CT is definitely the way to go.  If digikey does not work for you, maybe
someone knows a more local supplier.

I built a project just like this, it was a current controlled stopwatch.
The CT runs a resistor that ties into the PIC.  I believe I used a simple
diode restifier, however a precision rectifier would be more accurate.  I
did not need a calibrated range, I just wanted to turn the stopwatch on
reliably anytime there was a 1 amp or greater load and turn it off
whenever that load dropped below a half amp.  I adjusted the resistor and
A/D trigger point until my calibrated instruments said it worked right.

The other way to do this is to get a current controlled relay from SSAC,
google on SSAC Relays.  $70, no programming, CT included.

-- Lawrence Lile





Ian McLean <ianmm@OPTUSNET.COM.AU>
Sent by: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
02/19/2004 06:08 AM
Please respond to pic microcontroller discussion list


        To:     PICLIST@MITVMA.MIT.EDU
        cc:
        Subject:        [EE:] Current Transformers


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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