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
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
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
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
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.
... snip ...
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]:
Try the download section.
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]:
Try the download section.
The subject is finding the short, not fixing the short. ;)
--
Grant Edwards grante Yow! Did I do an INCORRECT
at THING??
visi.com
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
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
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
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Original message:
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.
Nice! I will give it a shot next time.
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
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" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
In the distant past I have used a milli-ohm meter for this, but alas, those 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.
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/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
do
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
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.Another good non-destructive technique.
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............... Forth based HIDECS Consultancy Mob: +44 (0)7811-639972 Tel: +44 (0)1235-811095 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
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