104 M ODEL AVIATION
ThIS coluMN will cover a meet announcement, the Electric
Connection Service, some USC reader inputs, using LEDs, and
writing to me.
The Mid-Winter Electric Festival, as presented by the Silent
Electric Flyers of San Diego (SEFSD) for the past several years, is
scheduled for February 16-18, 2001, at Mission Bay CA.
This meet is presented by Hitec RCD, SEFSD, and Model Airplane
News, and it will be loaded with events and demonstrations,
substantial prizes, supplier booths, and more. An added feature is two
days of Indoor flying in a nearby facility.
For more information, go to www.sefsd.org; contact Bill Everitt
at (760) 753 1055; Fax: (760) 633 2271; E-mail: [email protected];
or call Glen at (858) 747 6948, ext 310. Tell ’em Bob sent ya!
The electric connection Service (ECS) is a free column feature,
intended to help E-modelers hook up with other E-modelers in
their vicinity.
Dick Gum, 26828 Racquet Cir., Leesburg FL 34746-8083, is
seeking others to help him get started in E-power.
The ECS generally works very well, and I’m counting on this
edition to work well too.
Anyone anywhere who is seeking to be so connected, just write me!
Slow Reactions: Readers continue to react to the Universal Slow
Charger (USC) construction article (9/00 MA) with stories of
success, and with some questions. Some have even sent photos of
their completed USC’s, which are always nice to see!
Surprisingly, several readers have inquired about adding a peak
detector to the USC.
Folks, this is a slow charger—no peak detector is needed, or even
useful, because there is no peak to detect in the usual sense of the
word. Let’s review.
Our motor packs are routinely flightline-charged using fastcharge
techniques, so they are ready to use in 15 to 25 minutes.
This rapid rate of charge (i.e., high-current charge) is associated with a
certain risk; if the charging is not stopped when the pack’s power is fully
restored, pack damage is likely to occur.
In earlier years, fast-charge termination was done with a timer, set to
shut the charger down in some safe time. In recent years, the process of
charge peak detection has become precise and reliable, so now most
chargers automatically safely terminate charge by electronic means. All of
this is to prevent pack damage.
“Slow charging” (“overnight charging”) is entirely different. For
decades, it has been widely accepted as the rate needed to fully
charge an empty pack in approximately 14-16 hours.
It’s the charge process historically used with our transmitter and
receiver packs, but it can be applied to any pack, including motor packs.
The most commonly used rate (current) is the familiar 1⁄10th
capacity (“C/10”) value. One would charge a 500 milliampere-hour
pack at roughly 50 mA for the time specified.
It is generally accepted that although approximately 14 hours is
needed to properly charge an empty radio pack (or any other Ni-Cd
pack) at the C/10 rate, no real harm is done if the time exceeds this
slightly, because the charge current is relatively low.
Although it is not recommended as normal practice, you probably
won’t notice an immediate problem if such low-rate charging continues
for an extra day or so. This is much different from allowing high-rate
charging to continue for even a few minutes beyond the “full”
condition, where pack damage is essentially certain.
Pack behavior is different for the slow- and fast-charge conditions.
With slow or “overnight” charging, no significant pack-voltage
peaking occurs at the “full” condition because there is only slow and
slight internal pack temperature rise. (Please see this 4/00 column
for more in-depth discussion.) Therefore, one cannot readily apply
peak- detector techniques to terminate slow charging, nor would it
be of significant value anyway.
However, there is the time-switch approach for those who want to
end charging when slow charging is complete.
Many household timers on the market are
intended to turn appliances (lights, the coffee
machine, etc.) on and off; surely some could be
adapted to this charging application.
Most household and hardware stores carry a
variety of such timers; survey the marketplace to
find one suitable to your needs. Plug the timer into
an outlet, and plug the charger into the timer.
I cannot suggest a particular one, because I
don’t have any in use. My attitude is, if I
happen to forget and charging goes on for an
extra day at “C/10,” so what?
One reader, who inquired about using a
household timer with the USC, asked if any
pack discharging would occur back through the
USC circuitry once his timer shut it down. The
answer is a simple “no”!
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Author’s 12.1-ounce NiteLite has six 500 mAh, 280 motor, Jeti 05 ESC, Hitec 555, S-60
servos, Gunther prop, five internal high-brightness LEDs, bright white nose LED.
light-emitting Diodes (LEDs) are nifty little
semiconductor devices that most people have
seen or used sometime in their lives.
These “solid state lamps” come in a wide
range of colors, brightness, case and lens
styles, and prices. The most familiar types are
those not-too-bright ones used as indicators
and pilot lights in instruments and appliances.
I got interested in building a new night-flier,
based on LEDs deployed within a clear-covered
open balsa structure—it’s my new NiteLite.
I’ve been night-flying for more than 25
years, but always based on incandescent
lighting; this was a departure for me.
I bought some “high brightness” LEDs in
various colors, including red, blue, green, yellow,
orange, and white, then I began to experiment.
Wow! These LEDs have brilliant color, and
they can be painfully bright! (The best source I
have found for them is www.hosfelt.com,
which is a good company from which to buy.)
As I was happily showing off my new LEDequipped
NiteLite, I learned that many people
do not understand how to use LEDs—no matter
the application. Following is a primer.
LEDs are diode devices, so they have an
operating polarity; this means they have (+) and
(-) terminals. Since they are diodes, it’s best to
use LEDs by setting a current through them, not
a voltage across them—a key distinction, and
the most confusing point for most people.
Although some have operated some LEDs
connected directly across two Ni-Cd cells, it is
a risky approach; being diodes, a small voltage change will have an
associated large current change—a touchy and uncontrolled situation.
And because high-brightness LEDs are more costly, “risky” and
“uncontrolled” are good things to avoid—especially for flying in the
deep, dark dead of night!
The LEDs in my NiteLite are biased with approximately 20 mA,
as specified in the part descriptions. The power source is the motor
battery, which is also the receiver battery by virtue of the BEC ESC.
The pack is six-cells and 500 mAh, so the average voltage of it
during the flight is approximately 6.6 volts. Of course the voltage is
a bit higher immediately after charge and a bit lower at the end of
flight, but not to worry.
Catalog information for LEDs usually includes a voltage
statement. As presented, this information can be misleading; it
gives what appears to be the voltage applied to the LED. In
actuality, it is the voltage on the LED as a result of current
flowing through it.
Think of this as “the current comes first, then the voltage is
the result.” (It is somewhat like charging a battery with
current—not voltage.)
LED voltage does vary with the LED type, but is necessary
information in any case.
Many LEDs display a terminal voltage of 1.9 to 2.1 volts at a
specified current of 20 mA—a common number (but not the only
one). This tells us that if we have 20 mA flowing through this LED,
we can expect the LED to “use up” (“drop”) roughly two volts.
The diagram shows a complete LED bias circuit, consisting of a
battery, a resistor, and an LED. For everything to work, the battery
voltage must be greater than that needed by the LED. The resistor—
in series with both—is chosen to set the circuit (LED) current.
In the case of my NiteLite, using the average of 6.6 volts as the
battery voltage and knowing this example LED will “use up” two
volts, the circuit voltage “left over” is 6.6 – 2 = 4.6 volts.
That voltage appears (“drops”) across the resistor and, in fact, is
the number used to determine the resistor value, as follows.
Since 20 mA is the desired current, the needed resistor is given
by 4.6 volts divided by the 20 mA current—4.6/0.02 = 230 ohms.
(The 20 mA is expressed as amps here.) Standard resistor values are
220 or 240 ohms, and either will do.
The resistor power is resistor voltage multiplied by the current,
or 4.6 x 0.02 = 0.092 watts = 92 milliwatts. A standard quarterwatt
resistor is more than adequate.
Use this basic approach for the safe operation of any LED. Note
that the procedure is flexible.
Some LEDs (e.g., blue and white) have a three-volt terminal
voltage—use that number instead of the two volts in the
preceding example.
I have two two-volt LEDs in series in my NiteLite, so this
total LED voltage is 2 + 2 = 4 volts. If you do the arithmetic,
you’ll learn that the resistance needed here is much less than
with one LED.
It won’t work to use two three-volt LEDs in series, because the
associated six-volt drop is much too close to the average battery
value of 6.6 to derive a meaningful resistor value. Relatively
speaking, there must be sufficient circuit voltage “left over” on the
resistor for the current to be reasonably stable with the varying pack
voltage during rundown.
In my NiteLite installation, I have six LEDs in four circuits, and
a total lighting drain of approximately 80 mA—much less than the
motor current to fly the model!
I used white LEDs in the nose, red in fuselage aft section, blue
in front of spar, yellow aft of spar. And does it ever glow in the
deep darkness!
It’s okay to Write: All reader letters are welcome, and I answer
each one that includes a self-addressed stamped envelope.
Some responses are quicker than others, depending on the
topic (do I need to research something?) and my general work
load at any given time.
It’s best to write directly to me—not via MA. I do not use E-mail
for this purpose! I know some folks have real trouble with this, but
this is how it is. Okay?
So ends another column. Here’s wishing you a happy holiday
season and a great electri-flyin’ new year.
Don’t forget that Electrics fly just fine in the dead of
winter—and they have no associated cold, thick, and messy goo
to be cleaned up! MA
Full-depth spar, stick-rib structure allow different-color LEDs to light up front, back of
wing panels independently. LEDs are held by rubber grommets.
NiteLite 280 motor mounted in “V” block with thin auto trim double-stick foam tape.
Motor-attach method is now in use on four small airplanes—works great.
February 2001 105
