Capacitor Discharge Units of Various Capacities for all sizes of Layout BLOCKsignalling CDUs provide a high current pulse to operate points motors Protects against coil burn-out by limiting subsequent current Regulates charging current to prevent voltage fluctuations on the rest of the layout Recharging time typically 0.8 seconds Various sizes to suit all sizes of layout Versions available with Ready Led to indicate charge state Model railways which use points primarily use points motors 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 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. 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 may suffer voltage drop during the coil operation. The high currents can also damage switches. The back emf (reverse voltage) generated by the solenoid is sometimes sufficient to cause sparks and can contaminate the switch contacts that, after a time, can 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 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 by the switch being held for a long period, the current flowing through the coil (after the initial surge) will be less than 50mA so it will not damage to the coil.