RADIO CONTROL ELECTRICS - 2003/09

THIS COLUMN SHARES some reader reaction to Dump’r and
continues the topic of basic electrical terms and concepts which
began last month.
I introduced Dump’r in the June column with a photo and a brief
description. It is a versatile, home-brew battery discharger I
developed for my own use. It is intended to safely discharge 4- to
18-cell motor packs which I’ve charged for flight but did not use.
(I prefer not to store charged packs for a variety of reasons.) The
discharge current is a constant 500 mA. As I described, I intended
to build more of them, and now I have four Dump’rs in use.
As I write this, several weeks have passed since that June issue
came out, and reader reaction to Dump’r has been amazing!
Everyone who wrote requested that I make it available to all.
Given the substantial reaction, I have begun a construction article
that MA will publish in next month’s issue. Thanks to all of you
for showing so much interest and support!
I introduced the topic of electrical terms and concepts last
month, and it will continue now and in the future. This offering is
in response to a slow but steady flow of reader mail throughout
the years indicating the continuing need for basic electrical
information, understanding, and “how to.”
Last month’s discussion was an attempt to convey a feel for
terms and concepts such as voltage, current, and power. This even
included taking liberties with the pure technical definition of
these and related terms—solely for the purpose of making the
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Variety of meters include two analog multimeters, two digital
multimeters, panel ammeter, AstroFlight Whattmeter.
Compact Dump’r atop Revolt! fuselage, safely discharging 12-
cell motor pack. Dump’r shuts itself off when finished.
DVMs measure same battery voltage. Left set to 2 is overranging;
middle on 20 is just right; right on 200 has limited resolution.
Autoranging and manual DMMs. Former needs only function
selection; latter needs many more switch positions.
116 MODEL AVIATION
topic more comfortable. This theme
continues now.
Fortunately all common electrical
parameters are easy to measure, and
almost anyone can do so with minimal
investment and some insight. Having
interest and ability in this area can make
electric power all the more enjoyable for
those who want to pursue such detail.
On the other hand, it is unnecessary to
get into this stuff to enjoy Electric;
consider that the equivalent is unavailable
to the wet-power crowd! It’s similar to
with computers; some use them as tools
and others know all about what makes
them tick.
Let’s begin again with voltage. It is
physically measured with a voltmeter, but
nowadays one can be hard-pressed to find
a simple voltmeter. It’s far more common,
convenient, and practical to use a
“multimeter.”
Multimeters are “many meters” (i.e.,
many meter functions) in one box. They
typically include a voltmeter, a current
meter (ammeter), an ohmmeter (resistance
meter), and sometimes other less-familiar
functions. Function selection is usually
made by switch settings and/or multimeter
terminal connections.
There are two kinds of multimeters: the
classic analog (“moving needle”) and the
contemporary digital counterpart. The
former are getting harder to buy, but there
are still present-day applications for which
many prefer them. (Trend measurements
are easier to see with analog meters.)
Digital Multimeters (DMMs) display a
measurement result numerically; that is,
Analog and digital multimeters in “voltage” mode reading the
same seven-cell battery voltage, as discussed in text.
Series-connected analog, digital multimeters in “current” mode
reading same current through battery and resistor.
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122 MODEL AVIATION
displaying readable numbers so that the
need to look carefully at an analog needle
position is eliminated. Digital meters
typically offer greater reading resolution
(more significant numbers) than an analog
meter can.
Economy DMMs are so pervasive now
that they can be found at many local stores
such as RadioShack, hardware and autoparts
places, and often at variety stores. It
should be easy for most E-aeromodelers to
have a suitable instrument.
One photo shows several analog and
DMMs, a panel analog ammeter, and the
AstroFlight Whattmeter, which I will
discuss in the future. Other photos show
multimeters in comparative application.
Recall that voltage is electrically what
appears at your household outlets, the
auto-battery terminals, and at the terminals
of your radio and motor batteries. Voltage
sources never have current flow from them
unless something (a “load”) is connected.
Your flashlight battery has only voltage
and no current present while the flashlight
is off.
Anyone can measure a voltage level by
switching a multimeter to the “voltage”
function and connecting the two meter
leads to the unknown voltage-source
terminals. But to do so properly requires
one more determination: that of choosing
the meter “range.”
Most metering functions within a
multimeter have several ranges or scales
from which to select. A range is the full
scale value that can be displayed, and
analog and digital meters have ranges.
The most common scales found in
DVMs (Digital Voltmeters: the voltmeter
part of a DMM) include 200 millivolts (0.2
volts), 2 volts, 20 volts, and 200 volts.
Although these are typical, there are other
popular range values. I have DMMs with
full scale values based on 3, 4, and 2.
Manual meter-range selection is
usually made with a simple rotary switch,
although some DMMs feature
“autoranging.” Autoranging requires the
operator to select the function (volts,
amps, etc.), then the meter adjusts itself
for the proper range once a measurement
commences.
Some DMMs offer operator choice;
you can choose “manual” or “autorange”
modes of operation. Several versions are
shown in the photos. I generally prefer the
“fully manual” (nonautoranging) version,
at least for beginning hobbyists.
One photo shows manual analog and
digital multimeters set to read voltage and
connected to the terminals of a seven-cell
pack. The analog meter voltage range
selector is manually set to 10 and the
DVM is set to 20. These scales were
chosen from those available because I
know that a seven-cell pack voltage will
likely fit on these ranges. These are the
closest meter ranges available with which
to measure this expected voltage level.
In general, with manual range selection
it’s best to begin with the highest range,
then “downrange” as appropriate when a
reading appears. This guideline applies to
all multimeter functions and is especially
important with analog meters, where it’s
much easier to physically damage the
meter movement.
That might happen if too high an input
is applied to a range that is set too low
because the needle will “peg,” or
physically hit an upper movement stop.
DVMs will not be as easily damaged and
will simply display an over-range
indication such as flashing numbers or a 1.
Remember that in all of the preceding,
voltmeters of any kind measure voltage
across terminals (from one terminal to the
other). “Terminals” might be battery
terminals, motor terminals, or any others,
such as component connections in an
electronic circuit.
Unlike voltmeters, current meters
measure current flow (amps) through a
closed circuit. Recall that current flows
when a load—say a motor—is connected to
a voltage such as a battery. Conversely,
current never flows in an open (incomplete)
circuit—even in the presence of voltage.
The unit of measure for current is
amps. Current is measured with an
ammeter, and it could be an individual
panel meter movement (such as the one in
the photo) or a multimeter’s current
function.
Now that I am discussing two entirely
different kinds of measurement meters
(voltmeters and current meters), another
fundamentally important distinction can
be made: a voltmeter is typically a highresistance
device. That means a good
voltmeter takes (“steals”) little current
from the device or circuit under test, to
minimally disturb or influence the true
value being measured.
Ideally a voltmeter would have infinite
resistance and would take no current
while making a reading. Most DVMs
(DMMs set to the voltmeter function)
have a 10-megohm (10 million ohms)
input resistance.
On the other hand, a good current
meter has low resistance, and when it’s
connected in a closed-circuit path it uses
up little of the total available voltage. This
is sometimes called ammeter “insertions
loss” or “burden.” Ideally an ammeter
would have zero resistance and thereby
introduce no loss when in place in a
circuit.
Putting it yet another way, an ideal
current meter would require no voltage on
its own terminals to operate. This is the
“dual” of the voltmeter case: the ideal
current meter would “take” no voltage and
the ideal voltmeter would “take” no
current.
In practice, all current meters have
some burden, and in the case of DMMs
this is often equal to or slightly higher
than the multimeter’s lowest scale
voltage. Thus most DMM current
functions require (use) approximately 0.2
volts for a full-scale current reading.
That is, if a current meter were
indicating a reading of full-scale value (on
any range), it would be “robbing” 0.2
volts of the voltage available within a
circuit path. This would have the same
effect as lowering the source voltage by
this amount. Later in this discussion series
I’ll examine why this is so and how and
why the insertion loss of a current meter
may sometimes be significant to Eaeromodelers.
One photo shows a seven-cell pack,
one of each current-meter type, and a load
resistor—all connected in series, or “daisy
chain” fashion. The load could just as
easily be a motor, a light bulb, or some
electronic device such as a radio.
The most important point is that
current meters are connected in series
with the voltage source and the load. The
meters are displaying the current flowing
in the path; i.e., from the battery through
the meters to the load. This is a closed
circuit: a necessity for any current to flow.
The resistor in this demonstration
circuit is 510 ohms. The expected current
(voltage divided by resistance) is roughly
9 divided by 510, or approximately 0.018
amps or 18 milliamps, or mA. (“Milli”
means 1/1,000.)
As in the voltmeter case I described
earlier, a suitable ammeter range needs to
be selected. An autoranging DMM will do
this automatically, but manual
multimeters should be set to a high value
range before a test connection is made. In
the example shown, the analog multimeter
is set to the 50 mA scale (range), and the
DMM is set to the 20 mA (or 0.02-amp)
range.
This topical discussion will continue in
upcoming columns. For now, do
remember that voltmeters are connected
across terminals, and current meters
(ammeters) are connected in series with
the circuit path. Never connect a current
meter across a voltage source!
Thus ends another column. Please
enclose a self-addressed, stamped
envelope with any correspondence for
which you’d like a reply. Everyone so
doing does get one. Happy summertime
electric landings, everyone! MA
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