rickman 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?
> 

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

On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
<P...@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

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?

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.

On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
<P...@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

rickman wrote:
> On Jul 23, 12:40 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> rickman wrote:
>>> On Jul 23, 3:17 am, Neil <NeilKu...@worldnet.att.net> wrote:
>>>> rickman 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
>>>> If it is a lot of board, and they are not too expensive, Try more
>>>> 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.
>>> 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!
>> 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
> 
> 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?


> 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).

-- 
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

dalai lamah wrote:
> Un bel giorno rickman digitò:
> 
>> 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.

On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
wrote:
> dalai lamah wrote:
> > Un bel giorno rickman digit=F2:
>
> >> 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 d=
o
> > 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

On Jul 23, 4:17 pm, Paul Keinanen <keina...@sci.fi> wrote:
> On Wed, 23 Jul 2008 19:49:58 +0100, "Paul E. Bennett"
>
> <Paul_E.Benn...@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.

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

rickman 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.  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

rickman wrote:

> On Jul 23, 11:17 am, Mark Borgerson <mborger...@comcast.net> wrote:
> 
>>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.

It can be a problem if etchant is able to sneak into the plated hole,
you can guess what happens next!

-jg