On Thu, 12 Jun 1997 20:03:45 -0400 "Fred Tumbagahan, Jr."
<fstj@YESIC.COM> writes:
>Any ideas on how to measure current up to 200 amps (without using
>shunt)  must have good isolation from power. Maybe similar to clamp on
>ammeter or thru magnetic induction.  I will use the pic to measure
>this
>current.
>Thanks for the help.
>

        AC or DC?  For AC I use current sensing transformers from Torroid
Corporation of Maryland (phone +1 410 860 0300).  I take the secondary,
run it thru a full wave bridge, THEN put a terminating resistor across
the output of the bridge.  Putting the terminating resistor after the
bridge ALMOST eliminates nonlinearity due to diode drops.  The
transformer secondary current is ideally some fraction of the primary
current.  It, again, ideally, generates sufficient voltage to cause this
current to flow in the terminating resistor, even if it has to get over
diode drops.  The voltage across the terminating resistor is then run
through a low pass filter (probably just a series R and C to ground),
then into the A/D.
        For DC, there are similar devices but they use a Hall effect
sensor.  The name FW Bell comes to mind...  I haven't used these, but I
understand that there is a reference winding on the core and an op amp
adjusts current through the reference winding until the Hall effect
sensor detects 0 magnetic flux.  The output of the op amp is proportional
to the primary current.  This gets around nonlinearity of the Hall effect
sensor and nonlinearity of the core, since the system is adjusting for a
net zero flux.


Harold