Troubleshooting 'shorted' power rails

You can get a full thermal camera attachment for Android from the state store for $200, sometimes less like $15-20 off on sale from time to time:

Thermal imagers are getting affordable and are great to have around. Not only can they find shorts and bad parts, it's good to image a new design and see if anything runs hot.

Our test people use a thermal imager too, to spot unusual stuff.

One eternal problem is electronics is "where is the current going?"

Sometimes you can measure microvolt drops in traces and planes to answer that question. It would be great if we could see mag fields!

What was it?

But the cheap ones have low res. What do you think is minimum useful res?

Don't know, but what is important is that it can focus really close to the parts. Some imagers are fixed-focussed at infinity, useless for electronics.

Seek Thermal is quite adequate for the task.

In many cases, the thermal profile is low resolution. A heat source anywhere near anything else will warm that and 'fuzz out' any possibility of extremely high resolution.

If you get one, make SURE you get one with variable focus. The very early ones were fixed focus and can't get close enough. The newer ones have the SAME PART NUMBER and variable focus. Make sure the lens rotates.

There is mag field viewing film. And you can make your own viewer by dropping iron filings in a shampoo bottle. Crude but I've seen it demoed and work.

NT

oops, baby oil not shampoo

NT

Microvolt sensitive DVM's are cheaper, if not as quick. Any power trace that show a voltage drop points to the short, so it doesn't take long to get it localised. And you don't have to wait for the board to warm up.

HP made an AC current probe/tracer. In combination with the pulser, you can lay it on a trace and determine which way the current goes with the tracer. Easy on outer layers. Not so easy on inner layers or with multiple ground planes in the way.

Thermal imager is easier...and a good excuse to buy a new toy.

I had used a Tektronix DC current probe at one point in time to follow current.

Greg

It can be done. The old Kyread solution (spray=on to floppies or mag tape, it shows the tracks) is one way

and ferrofluid is another. The sheet gizmos (actually a lot of microspheres with loose grains inside) are fun to play with, but only old 8" floppies have coarse enough tracks to show up on those.

I'd have to be really desperate before I'd apply any kind of goo to my circuit board to find a short. I've always homed in on the short by following the voltage gradient with a sensitive voltmeter. It's easy if the current flows in discrete tracks, but I've also found hard shorts between full power planes on big boards full of ECL logic that way.

Jeroen Belleman

Right that's what I do, (measure DC mV with ~20 mA of current) My power supply tends to look dendritic and I don't know how well the DC trick works with power planes.

George H.

A real short can stand an amp or more of current. A 1oz copper sheet is about 500 uohms per square so you get lots of signal. The polarity and voltage pattern can lead you to a short.

If you order 1 oz copper, you'll rarely get 1 oz, so there's even more voltage!

We have all our PCBs bare-board tested, so we don't see PCB internal shorts any more.

Lately I put a 0.1 ohm resistor in series with every voltage regulator, so I can snoop supply currents. That has lots of uses.

The mag fields would be milligauss, way too low for iron filing sorts of tricks.

I remember the Shortsqueak. Like a logic probe. Worked on resistance, and changed pitch as you got closer to the short. I think Nuts&volts had something similar too. Cheers

Many smartphones, even cheap ones, come with three-axis Hall effect sensors built in nowatimes. With a free app you can always have a tool for sensing e.g. hidden wires in walls or whether an outlet somewhere is getting power

In fact it would probably be possible to write an app that lets you sweep the phone thru an volume of space and generate a 3D graphical representation of the field strength magnitude in that space. Hmm, maybe there's a market for that.