Half-Wave Lighting
In last month’s column, I briefly discussed half-wave lighting
)
and several situations where its use might be useful. Here is
how it works.
To connect a function in a half-wave lighting fashion, a rail con-
nection is used instead of the decoder power supply. Instead
of connecting the positive side of the load to the blue wire, it is
connected to, for example, the rail red wire.
In figure 3, you can see that the only thing between the track
and the blue wire is the power supply. Since the DCC track volt-
age (5) is positive about half the time and negative about half
the time, connecting a load between it and the negative termi-
nal generated by the power supply (through a function lead)
will have current flowing through the load about half the time.
With LEDs, this technique results in a slight reduction in light
output. Sometimes it is so little that you may not even notice.
Reducing your series resistor by one standard value (1K to 910
ohms, for example) will probably restore any lost brightness, if
necessary.
With bulbs, the light reduction is somewhat more pronounced.
However, most folks seem to be happy with the results, with
either bulbs or LEDs, with little or no changes.
Sound and the Audio Amplifier
The microprocessor in sound decoders is programmed to
know when to generate what sound, based on throttle setting,
momentum, load (measured by BEMF), and function inputs. It
calls upon sound waveform files stored in memory to allow it
to generate the appropriate sounds at the proper time.
DCC Impulses Column - 8
In a full-featured sound decoder, the microprocessor is very
busy. In addition to decoding the DCC data being sent to it, it
may be generating 5 or 6 sounds, calculating what pulses to
send to the motor and as many as 4 different function leads -
all at the same time. Talk about multitasking.
Once the sound waveform is created by the microprocessor, the
amplifier section of the decoder drives the attached speaker.
Keeping the decoders small limits the flexibility of amplifier
design. They are typically designed to work with a specific
speaker load (frequently 8 ohms). Higher impedance (say,
16 ohms) will work, but the volume may be diminished a bit.
Lower impedance (4 ohms) may damage the amplifier.
How fast am I going?
One of the more difficult things to do right is to generate the
proper sounds for the speed your loco is actually moving.
11
11: Tsunami decoder plugged into a Proto Heritage 2-8-
8-2 tender wired to the speaker in the shell.
MRH-Feb 2013
