Coil Switch Machines

TOPICS IN THIS SECTION

COIL MACHINE OPERATING BASICS
OPERATION OF TWIN COIL MACHINES
TYPES OF TWIN COIL MACHINES
TWIN COIL CURRENT CONSIDERATIONS
TABLE OF TWIN COIL DRIVE POWER REQUIREMENTS


COIL MACHINE OPERATING BASICS

Coil based switch machine are, as the name implies, based on a coil or more accurately a solenoid.  Technically the word "solenoid" is another name for a coil.  However in the industrial industry, solenoid also refers to a coil based machine that can create linear mechanical motion to do some kind of work.  The motion is very limited but can be very powerful.  The solenoid is the bases of making relays (contactors), electric valves or in our case a turnout switch machines.

To understand how the switch machine works, we need to understand how a solenoid works.  A solenoid consist of Terminals (or wires to reach the coil), a Coil, an Armature (Plunger or slug) and a frame (open or closed) to hold it all together.  A picture of a simple open frame solenoid is shown below:

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Construction of the solenoid is a coil wrapped around a hollow tube.  Inside the tube is the solid metal armature which is allowed to freely slide up and down the length of the tube.   However to prevent the armature from sliding completely out of the tube, some form of mechanical stops are included.

A solenoid's key to operation is the coil.  When we pass electricity or more accurately current threw the coil, the coil creates a magnetic field.  The more current we pass through the coil, the stronger the magnetic field gets.  The stronger the magnetic field, the more mechanical force is available to do work.

Iron based metal is attracted to magnetic fields.  The Armature is made of steel (iron).  When the coil is energized (turned on) with current, the armature is attracted to it and will slide within the tube up or down as required to reach the strongest concentration of the magnetic field within the tube.  The strongest magnetic field is found in  the very center of the coil in terms of the coils physical length.  For most solenoids, that will also be the very center of the tube.

But what happens to the armature when the coil is de-energized (turned off)?  The Armature will stay right where it is assuming gravity is not a factor. However, if the armature is left in the center of the tube, the armature will not move at all the next time the coil is energized!  The solenoid will not work.  What is required is some EXTERNAL force on the armature that will cause it to move out of the center of the tube.  Often part of that external force is integrated into the solenoid's tube in the form of an internal spring.  With the coil off, the spring will push the armature out one side of the tube until it hits a mechanic stop.  

However, such a solenoid design is not useful for a turnout.  Why?  

1) The spring will alway push the points back to one direction of the turnout when you turn off power to the solenoid!   That means to keep the given turnout thrown the other way, you will have to provide power to the solenoid constantly.

2) If you have to energize 50% of the solenoid on the layout to keep trains running, the amount of power required will force the use of a big power supply or powerpack.  That translates into costing more money in terms of the size (high power rating) of the power pack and the use of lots of electricity.

3) The coils, when continuously energized, will get hot which means they must be designed to handle the heat.  The will lead to a larger mechanical design to allow the heat to dissipate into the air.  More material means more cost.  In addition, scale modeling need small switch machines for both practical and cosmetic reasons and this will work against that goal.

How do you solve these problem?

Use a "Dual Solenoid" or "TWIN COIL" solenoid or switch machine.  
 

OPERATION OF TWIN COIL MACHINES

A picture of a twin coil open frame twin coil switch machine is shown.  
The picture/drawing is of a PECO PL10 switch machine.  What is nice about this picture is that you can actually see what is going on.  You can partially see the tube's open end on the ride side of the picture.  The two "green" coils (twins of each other) are mounted side by side such that the tubes of each coil are in perfect alignment with each other.    They twin coils share a common armature that slides in BOTH tubes at the same time without falling out of either one of them.  There is a gap or open space between the two coils. A solid metal rod (brass) is mounted through the armature and extends out both the top of bottom of the switch machine in this gap.  When the solenoid moves back and forth, so does the rod which in turn moves the turnout points.

What will happen if both coils are de-energized?  The armature is free to move between the two coils which can cause loss of point contact pressure against the stock rail.  To address that problem, switch machines include a soft "latching mechanism" the will hold the armature in the current position whatever that is.  Hence gravity or vibrations will not cause the armature to move out of place.

How does this address the problems above.

1) To address the spring problem, we now use these two coils to move the armature back and forth between them.  By energizing the coil that does NOT have the armature at it center, the armature will move towards that energized coil.  If we only energize one coil at a time, then the armature will now be located in the center of the LAST coil to be energized.  Hence the turnout point rails will now be in one position or the other.

2) The constant power requirement is eliminated since we have no spring.  Small power supplies or power packs will work since the only need to power one coil at a time and do so only momentarily.

3) No heat issues when the coils are not constantly energized.  Small switch machines now become possible.


TYPES OF TWIN COIL SWITCH MACHINES

There are two types of "Twin Coil" based switch motors and it based on the number of electrical connections you need to make the twin coil switch machine work.

2 TERMINAL TWIN COIL MACHINES

This type of machines has 2 terminals (wires) to drive both coils.  One terminal is common to both coils while the other ends of each coil are connected to their own diode.  The design is such that one diode is installed in series with with each coil.  The other ends of the diodes are tied together and connected to the second terminal.  The diodes are wire backwards to each other.  This means the only one given coil will be energized depending on the POLARITY of the DC POWER being applied to the machine.  Only one coil is activated at any given time and on only as long as needed to throw the armature.  The key point is that this twin coil switch machine only WORKS on DC.

3 TERMINAL TWIN COIL MACHINES

This type of machine has 3 terminals (wires) to drive the two coils.  On terminal is common to both coils while the other two terminals are connected to the opposite coils respectively.  Only one coil is activated at any given time and on only as long as needed to throw the armature in that coil direction.  Three Terminal twin coil machines are the MOST POPULAR because they will work on BOTH DC and AC power.  This gives one more options to find a suitable power source in terms of cost and power levels.  Although it is getting less and less true today than in times past, simple AC transformers are often less expensive to buy and use than the DC power supply equivalent.


TWIN COIL CURRENT CONSIDERATIONS

The energy it take to drive the coil varies with the size of the machine.   A good indication of the power required is directly tied to the physical size of the coil.  

Small twin coils machines are designed to drive very specific type of switch machines of a specific scale.  Hence the mechanical load on the coil is very well defined and the coil is sized to handle that specific load with maximum efficiency.

Big twin coil switch machines are more generic in that they can be adapted to work with almost any scale turnout/track switch.  Since they can be used with large scale track switches, the mechanical load can be high and as such the coil will need to consume more current to drive this large load*.  Many of these big switch machines also offer additional contacts that can be used to power routing, signaling ect.

*Remember a stronger magnetic field is required if the load it has to move is larger.  For a given coil design, a stronger magnetic requires more current in the coil.  See Coil machine basics section at the top to learn more.

Below is a table of switch machines and there current consumption/class of power.  
 

TABLE OF TWIN COIL SWITCH MACHINES POWER REQUIREMENTS

The key information is the "Power Class" which is divided into two categories.  Low and High.  This will tell you what are the compatible DCC Accessory Decoder will drive them as discussed here: Coil Accessory Decoders

Machine
Part
Numbers
 Picture
Coil
Resistance
(Ohms)
 Operating
Voltage
(Volts)
Current
@ 16V
(Amps)
Power  ClassContacts 
?
Notes
Atlas HO
Code 100
0052
0053

 
 
   <2A LowNo Left and right hand versions. Docks with Atlas snap track switches.
Atlas HO
Code 100
0065 
    NoUnder table design works with any Atlas HO turnout.  Same mechanism as 0052/3
Atlas HO
Code 100 
0066 
   LowYes DPDT
or two
SPDT
 Under table design works with any Atlas HO turnouts but also adds a set of electrically isolated DPDT contacts.  Contacts can be used to support live frog power, simple signals or power routing.
Atlas HO
Code 83 
0584
0585 
 
   LowNo Same machine design is used with Atlas "True Track" turnouts.
Atlas N2715
2716

 
   LowNoLeft and right hand versions. Works with Atlas 2700 series track switches. 
Atlas N
Code 55

2065 
    LowYes DPDT
or two
SPDT
Under table design works with any Atlas N code 55 turnouts but also adds a set of electrically isolated DPDT contacts.  Contacts can be used to support live frog power, simple signals or power routing. 
Atlas O

6098
 
   High Yes DPDT
or two
SPDT 
Under table design works with any Atlas O turnouts but also adds a set of electrically isolated DPDT contacts.  Contacts can be used to support live frog power, simple signals or power routing. 
 Atlas O
 3-Rail

6099 
    HighNo Heavy Duty
Atlas O
2 Rail
Code 148 
7098
7099
 
   HighNo 
Rix628-0004 
 
  16V to 24V  HighNo  
Roco       No  
AHM       No  
NJ   1.9 Ohms  8.4A High Yes DPDT
or two
SPDT
+
High
Current
SPDT 
 Blue colored coil covers
Kemtron     High
Yes DPDT
or two
SPDT

High
Current
SPDT 
 Red colored coil covers
Tenshodo       HighYes DPDT
or two
SPDT

High
Current
SPDT 
Black colored coil covers 
PFM




High
Yes DPDT
or two
SPDT

High
Current
SPDT 
Green colored coil covers
Peco  PL10
Digital
 10.9 Ohms  1.46A Low
NoGreen Colored Coils is the indicator of the switch machine.  The coil resistance was raised in an attempt to reduce the current requirements. 
Peco PL10      No