Excessive
Track Voltage for DCC.
After Christmas 04 I was asked to help
a modeller that had a problem with a Broadway Limited loco not running at all
that he had just received from Santa at BLI. All the loco would do when placed
on his Roco equipped layout was make a few beeps from the speaker but would not
move.
I placed the loco on my NCE equipped
layout, selected address 3 and it ran beautifully. Set up his Roco system on a
test track and tried, no movement, just the beeps, back on my layout and it
works fine.
Reading the operating instructions for
the BLI loco, it states that the QSI chip shuts down the motor control and
emits sound beeps when the track voltage exceeds 22.5 volts DCC. This was the
exact symptom that was happening here. My NCE DCC powered layout has 12.8 volts
DCC at the track.
I use a bridge rectifier to measure DCC
current on my layout (see it at Meters for DCC) but
this bridge rectifier reduces the track voltage by approximately 1.5 volts. I
wired in a bridge rectifier into one of the track feeders from the Roco system
and the loco operated normally, including the correct sounds. A much relieved BLI
owner.
DCC manufacturers have different
methods for adjusting track voltage. The recommended NMRA Standards for track
voltage for HO is 14.25 volts DCC. DCC systems that have adjustments for the
track voltage are: Digitrax uses a scale switch, Lenz use “jumpers” inside
their booster and NCE uses an internal potentiometer. Refer Instruction Manual
for details. If your system has no reference for voltage adjustment, measure as
per making a voltmeter at Meters for DCC.
This high voltage has had me thinking
and asking questions about what else could be effected. The below list itemises
what things are affected by high voltage.
Broadway Limited locos will not run
on greater than 22 volts DCC.
Operating
Soundtraxx DSD-100LC sound decoders will void warranty – see below.
These
decoders run really hot at 23 volts and will most likely cause premature
failure if this is not corrected.
All
decoders will run hotter with higher voltage.
The
voltage at the track determines the value of motor and function outputs.
Light
globes in locos designed for 12 – 14 will certainly glow brighter and will
eventually blow.
Other modellers have had the same
experience with high track voltage on entry level DCC systems and have fitted
diodes, similar to the arrangement below that I strongly recommend for users of
these DCC systems.
Unless you are sure that your DCC
systems outputs about 14 volts DCC, I would fit some diodes to reduce the
voltage as shown below.
With the diodes connected this way,
they are used as a voltage reducer since there is a approx .3 to 1.0 volt drop
depending on the type, across a semi conductor junction. Diodes only allow
current to be conducted in one direction. With this arrangement the current is
not rectified into DC as most people associate diodes to do. In this case as a
voltage reducer that is mainly constant as current increases. We could use a
resistor or lamp instead of the diodes, but as more current flowed the voltage
drop would increase too much, hence the use of diodes that don’t increase the
voltage drop as much. See Meters in DCC for bridge rectifier
explanations and examples.
I suggest to all DCC operators to fit
an ammeter to monitor layout current demands, so fitting a bridge rectifier to
do this, will reduce the track voltage.
Soundtraxx have a warning in their
DSD100LC decoder manual and feel quite strong about high voltage.
“CAUTION: The DSD-LC series of decoders
are designed to work at track voltages between 7.5 and 16 volts maximum. On
most command stations, this corresponds to a track setting of N or HO. Do NOT
use the O or G scale setting!
Operating your DSD-LC at voltages
greater than 16 volts will void your warranty, produce excessive heat and
possible permanent damage to the DSD.”
Andrew
Krassey wrote in the Australian Model Railway Magazine in the August 04 issue
and showed a method for reducing the voltage for entry level DCC systems such
as Roco and Atlas, using diodes similar to what I have shown below, for
excessive voltage that these systems place on the track for overstressing sound
decoders that require only 16 volts.
