Hi all,

Before anyone picks me up on this, I have been doing some more research on
current transformers.  I don't need to rectify the secondary of the CT.
Just put a 'burden resistor' (high watts, low resistance) straight across
the secondary winding - then read the voltage across this resistor, op-amp,
A/D input etc.

Have I got this right ?

Anyone know where I can get a suitable 50:5 CT from ?

Or is there a "better way" ?

Rgs
Ian

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Ian McLean
> Sent: Thursday, 19 February 2004 11:43 am
> To: PICLIST@MITVMA.MIT.EDU
> 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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