Electrics - 2012/10

It’s been a busy period since the last
column. I recently returned from a
trip out west to see my new (and
fi rst) granddaughter.
I fl ew with the pilots at Cherry Creek
State Park again this year in Denver. It’s
great to hang out with those people.
I’ve received quite a bit of mail
regarding high-voltage setups, so I
thought it would be a good time to
respond to some of those questions and
concerns.
Fire in the Hole!
It’s easy to become distracted at
the fi eld; people want to talk about
your airplane while you’re connecting
things, etc. I’ll admit that I enjoy a nice
conversation in the pits, but when I
cover events I try to remain cognizant
of how distracting I can be with my
camera and interview questions.
Unfortunately, I’m not always that
“aware” when I’m at my own fi eld and
in my comfort zone. At big events, I’m
more focused for fear I’ll screw up in
front of a crowd.
I received some photos from a good
friend of mine showing the results of
being distracted while getting ready
to fl y. This is not a man who is new to
electrics by any means; he is a wellknown
expert in our fi eld, but a small
distraction nearly
ended in disaster.
He had two 4S
packs connected in
series to power his
8S airplane. The
connectors are color
coded and he was
connecting a black to
a red between the two
packs, leaving a red
from one pack and a
black from the other
to connect to the
ESC.
During fi nal arming,
the gentleman was
asked a question,
which distracted him.
He grabbed the
two wires from the
pack and connected
them, forgetting that
he had already made
the series connection so he shouldn’t
be connecting any unmatched color
connectors. This created a dead short of
his 8S pack.
The fl ash was instant and impressive.
Because the pack burned the connectors
so completely and instantly, he was
able to grab the wires and pull the pack
from the airplane before additional
damage was done.
Surprisingly, the
packs did not
ignite beyond the
initial fl ash.
Fortunately, the
contacts in the
Anderson Power
Poles quickly
shorted out and burned off so the
circuit was broken. That left the burn
marks and melted plastic to deal with,
but he wasn’t injured.
How can this happen? Easily! Most
of us who use larger-size LiPos use
multiples to make up our 8S, 10S,
or 12S battery packs, by connecting
them in series. This was the case in my
example.
There are some simple steps we can
use to avoid a similar situation; here are
just a few to keep in mind:
• Stay focused! Ask visitors to wait a
moment while you complete the task,
then you’ll be glad to talk to them.
• Standardize your method of keeping
connections identifi ed.
• Be consistent in your methods
across all airplanes and helicopters.
• Double check everything before
making the connection.
• Check it again!
• It doesn’t matter what method you
use, but use something.
I’m guilty of not following all of my
own advice, but I’m trying to get better
after seeing the results of easy mistakes.
On my largest 12S setup, I have
color-coded connectors that are also
numbered with tabs. This means I always
plug the number one plug into the other
number one plug, the number two
into the other number two, and so on,
until it’s all connected. If the numbers
don’t match, they don’t get plugged in,
regardless of the color of the connector.
High-Voltage Danger
I think there are wars being fought on
the Internet over what constitutes “high
voltage” in our airplanes and helicopters.
I stay clear of all the arguments, but it
does make for some interesting reading.
It also confi rms my thought that
anyone with a keyboard can appear to
be an expert on a discussion forum. Be
cautious and do your research before
accepting anything you read there.
In the “old days,” AstroBob of
AstroFlight fame would say he’d never
make a motor/ESC system for more
than 36 cells (we were using Ni-Cds
at the time) because of Underwriters
Laboratories (UL) requirements and
various codes on usage.
Throughout the years, this was
debated and various interpretations of
the UL guidelines have seen this 36-cell
or 50-volt limit go by the wayside. Now
we regularly see
systems all the way
up to 15S LiPos.
My fully charged
12S system is
more than 50
volts when I fi rst
connect it.
I’m neither
an electrical
engineer nor a
lawyer. I’ll leave
the particulars of
what constitutes
high voltage to
them, but I have read and researched
more UL guideline papers than I care
to think about and there is a common
theme across several of them. Here is an
excerpt that appears in several different
papers regarding high-voltage DC
circuits:
“… a circuit fi re hazard is considered
to exist if either the voltage is greater
than 35 volts or the current is greater
than 0.2 amperes.”
I pulled this particular one from UL-
6500, but it is in several publications, so
I assume it is the basic guideline. There
are two important things to notice here:
voltage and current. The hazard doesn’t
come from a combination of both
parameters; it’s either/or.
I can’t think of a system we’re fl ying
that doesn’t draw more than 0.2 amps,
so in essence, everything we do is a fi re/
safety hazard.
Are we unsafe? That depends upon
us. If we consciously do things in a safe
and educated manner, then the answer
is “no.” If we become complacent or
careless in our thinking, then “yes.”
Many people don’t think about DC
voltage being dangerous because it is
so much lower than the AC voltage
we’re accustomed to at home, but it
has been clearly documented that it can
be equally dangerous. The best system
is only as good or as safe as the person
using it, so be aware and be careful.
Anti-Spark Devices
This is getting into Red Scholefield’s
end of the world, but because I’ve
received so many emails after showing
my big 12S, 99-inch airplane, I’ll
mention it briefly. The anti-spark device
I used in the Robinhood 99 is from
Xtreme Power Systems and sells for a
few dollars.
This device doesn’t remain connected
as others do; it is plugged in for a few
seconds to slowly charge the caps on
the ESC, then disconnected. As soon as
you disconnect it, you plug in the final
battery connector and all is well. If you
wait too long, the caps will self-discharge
and you’ll get the spark when you
connect your battery.
The only precaution I take with this
particular one is to insulate the bullet
connector when I disconnect it so it
doesn’t accidently short against anything.
It is a positive lead going through the
connector, so anything it might touch
that is negative would produce a short.
There are several places online that
explain how to make your own and they
are a simple device. I’m lazy and these
are inexpensive enough that it doesn’t
seem worth the effort for me to make
my own.
I think Red described several months
ago how to determine the requirements
of the resistor and make it.
Taking Off
That about covers it for this issue.
Regardless of whether you’re flying
3S-size setups or monster 12S setups,
you need to be careful. Don’t be afraid
to ask someone to wait a minute so you
can finish your task. Then you can visit
with him or her knowing everything has
been done correctly.