CDU1C BLOCK SIGNALLING CAPACITOR DISCHARGE UNIT.
For a basic, compact layout, where you only want to operate one or two points at the same time, then CDU1C CDU will probably suffice.
Model railways which use points primarily use points motor which creates a linear movement of about 5mm (1/4in) of the blades of the points.
To create this movement, the simplest device is the solenoid which consists of 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 and any other items using the same supply will suffer voltage drop during the coil operation.
The high currents can play havoc with switches.
The back emf (reverse voltage) generated by the solenoid is sometimes sufficient to cause sparks and weld the switch closed.
This will keep the current flowing through the solenoid and it will overheat very quickly.
A capacitor discharge unit (CDU) overcomes all these problems.
CDUs 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 so it will not do any damage to the coil.
Operation
AC INPUT voltage up to 16v OR DC INPUT voltage up to 24V