STALL MOTOR

SWITCH MACHINE

CONTROL

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ABOUT USING STALL MOTORS

 

How much power does a stall motor use?  Well, that is a tricky question to answer.  A typical Stall motors draws only about 20 mills (about 1/50 amp) at 12v when it is used so if they are thrown using push-buttons then the smallest power supply can be used.  It would even be possible to use the power from some other power supply already in use.

 

Stall motors can also be thrown by switches but this changes the power requirements dramatically.  Stall motors are designed to handle continuous power through switches, the switches giving a visual display of the turnout position.  This means that ALL motors are continuously drawing power.  The Amount of current of a single motor may be small but it adds up.  Fifty machines will draw about a full amp.  Driving a stall motor through solid-state circuitry and signals will add an even greater drain on power and almost double or sometimes triple the total power required.

 

One large advantage of a stall motor is that LEDs used for signaling can be connected in series with the stall motors.  The motors will act as the resisters required to limit the current through the LEDs and several sets of LEDs can be used.  The one thing to keep in mind here is that each LED that is connected in series with the stall motor causes a slight voltage drop of a bit more than a volt.  Stall motors are usually run at about 12v but will continue to operate quite reliably down to about 8v volts if the load isn’t too heavy.  This can be counter-acted by using a slightly higher supply voltage.

 

 

 

  

  

   

 

      BUILDING A UNIVERSAL CONTROLER

 

   

Building a controller for stall motors is not a difficult project.  There are several circuits published on the web using various chips of which the 556 is by far most used.  This design incorporates all the features of these circuits.  It can be controlled by computer or pushbuttons as well as switches.  Controllers can be chained for ladder tracks and complex track configurations.  There is also provision for up to (3) sets of signals.  The LTV846 chip is for a computer interface and can be omitted where no interface is required.

 

              

 

The schematic (Fig 1) shows how the operation of a stall motor can be further enhanced.  IC-1 represents some kind of rectifier that can be eliminated if the power source is DC. Only (1) IC-1 and C1 need to be used for the entire system.  IC-2 along with capacitors C2 and C3 can power clusters of up to eight controller circuits. Using a voltage regulator is a bit of overkill as the circuit will operate anywhere from 3 – 24 volts but it is cheap insurance to make sure things don’t go wrong. The main requirements of the power source is that the voltage can not drop below 3v during the AC ripple of the power supply and remain above 8v long enough to drive the motors.

 

The LM556 is essentially a dual LM555 Timer but in this case it is wired as a latch to boost the power output and to  “remember” whether the “normal” or “Reverse” button was pushed last.

 

The capacitor (C4) sets the circuit to a known or default “normal” state when power is first turned on.  Note that I have several jumpers in series and parallel so that signal lights can be inserted in various configurations.  Notice that the stall motor is essentially connected directly to the output of this chip and that the polarity across it changes with the state of the circuits.

 

The LTV846 interfaces to a computer and even then only to insure complete electrical isolation and protection from any transient currents that might be harmful.  It can be completely eliminated from the circuit if computer interface is not used.  This chip contains four sets of isolation circuits but only three of these circuits are actually used.  One is used to provide a signal to the computer which way a turnout is currently thrown.  Two more allow the computer to throw the turnout for to route desired.  Alternately, this device can also be used for additional signaling. Turnouts can be controlled using switches or buttons on a control panel, trackside switch stands that operate electrical contacts under the table, or under computer control.

 

This system is quite versatile. Multiple turnouts such as crossovers or double crossovers can be connected together.  A small “daughter” board containing a number of diodes can also be inserted at the push-button connections to control ladders in yards.  Like all solid-state devices, there is a small voltage drop across each diode that limits just how far a ladder can be cascaded in this manner but I have found it possible to cascade fifteen switches at a time with only an extra wire or two.  That should be enough for anyone.

 

 

  

   

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The basic circuit is fairly cheap and easy to build.  Parts cost about  two or three dollars per turnout (four if you add the LTV846) and are easy to find.  All parts can be obtained from DigiKey (http://www.digikey.com)

 

 

 

   

   

 

  

  

   

    NR44A

 

NR-44 stall motor control circuit for (1) turnout

   

   

              

SPECIFICATIONS for NR-44 Switch Machine control circuit using LM556 dual latch

Power Supply

12v @ 22ma (typical)

 

 

 

Motor

10v @ 18ma    (660 ohms)

gear ratio:  275:1

 

contacts

use 5, 6 and 7 to power points

Use 2, 3 and 4 for aux circuits

 

 

  

   

   

 

    

  

   

              

A small diode matrix “daughter” board, a separate project in itself, can be plugged into the control board so that multiple turnouts such as in a ladder track can be controlled from the push of a single button.  In general, connections A1 - A4 plug into the control board, the push-buttons are connected to B1 – B4 and connections C1 – C4 are chained to other controller boards to build the ladder or other multiple switch operations.  

 

    NR44B

                

  

      A1, B1

Ground connection

A2, B2

(not connected)

A3

Turnout Normal position (on control board)

A4

Turnout Reverse position (on control board)

B3

To Normal position pushbutton

B4

To Reverse position pushbutton

C1,C2

From controllers higher up on ladder used to control this unit

C3,C4

To controllers lower down on ladder than are controlled by this unit

          

 

   

   

   

 

This controls (3) switch machines and a power supply that can power up to 5 more switches   

       

 

   

 

   

   

 

 

 

Underside of typical Controller board with Power supply section to the right

  

 

   

 

 

 

 

 

The LTV846 contains four LED “emitters”, each paired with its own transistor.  There are many variations of this chip that offer different numbers of circuits and sensitivity, all of which will work quite well in this circuit.  The main point is that the main controller circuitry is completely isolated from the computer circuitry and thus forms a 100 percent safe interface. 

 

LTV846_B

       

 

   

  

   

ne556

           

 

The 556 chip used in this project is actually a dual 555 timer, an old, versatile and still commonly used part.  The two sections of this chip are wired to for a single two-state latch.

While there are several TTL chips that contain multiple latches for the same price, this CMOS chip has the advantage that it can operate over a wide voltage range of from 3.0v to 18v and more.  It also has the advantage of being able to both “source” and “sink” a higher current than a TTL series circuit.   

    

 

 

    

 

NR44A switch machine control board

IC-1

 

1-amp bridge rectifier

IC-2

LM7812

LM7812 - 12v Voltage regulator

IC-3

LM556

Dual latch timer (CMOS)

IC-4

LTV846

Quad opto-isolator

C1

 

100mfd @ 16v electrolytic capacitor

C2,C3

 

0.1 mfd 16v capacitor

C4

 

4.7 mfd 16v capacitor

R1,R2

 

100k 1/8w thin-film resister 

R3,R4,R5

 

1.5k  1/8w  thin-film resister

 

 

 

NR44B Ladder track daughter board

(all)

1N014BCT

small signal diode

 

   

   

 

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Model Railroading is fun in Connecticut.
Bob Van Cleef, MMR

Last update   7/25/2012