Envelope-to: dave@picks.force9.co.uk
Delivery-date: Thu, 15 Sep 2005 13:43:14 +0100
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 15 Sep 2005 13:41:44 +0100
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Subject: LF: RE: Current Transformers (off topic, but expertise probably here)
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Dear Andy, LF Group,

I believe steel has a B/H curve that is curved at low flux densities as well
as high, so you may not need to have much flux to get non-linear effects.
But the point is that, provided the current transformer secondary (240V
primary in this case) has much higher impedance than the "burden" load
resistor, the magnetising current will only be a small fraction of the total
current flowing in the load, and so non-linear effects will only cause small
errors.

Out of curiosity, I measured the primary impedance of some small mains
transformers around 6VA lying about at work - they seemed to fall into 2
categories, ones with L > 100H and impedance at 50Hz of several 10s of
K or more, and others with L of only 10 - 20H. I've no idea why they should
be so different - visually they were very similar, and the DC resistances
were similar at several hundred ohms. The impedance of the latter group is
only kilohms, so you could expect large errors if your current transformer
with 4k load falls into this category. I suppose using a much smaller burden
resistor would help, possibly with more primary turns to maintain the scale
factor.

Another thing is that the windings become parallel resonant somewhere in the
kHz range, so I doubt if the bandwidth would be very good if you are
expecting very spiky current waveforms.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Andy
Sent: 14 September 2005 12:47
To: rsgb_lf_group@blacksheep.org
Subject: LF: Current Transformers (off topic, but expertise probably here)

Can anyone help with this knotty annoying problem...

I wanted to make a 50Hz current transformer (CT) to measure the input
current waveform to a SMPSU.  I took a small cheap transformer from a wall
mounted PSU of about 6VA rating, and stripped off all the secondary winding
replacing this with one turn of thick wire to form the primary of teh CT.
The old 240V winding had a DC resistance of around 1k, but this should ( ??)
be irrelevant to CT operation...