Download - O scale trains

DCC For Traction Layouts
David Gairo
1
I was born and raised in Philadelphia when there were still a
lot of trolleys operating. I rode them all over the city. I have been
building and operating model trolleys since 1965, and I am very
much involved with the trolley museum in Scranton, Pennsylvania,
and one of the organizations that became a part of the volunteer
group there. With a background of operating and riding
the real thing for many years, I want to have my model trolleys
operate as much like them as possible.
I like city trolley operation with complicated trackwork in
the streets. A previous layout, measuring only six by eight feet,
had 21 blocks and 30 track switches incorporated into it. Before
digital command control (DCC), I was limited to carefully running
two trolleys at a time. This was accomplished with a lot of
block-toggle throwing. Now, with DCC, I can have a car pull
up behind another car at a car stop and have multiple cars at
an intersection at the same time. The limitation is the number of
throttles available and people to run them. Even that can be gotten
around with a computer!
A decoder is the controller for every powered unit on a DCC
layout. It recognizes commands addressed to that car (trolley or
locomotive) and controls its motor. For my O Scale trolleys, I use
HO decoders rated for 1.2 to 1.5 amps because most O trolleys
use HO motors in their power trucks. Even the older open-frame
motors (DC-60 size) will operate well on these decoders. I have
used decoders from most DCC manufacturers. I generally select
the basic models, as I do not require a bunch of functions since I
wire the car’s interior lights on all the time and only use the forward
and reverse headlight functions of the decoder. All decoders
include the two headlight functions.
Everyone who writes about selecting decoders tells you to
test the ”stall” current of your motor. I cannot remember ever
stalling a motor. I weight my cars to slip the wheels if the load
is too much. I then select a decoder that handles the ”normal”
motor load. I have not lost a decoder yet. You decide! By the
way, don’t forget that any lights driven by the decoder have to be
added to the total load.
Since I detail the interiors of my passenger trolleys, I usually
mount the decoder under the floor with double-sided tape. The
decoder gets enough air circulation, and most of the car’s wiring
is under the floor already. Photo 1 shows the bottom of a singletruck
car with the decoder near the left platform. This is a small
car, but it has little underbody detail so there is enough room
for the decoder. It is an open car with running boards and they
hide the decoder from view. Photo 2 shows the underside of
two similar Philadelphia cars. The car on the left has its decoder
under the front platform, out of the way of any detail. The other
car’s decoder is mounted between the center-door steps and
other underbody details. In both cases, the decoders cannot be
seen when the cars are on the layout. Photo 3 is the car with the
decoder under the platform. If you look really close, you may
2
3
be able to see a trace of yellow wire! (Yep! There are a few paint
chips on that car. I do run them and no Philadelphia trolley ever
went unblemished!) On freight motors and other large-motored
cars, I mount the decoder inside the car. These decoders are usually
larger, to handle more current.
Now we get to the wiring of the decoders (see Fig. 1). Trolley
models are typically wired to pick up power from the overhead
wire, and the poles are set up to reverse polarity when they are
changed. Pole-reverse required the motor to be isolated from the
car, and so does DCC. DCC-equipped cars can also use polereverse,
but only if they are operating on a DC layout. When
running with DCC, the decoder controls the direction. It doesn’t
matter which pole is up or what polarity is on the overhead. I
used to wire my cars for pole-reverse, but I’ve pretty much quit.
The few times my cars operate on DC, the layouts are in a loop
configuration. Since the cars always go one direction, the pole
reverse is not needed.
Basic connection of a DCC decoder requires only four wires,
two from the track and overhead to the red and black connections
on the decoder, and two to the motor from the orange and
gray of the decoder. All the other wires are for functions and
are optional. To have reversing headlights requires three wires,
white to the front, yellow to the rear, and the blue one is a common
(for all functions) return. What can we do with any extra
functions your decoder may provide Photo 4 is a Philadelphia
snow sweeper. I used a decoder function to turn on the motor
that runs the brushes. At train shows, it fascinates people when I
bring the car to a stop in front of them and then start the brushes.
I have also used extra functions to control marker lights and interior
lights, separate from the headlights. You do have to be careful,
though! Function leads can usually support a current of only
200 mA, and the maximum capacity of the decoder includes
any function load plus the motor. So, how do we handle a heavy
July/August ’07- O Scale Trains • 51