MODRATEC Useful Data (otherwise known as MUD) is a series of pages containing helpful information particularly relevant to rail modellers. This page offers ideas for controlling double-solenoid point motors (switch machines) from the auxiliary switches of MODRATEC Lever Frames.

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Operating Double-solenoid Point Motors using Standard Switches

Updated with the insertion of a new Circuit 1 - 19 June 2005.
Original Circuit numbers have been incremented.
Updated 21 May 2012 with note regarding distribution of power to multiple point controls.

MODRATEC has now designed an interface that embodies the principles expressed in this article. It is available from The MODRATEC Shop. See listing here.

Double-solenoid type point motors require a high pulse of current to produce the necessary magnetic force to throw the points. These devices are rated for short-term use only. Special arrangements are necessary to ensure that overheating does not occur. For example, they may be controlled by push-button (momentary contact) switches, passing-contact switches, or using a stylus (electric pencil) to make brief contact with a stud which is typically fixed to a mimic panel.

But, how practical is it to operate these point motors using conventional toggle switches?

And how can they be operated from a MODRATEC Lever Frame which has a simple single-pole single-throw (SPST) switch?

The following circuit ideas show one possibility of how this can be done. The circuits are simple enough to be easily constructed by someone with minimal electronic skills.

Circuit Idea 1
Circuit 1

Circuit 1 illustrates the basic principle employed. A capacitor is used to control the amount of energy that is passed into the operating coils of the point motor. A capacitor (once known as a condenser) is a device which can store electrical charge - it is basically conducting parallel plates separated by insulation. When it is connected to a voltage source, it accumulates electrostatic charge until the voltage between its plates is the same as the source voltage. Conversely, if the terminals of a charged capacitor are connected together, then the charge will dissipate into that connection. The switch in Circuit 1 allows the capacitor, C1, to be charged (R position) or discharged (N position). The current that flows to charge the capacitor flows through one coil of the point motor, while the discharge current flows through the other.

Although this is a perfectly practical circuit as it stands, there are some considerations. 1. You need access to all four terminals of the point motor coils. 2. You need four wires running to each point motor on your layout - two supply commons and two control wires. 3. You must use a DC power supply (which applies to all circuits on this page).

Circuit Idea 2
Circuit 2

Circuit 2 shows the next possibility. Kato users should note the following diagram:

3-wire v 2-wire

For those who need it, there is information about identification and orientation of components at the end of this article.

How it works: The point motor is operated from a DC supply (typically 22V). The SPDT switch is shown in the 'N' (normal) position where it is connected to the negative side of the supply. It can be switched to the 'R' (reverse) position where it connects to the positive side of the supply. C1 is an electrolytic capacitor (stores electric charge). D1 and D2 are diodes (electrical one-way valves). When the switch is moved to the 'R' position, C1 will be charged, and during that process current will flow from the positive of the supply, through the capacitor, through D1, and through the 'R' coil of the point motor back to the negative of the supply. Once the capacitor is fully charged, current will cease. When the switch is moved to the 'N' position, the capacitor discharges with current flowing from the positive terminal of the capacitor, through the 'N' coil of the point motor, and through D2 back to the negative terminal of the capacitor. Once the capacitor is fully discharged, current will cease.

Note that the diodes 'steer' the current through one or other of the point motor coils depending of the direction of that current. Note also that the amount of energy that can be dissipated by each coil is limited by the amount of energy that can be stored in the capacitor, thus, point motors are protected from overheating.

The size of the eletrolytic capacitor, C1, may need to be determined by experiment, but a value of 2200μF (micro-farad) is typical. It needs a voltage rating of at least 35V (you may get away with 25V if you're penny-pinching).

The 'steering' diodes, D1 and D2 are 1A* rectifier diodes, for example 1N4004.

A switch rated at 1A* will be fine.

[* The instantaneous initial current spike passing through these devices will usually be well in excess of 1A, but, because it quickly decays, the components are not damaged. Also, the most significant limitation for a switch is its ability to 'break' its rated current. In this circuit the switch will normally be breaking a current of virtually zero.]

A helpful spin-off from controlling point motors in this fashion is that, if the steering diodes are located at the point motor, only one wire is needed (in addition to the common) to connect each motor!

The power supply may be a conventional 'Capacitor Discharge Unit', or can be derived from the 15V or 16V AC supply generally used for point operation. The circuit below shows the arrangement. If there is a need to operate many points simultaneously, then the 10000μF option is advised.

22V DC Supply
22V DC Power Supply

AN IMPORTANT NOTE: When distributing power to multiple point-control circuits, it is important to isolate each circuit using diodes as indicated in the circuit below. 1N4004 diodes are satisfactory for this task.

Distributing the DC Supply
Distributing the DC Power Supply - example for controlling 4 points

If there is a requirement to indicate the orientation of a turnout on a mimic panel, then LEDs (Light Emitting Diodes) can be added using Circuit 3.

Circuit 3
Circuit 3

The labels X, Y and Z correspond to those marked in Circuits 1, 2, 4 and 5. [1K8 means a 1.8kΩ (1.8 kilo-ohms, 1800 ohms) resistor - 0.5 watt carbon or metal-film is suitable. Note that the LEDs are connected in reverse-parallel.]

Circuit 4
Circuit 4

Circuit 4 shows how to extend the principle to interface point motors to the Lever Switches on a MODRATEC Lever Frame. The Lever Switch is SPST. It is open when the Lever position is normal, and closed when the Lever is reversed. This on/off action is used to operate a relay having changeover contacts which perform the same function as the SPDT switch of Circuit 2.

D3 is a signal diode such as 1N914 or 1N4148, and is essential to prevent excessive arcing when the Lever Switch contacts open.

Circuit 4
Circuit 5

Circuit 5 works in a similar way to Circuit 4 but uses electronic switching to replace the relay.

The normally-open contact of the relay is replaced by the PNP power transistor BD682 (actually a darlington pair for those you like to know these things), and the normally-closed is replaced by the NPN power (darlington) transistor BD681. When the Lever Switch is open, the BD681 is turned on while the BD682 is turned off. The situation is reversed when the Lever Switch is closed. Although using power transistors, these do not need to be mounted on heatsinks.

D4 is a signal diode such as 1N914 or 1N4148.

The resistors should be 0.5 watt carbon or metal-film types.

Resistor Colour Codes:
1K0 - 4-bands: brown-black-red-[tolerance band]
1K0 - 5-bands: brown-black-black-brown-[tolerance band]
1K8 - 4-bands: brown-grey-red-[tolerance band]
1K8 - 5-bands: brown-grey-black-brown-[tolerance band]

Electrolytic Capacitor Orientation:
Although it is usual to mark the '+' terminal on circuit diagrams, it is more common that the '-' terminal is indicated on the actual device.

Diode Orientation:
The cathode (K) end of a diode is normally identified by a ring of contrasting colour.

LED Orientation:
The cathode (K) of a Light Emitting Diode is normally indicated by a shorter lead and/or a flat section on the body of the device.

Transistor Orientation:
The terminals of a transistor are identified in the diagram above.
For the BD681 and BD682 types, the lead configuration is shown below.
TO126 package
The following transistor types may be substituted for the BD681:
BD675, BD677, BD679, 2N6037, 2N6038, 2N6039, MJE800, MJE801, MJE802

The following transistor types may be substituted for the BD682:
BD676, BD678, BD680, 2N6034, 2N6035, 2N6036, MJE700, MJE701, MJE702


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