Re: Current Transformer problems

["Tesla list" <tesla-at-pupman-dot-com>]

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