Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 14 Sep 2005 14:45:01 +0100
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From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <000601c5b922$0e59f410$268cf8d4@standalone>
Date: Wed, 14 Sep 2005 14:44:59 +0100
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Subject: LF: Re: Current Transformers (off topic, but expertise probably here)
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Hi Andy, I am not up to speed with this these days (I am no doubt someone
will chime in with the right gen) but I recollect a value of 7 turns per
volt for mains transformer "rule or thumb" design which would be less that
2000 turns on the primary of a mains step-down. What is also the case is
that some "cheapy" transofrmers  (Altai ??) use as little as 5 turns per
volt. These work ok at 60Hz I believe but seem to get hot at 50Hz, even with
no load. I would think it unlikely that an economy  "wall-wart"  would be
designed at 20 turns per volt. So maybe your checks were in error ?? Current
transformers are available but are quite pricey. I notice that the one in
the RS cat shows 8 turn primary for 15A FSD but I guess that depends on the
core (toroidal) material.

Cheers de Alan G3NYK

----- Original Message -----
From: "Andy" <actalbot@southsurf.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: 14 September 2005 12:46
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
>
>
> The one turn generated about 0.3V RMS from a 240V input, although after
some
> prolonged testing as a CT, I realised I had got the turns calculation some
> way out, but thats by-the-way, (it was actually nearer 5000 than 4000).
> That suggested the turns ratio as approximately 4000 turns on the 240V
mains
> winding.  So for a first test, I used a 4k burden resistor with the
> intention of obtaining a resultant voltage output of 1Volt / Amp of
primary
> current.  That way,  with at the most 10V across it(corresponding to 10A
> input) the core must surely be way below saturation so should give good
> linearity - shouldn't it !!!.
>
> BUT, and I don't know how to account for it,  there was a very significant
> non-linearity .  Testing over a range of currents from 0.2A RMS to 9A, the
> slope of the  Vout / Iin graph shifted by nearly a factor of 2.
>
> That says 'core saturation' to me,  but I don't see how that can be.   If
> the core is saturating with just 9V across a winding designed for 240V, it
> doesn't say a lot for the device in a PSU.   Should reducing the value of
> the  burden resistor help matters?
>
> Can anyone shed any light on this?   I want a good linear current
> transformer that will work to 13A RMS that can be used for signal analysis
> of the current and voltage waveforms by feeding into an A/D and
calculating
> what's really happening when fed with typical mains supply inputs.  I
don't
> want to use a massive transformer if that is not essential!
>
> Andy  G4JNT
>
>
>