Basic Electricity And Control.pdf

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PRODUCTS
I
N F O R M AT I O N
S
TAT I O N
Basic electricity
and control systems
Electricity
K
nowing how to use electricity in
model railroading extends beyond
running trains. You’ll also want to
use electricity to power switch
machines, structure lights, streetlights,
and other accessories. Having a good
basic knowledge of electricity will help
you use these accessories with confi-
dence and improve the realism (and
fun) of your layout. Let’s start with
some basics.
Fig. 2 POWER PACK CONNECTIONS.
MRC’s Tech II has fixed DC terminals for
accessories. Power packs also include
variable DC (track power) and fixed AC
(accessory) terminals. When making
connections, always wrap wire clockwise
around the terminal screws.
PHOTOS BY JEFF WILSON
AC vs. DC
Standard house current
is alternating current(AC),
and although some model
trains (notably three-rail
O gauge, American Flyer S
gauge, and Märklin HO)
use AC, most scale model
trains use direct current (DC).
With AC, as the name implies, the
polarity constantly alternates at 60
cycles per second in the United States.
With DC the polarity is constant as the
current flows. Direct current allows us
to use polarity to control motor direc-
tion (very important in running trains),
something that can’t be done with AC.
Figure 1 shows how this works.
On a typical power pack, the variable
DC output goes to the track – this is what
is adjusted by the speed controller. Most
power packs also have fixed AC termi-
nals, and some also have fixed DC ter-
minals (see fig. 2), which put out
continuous voltage. These can be used
for accessories.
Power
supply
To trains or
accessories
3
0
3
12
0
12
Ammeter
Voltmete
r
Ammeter
connected in series Voltmeter
connected in parallel
Volts and amperes
It’s important to provide the proper
voltage to accessories, as too much
power can burn out bulbs and motors.
Electricity is measured using volts
(V) and amperes (A). I find it easiest to
understand each using the analogy of a
water supply pipe coming into your
house. Volts measure the force of elec-
tricity (the pressure gauge on a water
pipe) and amps measure the amount of
electricity (the water meter).
The water pressure (volts) to your
house remains constant, but the vol-
ume of water (amps) used varies
depending upon how many faucets you
have open at a time.
The electricity in your house is at
120V, much too high to use safely
Direction of travel
Fig. 3 METER CONNECTIONS
for model railroads. Transformers
reduce the voltage to the 12 to 18V
needed for trains and accessories.
For current, most modern locomo-
tives in HO and N scales use less than
.5A, while some older models draw
more. Engines in O scale and larger
models can use 1A or more. Small
devices such as light bulbs require little
current, and their current consumption
is often measured in milliamps (mA).
For reference, one milliamp is a hun-
dredth of an amp (1mA is .01A; 5mA
is .05A).
Most power packs and other power
supplies are rated in volt-amps (VA),
meaning “volts x amps.” To convert this
to the ampere ratings used by model
railroaders, divide the output rating of
the power pack by 12, which will give
you the pack’s amp output rating at
12 volts.
Light bulbs are probably the most
commonly used electrical accessories.
Most small bulbs draw roughly 10mA
(0.1A), while some microbulbs draw
less than 5mA.
How do you know if there’s enough
power for all of your accessories? Add
the current required for all accessories
and make sure the total is less than the
rated limit of your power supply. For
example, if you have a 1A power supply
Motor
Polarity
Motor
Polarity
Fig. 1 DC POLARITY
Direction of travel
ILLUSTRATIONS BY ROBERT WEGNER
Each bulb receives
full voltage
12V
12V
Optional switch
12V
12V
4V
Parallel
4V
Optional switch
4V
To other
lights
Single-pole, single-throw (SPST)
12V
Voltage is divided
among bulbs
Series
Fig. 4 TYPES OF CIRCUITS
you can use it to power ten 10mA bulbs
or 20 5mA bulbs.
You can determine the amount of
current a bulb or accessory is drawing
and how much voltage is in a circuit
with separate meters or a combination
volt-ohm-milliammeter (VOM) as
shown in the photo on page 54. The
proper way to connect a voltmeter and
ammeter in a circuit is shown in fig. 3.
Don’t use a VOM as part of a perma-
nent circuit, as the electronic compo-
nents of the milliammeter function can
be damaged by continuous use.
It’s a good idea to have a separate
power supply for accessories. With
small power packs, the drain of acces-
sories can slow your trains, and increas-
ing the speed of trains can dim your
accessory lights.
It’s also good practice to operate
bulbs at less than their rated voltage
(12V for a 15V bulb; 1.2 or 1.3V for a
1.5V bulb). This increases bulb life and
lowers the operating temperature,
important with plastic models.
Throwing switch turns power on or off
Single-pole, double-throw (SPDT)
Throwing switch directs power
to one of two circuits
Fig. 5 TOGGLE SWITCHES
Also, you can’t add additional bulbs
while keeping constant voltage.
Switches
You can turn accessories on and off
by turning the power supply on and off,
but a better way is with a switch. Fig-
ure 5 shows the two styles of toggle
switches used on most layouts.
They come in a variety of sizes and
styles, with the single-pole, single-
throw the simplest type (a basic on-off
switch). The number of poles refers to
the number of connections that can be
made. The number of throws is the
number of positions that the switch
can be turned to.
For more detailed information on
wiring, see
Easy Model Railroad Wiring:
Second Edition
by Andy Sperandeo
(published by Kalmbach).
The more comfortable you are with
wiring, the more animation and life
you’ll be able to bring to your layout.
1
Series and parallel
You can connect multiple acces-
sories or bulbs in one of two ways:
series or parallel. Figure 4 shows each
style, using light bulbs as an example.
Parallel connecting gives each bulb
the same voltage, regardless of how
many there are. Adding bulbs is easy by
wiring them to the main (bus) wires.
Connecting in series (end-to-end)
divides the voltage equally among all
bulbs. A disadvantage is that if one bulb
burns out, the whole string loses power.
Cab control
Common feeder
One block
Switch set to select cab B
to control this block
SPDT
toggle
switch
Common
connection
Insulated joiner (or gap)
Block
feeder
Insulated joiner
(or gap)
Cab A
Cab B
ILLUSTRATIONS BY RICK JOHNSON
Block 1
Block 2
Common rail
Block 3
Block 4
1
A
SPDT
Switches
B
A
2
B
A
3
B
A
4
B
A
5
B
Block 5
switch which can turn the power to that
block off. Since the switch is a double-
throw type it connects only one of the
cabs to the block at any one time. The
illustration at left shows how to connect
both cabs and a switch to a single block.
Of course, you need more than one
block on the layout. The schematic
drawing shows how to wire an oval
with a passing siding for two-train con-
trol. The wiring may look complex but
it’s simply a matter of repeating the sin-
gle block wiring as necessary.
You’ll need not only a cab but also as
many single-pole, double-throw (SPDT)
switches as you have blocks.
Another option is the Atlas Selector,
which is simply four SPDT slide
switches in a single case. Most model-
ers, though, prefer to have the switch
controlling a particular block mounted
on the control panel schematic, making
it easier to tell at a glance which block a
switch controls.
It’s a good idea to test each block as
you wire it up as crossed wires are the
biggest problem with cab control. And
be sure you color code your wiring as it
makes the initial wiring and any future
alterations or troubleshooting a whole
lot easier.
How it works
Once your wiring is complete place
one locomotive in block 1 and flip the
toggle for that block to cab A. Then
place a second locomotive in a differ-
ent block and flip that toggle to cab B.
You should be able to control the two
locomotives independently within the
individual blocks.
As the trains work their way around
the layout turn the next block to the
appropriate cab. Turn off the block
after you leave it or the next train enter-
ing the block will be controlled by the
“wrong” cab, leading to that familiar
cry of “Someone’s got my train!”
While command control (another
topic for a later time) takes care of that
problem, cab control is still a viable
option. For more details on layout
wiring see
Easy Model Railroad Wiring
by Andy Sperandeo (Kalmbach Pub-
lishing Co.), which includes a detailed
explanation of cab control and the asso-
ciated wiring.
1
Common connection
Cab A
Cab B
T
he only thing better than running
one train is running two at the
same time. Getting a single train to
run around the layout is usually a mat-
ter of simply running a pair of wires
between the power pack and the track.
But simply placing a second locomotive
on the track will mean both trains run
in the same direction at the same rela-
tive speed, so getting independent con-
trol of two trains will require some
additional wiring, what we commonly
call cab control.
Layout wiring
Cab control works by dividing the
layout into electrically isolated sections
of track, called “blocks.” You can do
this using plastic insulating rail joiners
or by cutting a gap in the rails and fill-
ing it with a non-conductive material
like styrene. Since you can isolate the
track sections by gapping only one rail,
the other rail is called the common and
is wired to both cabs.
Power wires, or feeders, from each
block are connected to an electrical
Wiring reverse loops
Fig. 2 LOOP
SHORT CIRCUITS
Short circuit!
Return loop with all-live turnout
Short circuit!
Return loop with power-routing turnout
PHOTO BY JIM FORBES
R
everse loops – tracks that turn
trains around – can add a great
deal of operating interest to a lay-
out, but they also require special wiring
that goes beyond connecting a pair of
wires to the track. Some modelers
avoid building layouts that have a
return loop or wye, thinking that the
wiring is beyond their level of expertise.
However, there’s no need to fear.
Although reverse loops require special
wiring, the job is not difficult and can
be done quickly.
What qualifies as a reversing track
section? The most common is the simple
return loop or “balloon track,” but wyes
and reversing cutoff tracks also qualify
as reversing sections and need special
wiring. Figure 1 shows four examples.
In some track plans the existence of
a return loop isn’t as obvious. To check
Without special wiring, turning trains on
loops or wyes on your model railroad will
cause an electrical short. Fortunately, wiring
a reversing section of track is easy. You need
only a little electrical know how, some wire,
and a toggle switch or two.
a plan, trace the path of a train around
various routes. If a train can wind back
to go the opposite direction on a track
without reversing the locomotive, then
there’s a reverse loop.
Without special wiring, a reverse loop
will result in a short circuit as fig. 2
shows. This is true regardless of
whether you’re using a single power
pack, a cab-control system with multi-
ple power packs, or Digital Command
Control [For more on reverse loops and
DCC, see DCC Corner in the August
2003 issue of M
ODEL
R
AILROADER
Magazine.
– Ed.],
and whether your
turnouts are either the all-live or power-
routing type.
The basic idea in wiring a reverse
loop is to isolate the loop itself using
insulated rail joiners to make it a
separate electrical block. Double-pole,
double-throw (DPDT) switches are then
used to change the polarity of the loop
track, thus avoiding a short circuit.
There are two simple methods of wiring
loops: One method uses a single DPDT
switch; the other uses two switches.
Reversing with one switch
Figure 3 shows the single-switch
method. It’s the simplest to wire; how-
ever, it can be difficult to operate
smoothly without bringing the train to
a complete stop.
Here’s how the single switch works:
First set the loop direction switch to
match the polarity of the main line,
allowing the train to enter the loop.
Next, stop the train on the isolated sec-
tion of track and throw the power
pack’s reversing switch, the loop
switch, and set the loop’s turnout to
Fig. 1 TYPES OF
REVERSING
TRACKS
Return loop or “balloon track”
Cutoff track
Fig. 3
SINGLE-SWITCH
METHOD
Turnout set
for diverging
route
Wye
Loop direction
DPDT switch
Figure-eight connector
Power pack
ILLUSTRATIONS BY RICK JOHNSON

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