Re: [TCML] Measuring Isecondary: Current transformer too close toprimary?

["Carl Noggle" <cn@xxxxx>]

It would probably work to use a Pearson current xfmr.  They are designed for 
pulse work and are accurate and robust.  They connect to the scope through a 
coax for low spurious pickup.  They are sort of expensive, but worth it.

    http://www.pearsonelectronics.com/products/current-monitors

The model 2100 would be a good bet.  It could also measure the primary 
current waveform.  I have a 411, but the 1/2" center hole is a problem. 
Might try it anyway.

---Carl






-----Original Message----- 
From: Andreas Hahn
Sent: Saturday, September 01, 2012 2:00 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Measuring Isecondary: Current transformer too close 
toprimary?

That's a valid point -- though the transformer is quite small relative to
the distances involved (just under 1.5cm diameter toroid, vs 5cm+ distance
to the nearest primary lead). But yes, magnetic shielding would seem to be
the solution...


On Sat, 1 Sep 2012, jhowson4@xxxxxxxxxxx wrote:

> Wouldn't the fields from the primary canceling out be dependent on those 
> fields being uniform though?
>
> The primary is a spiral, so you have some non uniformity from that effect 
> (offset circles)
> But more probable is the primary inner lead, which would probably run 
> "roughly" parallel to the wire coming from the base of the secondary.
> Add on top of that the fact that primary currents are usually orders of 
> magnitude higher than that of the secondary.
> Seems reasonable to me that the proximity of the primary would be a major 
> concern.
>
> Perhaps you can put the current transformer inside a hunk of thick iron 
> pipe to try and eliminate some of these magnetic effects?
>
>
>
> Thanks,
> John "Jay" Howson IV
>
>
> "Why thank you, I will be happy to take those electrons off your hands."
>
> ----- Original Message -----
> From: "Andreas Hahn" <andreas.hahn@xxxxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Saturday, September 1, 2012 2:58:02 PM
> Subject: [TCML] Measuring Isecondary: Current transformer too close to 
> primary?
>
> Looking through the archives for current transformer theory, I found this:
> http://www.pupman.com/listarchives/1999/February/msg00081.html
> "I was using a current transformer and (this time) a usual scope 10:1
> probe.
> As I was about 40 cm from the primary, probably the magnetic field induced
> by the primary was stronger than the field induced by the secondary base
> lead passing through the same transformer.
> The result was that I measured an attenuated version of the primary
> current, NOT the secondary current."
>
> Is this actually possible? Since the primary windings do not pass through
> the current transformer toroid, my understanding is that any fields from
> the primary would cancel out, and therefore little or no influence from
> the primary should show up in the secondary of the CT.
>
>
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