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Author Topic: How to measure high current pulses without the noise?  (Read 3381 times)

Offline pomodoro

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How to measure high current pulses without the noise?
« on: July 22, 2015, 02:20:27 PM »
Guys, I need help. I've read a mountain of research papers from the wire explosion days and have tried to make my own sensors for measuring fast pulses of 100A plus from discharging 4-10 uF caps at 1000V plus into inductors.

I have tried building a Rogowsky Coil with coax, as these are supposedly the most noise immune sensors, but have found that pulses from the discharge enter the oscilloscope even when the sensor is a few feet away from the discharge.
Not only that, but a coax cable, terminated with 50 ohms at the scope and soldered short at the other end still picks up the discharge.

Even a bnc hookup coax simply connected to the scope picks up a pulse.
But without a connection to the input, the scope pick up nothing.

I cant figure out how these guys in the 60's faithfully measured kiloamps , which gave only a few mV from the Rogowsky coils, and here I am picking up 100's of mV with a shorted coax.

How do  I make something totally immune to pulses that are not from a current flowing through it , yet retains a good bandwidth?

Free Energy | searching for free energy and discussing free energy


Offline MarkE

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Re: How to measure high current pulses without the noise?
« Reply #1 on: July 23, 2015, 10:13:23 PM »
Guys, I need help. I've read a mountain of research papers from the wire explosion days and have tried to make my own sensors for measuring fast pulses of 100A plus from discharging 4-10 uF caps at 1000V plus into inductors.

I have tried building a Rogowsky Coil with coax, as these are supposedly the most noise immune sensors, but have found that pulses from the discharge enter the oscilloscope even when the sensor is a few feet away from the discharge.
Not only that, but a coax cable, terminated with 50 ohms at the scope and soldered short at the other end still picks up the discharge.

Even a bnc hookup coax simply connected to the scope picks up a pulse.
But without a connection to the input, the scope pick up nothing.

I cant figure out how these guys in the 60's faithfully measured kiloamps , which gave only a few mV from the Rogowsky coils, and here I am picking up 100's of mV with a shorted coax.

How do  I make something totally immune to pulses that are not from a current flowing through it , yet retains a good bandwidth?
Yours is a basic shielding problem.  Noise can get into your oscilloscope through several paths.  One path is cross talk between oscilloscope channels.  Another path is whatever exposed loop area is at the end of your coax.  A third is the less than infinite shielding effectiveness of braided coax.  A fourth is the effective antenna formed by the AC power cable. 

If you shield the experiment by putting it in an RF screen room then junk won't get into your oscilloscope through the AC power.
If you can't do that, you can put as many ferrite clamp on filters over the coax and AC power leads as possible to mitigate issues with the cables.
You can troubleshoot using just one oscilloscope channel to eliminate cross talk issues while you fix the other problems.



Offline minnie

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Re: How to measure high current pulses without the noise?
« Reply #2 on: July 23, 2015, 10:25:46 PM »



 Sounds like you could use a Krytron!!
            John.

Offline pomodoro

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Re: How to measure high current pulses without the noise?
« Reply #3 on: July 24, 2015, 02:15:09 PM »
Cheers Mark, your idea of using those ferrite noise suppressors worked quite well. I used some from old monitor cables.

Unfortunately the Rogowsky coil simply had way too little output and was swamped even by the greatly reduced noise.  However, the same ferrite toroids came in handy as I used one to make a current transformer instead of the Rogowsky coil.  Similar to the Pearson coils but at no cost compared to $ hundreds.

 This simple device I made has a lower 3db limit of 2.5kHZ and the upper beyond 5Mhz. It outputs 1V per amp and is very linear .  This huge voltage output  is not affected by any stray pulses and easily measures 300A , which is more than I need for my experiments.


Offline MarkE

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Re: How to measure high current pulses without the noise?
« Reply #4 on: July 24, 2015, 02:59:08 PM »
Cheers Mark, your idea of using those ferrite noise suppressors worked quite well. I used some from old monitor cables.

Unfortunately the Rogowsky coil simply had way too little output and was swamped even by the greatly reduced noise.  However, the same ferrite toroids came in handy as I used one to make a current transformer instead of the Rogowsky coil.  Similar to the Pearson coils but at no cost compared to $ hundreds.

 This simple device I made has a lower 3db limit of 2.5kHZ and the upper beyond 5Mhz. It outputs 1V per amp and is very linear .  This huge voltage output  is not affected by any stray pulses and easily measures 300A , which is more than I need for my experiments.
Pomodoro, I am glad the ferrites did the trick.  I would take the transformer down so that the output remains under scope input amplifier maximum: typically ~+/-40V.  I would aim for +/-10Vpp for your hottest signal into 50 Ohms which for a sine wave would limit the dissipation in the oscilloscope front end to ~1W.

Free Energy | searching for free energy and discussing free energy

Re: How to measure high current pulses without the noise?
« Reply #4 on: July 24, 2015, 02:59:08 PM »
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Offline fritz

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Re: How to measure high current pulses without the noise?
« Reply #5 on: July 24, 2015, 03:16:34 PM »
Hi,


I would use similar setup used for EMV measurements.
1) large grounded sheet metal plane as reference plane
2) strip grounded oscilloscope on the reference plane
3) isolated differential probe to convert differential signal to unbalanced scope input
http://www.tek.com/oscilloscope-probes-and-accessories/differential-probe-high-voltage
4) A shunt made out of konstantan or similar sheet metal with twisted pair from the ends coupling to the differential probe
5) ground the shunt (only grounding point of setup) in the middle as symmetric as possible
6) Using capacitors with symmetric parasitic capacity to ambient potential (don´t use metal cap types - because one end has more capacity to ground than other)
7) Try to arrange your setup in a way that impedances regarding ground (reference plane)on both shunt ends  are pretty the same (if you want to measure Mhz - this impedance should work in this range also)


-> Try to get as minimal common mode disturbancies as possible.


Thats straight forward - and should be no problem with few 100kA
For shunt construction - you should think about total energy consumed, thermal capacity of shunt (....)


rgds.






Offline MarkE

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Re: How to measure high current pulses without the noise?
« Reply #6 on: July 24, 2015, 03:23:41 PM »
Hi,


I would use similar setup used for EMV measurements.
1) large grounded sheet metal plane as reference plane
2) strip grounded oscilloscope on the reference plane
3) isolated differential probe to convert differential signal to unbalanced scope input
http://www.tek.com/oscilloscope-probes-and-accessories/differential-probe-high-voltage
4) A shunt made out of konstantan or similar sheet metal with twisted pair from the ends coupling to the differential probe
5) ground the shunt (only grounding point of setup) in the middle as symmetric as possible
6) Using capacitors with symmetric parasitic capacity to ambient potential (don´t use metal cap types - because one end has more capacity to ground than other)
7) Try to arrange your setup in a way that impedances regarding ground (reference plane)on both shunt ends  are pretty the same (if you want to measure Mhz - this impedance should work in this range also)


-> Try to get as minimal common mode disturbancies as possible.


Thats straight forward - and should be no problem with few 100kA
For shunt construction - you should think about total energy consumed, thermal capacity of shunt (....)


rgds.
The common mode rejection of those probes is down to ~30dB at a couple of MHz and further degrades to their bandwidth limit.  Armoring the power and probe cables with clamp on ferrites is often the only effective way to keep common mode noise from getting into the oscilloscope vertical amplifiers without putting the EUT in a screen room.  The big reference plate under the scope that you suggest is sort of a DIY screen room that is not fully enclosed.

Free Energy | searching for free energy and discussing free energy

Re: How to measure high current pulses without the noise?
« Reply #6 on: July 24, 2015, 03:23:41 PM »
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