nuckolls.bob(at)cox.net
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 Posted: Fri Aug 15, 2008 6:59 am Post subject: Measuring "wiggly" currents . . . |
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At 11:59 AM 8/14/2008 -0400, you wrote:
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If you need to know actual current draw, check out
www.sears.com/shc/s/p_10153_12605_03482369000P. DC clamp-on ammeter, works
well for this kind of situation, not too expensive. Just be sure you're
clamping around the wire that you really want to read. It's easy to get on
the wrong side of a terminal. (Don't ask how I know -- I know!)
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This brings up a topic that I've been meaning to do an article
on . . . but not all ammeters are the same . . . and some are
decidedly unsuitable for some kinds of measurements.
When we're sizing batteries and alternators, the REAL questions
go to deducing the amount of ENERGY we need to store and generate.
You can buy ENERGY meters but they are decidedly more expensive
than the common mulitmeters and very few folks really need to
make detailed energy studies.
To review, energy is power over an interval of time. Like
the Joule is 1 watt for one second, i.e. 1 watt-second.
You pay for killowatt-hours recorded on the meter at the
back of your house. While an alternator may be rated at
60A and 14v (840 watts) the energy you need from it is
spread over a period of a flight and might turn out to
be something like 600 watt-hours. A tidy number that
accumulates total consumption that doesn't care if the
current draw was 210 amps for seconds, 5 amps for minutes,
16.2 amps most of the time an 31 amps for a few minutes.
Making measurements to deduce energy requirements can
have some pitfalls . . . from two major sources. Accuracy
of the instruments at the levels of interest and response
of the instrument for integrating the true energy measurement
from a reading that is anything but steady.
Light bulbs, fans, radios, etc. tend to have steady
current draw requirements. Their energy requirements are
pretty accurately measured with the product of volts
x amps x time. some devices in the airplane do not
present steady, current draw signatures. Placing an
ammeter in series with these devices is problematic.
First, digital multimeters have some electronics that
does an analog to digital conversion for display. This
is a stone simple task . . . when the stimulus to be
measured is steady. But depending on the instruments
choice for a/d processing, currents that wiggle can
cause erroneous if not wild swings in displayed readings.
Systems like electronic fuel injection, pulse pumps,
ignition systems, and strobes tend to demand current
from the system with jumpy characteristics. Years
ago I wrestled with these measurements by crafting
an operational amplifier integrator that was fed by
an ammeter shunt in series with the measurement of
interest. The first time I did this was to deduce
the batter size needed for this machine:
http://aeroelectric.com/Pictures/Misc/Mobilizer_3s.jpg
During a patient load-unload cycle the current draw
of the various motors was all over the place. But
the integrator would sum the current values per
unit time whereupon I could multiply by battery voltage
and deduce watt-seconds of energy per patient handling cycle.
It's exactly this kind of measurement philosophy that's
needed to deduce energy requirements for the "jumpy"
systems aboard your airplane.
This has always been a bit of tone in my shoe when
dealing with suppliers of accessories to aircraft.
The product data sheets give max current draws (sizes
wire and breakers) but for things like strobes
and pumps, there was no integrated total-energy
value (sizes batteries in the alternator out mode).
How does all this relate to the topic at hand?
First, it is unlikely that a clamp-on DC ammeter
of the type cited will be very useful for measuring
current draw of an EFCI. First because the measurement
of interest is large compared to the accuracy of
the device. If it has a published accuracy of
say 3% of full scale for a 50A scale, that's
plus or minus 1.5 amps. Furthermore, the electronics
within the instrument may not handle the wiggly
nature of current draw for injectors that get
pulsed at 10A for milliseconds and once per
cylinder-firing.
A modern version of the instrumentation I described
above would be useful. A 10A, 50Mv shunt would
provide the current sample. We would want to
put a gain of 100 on the 50Mv signal to get it
up to the 5 volt range. The output of the gain
stage would drive an integrator with a low dielectric
absorption capacitor (like a Panasonic ECQE1106K).
With a 1M integrating resistor and 10uF capacitor,
our amplified shunt signal would drive the charge
on the capacitor to 5 volts in 10 seconds. This
is the order of magnitude for integrator gain
and time constant useful to our tasks. See:
http://aeroelectric.com/Pictures/Schematics/Ammeter_for_Wiggly_Currents.pdf
Hooking this shunt in series with your wiggly
system power, let us assume that you get
a reading of 3.6 volts at the end of ten seconds.
We know that 10A for 10 seconds raises our instrument's
output to 5 volts. The reading of 3.6 volts represents an
integrated current value of 10 x 3.6/5.0 or 7.2
amps. It matters not how much the measurement
thrashes around, as far as your BATTERY is concerned,
it needs to deliver 7.2A x 12v x 2 = 173 watt-
hours of energy to keep that puppy running for
two hours.
When you have the integrated value of current
draw for ALL endurance loads, you're prepared
to go into the battery performance charts and
select a battery that meets your design goals.
I know this is a lot of info that most folks
don't need. But if anyone is interested in getting
the real numbers on their system's performance,
this kind of tool is quite useful. But be aware
also that the plain vanilla ammeters have
limitations for accuracy at levels of interest
and performance for dynamic (wiggly) loads.
Bob . . .
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