Alan,

I'm pretty sure that he is correct. There is no physical law which
says that the secondary current equals the primary current times the
inverse of the turns ratio. That is an approximation which is valid
for a certain range of load impedances. Fundamentally, you are
producing some changing flux in the secondary. This will produce a
given open-circuit voltage according to Faraday's Law. As you load it
down, and allow current to flow in it, that current will act to reduce
the flux change. As you approach a short-circuit (load impedance much
less than the reactance of the secondary) in the secondary, the
current will begin to match the approximation you mention.

Sean


On 3/20/07, Alan B. Pearce <A.B.Pearce@rl.ac.uk> wrote:
> >> Umm, I think you have a problem with mixed units.
> ...
>
> >I am talking about measuring the magnetic field produced by current
> >in a conductor.A voltage will be induced in the terminals of a winding
> >wounded around the toroid when the magnetic flux is changing in time.
> >I will process this induced voltage to the project requirements.That
> >is it.No need to deal with the current coming out of the secondary coil.
>
> Umm, wrong. The magnetic field induces a current in the coil, and the
> voltage you get out of that will depend on what resistor you put across the
> coil.
>
> This resistor will have a temperature co-efficient that will almost
> certainly not meet the accuracy spec you are being asked for.
>
> The various current probes we have around here all suffer from some form of
> temperature drift, and the best ones talk in terms of accuracy of about 2%,
> which is 10^2 worse than you are asking for.
>
> Then as Vasile points out, you will not get a consistent magnetic field in a
> clamp on system. The only way to do that is to thread a wire through a
> permanently closed toroid. The changing air gap will cost you 1-2%, again a
> couple of orders of magnitude worse than you are wanting.
>
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