Re: gEDA-user: Transformer as voltage transducer?

[John Doty <jpd@wispertel.net>]

On May 25, 2007, at 7:21 PM, Randall Nortman wrote:

> I would like to measure mains voltage (110-240V, 60Hz), but I want my
> measurement circuit to be isolated from the voltage being measured.  I
> am going to be sampling the waveform at a high sample rate (relative
> to the 60Hz waveform being measured) and comparing that with the
> current on the same lines with (near-)simultaneous measurement.  So it
> is important to me that the voltage waveform not be distorted or
> phase-shifted, and the voltage I see should be related to the source
> voltage by a simple linear ratio.
>
> Seems like a simple transformer will do it, in theory.  But I'm not
> sure about how they will work in the real world.  Do transformers
> distort the voltage waveform or phase?

Some. Cores are nonlinear, there are ohmic losses, eddy current  
losses, magnetization inductance, leakage inductance, capacitance, ...

For a stiff voltage drive and a light load, you'll probably find the  
distortion minimal. But you haven't stated a quantitative requirement  
and transformer manufacturers generally don't write specs in a way  
you can relate to here, so you'll want to to get a transformer and  
make some measurements, I suppose.

>   What happens if I put a load
> on the secondary of more than a few nA?

For a small power transformer you'll probably find the difference  
between no load and 1 mA difficult to measure.

>   What non-linearities are
> there?

Mainly the nonlinear hysteresis curve of the core.

>   What sort of transformers are going to give me the best
> response?

Hard to say. Again, you haven't stated quantitative requirements, and  
the usual specs won't tell you anyway. So, I suggest just getting a  
cheap power transformer, trying it (look at the AC in and out on a  
scope, say), and if it isn't good enough come back here with details  
of the problem.

>
> Or if the transformer idea is awful, any other ideas for isolated
> voltage transducers?

A transformer is probably fine. Alternatively, google "isolation  
amplifier". These are good for sensing current (through a small  
resistor) too.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@noqsi.com




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