In the first case, wrong parts stuffed. Digikey switched labels on the bags, and the assembly house missed it.
The second board had Vcc & ground swapped on an ICs. I'm not sure if the designer did that, or the layout guy.
In both cases the result was a diode-drop from power to ground, and power supplies that wouldn't power up.
Cheers, James Arthur
I actually posted that here some time ago. But I've gotten along fine without one. Besides, I don't really need Google-candy tracking me just so that I can do electronic stuff.
A 'smart' phone and all that baggage seems like vast overkill when a $14 thermometer does 85% of the same thing, and without even spying on me .
Cheers, James Arthur
power supplies that wouldn't come up above a diodedrop, and I was able to use a trick devised for the first one tosolve the problem on the second one , too --I set a current-limited supply to ~30mA & left the boardto stabiliz e thermally. I then looked for the 'hot' partswith an IR thermometer. Work ed like a charm.Temp. rise was about 1oC @ 20mW. The IR thermometer? $14we ll-spent.Cheers,James Arthur
One of my most-treasured tools is a buzz-box I made ages ago from an LM3909, that lets you easily hear the difference between a short, an ohm, and tens of ohms.
But that wouldn't have been much help here.
Since it was acting like a diode, I considered using freeze spray and watching the voltage until I hit a part that made the rail wiggle appropriately.
Cheers, James Arthur
One pixel worked decently well for me.
YMMV.
Cheers, James Arthur
But a microvolt DVM can't track down a diode-to-ground, which was the problem.
Cheers, James Arthur
I've got a pal who'd loan me his FLIR, but it was a lot faster this time just to whip out the 1-pixel wonder.
TDR might be good for finding shorted planes. But boards and parts are generally so good these days, I've never had that happen.
Cheers, James Arthur
If you pump enough current into the short, the clues from measured voltage drops are higher. A potential problem with thermal debugging is, the guilty short may not dissipate much heat.
--
Thanks,
- Win
Yeah I'm pretty good at that.
But seriously how can a TDR find a short on a plane?
Launch the pulse from two sides of the plane. The delay time to reflection lets you draw a circle around each of the launch points. Where the circles intersect should be the local of your short.
If the circles interesect at two points, launch a third pulse from yet another point on the edge.
-- Bill Sloman, Sydney
Yes, it can. If starting from the V+ rail, use the DVM to measure drops at each fork, and see which one is carrying current. Voltages on the ones that aren't don't change. Eventually this narrows down to the offending item. As you progress past the item, the voltage drops stay the same, revealing the spot.
--
Thanks,
- Win
Or launch a pulse from one point, and look at the arrival at two places. Like a Rotman Lens. I'd love to see some discussion of those here - has anyone implemented one?
Clifford Heath.
TDRing a plane wouldn't be useful to find a short. A pcb plane is a very lossy transmission line/capacitor with some weak edge and corner reflections.
On a loaded board with parts and bypass caps, it would be even worse.
Besides, your board would have to have multiple SMA connectors or equivalent into all its planes.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Just leave some probe points, solder coax to them. The first 0R reflection should be from the short to the two probe points, and the time differential would allow some triangulation. You just need to zero out the respective delays on two ports.
Have you tried a TDR setup with one Tx and more than one sampler? I did a similar thing with ultrasound once, it was surprisingly effective.
I certainly understand the theory, I just don't see this application of it.
Let's see -- my test current was 30mA, the most I dared without risking a board full of pricey ICs. That'll make roughly 12uV across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane. Doesn't that sound like a bit of a handful to hand-probe?
It would help if the fault were at constant temperature, but it isn't. Vf ramps down as the component warms up from the dissipation.
I suppose one could let the board thermally stabilize, then follow the uV drops, being careful not to touch and warm one's test probes.
In my first case there were five faults (five wrong components stuffed), which would be mighty fancy tracking / simultaneous equation-solving with a voltmeter of any kind.
Hitting it with freeze spray while watching Vf would be fast and fun. A -10C step would be easy to see (maybe even a good time to drag out an old-school analog VOM :-).
Cheers, James Arthur
I figured one could just bang the plane in X, then Y, then get the fault's coordinates from timing the early reflections. Unfortunately John Larkin, TDR maestro of the known universe, just dumped an ocean of cold water on that notion.
:-)
Cheers, James Arthur
It's a lot easier if you use a proper four-terminal milli-ohm meter with a sinusoidal or chopper bidirectional current source and synchronously rectify the differential voltage signal between the potential leads.
Hook up the current leads where you like, and use the potential leads to see where the current is flowing. A shorting diode will only carry current one way, so you only see half the voltage swing, but that should be plenty
Not a great deal.
That's where AC excitation and synchronous detection helps - thermal time constants are relatively long and the accompanying microvolt thermocouple voltages don't vary far enough to show up a signal.
Persistence would do it. There would be tracks/paths where current was flowing (in a measurable direction) and the rest.
That would work to some extent. Ten degrees Celcius is 20mV on Vf.
Forward diode drops do tend to be all over the place, so one of your five "wrong component" diodes is likely to be carrying most of the current and you'd have to freeze that one to find it, then fix it and move on the next.
-- Bill Sloman, Sydney
John Larkin has some interesting ideas about his place in the known universe, some of which suggest that he knows less about the known universe than he thinks, which is to say the universe he knows about may be smaller than the real one.
-- Bill Sloman, Sydney
Hi James I like your technique. It's too bad we don't have better built in heat sensors (like a snake). (Our lips are fair... but almost kissing a pcb is going to get one looks from colleagues. :^)
With the DC current idea, could you try and track it down by looking at the voltage drop along the ground current return line? (Right... maybe uV's as you guesstimate above.)
George H.
Well, _my_ PCBs are almost that good looking. (But not quite.) ;)
A chopamp running at some very large gain driving an audio VCO and speaker might be useful. It would be more useful with some diodes across the feedback resistor to extend the range to higher voltage drops. Have to try that one of these times.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Don't be shy, pump in an amp or two from a power supply set to make
0.5 volts max. Then use a DVM with uV resolution. Probe any pairs of points where the plane is accessable, parts or vias, and see which direction the current is flowing.But personally, first I'd crank in a lot of current and thermal image.
Multiple shorts calls for amps and thermal imaging.
Fire your assembler!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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