Download - O scale trains

AC
IN
AC
IN
RAIL
1
FROG
RAIL
2
RECT.
RECT.
RECT.
RECT.
Figure 3
CAP.
.1
CAP.
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Figure 4
CAP.
.1
CAP.
.1
CAP.
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LM
317Z
RES.
68
LM
317Z
RES.
68
LM
317Z
RES.
LM
317Z
RES.
LEDs
LEDs
LEDs
LEDs
want to control incandescent lights, like the grain-of-wheat
bulbs that go into marker-light assemblies. They can be driven
by a current-limiter also, which will prolong their life.
Now, back to the question of flicker. A locomotive tolerates
dirt and gaps in the track by having a flywheel to carry it over
the bad spots. The electrical equivalent to a flywheel is a capacitor;
it stores up electricity during the good times and gives it
back during the bad times. A capacitor needs DC (and we just
happen to have DC after the bridge rectifier), so we just need
to hang a big capacitor across the power leads and we can say,
”goodbye flicker”. I added a 10,000 micro-farad capacitor to
my lighting circuit, and the flicker disappeared. It even went
over insulated frogs without blinking off and on again.
That really isn’t a good solution for several reasons. The
capacitor takes in electricity in such big gulps that it can cause
arcing between the wheels and rails. This effect causes new bad
spots for electrical transfer. The capacitor can also upset command
control systems, making the system think that a short circuit
occurred or that a reversing loop should be switched. We
need to limit the amount of current that the capacitor can draw.
Well, we’ve just been talking about current-limiters. As it turns
out, we need another one in front of the capacitor, one with
higher current capacity.
Figure 3 shows the same unit with one rectifier omitted and
the upper half feeding the lower half. The first unit is charging
up a capacitor to prevent flicker. It has a 12-ohm resistor to
limit current to 0.1 ampere, the maximum current that the tiny
LM317LZ can handle. The second unit has the usual 68-ohm
resistor to limit LED current to about 0.02 ampere. I used a 47-
microfarad capacitor (C1) and it removed most of the flicker.
When I used a 1000-microfarad capacitor, it removed all of the
flicker, but it is physically larger. If you want to use more current,
you can replace the first 317 with the big brother version
LM317AT. It can handle up to 1.5 amperes, if you use a smaller
value of resistor. Its leads fit into the same holes in the board
as the smaller unit, but it sticks up higher. This circuit is only
practical with the full track-voltage that comes with command
control systems, because it takes up about five volts, leaving ten
volts or so for the lights. This version does require some more
space for the bigger capacitor.
Now, I would like to consider turnout signal lights, not car
lighting. This is a completely different application. A prototype
railroad will have some indicator telling which way a turnout
is set. It may be a simple mechanical signal that is a part of the
turnout controller, or it may contain electrical signals. Electrical
signals are popular in model railroads. They add some pizzazz,
but also the engineer (who is not actually on location in
the locomotive) can see the turnout status. These are controlled
by extra contacts on the turnout actuator. If you use (computer
friendly) turnouts that have points that are always electrically
connected to the adjacent rail and a frog that is isolated and has
its power switched with the turnout, then you get signal power
for free. Just use the voltage between the frog and the two siderails
to light the appropriate signal lights.
Figure 4 shows how two rectifiers are connected together and
fed from the two outer rails at a turnout and the isolated turnout
frog. Then, as the turnout is thrown and the frog voltage switched,
the two circuits power the appropriate LED signal. I use three
signals, one for each path into the turnout. They have green and
amber lights at the ”points end”, green and red lights at the other
end, and amber and red lights at the diverging track end.
In this installment we used the following additional parts:
1 Bridge Rectifier (1 amp 100 volt Digi-Key W01G/1G1
or equivalent)
Electrolytic Capacitors (47 to 1000 microfarad, 35 to
50 volt)
Perf-Board for mounting (1-7/8” x 2-7/8” Radio Shack
276-149 (can be cut down))
This universal controller is small and inexpensive, but it is
awkward to solder together all those little parts. I am making
printed circuit boards for this unit because they simplify the
assembly so much. I would be interested in knowing if anyone
else is interested in these PC boards.
u
July/August ’07- O Scale Trains • 49