Capacitor Discharge Unit CDU1C CDU2C CDU4C

BLOCKsignalling CDU1C CDU2C CDU4C Capacitor Discharge Unit (CDU)

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 0.05A so it will not damage to the coil.


Which CDU you will need depends on several factors.

It is not possible to say how many points can be operated from one CDU, as all of these factors below will have an influence:


1. The power supply voltage to the CDU, and hence the charge held in the capacitors.

2. The distance of the points motor from the CDU.

3. The size of the interconnecting wiring.

4. The make and type of points motor and its condition.

5. The scale used for modelling the layout.

6. The capacity of the CDU.


BLOCKsignalling has three capacities of CDUs available, each suited to different sizes of layout.  There is an overlap between the capabilities of each, so if you are planning to expand your layout, or wish to operate several points simultaneously, the CDUs with two or more capacitors are recommended.

BLOCKsignalling CDU1C

For a compact layout, where you only want to operate one or two points at the same time, then our basic CDU will probably suffice.

BLOCKsignalling CDU2C

For a larger layout where now, or in the future, you may wish to operate several points at the same time and these may be some distance from the controller, then our 2C CDU is probably more suitable.

BLOCKsignalling CDU4C

For the ultimate in performance and future proofing, then our 4C CDU is probably the most suitable.

BLOCKsignalling CDU4R

The BLOCKsignalling CDU4R has a red LED on the PCB that lights when the capacitors are charged to 15V and above. Knowing the state of the charge on the capacitors is useful to confirm correct supply connection and to show that the capacitors are charged sufficiently.  The led will also indicate if a points switch has inadvertently been left operated, as the LED will not re-light after the points operation. 




The CDU inputs can be connected to any DC power supply up to 24V or AC power supply up to 16V. The most common connection point would probably be the 16V AC auxiliary output usually found on model railway speed controllers.  Old laptop power supplies can also be suitable as many have a 19.5V DC output.  With DC supplies, be sure to connect them with the correct polarity to the CDU.

The minimum supply voltage is 12V AC or 15V DC, as below this level the CDU will no longer improve the action of the points.

The output terminals are connected to the points switches, with the CDU negative output generally connected to the common, centre point, of the points motors.

Capacitor Discharge Unit Wiring Diagram

When points motors are operated they draw a large current, generally around 4A.  This high current is necessary to allow the points to move positively and snap to their new position.

To allow this current to flow, wiring with low resistance is necessary.  The following sizes are recommended:

Runs up to 2m (6ft): Wire size 16/02mm.  If operating several points at the same time, and using a common return wire, the return wire should be 24/0.2mm.

Runs over 2m (6ft): Wire size 24/02mm.  If operating several points at the same time, and using a common return wire, the return wire should be 32/0.2mm.



CDUs with remote Ready Leds

Some BLOCKsignalling CDUs wired leds, designed to provide an indication on a control panel that the CDU is fully charged.

It is important to wire the led connections correctly.  The red wire is connected to the Ready+ terminal and the Black wire is connected tot he Ready- terminal.

The led can be mounrted through a 5mm hole drilled in the control panel.  The wiring to the led can be extended if necessary.


Other Ideas

Use general purpose diodes (1N4001-1N4007) to switch several sets of points simultaneously.

Capacitor Discharge Unit Diode Matrix