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.
Il 23/07/2008 17.53, rickman ha scritto: [snip]
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.
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
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
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
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
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
Un bel giorno rickman digitò:
Tsk, amateurs. Use a 12V battery car instead, and let monsieur ampere do his job. :)
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.
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
Hi Rick.
Must be the stuff marked under the name Ferro-Liquid.
Fun stuff for sure..
Nils
Hah! - no fun - AMPS...
Geo
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.
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
That's going to cost you in winter.
Why is everyone concerned about Radon? Does it really occur a lot?
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).
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.
o
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
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
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
It can be a problem if etchant is able to sneak into the plated hole, you can guess what happens next!
-jg
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