Talking Electronics

Item Description
Capacitor Discharge Unit for Model Railways. MR-CDU2.

Every model railway has points. I don't consider a layout to be complete without at least one. Without them, the layout is a train set!
Many of these points are switched remotely due to their distance from the operator or inaccessibility in tunnels etc. Their method of control is usually electrical and up to now a number of problems have been associated with these circuits. They had the tendency to overheat the solenoids and even burn them out. If this happened, the points, and even the track, could be damaged. The control of a set of points is simple. Electrical energy is converted to mechanical movement via a solenoid actuator. This device is called a 'POINT MOTOR.' They are mounted under or near the point in such a way that the movement sets the blades of the points for one direction or the other.
This involves a linear movement of about 5mm (1/4in). To create this movement, the simplest device is the solenoid. It is simply a coil of wire wound on a former. Inside the former is an iron actuator or slug which can be pulled into the coil when the power is applied. By placing two of these coils end-to-end, a forward and reverse motion can be created. These arrangements are called 'Switch Machines' or 'Point Motors'.

These two-solenoid point-motors are usually switched by short pulses of electricity. The pulse length is often determined by the operator or by a simple spring-loaded switch.
But there are several problems with this arrangement. Point motors require a considerable amount of current for their operation. This means the full capacity of the transformer will be needed. Any other items using the same supply will suffer.
There are other problems too. The high currents will play havoc with switches. The back emf (reverse voltage) generated by the solenoid is sometimes sufficient to weld the switch closed. This will keep the current flowing through the solenoid and it will overheat very quickly.
This CAPACITOR DISCHARGE UNIT overcomes all these problems. Capacitor Discharge Units (CDU) supply a high current 'burst' to the solenoid. This current burst is over by the time the switch contacts open, thus eliminating back emf across the switch contacts. Should a solenoid be left in circuit, the current flowing through it (after the initial surge) will be less than 50mA. This won't even be enough to warm the coil!

The Capacitor Discharge Unit.
 

Wiring the Capacitor Discharge Unit to your point switching system.
The unit can switch many points at one time, but really depends on the size of the points. Multiple points can be run from the one controller.


 

An example of a case mount push button that can be used.
You can mount them on a Jiffy box with the LED and label them.

Connecting the unit to your already existing point controls is easy. Disconnect the wires of your existing system from the transformer and connect them to the output of the CDU.
With the Push buttons between the output of the CDU and the actual point motors.
Connect the input of the CDU (marked 15volts) to the transformer. The system is now ready for operation. The input can be an AC source.
The LED can be placed on the main control panel of the layout to indicate the condition of the unit. The LED will light to indicate when the unit is ready. When a point is operated, the LED will extinguish, then come back on when the capacitor charges.
If it remains extinguished, it indicates a fault is present and the solenoid may be still in circuit. No other points can be operated until this is fixed, but at least the solenoids will not be damaged!