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Stall Motor Switch machines such as the Tortoise slow motion
mechanism have been a wonderful advance in Model Railroad
technology. Their cost is may be
a bit more than a conventional twin-solenoid however they have built in
contacts for routing power and signals and they have every bit as much
power to throw a turnout and hold the points firmly against the stock
rails with as much or greater reliability with far less current. They can also be driven with push-buttons, switches or interfaced directly with
computer type circuitry. Most
importantly they do not fall apart over time as solenoid machines and
the power requirements are much less critical
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So what is inside the
machine? Let’s
take a look. Note the actuator
arm in front and the contacts that wipe across the PC board an act as a
DPDT switch. The motor is mounted directly on the PC board and the drive
gears can be seen in back.
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Here we have removed the
circuit board with the motor to expose the gearing. There is a 275:1
gear ratio that amplifies the motor torque to more than enough
to throw a turnout.
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The motor itself is quite
simple with only (3) armatures and (2) simple brushes. The field magnet is the thin black
band outside the armatures.
The resistance of the motor is around 650 ohms (there is some
difference between running and stalled) and can be
used to limit the current through a LED without any
resisters.
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The one drawback of the
Circuitron machine is that centering the points can become quite
critical when electrically feeding a frog through the contacts. Note the narrow “deadband” area in
the center. Shorts may occur
between the turnout points against the stock rail and the position of
the wipers. If the cenering is too far off. This short is usually of extremely short
duration but it can be dangerous to modern DCC or other control
circuits that may be present in track voltages. To correct this I open the motor
(yes, this voids the warranty) and widen the gap between pins 5, 6 and
7. I use these contacts because
it is more tolerant of errors
The best way to avoid this
problem is to scribe through the foil to increase the deadband
area. Yes, this voids the
guarantee but the improved ease of installation is worth it.
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We are now ready to install
the modified machine. There are
8-pin edge connectors that can be used to
allow easy removal without unsoldering but I find it easier and much
cheaper to use headers. Note
that the smaller header is for the two motor wires that drive the
machine while the other header contains the six wires for the DPDT
contact configuration.
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A word about
layout wiring the easy way. The North River
is built on a series of inverted “T” beam with
the first six inches from the front dedicated to the wiring. This view shows the front fascia
removed exposing the 1x2 platform where the machines are mounted, two
machines, and a few connections.
The only “under-the-table” that needs to be done is the
mechanical connections to the turnouts and any feeder connections to
the track etc.
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This mounting bracket is used to quickly mount a machine to the benchwork.
Note the Vertical plate to the left has been
pre-drilled for the machine while the block to the right
provides a solid area to screw the bracket to the benchwork. Screws are used to allow easy
repositioning should the need arise.
This allows most of the mounting work to be done at a convenient
workbench
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Another close-up of mounted
switch machines
CLICK HERE for Controlers
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