On 7/4/2018 7:20 PM, Jeff Liebermann wrote:
On Tue, 03 Jul 2018 12:36:23 -0700, mike wrote:

On 7/3/2018 9:43 AM, Jeff Liebermann wrote:
I built a crude current
probe using a Hall effect device at the end that is useful for finding
which device is sucking the most current.

I've been considering doing that.
Can you share details about which parts you used?
What kind of sensitivity can you get?
Thanks, mike

I tried to find the one I built but couldn't. So, I do it from
memory.

The "probe" is a wooden coffee stirring rod that I stole from the
local coffee shop. Stuck onto the end was a linear Hall effect device
in a TO-92 package. As I recall, it was a TI DRV5053:
http://www.ti.com/product/DRV5053
http://www.ti.com/product/DRV5053/datasheet
http://www.ti.com/lit/ds/symlink/drv5053.pdf
It comes in several sensitivity ranges. Methinks it was EA or +45
volts/Tesla. That's not very sensitive, but more on that later.

The schematic was very simple. See 8.2.2. on the data sheet or:
http://www.ti.com/product/DRV5053/datasheet/application-and-implementation#SLIS1534512
The output went to crude x1000 LM358 DC amplifier and then to my TEK
2247A scope on the lowest 2mv/div scale.

The reason I threw this together was that I was trying to fix a
no-name Chinese solar inverter that had about 32 IGBT devices in
parallel in 4 strings. One device was probably shorted, but I
couldn't tell by probing. I also couldn't run it long enough to check
which device got warm. Checking the gates showed nothing useful. So,
my last resort was to try and measure the collector current without
breaking any wires. Looking for a switching waveform on the collector
would have been so easy, except that some protection circuit turned
off the drive oscillator because of the high current. I tried an
inductive pickup, but it just picked up junk from the adjacent working
devices.

I had just enough exposed collector lead accessible to sense a
magnetic field. I had a sample DRV5053 device, so I quickly threw it
together on the wooden stick and looked for some indication of a
magnetic field. After some tinkering, I could just barely see the
current on the scope. Walking it down the line, I found that 3
devices had shorted. I removed the shorted device but did not
initially replace them. It worked. I then made a guess as to some
potential replacements, actually found some in my pile, and proclaimed
the inverter to be repaired (until some better parts arrived in the
mail). Note that this repair was done from start to finish in about
an hour.

The key here was that it was only going to work with fairly high
currents. Judging by my automotive magnetic field ammeter, the
inverter was sucking about 30A with the blown devices which was
probably distributed as 10A per device. So, how much sensitivity will
it take to get a 1 division (2mv) change on the scope? Plugging into
a handy Ampere's Law calculator at:
https://getcalc.com/physics-amperes-law-calculator.htm
for 10A at 2mm radius (my guess), I get a 4x10^-8 Tesla field. The
Hall effect device has a sensitivity of -90V/T for an output change of
a fabulous:
4x10^-8T * 45V/T = 1.8 uV
with a gain of x1000, that becomes 1.2mv, which I could just barely
see under the noise and garbage on the scope.

So, you're welcome to build such a device, but I don't think it will
be a troubleshooters dream current probe, that can measure DC currents
without breaking the line. You might want to look into IMC
(integrated magnetic concentrator) type Hall effect structures. By
grabbing more of the magnetic field and concentrating it into the tiny
Hall effect chip area, it produces a higher reading and therefore a
higher sensitivity. The problem is that the pickup toroid or core
needs to go around the wire, which defeats the basic purpose of the
no-contact current probe. However, I suspect that some kind of probe
can be contrived with will sense more of the magnetic field than the
tiny chip, which will help with the sensitivity problem.
"Measuring Current with IMC Hall Effect Technology"
https://www.sensorsmag.com/components/measuring-current-imc-hall-effect-technology


Thanks,
Looks like that's not gonna be worth the trouble for me. Too little
sensitivity.
I had some success with AC fields using the tiniest torroid core
I could find. I cut a slot in the core and wound it full of tiny wire.
Took about a dozen tries to cut a slot without breaking the core...good
times...
In your case, proximity to other strong fields would be an issue.

For stuff that's actually shorted, I've had great success using the
HP current tracer and pulser. I can stick the pulse on a trace and
tell which direction the current is going. Also useful finding
bad caps.