Hi,

I posted this earlier, but it never appeared on the list - in fact until
Jinx just posted then, nothing seems to have hit the list since about 10
hours ago.  Has PICList been off the air for awhile ?

Anyhow, I'll repost my question now.

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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