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 ;)

-jg

Paul Keinanen wrote:
> 
> 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

rickman wrote:
> 
> 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

rickman wrote:
> 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

rickman wrote:
> On Jul 23, 6:17 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> 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.
> 
> 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.

rickman wrote:
> 
... 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.

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.  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.

-- 
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: <http://cbfalconer.home.att.net>;
            Try the download section.

On 2008-07-23, Joerg <n...@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. ;)


-- 
Grant Edwards                   grante             Yow! Did I do an INCORRECT
                                  at               THING??
                               visi.com            

On Wed, 23 Jul 2008 14:44:20 -0700, Joerg
<n...@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
 

CBFalconer wrote:
> 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