Re: [SI-LIST] : voltage measurements using a current probe

Charles R. Patton (patton@dt.wdc.com)
Tue, 14 Sep 1999 15:38:06 -0700

Doug,
Now I am confused. You mention, "...the current probe should be well shielded if constructed properly and as such would not respond to E fields on the conductor," which I fully agree with, but when this thread started I went to
your site and as I recall, the tech note I thought you were referring to was:
Technical Tidbit, August 1999: The Paperclip Magnetic Probe
at
http://emcesd.com/tt080699.htm
No shielding is shown on this probe, which is why I was throwing out the directional coupler thought. When I build bench-top current sense loops of this style, I will often construct them from small diameter coax such as RG-174 or
178 and tie the center conductor back to the shield such as: (use a fixed space font such as Courier for the following)
-----------------------------+
----------------------------+|
------------------o--------+||
| |||
COAX ||| |||
||| |||
||| |||
|||-------+||
|+---------+|
+-----------+
Which of course does the shielding while not shorting the turn.

Did you have another note you were referring to that I missed?

Regards,
Charles R. Patton

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Charles R. Patton, Sr. Princ. Eng
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Douglas C. Smith wrote:

> Hi Charles,
>
> Interesting thought, however the current probe should be well shielded
> if constructed properly and as such would not respond to E fields on
> the conductor. Not sure if I am missing something.
>
> What I am measuring is just the series drop along the wire. The
> voltage between the wire and ground or the probe should not affect the
> measurement.
>
> Does this seem reasonable?
>
> Thanks,
>
> Doug
>
> "Charles R. Patton" wrote:
> >
> > Doug,
> > I have a question re your "voltage measurements using a current probe"
> > after looking at the tech note. It would seem to me that the method is
> > suspect as it is exactly the same configuration as used for directional
> > couplers. So if there is an accompanying voltage swing, as there would
> > normally be for most signal wires, then how would you avoid confusing
> > the capacitive voltage coupling from the inductive di/dt coupling? It
> > would seem that twisting the probe 180 degrees would discern that the
> > effect is taking place, just as it does in directional couplers (i.e.,
> > Bird wattmeters) but without knowing the voltage/current ratio on the
> > line (in effect the line impedance) then I would guess that it would be
> > hard to extract the two simultaneous effects.
> > Sincerely,
> > Charles R. Patton

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