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CDU Capacitor Discharge Unit


Capacitor discharge unit CDU for switching solenoid point motors

Most types of solenoid point motors require a large current to switch them. A typical transformer can only supply enough current to switch one or perhaps two point motors at the same time. If the AC power supply is from a controller you may notice a train powered from this controller slowing when the point is changed. The purpose of capacitor discharge units is to store up electricity in a capacitor. The electrical charge is released, on throwing a switch, as a burst of a much larger current than the transformer can supply directly and so switch the point motors reliably. Think of a bucket under a dripping tap. There is only a small flow of water from the dripping tap but when the buckets has filled then tipping out the contents of the bucket give a large flow of water forr a short time

The CDU will also protect point motors from being burnt out by a fault in the switching circuit. After the CDU has discharged it will not allow recharging or any current to flow until the switch reopens.

Both the standard power and high power CDU may be operated from any AC voltage up to 24 volts.Using a high voltage increases the power from a CDU. Operating from 24 volts instead of 12 volts gives 4 times the power. (24 volts can be obtained by connecting two 12 volt transformer windings in series). Generally a voltage of around 16 Volts AC is used to power the CDU. The Capacitor Discharge Unit can also be powered with DC. However 12 Volts DC provides too little power to move the points reliably. We stock 24 volt DC power supplies for powering CDUs. Too high a voltage can also be a problem as the point motor may move the point over with so much force the point tiebar is eventually shaken to bits. Note that whether you power the CDU from AC or DC the outuit of the CDU will be DC.

High power CDU

Our high power CDU allows 6 or more point motors to be operated simultaneously when powered from !6 volts AC.

wiring peco hornby seep point motors

The diagram shows three Point Motors powered by a CDU, each Point Motor being switched by its own momentary toggle switch. Momentary toggle switches are spring loaded switches. The centre solder lug is disconnected from the outer ones when the toggle is released and the spring returns it to the upright/centre position. When the toggle is moved to either side the centre solder lug will connect with one of the outer solder lugs connecting the red wire to one of the brown wires and so causing the CDU to supply a current through one or other of the coils on the point motor (the point motor has two coils one for each direction coloured in red. This current will make the point motor move up or down depending on which coil is energised. Motors. The point motors shown have four terminals. Some point motors such as SEEP have three. This is because the manufacturer has connected together one end of each coil to give a common return (blue wire) connection.

Wire Size

Because the point motors require a large current then there will be a voltage drop if very small diameter wire is used. This is due to smaller diameter wire having a higher resistance than larger diameter wire. The longer the wire the greater this voltage drop will be and it may result in the point motor switching erratically or not at all. For switching a single point 1.4 amps wire generally is fine but if you want to switch a number of points you may need 3 amp rated wire particuarly if they are a distance from the CDU.



SIZE Length x Width 2.8 x 1.35 inches 71 x 34 millimetres