Circuitron Tortoise stall motor switch machine

 

STALL MOTOR

SWITCH MACHINE

INSTALATION and CONTROL

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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

 

 

   

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.

   

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.

 

 

   

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. 

    

 

   

    

    

   

    

 

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 centering 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. 

 

    

      

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. 

      

  

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.

 

  

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

   

 

     

    

Another close-up of mounted switch machines

 

 

 

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Bob Van Cleef, MMR

Last update   7/25/2012