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Decoders usually come with varying amounts of function
wires for control of various lights and other electrically-operated
devices on the loco and train. Most of the them also have a blue (common)
wire that provides full-wave power for the function wire to use.
Because of size constraints, some decoders come without
the blue wire. In this case, power from one of the rails is used to
power the device. Further, some people are tempted to wire lights
this way, because of the way the bulbs are designed into the loco,
even when the blue wire is available.
Wiring this way is called Half-Wave. This term is
used because it describes the portion of electricity a function gets
when wired to only one rail. So what does full-wave/half-wave really
mean?
The track provides square-wave AC power. One rail
has one half of the "full-wave" of that power, the other
rail has the other half. A full-wave bridge rectifier inside the decoder
converts this AC power to DC. The blue wire is connected to the "+"
terminal of that bridge rectifier. When the function wire sinks that
power, it is sinking full-wave power. If you wire a bulb or other
device directly to one rail, instead of the blue wire, that device
will only be getting half of the full AC wave - hence, Half Wave.
Half-Wave wiring is NOT recommended unless there
absolutely is no other way. Because the device (bulb) will be getting
power from only one rail, that power will vary if an analog loco is
run at the same time. Even though you're controlling an analog loco,
the manipulation of track voltage for that analog control will affect
all Half-Wave-powered lights.
When controlling an analog loco, power on the rails
is manipulated in a way that will increase and decrease voltage between
the rails. To make an analog loco go one way, one rail increases while
the other decreases. To make it go the other way, the other increases
while the first decreases. This means that the rail the half-wave
bulb is connected to will determine whether the bulb gets brighter
or dimmer as the analog loco's speed is increased and decreased. |
However, even if your decoder doesn't have a
blue common wire for full-wave connection, you can still wire for
Full-wave, as shown in the illustration at right. |
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| Connecting the bulbs with two diodes like this simulates
the "+" terminal of the bridge rectifier. Since the function
wires sink that power through the "-" terminal of the bridge
rectifier, it's exactly the same as if the decoder had a blue wire.
Any type of diode that can handle the voltage and
amperage of the functions (bulbs) will do. Notice that the stripe
on each diode is away from the rails. This allows the voltage from
one rail to feed the bulbs when positive is on that rail, and voltage
from the other rail to feed the bulbs when positive is on that rail
- both halves: Full-wave. Remember, the rail has AC voltage, which
means polarity alternates back and forth between positive and negative. |
