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A Brief History: HO Racing Power Supplies
- Carl Dreher, April 11, 1996
The DC (direct-current) power supply provided in an HO racing set consists
of a step-down transfer, which reduces the approximately 120 volt AC
(alternating current) house line to a safer 12-18 volts AC, and a
rectifier to turn the AC to DC. They
also always include a fuse of some kind for safety.
In many cases the fuse is the transformer itself, which is designed
to
burn out on overload. (Cheap
for the manufacturer, expensive for you.)
An AC voltage looks like this: (bad ASCII art!)
Notice
that although the voltage goes positive at some point, the AVERAGE voltage
is zero, because the upper and lower half-cycles cancel each other.
Your
DC permanent-magnet motor cannot run on AC!
It requires a DC voltage
that
looks like this:
Running
a DC motor on AC is a recipe for a burned out armature. A rectifier
circuit is built of diodes, which are a solid-state "one-way
valve", allowing current to flow only in one direction.
Very cheap power supplies (and I use cheap in all senses of the
word) perform what is called "half- wave" rectification.
The diode is used to remove the negative side of the AC, giving
this:
Notice now that although the voltage still isn't a straight-line DC, the
AVERAGE voltage is now some positive value, which your motor can
use. However, your DC motor isn't happy about this, because every
time the current rises and falls, the magnetic field in the armature pole
also rises and falls, sometimes in direct opposition to the direction
necessary to keep the motor turning.
The result is a motor that runs slower and worse, hotter.
In the early 1960's, diodes big enough to handle the current drawn by a
slot-car motor were expensive, so half-wave rectification was used.
Now, diodes
are very inexpensive, so a slightly more complicated rectification circuit
is used. It is called a
"full wave rectifier", and gives:
The AVERAGE of this waveform is a higher value than before, which means
your motor runs faster, bit it still suffers from all those humps of
rising/falling voltage. Unfortunately,
this is where most commercial HO slotcar power supplies stop.
OK, the power-supply lesson is over.
What can we do about this?
These
best solution is to run on batteries.
Batteries product pure DC.
Unfortunately, they also are messy and in the case of car batteries, can
produce hydrogen gas, which is explosive.
Despite this, I used car batteries very
successfully for years, combining a 12V and 6V to give an 18V
supply. Car batteries are inexpensive.
Six volt batteries are harder to find, since no
car has used 6V for over 20 years. However,
fork-lifts use them, and many battery
specialty stores can get you one. Also,
stores that cater to the antique
car trade are a good source. You
will also need two small battery trickle
chargers, one 12V and one 6 volt, to keep the batteries charged.
(Available from motorcycle shops.) You
can also use a regular, large charger
to top-up the battery between racing-heats.
HERE'S
AN IMPORTANT POINT: DON'T use
a large battery charger on the batteries
when you are racing. These
can induce a full-wave AC ripple voltage
on the DC battery, which defeats the purpose of using batteries!
Gell-cell,
or "sealed" batteries are also available, but much more
expensive and
(I believe) they take a bit longer to recharge.
If you don't want to go to
batteries, these is another solution.
The goal is to
fill out those valleys in the full-wave rectifier waveform.
You can do this
very easily with a capacitor. A
capacitor is nothing more than a charge storage
"bucket". When the
voltage is high and the power supply is providing current
to your motor, the bucket fills up. When
the voltage is low and the power-supply
cannot meet the demands of the motor, current is drawn from the bucket.
With a big enough capacitor, the waveform looks like this:
The
average voltage is now higher and there is less ripple to the voltage.
Your motor will run faster and cooler.
(Well, that isn't exactly true.
Since the average voltage is higher, you will be feeding more power to
your motor and it will run hotter. But
for a given speed, as set by your controller
and racing hand, the motor will run more efficiently and waste less power
turning the AC ripple into heat.)
OK,
so how do I get these magic capacitors?
Radio Shack, where else!
Capacitors
are rated in micro-Farads (abbreviated uF) and volts.
FOR SAFETY,
YOU
SHOULD CHOOSE A CAPACITOR THAT IS VOLTAGE-RATE 100% GREATER THAN THE
VOLTAGE
OF YOUR POWER SUPPLY. This is
important!!! Using a
low-voltage capacitor
will cause it to overheat and explode.
A standard rating is 35 volts,
which is perfect. Don't use
less.
You want to buy the maximum uF's you can get.
The more the better. 5000
and 10,000
uF capacitors are easily available. Don't
bother with anything less than
1000. Furthermore, if you
wire capacitors in parallel, the uF rating adds,
that is, two 5000 uF caps wired in parallel gives 10,000 uF.
To wire them to your power supply and gain all their benefits, simply
connect the positive side of the capacitor to the positive side of the
power supply and the negative side of the capacitor to the negative side
of the power supply. That is,
it goes ACROSS the power supply, NOT in-line with a wire going
to the track.
WARNING!!!
WARNING!!! WARNING!!!
WARNING!!!
WARNING!!!
WARNING!!!
LARGE
CAPACITORS HAVE A POSITIVE AND NEGATIVE SIDE, JUST LIKE A BATTERY.
THE POSITIVE SIDE OF THE CAPACITOR MUST GO TO THE POSITIVE SIDE OF
YOUR POWER SUPPLY. IF YOU
REVERSE THE CONNECTION, THE CAPACITOR WILL OVERHEAT AND EXPLODE
WITH THE POWER OF A LARGE FIRECRACKER.
YOU CAN BE SERIOUSLY INJURED!
DOUBLE
CHECK THIS BEFORE YOU TURN ON THE POWER SUPPLY!
Try
this and I think you'll be surprised what a difference it makes.
---------------------------
One
last funny story: Back
when most HO tracks were powered by the cheap power
supplies that came with racing sets, someone had the bright idea of