You can also have a look at  the hall effect current sensors from allegro.
http://www.allegromicro.com/sf/0750/

Regards
/Ake

-----Ursprungligt meddelande-----
Fr=E5n: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]F=F6r Ian McLean
Skickat: den 19 februari 2004 01:43
Till: PICLIST@MITVMA.MIT.EDU
=C4mne: [EE:] Current Transformers


I would like to measure AC current using PIC A/D but am not sure what wou=
ld
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 rang=
e
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 recti=
fy
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 t=
o
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 curren=
ts.

Alternatively, how hard would it be to wind my own CT for this purpose, o=
r
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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