RADIO CONTROL ELECTRICS - 2003/11

100 MODEL AVIATION
THIS COLUMN announces one meet, has two Electric
Connection Service (ECS) offerings, describes a mishap you’ll
want to avoid, reminds you of one E-resource that is good and
getting better, and continues the ongoing discussion of basic
electrical terms and concepts.
Don McGillivray, 13371 Sylvan Ave., Fort Myers FL 33919;
Tel.: (239) 481-0063, wrote to tell me about the Fourth Annual
Southwest Florida All-Electric Event scheduled for Saturday and
Sunday, November 8-9, 2003, in Cape Coral, Florida. The Cape
Coral R/Sea Hawks sponsors the meet; you can visit the club’s
Web site at www.rseahawks.org.
Don described the flying site as having a 500-foot paved
runway and grass within a large, open field. There are shelters
and unlimited parking. Sunday noon is the time for an All-
Up/Last-Down event—the only one described on the mailer. It
includes detailed driving directions to the field, and you can
request a copy from Don. Do be sure to tell him that Bob sent
ya!
Jack Green, 2914 Yale Dr., Janesville WI 53548; Tel.: (608)
754-6335; E-mail: [email protected], wrote to let others know
that he has just moved to this location and is eager to find Eothers
in the area. Jack is a member of a largely wet-power club
and would like to find more people nearby who enjoy Electrics.
Jim Samuel, 81 Meer Dr., Upper Holland PA 19053; Tel.:
(267) 994-3195; E-mail: [email protected], is a newcomer to
Radio Control and is interested in E-power. He wants to hook up
with other E-fliers in his area, so, as did Jack Green, he inquired
about this column’s ECS.
Now it’s your turn. E-fliers in Jack’s and Jim’s vicinities can
get in touch with them, then everyone can have more fun sharing
the quiet joy that E-power brings! If you are seeking Eassociation
in your area, please avail yourself of this exclusive
ECS. Send me all relevant info and I’ll include it in a future
column. The ECS is available free to individuals and clubs.
“Duh” is about the only expression I can think of to describe the
blunder which resulted in the “ouch” shown in a photo. I’m
offering this experience as an example of what not to do!
As I often do, I was carrying an armload of E-stuff from the
car into the house. However, this time within a few steps I heard a
snappy “crack,” quickly followed by a “sizzling” sound, quickly
followed by severe hurt. The latter made me drop almost
everything that was cradled in my arm, then scramble to get one
hot watchband off of my wrist!
You can probably guess what happened, but just to be sure, the
battery you see in the photo managed to get across the all-metal
stretch watchband, which, in turn, got very hot, resulting in the
burn on my arm.
A three-segment length of the watchband shorted across some
of the cell interconnects, and if you look closely you can see one
larger and one smaller weld mark on the metal. The “crack” sound
came at the time of contact, and the sizzle was the reaction of my
sweaty arm under the hot metal watchband; a sort of “minifry”
took place.
I suppose I need not say more about this, except that I can
imagine the same sort of thing happening with bracelets and rings,
and maybe even neck chains, so please be advised. My arm and
watchband are okay; each is just a bit scarred.
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Mishap described in text. The battery was shorted by the
watchband, which got hot and cooked author’s arm underneath!
Current shunts include three commercial and two homemade
assemblies. Variety permits matching shunt to job at hand.
Battery under 5-amp charge. Large DVM reads battery voltage;
small DVM reads shunt voltage; Astro Whattmeter reads all.
I’ve written about this before, but I have to
mention it again; the magazine Quiet Flyer
is one terrific E-resource! It seems as though
every issue that comes out is better in some
way than the previous one—which was
probably one fine issue in its own right.
Quiet Flyer offers something electric for
everyone; each month typically contains
beginner stuff all the way through to rather
heavy technical material. Quiet Flyer’s
regular columnists can be counted on month
after month to provide a great deal of good
stuff, and those are what I tend to read first.
Then there are individual contributors with
varied reviews, how-tos, meet coverage, and
the like.
It is a worthwhile magazine, and I
encourage you to consider buying it. Check
out the ad in Model Aviation or contact
Quiet Flyer at (866) 627-0456.
The August, September, and October
columns included segments of an ongoing
discussion about basic electrical terms,
concepts, and some how-tos. So far this
discussion has briefly covered common
terms such as voltage and current and how
they combine (multiply together) to produce
power. There was early discussion of how to
measure these quantities, including some
“dos” and “don’ts” with common
multimeters.
One graphic in last month’s column
illustrated the use of voltmeters and current
Al Painton (Painton MO) likes E-powered Scale, and he routinely mounts battery packs
underside for easy separation/ejection in the event of sudden stops!
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in Ontario!
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AMA Convention, January 9-11, 2004 at
the Ontario Convention Center in sunny
Ontario, California.
Stop by and visit with Model Aviation
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Hope to see you there!
November 2003 101
meters (ammeters) within a simple
motor/battery circuit. Emphasis was placed
on the fact that voltmeters are always wired
across device terminals (e.g., battery, motor,
or other component terminals) and
ammeters are always wired in series within
a closed circuit.
It was also noted that because many of
our power systems operate with rather
“high” currents, the direct use of a
multimeter ammeter function could in some
physical ways be problematic. In such cases
it’s much better to use a current shunt, then
use the voltmeter part of a Digital
Multimeter (i.e., the Digital Voltmeter
[DVM]) to read the shunt voltage. This
approach was also illustrated in last
month’s graphic.
One photo this month shows five
variations of shunts. The massive-looking
one (top, center) is often called an
“instrumentation shunt” or a “panel
shunt.” The one shown is a “2 mV/Amp”
shunt. This means that every amp that
flows through it (through the two short
leads with Anderson connectors) produces
2 millivolts (i.e., 0.002 volts) across the
metering leads (the output at the dual
banana plug), which connect to a DVM.
Inherent in this description is the fact
that this shunt resistor is 2 milliohms—a
very low value. Good shunts (and
dedicated current meters) need to be of
low resistance to not “rob” voltage from
the circuit under test.
But there can be a downside if a shunt
resistance is too low. If this one were used
in a park flyer with a motor current of 2
amps, the resulting voltage would be 4
mV. Since most DVMs have a lowest
voltage scale of 200 mV, this low-value
reading would be more affected by meter
102 MODEL AVIATION
accuracy and resolution error. More about
this follows.
To the right of the large shunt is an old,
homemade 10 mV/Amp shunt. The
Anderson connectors carry the circuit
current, and a voltmeter is connected to the
test points (the white banana jacks). The
construction and calibration of this shunt
was described in detail in the April 1986
(that’s right—approximately 17 years ago!)
column.
The two compact-looking coiled-wire
shunts are products of Steve Kowalski and
are available from New Creations R/C, Box
496, Willis TX 77378; Tel.: (936) 856-
4630. As can be seen, one type is available
with Anderson connectors and another is
available with AstroFlight connectors.
Either is simply inserted in a motor circuit,
and a voltmeter is connected to the shunt’s
test points. These are 1 mV/Amp shunts, so
a voltmeter will read 1 mV for every amp of
current flow. The use of these particular
shunts was illustrated in the January 1993
column.
The remaining shunt, at the bottom left
of the photo, is another homemade device
intended for use with my “smaller”
airplanes. It consists of a precision 10-
milliohm, 2-watt wire element resistor
(Mouser Electronics item 588-12FR010).
Thus when used as a shunt, it produces a
10 mV/Amp output to a voltmeter. In this
case, the current path connectors are Deans
and plug right into some of my “park flyer”
power systems. The attached voltmeter
leads, soldered to each resistor lead, then
route to a DVM.
I’ll use this last shunt example as a way
to illustrate some electric arithmetic. As
described, the resistor is a 10-milliohm, 2-
watt resistor. How much current can this
shunt safely be used for?
To determine the answer, recall that
power (watts) is a product (multiplication)
of current and voltage; in this case it’s the
current flowing through the shunt resistor
and the voltage appearing across the resistor
as a result of the current flowing. But that
voltage is itself related to the current and the
resistor by the expression V = I X R, where
“V” is voltage, “I” is the common alpha
character used to designate current, and “R”
is resistance in ohms.
So the power in the resistor is given by P
= I X V = I X (I X R) or P = I X I X R or “I
squared R.” From this we can determine the
maximum current allowed without
exceeding the power rating of this shunt
resistor, thus 2 (watts) = I X I X 0.010
(ohms), or I squared = 2/0.01 = 200. The
maximum allowed current I = square root of
200, or roughly 14 amps.
Clearly this shunt is useful for up to
typical Speed 400 power systems in which
motor current is often 10 amps or so at the
most. On the other hand, if this were used
with currents exceeding 14 amps, the shunt
resistor would be overdissipated. All shunts
have a rated power-handling capability; they
have some upper current limit of
application.
Now that a 10 mV/Amp shunt has been
described, this would be a better choice for
that 2-amp park flyer power system I
mentioned earlier. This is because 2 amps
flowing in a 10-milliohm shunt would
produce 20 mV, and this value, being
farther “up the scale” on a 200 mV range,
would be less affected by basic meter
accuracy and resolution limits.
The “flip side” is problematic as well.
That same 10 mV/Amp shunt used with a
motor current of 30 amps would result in a
300 mV voltage reading and a 30 x 30 x
0.01 = 9-watt power dissipation in the shunt
resistor.
This over-range voltage and the
damaging dissipation are all wrong. A 1- or
2-milliohm shunt would be much better. It’s
all a matter of using the right tool for the job
or, in this case, choosing the appropriate
shunt for the expected circuit current level.
So much for this month’s column. The next
one will be in the January 2004 issue since
the December Model Aviation is dedicated
to Nationals coverage.
Please include an SASE (self-addressed,
stamped envelope) with any correspondence
for which you’d like a reply. Everyone so
doing does get one.
In the meantime, here’s wishing
everyone many happy E-landings! MA