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'Current Measuring'
1997\06\12@201330 by Fred Tumbagahan, Jr.

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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.

1997\06\12@202629 by Engineering Department

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> From: Fred Tumbagahan, Jr. <spam_OUTfstjTakeThisOuTspamYESIC.COM>
>
> 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.

Check Allegro for Hall Effect transistors.  Make a coil and measure
magnetic
field.

Win Wiencke
Image Logic Corporation

1997\06\12@204304 by Michael S. Hagberg

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circuit cellar took a ferrite bead and wrapped wire around it then passed
one of the supple (200amp) wires through the center. they did it with 20amp
circuits but the same applies. this creates a sine wave proportional to the
amperage.

michael

1997\06\12@210624 by Mike Keitz

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On Fri, 13 Jun 1997 16:50:03 -0400 Engineering Department
<.....imagelogicKILLspamspam@spam@IBM.NET> writes:
>> From: Fred Tumbagahan, Jr. <fstjspamKILLspamYESIC.COM>
>>
>> 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.
>
You didn't say whether it is AC or DC current.  If it is line-frequency
AC, use a current transformer.  The good-isolation, 200 A type is widely
manufactured for utility meter use.  Look at the utility meter for a
store, church, or other building that uses a fair amount of 3-phase
power.  You'll see that (unlike a household meter) only small wires go to
the meter.  The heavy currents used by the building is transformed to
small currents which drive the meter by current transformers.  These are
egg-shaped devices that the heavy power conductors pass through.  Usually
they are installed high up on the side of the building or a pole.

The single conductor passing through the center is a one-turn primary.
The secondary has many turns and a toroidal iron core encapsulated in
plastic.  A 1:N transformer increases the voltage by a factor of N (the
conventional use of a transformer), but it also divides the current by a
factor of N.  So if the transformer is 1:1000, a 1000A current in the
primary will cause 1A to flow in the secondary.  A current transformer
should work into a load close to a short circuit.  This keeps the loss of
voltage in the primary wire to practically zero.  Leaving the secondary
leads open is especially bad since the secondary voltage will run away.

1997\06\12@215106 by www.aeug.org/~chip/

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On Thu, 12 Jun 1997, Fred Tumbagahan, Jr. wrote:

> 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.

 For the current-measuring front end of our three-phase power systems, we
use CTs we make in-house.  They're easy to make (and easy to find
pre-wound), just wind 1,000 turns of #34 AWG magnet wire on a Magnetics
brand core of material 4N or so.  This will provide a C.T. that (when the
load wire passes through the toroid) convert 0-5 Amps into roughly
0-12VAC.  Rectify and filter the AC, use a voltage divider (or trimpot of
2K-5K) to scale the voltage to desired range.

 Since you want to measure up to 200A, buy a 200A-5A C.T. pre-potted into
a nice case.  Send its output through the 5A C.T.  There you go. ^^
I can provide a model # on request.

 On a similar note, I've just completed a PIC(12C508) project that
operates as a combination phase-sequence detector and time-delay
processor.  This replaces a $100 circuit board we were making to
previously protect the SCR gates (and load fuses) from the squirreliness
of power-on transients.  Nice little chips, these piccies..

 --Scott

/**/

1997\06\13@001321 by Harold Hallikainen

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On Thu, 12 Jun 1997 20:03:45 -0400 "Fred Tumbagahan, Jr."
<.....fstjKILLspamspam.....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

1997\06\13@004613 by Steve Hardy

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> From: "http://www.aeug.org/~chip/" <EraseMEchipspam_OUTspamTakeThisOuTAEUG.ORG>
>
> On Thu, 12 Jun 1997, Fred Tumbagahan, Jr. wrote:
>
> > 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.
>
>   For the current-measuring front end of our three-phase power systems, we
> use CTs we make in-house.  They're easy to make (and easy to find
> pre-wound), just wind 1,000 turns of #34 AWG magnet wire on a Magnetics
> brand core of material 4N or so.  This will provide a C.T. that (when the
> load wire passes through the toroid) convert 0-5 Amps into roughly
> 0-12VAC.  Rectify and filter the AC, use a voltage divider (or trimpot of
> 2K-5K) to scale the voltage to desired range.

No.  A current transformer does _not_ convert current into voltage (a
resistor does this).  The secondary of a CT must be operated with low
(preferably zero) voltage, and the current measured.  This can be
achieved, and converted to a voltage, by feeding into the summing node
of an inverting op-amp, maintained at virtual ground.  The op-amp would
need a current output capability equal to the peak secondary current to
be measured.

'Just' wind 1000 turns on a toroid?  No thanks!  If you really needed
to make your own CT, I would use, say, two in series with a more
practical turns ratio of 50:1, giving a nett ratio of 2500:1.

These days, it's better to use a hall-effect sensor module, which has
the advantage of DC-200KHz operation and simpler interface.  These
are available in a 200A range.

Regards,
SJH
Canberra, Australia

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