Ian,
Try Newark Electronics:
http://www.newark.com/NewarkWebCommerce/newark/en_US/support/search/searchResults.jsp?action=0&First=0&QText=current+transformer

CT's are used a lot in the power distribution industry.

Some things about CT's:

1. Never, *EVER*, leave the secondary un-terminated while in use!! Lethal
voltages will appear across the secondary. Several thousand volts can appear
across the secondary with very modest primary currents. Arcing is what
usually limits the secondary voltage. Instrumentation CT's usually have a
spark gap to preclude insulation failure.

2. The secondary burden resistor should be as close to a short circuit as
possible. Under an Ohm is a goal.

3. The secondary should be shunted by a bi-directional TVS diode (1.5KExxx
type works well) in case the burden resistor opens and to soak up starting
current surges when you turn on the product. Pick a comfortable voltage for
whatever you are driving with the CT for the TVS. I usually use the 6.8V
version.

4. Rectifying the output directly is not a good idea, especially with a half
wave rectifier. There are two reasons for this. The diode will leave the
secondary unterminated for half of the cycle, which will overvoltage zap the
diode if a burden resistor is not used before the rectifier. The other
problem is the diode offset voltage contributes a large error at low
currents, even with Schottky diodes. An op-amp "perfect rectifier" circuit
is recommended for pre-pic detection.

Regards,
Edward Gisske, P.E.
Gisske Engineering
608-523-1900
gisske@offex.com
----- Original Message -----
From: "Ian McLean" <ianmm@OPTUSNET.COM.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, February 18, 2004 6:43 PM
Subject: [EE:] Current Transformers


> I would like to measure AC current using PIC A/D but am not sure what
would
> be the best way to do it.
>
> I need to measure current at line voltage and frequency (240VAC, 50Hz).  I
> would like to be able to measure up to 50A current (yes, I know it is big,
> 12kW max., but that is what I need), and convert that into a 0-5V DC range
> for the PIC A/D, so I can display the current on an LCD display.  I have
no
> problem with the PIC side, A/D, LCD, etc., but am not sure how to do the
> current conversion.  I have done DC current reading on the PIC before up
to
> 20A, simply by using a sense resistor and op-amp, so I am half way to
> knowing what I have to do.
>
> I figured I should probably use a CT (current transformer) in series with
> the hot wire of the AC supply, that does a 50A:5A ratio conversion, but
> having trouble finding one of these.  Can anyone point me in the right
> direction ?  On the secondary side of the CT, I would then 1/2 wave
rectify
> the output and pass the resulting 0-5A DC through a sense resistor in
> parallel with the CT secondary winding, take the voltage drop across the
> sense resistor, amplify it with a general purpose op-amp, and feed that to
> the PIC A/D.  This is the only way I know (or at least think I know)to do
> this, and it does seem a rather roundabout way of reading large AC
currents.
>
> Alternatively, how hard would it be to wind my own CT for this purpose, or
> can someone suggest a clever alternate solution ?
>
> PS: This is for a personal project.  Accuracy is not paramount.  If I can
> get it accurate to within an amp or two, I am happy.
>
> Rgs
> Ian.
>
> --
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>

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