Try www.hobut.co.uk
Current transformers ect

Regards Steve...


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Ian McLean
Sent: 19 February 2004 00:43
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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