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*Tricky* current measurement.

F

Fred Bartoli

Jan 1, 1970
0
I've some requirements for some challenging voltage measurement (I know,
subject said current, but see later). Namely:
- up to+/-4kV pk WRT ground
- % order accuracy up to 50kHz
- duty cycle max 10%

'til now, no pb.

Then the fun part:
- 80dB CMRR up to 100kHz
- 60dB CMRR up to 10MHz (yes!)
- the signal can go down to 4V and yet 1% accuracy is required (which is
an overall 16b accuracy)

No pb with power drawn by the probe but they don't want maintenance,
periodic cal and so on...

We are politely (well not so politely) asked to do this à la 2x
compensated dividers then diff amplifiers. Yeah...


OK. Now I have to backup that crap with a working solution, or as near
as possible to the specs.

My first thought was a load resistor (told you it was current
measurement:) and two current shielded xfrmrs (two to reject the CM
currents from load to shield) for the BW, and a servoed DC-LF
compensating current, all that almost working like the Tek probe.

Unfortunately, looking closer, the LF-DC part isn't going to be that
easy (mainly Hall sensor noise, offset and low sensibility...) :
- the load can't go much below 100K (10W on average) giving a 40mA-ish
pk current (plus the needed HF zero).
- Hall sensors seems to be somewhat noisy
- can't use multi turn primary, due to the then uncontrollable
parasitics (still CMRR)

Thought about fluxgate but the switching frequency will be well into the
signal BW, so it's probably not OK. Plus I can't think of a simple way
to implement a gate suitable to the design (100, maybe 200 units made).


Any clever idea welcomed.
 
M

MK

Jan 1, 1970
0
"Fred Bartoli"
I've some requirements for some challenging voltage measurement (I know,
subject said current, but see later). Namely:
- up to+/-4kV pk WRT ground
- % order accuracy up to 50kHz
- duty cycle max 10%

'til now, no pb.

Then the fun part:
- 80dB CMRR up to 100kHz
- 60dB CMRR up to 10MHz (yes!)
- the signal can go down to 4V and yet 1% accuracy is required (which is
an overall 16b accuracy)

No pb with power drawn by the probe but they don't want maintenance,
periodic cal and so on...

We are politely (well not so politely) asked to do this à la 2x
compensated dividers then diff amplifiers. Yeah...


OK. Now I have to backup that crap with a working solution, or as near as
possible to the specs.

My first thought was a load resistor (told you it was current
measurement:) and two current shielded xfrmrs (two to reject the CM
currents from load to shield) for the BW, and a servoed DC-LF compensating
current, all that almost working like the Tek probe.

Unfortunately, looking closer, the LF-DC part isn't going to be that easy
(mainly Hall sensor noise, offset and low sensibility...) :
- the load can't go much below 100K (10W on average) giving a 40mA-ish pk
current (plus the needed HF zero).
- Hall sensors seems to be somewhat noisy
- can't use multi turn primary, due to the then uncontrollable parasitics
(still CMRR)

Thought about fluxgate but the switching frequency will be well into the
signal BW, so it's probably not OK. Plus I can't think of a simple way to
implement a gate suitable to the design (100, maybe 200 units made).


Any clever idea welcomed.

Hello Fred,

This will only help a bit because these people make high current sensors:

www.danfysik.com Ultrastab 867-200I transducer.

200A full scale but errors and noise in the 10 ppm range .

If you wind more turns round it the sensitivity goes up but new errors may
come in. If it worked with 100 turns it would be 2 A full scale so errors in
the 20uA region which is 1% of 2mA which would be fine if you could afford
2A full scale current (only 8kW !! :)
With your 40mA at 4kV max you are looking at an equivalent voltage error of
2V which is about 50 times outside your budget.

However you may get some ideas from what they can achieve and the way they
do it - you just need a 4A full scale device and you're there.

Good luck.

Michael Kellett

www.mkesc.co.uk
 
T

Tim Williams

Jan 1, 1970
0
What's wrong with a current transformer? DC capacity?

Maybe you could mix a limited-bandwidth (less noise!) Hall effect signal
with the transformer's signal output?

Tim
 
F

Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :
--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"Fred Bartoli"



What's wrong with a current transformer? DC capacity?

Maybe you could mix a limited-bandwidth (less noise!) Hall effect signal
with the transformer's signal output?

A current transformer alone won't do. You need two, to compensate for
the sensed wire parasitic current to shield/gnd. And then you still have
to be very cautious: with a total 10p across the 100K load, you still
need less than 10fF parasitics imbalance to achieve the 60dB CMRR.
Can't go much over 10pF there because dV/dt are 20-50V/ns which is
already 500mA. Still, 100pF requires less than 0.1pF imbalance, easing
things a bit, but at a 5A switching transient cost.

Otherwise it's more or less what I was thinking about, but in a better
way: if you just mux then you have some gain accuracy pb at crossover
and most probably some non linearity due to the core hysteresis.
Using FB to close a current compensating loop around the core takes care
of most of this.
But Hall sensors are... noisy. Just think:
A 20mm dia toroid, mur=10K, will only give 8mT flux density at my
4kV/40mA peak current.
But 1% of 60dB down is 100dB, so I want less than 80nT rms noise in the
bandwidth in which the Hall sensor will take over, not accounting for
the higher frequencies amplifier part.
And CT, even shorted, have limited LF response so I can't make the Hall
sensor branch crossover too low.

Oh, and I forgot to tell: 10MHz -3dB response, but that part is POC with
CTs.
 
C

Chris Jones

Jan 1, 1970
0
Fred said:
I've some requirements for some challenging voltage measurement (I know,
subject said current, but see later). Namely:
- up to+/-4kV pk WRT ground
- % order accuracy up to 50kHz
- duty cycle max 10%

'til now, no pb.

Then the fun part:
- 80dB CMRR up to 100kHz
- 60dB CMRR up to 10MHz (yes!)
- the signal can go down to 4V and yet 1% accuracy is required (which is
an overall 16b accuracy)

No pb with power drawn by the probe but they don't want maintenance,
periodic cal and so on...

We are politely (well not so politely) asked to do this à la 2x
compensated dividers then diff amplifiers. Yeah...


OK. Now I have to backup that crap with a working solution, or as near
as possible to the specs.

My first thought was a load resistor (told you it was current
measurement:) and two current shielded xfrmrs (two to reject the CM
currents from load to shield) for the BW, and a servoed DC-LF
compensating current, all that almost working like the Tek probe.

Unfortunately, looking closer, the LF-DC part isn't going to be that
easy (mainly Hall sensor noise, offset and low sensibility...) :
- the load can't go much below 100K (10W on average) giving a 40mA-ish
pk current (plus the needed HF zero).
- Hall sensors seems to be somewhat noisy
- can't use multi turn primary, due to the then uncontrollable
parasitics (still CMRR)

Thought about fluxgate but the switching frequency will be well into the
signal BW, so it's probably not OK. Plus I can't think of a simple way
to implement a gate suitable to the design (100, maybe 200 units made).


Any clever idea welcomed.

Why not use an amplifier and ADC, all floating at the high voltage, and then
clock out the output into a fibre-optic link. The ADC and fibre optic
transmitter (and probably a microcontroller) would need some power but
whether that is difficult depends on constraints of cost etc. (e.g. solar
cells + bank of infrared LEDs can transfer a few milliwatts, or for more
power a carefully constructed transformer.)

Chris
 
J

jasen

Jan 1, 1970
0
Unfortunately, looking closer, the LF-DC part isn't going to be that
easy (mainly Hall sensor noise, offset and low sensibility...) :
------------------

sounds like Massive Prong.
Any clever idea welcomed.

except for the above, I'm all out.
 
T

Terry Given

Jan 1, 1970
0
jasen said:
------------------

sounds like Massive Prong.




except for the above, I'm all out.

ROTFLMAO!

Cheers
Terry
 
J

joseph2k

Jan 1, 1970
0
Fred said:
Tim Williams a écrit :

A current transformer alone won't do. You need two, to compensate for
the sensed wire parasitic current to shield/gnd. And then you still have
to be very cautious: with a total 10p across the 100K load, you still
need less than 10fF parasitics imbalance to achieve the 60dB CMRR.
Can't go much over 10pF there because dV/dt are 20-50V/ns which is
already 500mA. Still, 100pF requires less than 0.1pF imbalance, easing
things a bit, but at a 5A switching transient cost.

Otherwise it's more or less what I was thinking about, but in a better
way: if you just mux then you have some gain accuracy pb at crossover
and most probably some non linearity due to the core hysteresis.
Using FB to close a current compensating loop around the core takes care
of most of this.
But Hall sensors are... noisy. Just think:
A 20mm dia toroid, mur=10K, will only give 8mT flux density at my
4kV/40mA peak current.
But 1% of 60dB down is 100dB, so I want less than 80nT rms noise in the
bandwidth in which the Hall sensor will take over, not accounting for
the higher frequencies amplifier part.
And CT, even shorted, have limited LF response so I can't make the Hall
sensor branch crossover too low.

Oh, and I forgot to tell: 10MHz -3dB response, but that part is POC with
CTs.

Perhaps a comapany called "Ion Physics" has some CT type sensors that can do
something close to what you want.
 
R

Robert Baer

Jan 1, 1970
0
joseph2k said:
Fred Bartoli wrote:




Perhaps a comapany called "Ion Physics" has some CT type sensors that can do
something close to what you want.
Some work at SLAC was done in that regard years ago; i forget even
the order of magniture that they were able to achieve, but suffice to
say they pushed existing technology a fair amount.
Application? Measure the (pulsed) beam current.
 
W

Winfield Hill

Jan 1, 1970
0
Fred said:
I've some requirements for some challenging voltage measurement
(I know, subject said current, but see later). Namely:
- up to+/-4kV pk WRT ground
- % order accuracy up to 50kHz
- duty cycle max 10%

'til now, no pb.

Then the fun part:
- 80dB CMRR up to 100kHz
- 60dB CMRR up to 10MHz (yes!)
- the signal can go down to 4V and yet 1% accuracy is required
(which is an overall 16b accuracy)

The 16-bit measurement rate would be under, say, 200kHz? Why
does the CMRR spec go to 10MHz, or are you saying that's an
overall spec that can be met in part with a sharp signal filter?
 
F

Fred Bartoli

Jan 1, 1970
0
Winfield Hill a écrit :
The 16-bit measurement rate would be under, say, 200kHz? Why
does the CMRR spec go to 10MHz, or are you saying that's an
overall spec that can be met in part with a sharp signal filter?

My bet is that the spec is coming from a nice anti-cooperative working
environment.

A (probably unexpectedly) sold a rough prototype, with as much
carelessness designed in as possible (IOW a "demonstrator"), to B under
the pressure of C (if you B still want to sell us in future, go have a
look at what A do) without clear understanding of all the implications.
Then D got a contract to ('just') industrialize the product.
Now that A sold its s..t and that B almost don't understand how it works
(I mean the fine important details), specs are flying over in an attempt
to making all that falling in working order and also probably to cover
one's ass as much as possible.
And as A and B almost don't speak to each other anymore, and A refuses
to speak to D, and B pretend to not being able to disclose some
information, then having the real info is, hem, well hem...
And this is taking interesting proportions.

So, despite that I think the sought CMRR is totally irrelevant we have
to comply to what we are given.
So yep, the CMRR has to be as asked, not a by product of some after
processing.

For the pb I think I've now come up with a nice working solution but it
probably won't see the light since B has now decided to engaged into the
2 x compensated dividers way and some other 'interesting' problems are
pointing their nose.
 
W

Winfield Hill

Jan 1, 1970
0
Fred said:
Winfield Hill a écrit :
The 16-bit measurement rate would be under, say, 200kHz? Why
does the CMRR spec go to 10MHz, or are you saying that's an
overall spec that can be met in part with a sharp signal filter?

My bet is that the spec is coming from a nice anti-cooperative
working environment.

A (probably unexpectedly) sold a rough prototype, with as much
carelessness designed in as possible (IOW a "demonstrator"), to
B under the pressure of C (if you B still want to sell us in
future, go have a look at what A do) without clear understanding
of all the implications. Then D got a contract to ('just')
industrialize the product. Now that A sold its s..t and that
B almost don't understand how it works ... [ snip ] [ snip ]
For the pb I think I've now come up with a nice working solution
but it probably won't see the light since B has now decided to
engaged into the 2 x compensated dividers way and some other
'interesting' problems are pointing their nose.

Sheesh!
 
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