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Re: Current Transformer problems



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Indeed... and for precision measurements, what you should do is put in a
current to voltage converter (zero input impedance configuration with
wideband op amp), although the series resistance of the wire and the ferrite
loss will perturb it somewhat.  For run of the mill measurements, a suitable
low value resistor (50 ohms sounds nice) feeding a high impedance sensor
(oscilloscope) will probably work fairly well.  If you are worried about
waveshape, then you need to do some analysis because of the various RC time
constants involved, or set up suitable compensation circuits (as used in a
scope probe) to make it into an appropriate terminated RC voltage divider.

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, November 27, 2001 11:57 AM
Subject: Re: Current Transformer problems


> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
>
> Hi Matt,
>
> On 27 Nov 2001, at 7:46, Tesla list wrote:
>
> > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <Mddeming-at-aol-dot-com>
> >
> > In a message dated 11/26/01 11:28:06 PM Eastern Standard Time,
> tesla-at-pupman-dot-com
> > writes:
> >
> >
> >
> > >
> > > Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> > >
> > > Hi Matt,
> > >
> > > The current transformer needs a low value resistor across it.  Like 10
> ohms.
> > > The voltage across the resistor represents the transformed current.
> > >
> > > Without a resistor, you get high voltages with all kinds off odd
signals.
> > >
> > > Cheers,
> > >
> > >         Terry
> > >
> >
> >
> > 11:45 PM EST
> > Hi Terry,
> > I didn't have any small resistors available tonight, so I tried a quick
&
> dirty
> > 620-Ohm one. That fixed the problem 80% - Great. HF ringing is all gone
but
> > still has some overtones. Will try different value resistors tomorrow.
> > Thanks a million,
> > Matt D.
>
> There is only one value to use: the one you need to use. You need to
> know the secondary turns *exactly*. "About 150" isn't quite good
> enough. You converting a current into a voltage in order to measure
> it.
>     Example: Np = 1 turn, Ns = 150 turns, burden resistor = 150 Ohms.
> Suppose 1 Amp flows in the primary. Then 1/150 Amps through the 150
> Ohm resistor will give 1 Volt across the resistor.
>
>       The resistor really needs to be as low a value as gives a
> useful output voltage for the current being measured so that flux
> cancellation in the transformer core is close to complete.
>
> Malcolm
>
>
>
>
>