"!
>>
>> Any ideas on how to find and fix this short?
>>
>> Rick
>
> I've described this methode before. Most people seem to think
> that a method of finding the short requires feeding current through
> the short. My method uses a different idea.
> Put your supply across the 12v line, from one end to the other,
> in current limit. The line then has a small voltage drop across it.
> Place a DVM lead on the ground. The location isn't important
> since there is no current in the ground.
> Use the 200mv scale. This works best with a 4 or 5 digit meter.
> With the other lead, probe along the 12V line. When the meter
> reads closest to zero, you are at the location of the short.
> I have used this method many times when other methods
> seem to fail.
> The idea is to consider the 12 v line to be like a resistor with
> a tap on it. The ground it the tap. With the second lead as
> another tap, when both taps are at the same location along
> the resistor, there is 0 volts.
> Dwight
rickman
> On Wed, 23 Jul 2008 14:44:20 -0700, Joerg
>
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
Spehro Pefhany
wrote:
In general, IMHO, it's best to use a beefy current source limited to
250mV or so. Then you can put it pretty much on any two points on a
board without blowing things up (well, except very low resistance
fuses, very low value resistors, perhaps some very low resistance
coils). You won't even be outside the specs on most chips, where
+/-300mV is typically allowed.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Not Really Me
"Paul Keinanen" <kein...@sci.fi> wrote in message
news:4k3f84dp3felbt385...@4ax.com...
don't seem to be in everyone's test cabinet. Also, that didn't always find
a big short.
Other than blowing the crap out of everything on the board with AC, a 12V
supply that won't current limit might work.
Joerg
> On 2008-07-23, Joerg <notthis...@removethispacbell.net> wrote:
>
>>> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
>>> his job. :)
>> 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.
>
> The subject is finding the short, not fixing the short. ;)
>
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" :-)
Joerg
> On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> dalai lamah wrote:
>>>> Many posts indicate that I should expect the something to be getting
>>>> warm from the 1 Amp of current. Yes, the power supply is getting
>>>> warm. The voltage on the board is only 5 mV max. That equates to 5
>>>> mW. I think it would take a very sensitive device to see the
>>>> temperature rise from 5 mW of power.
>>> Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do
>>> his job. :)
>> 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/
>>
>> "gmail" domain blocked because of excessive spam.
>> Use another domain or send PM.
>
> 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.
>
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.
dke...@hotmail.com
wrote:
> rickman wrote:
> > On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
> > wrote:
> >> dalai lamah wrote:
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
Paul E. Bennett
> On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
> <Paul_E....@topmail.co.uk> wrote:
>
>>In reviewing some of the other replies I noticed that no one had yet
>>(at the top level of the tree) suggested using a signal injector and a
>>trace probe. By injecting a current driven signal onto the power rail
>>and ground you can prod around with the trace probe and determine where
>>the short is by the signal level dropping nearest the location.
>
> What kind of signal would that be ?
The current injection probe I used would put in a square wave at about
1MHz or 10kHz (switch selectable). The trace probe could find this
signal on a shorted line and give an indication of how big it was. When
you got very close to the short you would actually lose the signal.
Works best if the return is on a ground plane but can cope with other
layout practices as well. The probes usually come as pairs.
> Using a variable frequency RF-generator you should be able to
> determine when the tracks and the short is resonated e.g. at 1/2 or
> 1/4 wavelength. After determining the PCB material and hence the
> velocity factor, you should be able to determine the distance from the
> feedpoint.
Another good non-destructive technique.
> One other method would be to inject a sub-nanosecond pulse edge and
> determine, how long it takes for the reflection from the short to
> return back to the generator.
This is also good and has been used to locate the shorts in underground
cables.
[%X]
At least I have got some discussion on the non-destructive methods going.
I know it seems like no fun but there are times when a non-destructive
approach is needed. After all, having to rebuild a board that is charred
is not easy and charcoal is a bugger to solder to. ;>
--
********************************************************************
Paul E. Bennett...............<email://Paul_E....@topmail.co.uk>
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
Paul Carpenter
Paul_E....@topmail.co.uk says...
.....
> [%X]
>
> At least I have got some discussion on the non-destructive methods going.
> I know it seems like no fun but there are times when a non-destructive
> approach is needed. After all, having to rebuild a board that is charred
> is not easy and charcoal is a bugger to solder to. ;>
But most engineers are just like children, only really like things that
make noises, flash or better still explode. Just like trying to teach
science to teenagers.
--
Paul Carpenter | pa...@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/> PC Services
<http://www.pcserviceselectronics.co.uk/fonts/> Timing Diagram Font
<http://www.gnuh8.org.uk/> GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/> For those web sites you hate
Joerg
It most likely wouldn't work very well. A really wide trace of plane
smears out the location too much.
With most of us being in the Wild West I am surprised nobody came up
with the obvious: Take a Smith&Wesson and blast holes into it until the
current drops. That's where the short is. Or rather, was. Just kidding ;-)
Paul Keinanen
<Paul_E....@topmail.co.uk> wrote:
>Paul Keinanen wrote:
>
>> On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
>> <Paul_E....@topmail.co.uk> wrote:
>>
>>>In reviewing some of the other replies I noticed that no one had yet
>>>(at the top level of the tree) suggested using a signal injector and a
>>>trace probe. By injecting a current driven signal onto the power rail
>>>and ground you can prod around with the trace probe and determine where
>>>the short is by the signal level dropping nearest the location.
>>
>> What kind of signal would that be ?
>
>The current injection probe I used would put in a square wave at about
>1MHz or 10kHz (switch selectable). The trace probe could find this
>signal on a shorted line and give an indication of how big it was. When
>you got very close to the short you would actually lose the signal.
>Works best if the return is on a ground plane but can cope with other
>layout practices as well. The probes usually come as pairs.
So I assume this uses the inductive reactance (instead of resistance)
to create a voltage gradient when the test current flows along the
line ? That 1 MHz test frequency sounds quite low, since a PCB track
would have XL about 0.01 ohm/mm @ 1 MHz, so the XL for a 100 mm track
would only be about 1 ohm. The signal generator output impedance would
have to be well below the typical 50 ohms in order to create a
significant voltage at the feed point.
Of course the probe can contain a high gain RF-amplifier to get
meaningful reading. Using RF would eliminate the problems of measuring
very low DC voltage gradients along the line (such as amplifier offset
drift and galvanic voltages caused by dissimilar metals).
Paul
David R Brooks
capacitance as through inductance. You might get many false positives.
I seem to recall H-P many years ago, sold a probe with a Hall effect
device in the tip. You could put a small, controlled DC into the short,
& see which traces it was flowing through. Maybe such a device could be
made up today: there are small Hall-effect ICs around.
Paul Keinanen
Capacitive coupling can be an issue if the probe has a high input
impedance. A probe using a small pickup coil for measuring the
magnetic field created by the RF_current_ or measure directly the
voltage gradient along the PCB track inductive reactance, could have a
low input impedance and the capacitive coupling should not be an
issue.
Paul
Wim Lewis
rickman <gnu...@gmail.com> wrote:
>I seem to recall a magnetic liquid with some very unusual properties.
>I don't recall the exact form, but in all of the images I saw it was
>intact and 3D. It would illustrate some very odd fields, one caused
>it to look like a spiky soccer ball. Anyone know what it was?
Ferrofluid --- basically a fine suspension of iron particles in oil,
I think. The "spiky soccer ball" is pretty characteristic of what it looks
like near a magnet. I think the spikes are not originally from the shape
of the field, but because the surface is somewhat unstable in a strong
enough field.
--
Wim Lewis <wi...@hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1
Jim Granville
> Jim Granville wrote:
>
>>rickman wrote:
>>
>>
>>>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?
>>
>>Sounds ideal - Forget the diode, and find a variac :)
>>
>>If the AC is under 500mV pk, it is unlikely to damage anything
>>and if your 5mV/1A number was right, 500mV will be reached at
>>~100A - something should happen before you wind up that far ;)
>
>
> Actually Variacs are transformers, and can supply lots of current.
> If you can find an old filament winding transformer (About 6.3 V at
> 2A) its output is strictly limited by the number of maggots that
> can run around in its core.
Rick alredy has a transformer (1VAC), so variac is to allow some
simple control on the output voltage, whilst keeping a nice low
impedance.
A nice low-tech solution, but still with control.
-jg