Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?

On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?


Problem I can see is that the hall effect IC, which I presume is the
device suggested for the "probe" is likely to be a switch rather than an
analogue sensor.

There are some cheap sensors about:
http://arduinosensors.com/index.php/...th-an-arduino/

Lee wrote:
On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?


Problem I can see is that the hall effect IC, which I presume is the
device suggested for the "probe" is likely to be a switch rather than an
analogue sensor.

There are some cheap sensors about:
http://arduinosensors.com/index.php/...th-an-arduino/


The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

On Sun, 07 Feb 2016 22:14:39 +0000, Bob Minchin wrote:

The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

On 08/02/16 00:24, Chris wrote:
On Sun, 07 Feb 2016 22:14:39 +0000, Bob Minchin wrote:

The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

No 'of' with 'comprise...'

A consists of lots of B's.
A comprises lots of B's.

GrammarNazi ;-)



--
If I had all the money I've spent on drink...
...I'd spend it on drink.

Sir Henry (at Rawlinson's End)

On Mon, 08 Feb 2016 08:06:24 +0000, The Natural Philosopher wrote:

On 08/02/16 00:24, Chris wrote:
On Sun, 07 Feb 2016 22:14:39 +0000, Bob Minchin wrote:

The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

No 'of' with 'comprise...'

A consists of lots of B's.
A comprises lots of B's.

GrammarNazi ;-)

I hate it when people get that one wrong. The university PR people had
this error on theor website the other day.

On Monday, 8 February 2016 08:06:27 UTC, The Natural Philosopher wrote:
On 08/02/16 00:24, Chris wrote:

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

No 'of' with 'comprise...'

A consists of lots of B's.
A comprises lots of B's.

GrammarNazi ;-)

obviously it's Bs not B's.


NT

On 08/02/2016 00:24, Chris wrote:
On Sun, 07 Feb 2016 22:14:39 +0000, Bob Minchin wrote:

The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

Any reasonably new drive - say 10GB up - will use a magnetoresistive
read head. It's resistance changes with magnetic field strength.

I've used several different magnetoresistive devices, but not from disk
drives. The newest devices can be very sensitive - www.nve.com makes
several types, again, not for disk drives.

They have an application note which may be of interest...

http://www.nve.com/Downloads/SB-SA-0...MR_Sensors.pdf

"GMR Sensors" - Giant magnetoresistance - is a good search term.

Cheers
--
Syd

Syd Rumpo wrote:
On 08/02/2016 00:24, Chris wrote:
On Sun, 07 Feb 2016 22:14:39 +0000, Bob Minchin wrote:

The read/write head from a disc drive would be a good start to
experiment with an AC current probe.
DC will be harder and need a linear Hall sensor.

Yeah, that's it: the r/w head. Does anyone know what these things
comprise of?

Any reasonably new drive - say 10GB up - will use a magnetoresistive
read head. It's resistance changes with magnetic field strength.

I've used several different magnetoresistive devices, but not from disk
drives. The newest devices can be very sensitive - www.nve.com makes
several types, again, not for disk drives.

They have an application note which may be of interest...

http://www.nve.com/Downloads/SB-SA-0...MR_Sensors.pdf


"GMR Sensors" - Giant magnetoresistance - is a good search term.

Cheers
It is possible that the electronics used to interface to the magneto
resistive sensor in the HDD might be usable but could well be too
optimised to the application and so not linear enough. The interface and
maybe the sensor itself might only have a narrow bandwidth linked to the
data rate off the disk which will be fixed.
If you roll your own interface, the key will be optimal matching to the
impedance of the sensor to get best signal to noise as well as some form
of equalisation. Something like a RIAA curve turntable pickup preamp
might be a good starting point.

After a while you might be excused for starting to think that what you
get in a scope current probe is actually quite good value!

Much interesting info here that might be of use:

http://www.intersil.com/content/dam/...rs/amplifiers/
current-sense-measurements.pdf

On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?

Cut the track with a scalpel, scrape the coating off, solder an sm
resistor across the break and measure the voltage drop. Obviously 1 ohm
will give 10mV at 10mA and 10mV may not matter, depending on your
circuit. When finished, take off the resistor and solder across the cut.

Messy, but cheap and potentially (!) accurate.

Cheers
--
Syd

On Wednesday, 10 February 2016 09:41:37 UTC, Syd Rumpo wrote:
On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?

Cut the track with a scalpel, scrape the coating off, solder an sm
resistor across the break and measure the voltage drop.

but that doesn't give you non-contact probing.

Make measurements via non-contact probing



Obviously 1 ohm
will give 10mV at 10mA and 10mV may not matter, depending on your
circuit. When finished, take off the resistor and solder across the cut.

Messy, but cheap and potentially (!) accurate.

and not very practical for those needing this type of instrument.


Of course if anyone has a better method then they can do it can't they.

Then there's those DIYer's that when they can't find the crewdriver a hammer does the same job.

On 10/02/2016 10:34, whisky-dave wrote:
On Wednesday, 10 February 2016 09:41:37 UTC, Syd Rumpo wrote:
On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component in
the disk drive, perhaps?) and there may also be a sensor in the power
supply section that deals with power factor correction, I gather. I need
something that can sense down to say 10mA or better that's (ideally) fine
enough to sample individual pcb traces a few mm apart.
Any ideas?

Cut the track with a scalpel, scrape the coating off, solder an sm
resistor across the break and measure the voltage drop.

but that doesn't give you non-contact probing.

**** me, you're right! I hadn't thought of that!

twaddle snipped

Cheers
--
Syd

On Wed, 10 Feb 2016 10:47:06 +0000, Syd Rumpo wrote:

On 10/02/2016 10:34, whisky-dave wrote:
On Wednesday, 10 February 2016 09:41:37 UTC, Syd Rumpo wrote:
On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component
in the disk drive, perhaps?) and there may also be a sensor in the
power supply section that deals with power factor correction, I
gather. I need something that can sense down to say 10mA or better
that's (ideally) fine enough to sample individual pcb traces a few mm
apart.
Any ideas?

Cut the track with a scalpel, scrape the coating off, solder an sm
resistor across the break and measure the voltage drop.

but that doesn't give you non-contact probing.

**** me, you're right! I hadn't thought of that!

If you're going to break the track, why bother with soldering in a sense
resistor? Might as well just stick an ammeter straight across the bridge!

On 10/02/2016 12:26, Cursitor Doom wrote:
On Wed, 10 Feb 2016 10:47:06 +0000, Syd Rumpo wrote:

On 10/02/2016 10:34, whisky-dave wrote:
On Wednesday, 10 February 2016 09:41:37 UTC, Syd Rumpo wrote:
On 07/02/2016 18:21, Chris wrote:
Current probes for oscilloscopes are nice to have, but very pricey. I
vaguely recall someone somewhere sometime saying that you can make a
pretty good one up from certain computer scrap parts (some component
in the disk drive, perhaps?) and there may also be a sensor in the
power supply section that deals with power factor correction, I
gather. I need something that can sense down to say 10mA or better
that's (ideally) fine enough to sample individual pcb traces a few mm
apart.
Any ideas?

Cut the track with a scalpel, scrape the coating off, solder an sm
resistor across the break and measure the voltage drop.

but that doesn't give you non-contact probing.

**** me, you're right! I hadn't thought of that!

If you're going to break the track, why bother with soldering in a sense
resistor? Might as well just stick an ammeter straight across the bridge!

Yes, you could, depending on the circuit. The resistor is maybe easier
- it doesn't matter if the probes drop off or are intermittent.

Cheers
--
Syd