Electrics-2013/04

The Castle Creations
Phoenix line of ESCs
clearly labels the
type and rating of
the BEC.
Stand-alone BEC
units can save you
weight and ensure
adequate power.
What type of BEC meets your needs?
This month I’ll look at what a
BEC is and give you some basic
information about it. I’ve also
received questions about throttle hold
that I use for fi xed-wing models, so I’ll
touch on that too.
BEC
A BEC is a Battery Eliminator
Circuit—plain and simple. What this
means is rather than using a separate
4.8-volt or 6.0-volt battery to power a
radio receiver, a BEC is used to provide
power to the airborne radio.
The BEC will pull power from the
motor battery, step it down to the
appropriate voltage, and use it to power
the receiver and servos. Most of the time
the BEC will be part of your Electronic
Speed Control (ESC), but there are
stand-alone units, too. Which one is best
depends on your application.
Linear or Switching?
This confuses many people and it’s no
wonder, given the various explanations
on the Internet. Most of the smaller
ESCs tend to use linear regulators
because they are cheaper to produce and
work well for low-power setups drawing
little current.
The voltage is stepped down through
a simple resistance method and the
waste product is heat. Linear is only
appropriate for approximately 12 volts
maximum, and at
most three analog
servos, unless the
manufacturer
indicates
otherwise.
A switching
BEC operates
much like a tiny
speed control,
switching power
on and off at high frequency to step the
voltage down to a safe level for your
receiver and servos. The switched output
is then fi ltered to
provide steady
voltage with little
ripple.
There is
minimal waste
and consequently,
little heat is
generated. The
signifi cant benefi t
is the more stable
power provided
with the ability to
stand up to higher
demands.
Some switching BECs can handle
20 amps or more of surge without
failing, whereas a linear regulator will go
into safety shutdown because of heat.
Switching BECs can also make use of
a much larger input battery without
danger of overheating. Some stand-alone
BECs can be used with packs up to 12S.
Although BECs are rated in amps, be
aware that watts are the limiting factor.
Why Not Just Use a
Receiver Battery?
Weight is a precious commodity when
fl ying electrics—the smaller the airplane,
the more important it becomes. Using a
BEC—even a stand-alone version—saves
some of that precious weight for other
things such as a larger-capacity motor
battery or scale detailing.
Equally important is that you know
that you’re going to take off with a
freshly charged motor battery and
you won’t have to worry about the
state of your receiver battery. If you’re
coming from a glow engine background,
how many times have you forgotten
to charge the receiver battery until
you were ready to leave for the fi eld?
Problem solved.
Have High Demands?
If you require a large motor battery
Stick with name-brand ESCs that label the
BEC’s type and rating.
The author’s JR
9503 doesn’t offer
throttle cut for
airplanes, but it’s
easy to program a
mix to do the job.
All radios should
have similar
mixes for you to
define.
and you are using several servos, then
you might benefit from using a standalone
BEC unit such as the Castle
Creations BEC Pro. These can be wired
to handle a 12S pack and programmed
to provide your choice of voltage to the
radio.
Best of all, they will handle a
momentary surge of as much as 20
amps. You don’t want to plan for that
level, but it’s a great safety net just in
case, and the unit only weighs roughly
an ounce. Most separate BECs can be
programmed for the output voltage and
the Castle BEC Pro is programmable
from 4.8 to 12.5 volts.
Helicopter Considerations
Be sure to check the allowable voltage
for your gyro/tail servo combination
before programming your BEC output
voltage. Most servos will handle 6 volts,
but many of the gyro combinations
won’t.
Throttle Hold
Helicopter pilots are accustomed
to this, but most fixed-wing fliers are
not. If you aren’t familiar with what
it does, very simply (with the flip of
a switch), you can kill the throttle
and regardless of what you do with
the throttle stick, the motor stays
dead.
For electric-powered airplane
pilots, it’s a huge safety feature.
You can arm your airplane and by
using the switch, add the safety of
disabling your throttle stick.
My Spektrum DX7s makes this
option available for helicopter
and fixed-wing programming,
but most radios don’t. Because
each transmitter is different, it’s
impossible to walk you through
specific steps to enable it.
Check to see which switch on
your radio is used for helicopter
throttle hold/cut and define a mix
so that the switch will move the
throttle to zero when activated. Test
it without your propeller attached to
be sure it works properly. This extra
safety step should be a standard.
Taking Off
I hope I cleared up some questions
if you are unfamiliar with BECs and
how they are used. Next time I’ll look
at some upgrades for electric ducted
fans that I’ve been playing with.
The Spektrum DX7s makes it easy to set up throttle cut for your airplane.
SOURCES:
Castle Creations
(913) 390-6939
www.castlecreations.com
Spektrum RC
(800) 338-4639
www.spektrumrc.com