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PICList Thread
'Current sensing again'
1999\07\23@002649 by Bob Drzyzgula

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As I've mentioned before, I'm working on an application
to monitor a bunch of computer and communications systems
in a data center environment. Most of the sensor designs
have been straightforward, but there's one that sort of
eludes me -- I have some ideas but I'm not 100% sure that
I'm not missing something more straightforward -- and I'd
appreciate y'all's thoughts on this.

I have 35 full-height (40U) 19" machine racks, each
of which has, at this point, a single single-phase,
120VAC, 20A dedicated circuit that is delivered through a
Pulizzi Z-line power controller.  (the 3302, picture
at http://www.pulizzi.com/p_ip3300.html, and data sheet
at http://www.pulizzi.com/ipc3xxx.html).  Each 20A circuit
is terminated in a single L5-30R twist-lock recepticle in
the raised floor plenum near the rack, and the Pulizzi
controller has an L5-30P plug on the end of a long,
rubber-jacketed supply cable; all power into the rack
has to come out of the Pulizzi.

What I'd most like to do is monitor the current draw from
each 20A circuit. At a minimum, this probably means interposing
some sensing device between the Pulizzi L5-30P and
the L5-30R in the floor so that I can separate out the
hot conductor and measure the current on it. I very much
do not want to have to open the Pulizzi and modify it;
any sensing strategy needs to leave the controller -- including
the power cable -- intact.

It would be icing on the cake if I could also, at low cost,
have an additional eight sensors to monitor the current
on each leg of the fan-out, but that information would
be much less useful on a routine basis than the
aggregate total.

I don't really need a lot of resolution on this, the 256
counts on an 8-bit ADC are more than sufficient; really
if I even had 1A resolution it would be OK, 0.1A
resolution would be great. What I expect to do is
routinely poll every few minutes and trigger an alarm if the
load exceeds 18A or some similar threshold. I'd also
like to be able to do a continuous poll on request to
generate a strip-chart output on demand if we want to,
say, watch the effect of powering up a new or balky
system.

The board I'm working on uses a PIC16C76, and there's
plenty of ADC ports availble for this, so all I really
need to do is to get that AC current turned into a voltage
input for the PIC. The obvious solution, to me, is to use
some sort of current transducer. Right now, I've got
some of the PC-50 units described here:

fwbell.com/Catalogs/Sensors/Current_Sensors/General_Information/4-20_mA_d
c_Output/PC_PCS/pc_pcs.html

However, the biggest drawback to those, IMHO, is the
need for the 24V power supply to drive the current loop
(thanks to everyone for helping me figure out how to
use these). Virtually everything else I'm doing will
run off just 5V.  There are probably other good transducers
that don't need all those volts, and some which may not
require any power supply at all. I'd love to hear any
suggestions along these lines. I'd also love to hear about
other ingenious ways to accomplish the same goal. I am
concerned a little about the linearity of the output,
and the ability to straightforwardly and accurately
(although not particularly precisely) translate the sense
voltage into an amperage reading. The solution also has
to be able to operate 24x7, ad infinitum, and should also
have a decently small response lag and conversion time,
so that I can get at least 1 or two samples per second.

You get the idea, sorry I'm so long winded about this
kind of thing.

Thanks,
--Bob

--
============================================================
Bob Drzyzgula                             It's not a problem
spam_OUTbobTakeThisOuTspamdrzyzgula.org                until something bad happens
============================================================

1999\07\24@070255 by Matt Roberds

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>From: Bob Drzyzgula <.....picKILLspamspam@spam@DRZYZGULA.ORG>
>Subject: Current sensing again

>What I'd most like to do is monitor the current draw from
>each 20A circuit.

20 cheap analog AC ammeters, little bits of black cardboard
glued onto the meter needles, and LEDs and phototransistors
to detect when the meter gets above a certain point?  No?  :)

How about this, run the power through several relays in
parallel.  In series with each relay is a fuse...20A for
the first relay, 15A for the second, 10A for the third...etc.
Your PIC monitors the input power (after all the relays)
and drives the relay coils.  First you turn on the 20A
relay...then the 15A relay, then turn off the 20A...then
turn on the 10A, turn off the 15A, etc...eventually you
will blow a fuse and lose power, at which point you know
the draw is somewhere between the last two ratings you
used.  Sort of like an R/2R resistor ladder from hell.

>There are probably other good transducers
>that don't need all those volts, and some which may not
>require any power supply at all. I'd love to hear any
>suggestions along these lines.

It looks like you're using an industrial-control type
4-20 mA output sensor of some kind.  That will work, but
it's sort of like a 10k linear pot with an RS-232 interface.
:)

I always thought that current transformers just produced
an AC voltage all by themselves.  Essentially, you are
wiring a transformer with a very-small-voltage primary
in series with the power to the device of interest, and
looking at the voltage generated on the secondary.  You
might want to rectify it before shoving it at a PIC, but
I don't think you'd have to do much more.

I have an All Electronics ( http://www.allcorp.com/ )
catalog that lists a current sensor.  It's rated for 1-100
KHz, so it may not be as good at powerline frequencies,
but it claims: "can read currents as high as 35A with
a sensitivity of 10mA to 1A per volt, depending on the
terminating resistor (200 ohms nominal)."  It appears to
just have two pins, and a hole in the middle that your
wire would be routed through.  The model given is
"Falco #CS200".  Price is $2 each.  I don't work for them,
but I have ordered from them in the past and been happy
with the service.

You may want to try building a "splitter" for the hot and
neutral wires as you mentioned, then just trying a few
dozen turns of small-gauge (24 or less?) insulated wire
around the hot wire, and see what that gets you.  If
you won't want to build several splitters, you *may* be
able to locate your current transformers in a handy
junction box, or at the main breaker panel...check code/an
electrician on this, though.

I know I saw a PIC-based power meter project recently...
I think it may have even been in a Microchip app note.
Anyway, they were measuring the voltage drop across
a "resistor" - a known length and gauge of wire whose
(low) resistance was known.  I think they were
measuring amps and fudging a little by saying P = V x I
which is not always strictly true for AC.  But you might
be able to adapt the same sensing technique.  If you
used an 0.01 ohm resistor (yes, you can get them
this low - check Mouser), it would drop 0.2V at 20A
and dissipate 4 watts doing so.  You'd probably want
to isolate this from your circuit though...maybe a
small 1:1 transformer or even an optocoupler would
do it.  It would be fairly linear with current...
0V at 0A, 0.1V at 10A, 0.2V at 20A.  You can get
a better range by going to a bigger resistor, but
then you "lose" more voltage as heat.  (0.1 ohm
resistor gives you 2V at 20A but also must dissipate
40 W).

I hope this helps!

Matt Roberds
mroberdsspamKILLspamworldnet.att.net

1999\07\24@101433 by Anne Ogborn

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Caddock  (Available through allied avnet)
makes an SR10 current sensing resistor for $2.91

This is a 4 terminal resistor. The idea is that there
are two heavy terminals to pass the current through, and
off to the side are two sensing terminals. The resistance between
the sensing terminals is accurately controlled.

Couple this to a hi-Z in op-amp through a couple hi-ohm resistors
and use to PWM a optocoupler.

or, if money's no object, you can buy clamp on inductive current
sensors for $95 ea frm FW Bell.

Alltronics is nice folks, but it's a big warehouse full of
junk they got from somebody. They usually have no idea what
it is or what it does.

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html

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