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DCC Impulses Column - 2
2
2: Opposite rail track pickup added to an O-scale
locomotive.
Variations in track height can pick wheels off the track and dis-
rupt power, as can out-of-gauge wheels or track.
The problem is enhanced if your loco doesn’t have all-wheel
power pickup. Sometimes the wire to a truck breaks, leaving
those wheels not connected to the decoder. Some locomo-
tives are designed with only a few wheels picking up power: for
example, brass steamers, where the loco usually picks up from
the right rail and the tender from the left.
What about opposite rail pickup?
Personally, I recommend adding pickups to locos that are not
designed for all-wheel pickup. Nothing beats having all the
wheels bringing power to you. However, even then disrup-
tions will happen.
Figure 2 shows what I did with an O-scale brass steamer to pick
power off the drivers. The axles pick up off the right rail and
keep the frame at the right-rail voltage. I installed a printed
circuit board that was insulated from the frame of the loco and
added phosphor bronze pickup wires to rub on the tires of the
drivers. The tires were in contact with the rail, but were insu-
lated from the rest of the wheel.
As you can see, I didn’t try to pick up off all five drivers, but
just the middle three. The tender originally picked up off the
left rail only. Similar wipers were added there to pick up off the
right rail, too. These additions were enough to keep the loco
running smoothly without adding any energy storage.
Enhancing the contact points on your loco is always a good
thing, in my mind. Once the loco itself is as good as it can be,
then it is time to look at onboard energy storage.
Evolution of decoders and NMRA
recommended practices
Before sound, decoders had pretty simple power supplies.
There was a bridge rectifier to make the DCC waveform into DC
and a small filter capacitor to smooth the DC out and, perhaps,
a small capacitor to suppress the spikes coming from the DCC
waveform. Energy storage was left to the flywheel, just like in
the DC world.
Then along came sound decoders. Early versions had a bit
more storage in the decoder to keep the sound processor run-
ning when the track voltage varied a bit, but nothing fancy.
Manufacturers began to realize that, when the processor ran
out of power, the result was not realistic. While the flywheel
kept the loco moving, a diesel, for example, might go through
the start-up sequence while running down the track. UGH!
The manufacturers responded by added slightly larger storage
capacitors to their sound decoders. SoundTraxx, for example,
even provided a way for modelers to add a capacitor to their
venerable DSX series of sound-only decoders
.
Even the few hundred microfarads (µF) that were used early
on exceeded the NMRA recommended practices for the pro-
gramming track. A change to RP-9.2.3 in 2006 relieved some
MRH-Mar 2013
