> From:         Ian McLean[SMTP:ianmm@OPTUSNET.COM.AU]
> Sent:         Thursday, February 19, 2004 7:08 AM
> To:   PICLIST@MITVMA.MIT.EDU
> 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.

The typical current transformer is made from a toroid core. The primary is
a single wire passing through the center hole of the toroid. A secondary
winding is wound of N turns wrapped around the toroid in the conventional
manner. If the secondary is shorted, the current in the secondary will be
1/N of the current in the primary. A small value resistor can replace the
short; this provides for sensing the secondary current. The insulation
on the primary wire, and on the plastic transformer case, if there is
one, provides excellent voltage isolation.

This type of transformer is a high pass filter; the 3db point is located at
a frequency of

f(cutoff) = R / (2 * PI * L)    PI = 3.14159...

where L is the inductance of the secondary winding, and R is the total
resistance of the secondary winding Rs and the terminating resistor Rt.
Since this sum will never go to zero (the winding resistance will always
be there), the cutoff frequency will approach a non-zero limit as the
terminating resistor approachs zero. This frequency is characteristic
of the transformer, since Rs and L are determined by the properties of
the core and the number of secondary turns.

Proper choice of the core is necessary if the cutoff frequency is to be
brought down below line frequency. Note that decreasing N in order
to decrease Rs actually decreases L by N squared, since L goes
like the square of the number of turns. This increases f(cutoff). A
large number of turns, on the other hand, is a problem with toroids,
especially if you are winding your own.

Commercial current transformers consist of a toroid with a machine
wound secondary. N is important, because it determines the scale
factor. Smaller values of N pass larger current in the secondary. For
reasonably small primary current, the secondary current can be as
small as several milliamps with sufficiently large N. This opens the
door for using a virtual ground to sense the current, reducing Rt to
zero.

The smaller transformers are used to sense primary switch current
in switching power supplies. Typically, these are less than 1 inch
in diameter, with the toroid mounted in a plastic case. The largest
value of N I have seen is 1000, with 100, 200, and 400 more
common. For 50A use, the minimum core size will most likely be
determined by the size of the primary wire.

John Power

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