Electronics 2003/09

98 MODEL AVIATION
FORTUNATELY FOR US Radio
Control fliers, because of the tremendous
demand for rechargeable batteries brought
about by the equally tremendous use of
battery-powered devices such as cell
phones, broadcast receivers, compact-disc
and tape players, laptop computers, etc. in
general use, we now have batteries that are
greatly improved compared to those of
even a few years ago.
Of course, rechargeable batteries are no
better than the equipment used to
revitalize them. Although the battery
chargers that come with today’s radio
systems do the job, there are more modern
and more efficient chargers on the market.
There are so many more modern chargers
that, coupled with the latest battery
chemistries, the proper choice of a battery
charger can be confusing. But that is
another story.
However, since many of these chargers
are intended for field use and must be
powered by 12-volt batteries, a separate
AC (wall socket) power supply must be used to provide the 12
volts required. Such power supplies are available, within the
hobby market and through general electronics suppliers, but those
with the necessary high-current capability are somewhat costly—
especially for those who have only occasional use for such an
item.
That was the bad news, and here is the good news. Actually, it
couldn’t be “gooder.” There is a 12-volt-plus power supply that in
many cases will cost you nothing other than a few minutes of
work with your soldering iron. We are going to convert a common
computer power supply into a voltage source for your DCpowered
charger.
Don’t panic; this will require no real electronics experience—
merely the need to follow instructions. And with the proper
choice of one component—a resistor—you have some control
Eloy Marez
E l e c t r o n i c s
2626 W. Northwood, Santa Ana CA 92704
ATX computer power supply described in
text uses standardized four-pin
connectors for all system accessories.
Readily available computer power supply
can easily be converted into power
source for 12-volt DC-powered field
charger.
The wires (most of three basic colors) are connected in parallel;
they go to common connection points on component board.
Recommended way to terminate power supply’s output:
RadioShack Multipurpose Posts. But insulated fittings will work.
The many output wires and plugs on the original supply can be
intimidating, but they are removed for our use.
September 2003 99
over the exact output voltage. Most DCpowered
chargers will accept, and some
even like, a voltage slightly higher than 12
volts. My adaptation, which I will
describe, is set for 13.20 volts, which is a
recommended value.
Quite a bit of standardization exists
within the computer industry, at least
physically, so that subcomponents by
different makers will fit into the available
space and be secured with similar fittings,
and most everything is plug-in compatible.
This applies to power supplies, which
have evolved into what is probably the
most common, known as the “ATX.”
There are some variations, but the most
common ATX is a louvered metal case
with an internal fan, an AC power
connector, an on-off switch, and many
wires coming out. It is approximately 6.1
inches long, 5.7 inches wide, and 3.5
inches deep. It is what is known in the
power-conversion world as a “switching”
power supply.
The other, more traditional power
supply is now called a “linear” unit. Most
bench-type supplies available and in use
for RC applications are this type. They
will do the job, although they are
somewhat heavy, and in purely electrical
terms they are not as efficient, but that is
not a critical consideration for our use. A
switching supply will be roughly 35%
more efficient (input to output) than a
linear supply of comparable voltage and
current.
The switching supply is a rather
complex device, with a parts count
probably 100-fold more than that of a
linear supply. We don’t need to know ours
intimately—just that it works. However, to
explain the “switching” part, in these
systems the output voltage is sensed and
the input is “switched” on and off as
required to maintain a constant output.
As I stated, ATX power supplies are
created pretty much equal—except for
their current ratings. They have two
outputs, at 5 and 12 volts; we are
interested in the ampere rating of the 12-
volt side.
No guesswork is necessary; every ATX
supply that I have seen has been clearly
labeled similar to the one in the photo,
which tells us that this particular one is
good for 15 amperes on the 12-volt side.
This is at barely 3 pounds weight; a linear
supply with the same ratings would weigh
at least 10 pounds. For our use I
recommend at least an 8-ampere rating.
These computer power supplies are
plentiful and inexpensive. I believe that is
because computers become obsolete
seemingly the same week they come on
the market, or maybe it is because it is
cheaper to buy a new computer than to
have one serviced.
These supplies can often be found new,
at bargain prices, at electronics suppliers. I
purchased the one in the photos in its
original packing for $5. I mentioned
“free”; try your local computer-service
centers. I tried two of them; one gave me a
supply and the other practically begged
me to take a box full!
Once you have acquired your supply,
new or used, it has to be checked for
operation! Other than checking the
internal fuse, don’t even think about trying
to repair one that has died. You will see
how complex they are as soon as you take
off the cover. Besides, they cost so little.
If you obtain one from a service center,
they are set up for go/no-go testing and
will probably do so for you. If you have to
do this testing yourself, connect the load
to be described later and measure the
output voltage. Read on for how.
After confirming that the supply is
working normally, let it sit for half-hour
or so. You will have to remove the
component board from the case, and
although only low—and completely
safe—voltages are involved, there may be
a capacitor or two that will hold enough
voltage to cause you a surprise if you
wander across its contacts.
These voltages will bleed off in a few
minutes. And I hope I don’t have to
mention that you should be 110% sure that
it has been unplugged from the wall socket
before you remove the cover!
As you can see in the photo, the output
of the ATX power supply is a virtual rat’s
nest of wires! Don’t let them intimidate
you. Upon closer inspection, you will
learn that all except for one are duplicates;
i.e., they come from the same place, do the
same thing, and are interchangeable.
They all terminate in four-pin plug
connectors except for one that has at least
double that number and, of course, a larger
plug. The larger connector plugs into the
“mother” board, which is the main circuit
in the computer; the others power the
various drives and accessories.
The basic wire colors, which are
yellow, black (two wires), and red (as
shown in the accompanying sketch),
terminate at all of the connectors—all the
small ones and the one larger one. After
removing the entire component board
from the case, you are ready to perform
the necessary surgery.
When removing the board, you may
run into some mechanical differences from
one power-supply designer to another. The
other connections—the AC input, the onoff
switch, and, if installed, a switch to
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100 MODEL AVIATION
select either a 110 or 220 AC input supply
voltage—may have plug-in connections on
the board or they may be soldered in.
If it’s the former, simply unplug them.
If it’s the latter, they are held in place on
the case with screws. Remove them and
free the items. The board can now be lifted
from the case.
Your first task is to remove all of that
excess wiring. You will need to leave two
of the black (negative) wires and two of
the red (positive 12-volt) wires, and
unsolder and open the hole for one each of
the black and yellow (positive 5-volt)
wires. All of the others can be snipped off
close to the top of the board.
To ease handling, you can now cut the
remaining black and red wires to 6 inches
long. There may be other differentcolored
single wires terminating at the
large plug. They are there for some
special function in a particular computer;
they have no application here and can be
removed.
Now your only decision rears its ugly
head! These supplies require a load on the
5-volt side before generating that
specified on the 12-volt side. This load is
automatically applied within the
computer by any of the drives or
accessories connected. In our application
we have to fool it and apply an artificial
load.
Some confusion exists about this load.
Generally we find that a 1-ohm, 25-watt
resistor or an automotive light bulb, for
example one known as an “1156” type, is
recommended. Either will work, but ...
With 5 volts applied, a 1-ohm resistor
will allow 5 amps of current to flow, for a
resulting 25-watt power consumption.
The 1156 bulb is rated at 2 amperes as its
designed 12-volt input, which calculates
to 6 ohms effective resistance, and which,
in this 5-volt application, will be drawing
more than 12 watts of power.
You may hear that the bulb is a better
choice than the resistor since it will also
serve as a pilot light, letting you know
that the supply is operating. And you can
September 2003 101
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read by it if it’s dark. Although either will
work, either will waste significant power,
which we need to reduce wherever
possible.
But more important, the bulb and
resistor also produce a great deal of heat,
the dissipation of which has to be
considered. And my tests have proven, at
least with the two units tested, that such
low resistances are not absolutely
necessary.
The value of this 5-volt-output-load
resistor has an effect on the output voltage
on the 12-volt output, and it must be
considered in your choice. Since some
design differences do exist from one supply
to another, cut-and-try is in order. But fear
not; it is simple, and the required resistors
cost less than 50¢ in most cases.
My experimentation revealed that
resistances much higher than the 1-ohm or
light bulb will provide the necessary load
with much less heat generated. As stated,
my choice of 13.20 output volts required a
68-ohm resistor. Applying Ohm’s law
again, it consumes less than 1⁄2 watt.
A heatless-producing 5-watt resistor is
more than adequate, and it makes
connection much easier. It now sits right on
the board, soldered into the two holes you
previously opened, from which one
yellow and one black wire were removed.
Lower-value resistors will raise the output
in small steps.
If you have difficulty getting to an
electronics supplier, RadioShack has a 50-
ohm, 10-watt resistor (product 271-133) that
will do the job. There is more than enough
room for the slightly larger 10-watt unit.
There’s just one more step: to provide a
way to safely and securely make the charger
connections. My choice was RadioShack
Multipurpose Posts (product 274-661), to
which one can readily attach plain wires,
banana plugs, etc.
My particular power supply had two
openings; one was unused and the other
provided an outlet for all of the original
wires. I centered and mounted the
connectors on a piece of plastic and secured
it to the metal case with a machine screw.
Do you remember those two 6-inch
black and two red wires you left connected
to the board? Since it is rather small wire,
intended to carry low accessory currents,
you are simply going to double them to
more efficiently conduct the greater
amounts to be used.
After mounting the connectors in place,
solder the two black wires to the black
connector, and the red wires go to the red
connector. If you get them mixed up, your
charger will not like it! Test the output
voltage at the output connectors and you are
finished!
See? It wasn’t all that bad, and now you
have an inexpensive power supply that will
meet all the needs of whatever charger you
have or choose to purchase. One last thing
to remember is that with the exception of
the 110VAC input power, there are no
dangerous voltages inside these power
supplies. But never forget that 110 volts can
put an end to your RC flying! MA
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