nuckollsr(at)cox.net
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 Posted: Fri Jun 22, 2007 12:26 pm Post subject: Digital Ampmeter (P.S.) |
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At 03:05 PM 6/22/2007 -0600, you wrote:
<nuckollsr(at)cox.net>
At 04:48 PM 6/22/2007 +0100, you wrote:
I installed a 50 mV shunt in the AWG 6 wire coming from the
alternator, to measure Amps "produced" by it, and I don't want
to put an analog Ampmeter in my pannel. Can someone please indicate
me a source and a model of digital ampmeter to connect to my shunt.
TIA
Carlos
It's not a simple answer. Unlike the ordinary analog instrument,
a digital instrument cares about how much voltage is on the shunt
with respect to ground. The instrument is trying to deduce and display
a voltage on the order of millivolts while shunt is riding above ground
by about 14,000 millivolts and has a lot of 'noise' on it. The ability
to ignore the big picture while staying accurately focused on the little
picture is not a trivial task.
Most manufacturers get around it by using the hall effect sensor
which doesn't care how much voltage is on the sensed wire . . . it
cares only about the magnetic field around the wire that results
from the flow of the current to be measured.
If you're dead set against an analog (pretty cheap at $50) like
https://matronics.com/aeroelectric/Catalog/AEC/9007/9007.html
then in order to use a digital instrument, you'll need to
find one that can either run from your 14v bus and resolve
a millivolt signal riding on 14v common mode or select an
LCD instrument that runs from a 9v battery (just leave it on
all the time. Battery lasts for months!) and doesn't worry
about the 14v common mode thing. An example of the later
instrument can be seen at:
http://www.mpja.com/download/12205me.pdf
These are inexpensive but you'll note that the input
signal must lie within plus or minus 1.0 volts of
the instrument's ground (Com Mode Voltage).
Now, there ARE ways that one can craft a shunt signal
processing amplifier using a device with a very high
common mode capability. And example of such a part is
the Analog Devices AD628
http://www.analog.com/UploadedFiles/Data_Sheets/AD628.pdf
Note that this puppy is designed to live in a very
large common mode range of over plus/minus 100 volts.
This is the type of device employed by folks to choose to
offer digital displays driven by shunts. However, most
digital suppliers no probably use hall-sensors and step
over the common mode limits.
I thought JPI or Electronics International had stand
alone ammeters and voltmeters but a quick check of their
websites didn't turn anything up. Perhaps someone else on
the list will have some suggestions. It seems the trends
are toward multifunction instrument packages that include
volts and amps. Stand alone instruments are going to be
increasingly difficult to find.
The easiest thing to do is bite the bullet and
go the stone-simple analog instrument. You can spend
a lot of time and a lot more money getting to the
digital solution.
P.S.
If you wanted to shed the shunt and roll your own hall-effect
loadmeter, consider the parts from AmpLoc at:
http://amploc.com/PRO%20Series.pdf
You could combine one of these sensors with an inexpensive
DPM . . .
http://mpja.com/productview.asp?product=16179+ME
. . . and probably craft a loadmeter for under $40 in materials
and take care of the common mode problem at the same time.
Bob . . .
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