nuckolls.bob(at)cox.net
Guest
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 Posted: Sun Apr 13, 2008 8:20 am Post subject: Shunts (broken link fixed) |
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<nuckolls.bob(at)cox.net>
At 03:05 AM 4/13/2008 -0400, you wrote:
| Quote: |
<echristley(at)nc.rr.com>
gmcjetpilot(at)yahoo.com wrote:
>The AMP meter is really a VOLT METER. Yep it has nothing to do with AMPS.
>(There is an AMP and VOLT relation).
>
Aahh! But is it? Maybe the VOLT meter is really an AMP METER!!
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| Quote: | If using a swinging arm type meter, the voltage across the inputs drives a
current that energizes an electromagnet, the forces the deflection.
You're using current to measure a voltage, that's caused by the current
across the shunt, which is caused by a voltage.
Aahiiieee!!
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A very astute observation sir. Allowing a coil
of wire to move within the field of a permanent magnet
was the brainchild of a fine fellow named D'arsonval
who conceived the design about 130 years ago. There's
an excellent description of the details of this idea
offered over several pages beginning at:
http://www.tpub.com/neets/book3/7.htm
Hit the "-> next" buttons to access the series.
One may also craft a moving magnet instrument . . .
a good example is Van's ammeter. Unlike the more
fragile and difficult to calibrate moving coil
movement typical of:
http://www.aeroelectric.com/Pictures/Instruments/Loadmeter_2.jpg
Vans ammter shown at:
http://www.aeroelectric.com/Pictures/Instruments/VAM40_Interior_View.jpg
moves a barrel shaped magnet in the center of TWO coils
positioned at right angles to each other. This movement
doesn't need springs and is NOT calibrated with respect
to current flowing in the coils. In this case
two coils are excited with a current proportional
to the sine and cosine of the applied stimulus. The magnet
and pointer are often free to rotate 360 degrees. Van's
instrument is "pegged" at -45 amps and has a light spring
that drives the pointer to the peg when the instrument is
powered down. But you could remove the peg and spring.
The pointer would rotate freely and still properly indicate
the mechanical result of the ratio of two currents applied
to the coils. This style of meter movement can be used to
drive something like a compass card for displaying a
heading not unlike that offered by the DG.
This instrument is interesting because it can be
electronically conditioned to display a host of stimuli
and it takes advantage of the full rotational scale
of the instrument. D'arsonval movements get VERY
hard to build beyond 90 to 110 degrees of pointer
swing.
Because they deal with the results of a magnetic
interaction between coil(s) of wire and fixed magnets
both instruments are indeed displays of the effects
of CURRENT.
However, if one considers the internal resistance of
the instrument as being part of the overall circuit,
the raw device can be used to display the effects
of very small voltages. A good example is the classic,
non-amplified CHT instrument. An example is shown
here:
http://www.aeroelectric.com/Pictures/Instruments/CHT%20Gauge.jpg
This puppy reads full scale when something on the order
of 7 MILLIVOLTS is applied to its terminals. This is
the voltage generated by K-type thermocouple wire
at 350F. In this case, the moving coil can be crafted
of few turns of relatively large wire (low resistance)
because while a thermocouple doesn't offer much VOLTAGE,
it is a very low impedance source of energy and can
produce considerable CURRENT at the relatively small
voltage.
All moving coil devices are dependent upon the force
against a spring which is proportional to the current
flowing in a coil of wire. Of course the coil has resistance
of its own so any basic meter movement calibrated for
the measurement of a current may also be said to be
a "voltmeter" . . . typically, rudimentary movements will
indicate full scale with voltages on the order of 10
to 200 millivolts. The instruments designed to work in
conjunction with shunts are generally crafted so that
the CURRENT induced in the coil produces full scale
torque when 50 milliVOLTS (standard shunt calibration)
is applied to the terminals.
Van's instrument is fitted with electronics such that
the stuff sensed at the input terminals is indeed
a voltage . . . the considerable current required
to position the pointer comes from the electronics
power supply and not from the voltage stimuli being
monitored.
I've done some preliminary testing on the Van's
ammeter . . . by modern aviation design goals it
is exceedingly sensitive to local RF fields. I'm
working on an article to explore this condition.
I'll also see if I can craft an easily fabricated
filter or shield as a workaround for this product's
shortcomings.
Bob . . .
--
Date: 4/12/2008 11:32 AM
incoming mail is certified Virus Free.
Checked by AVG.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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