Beefy CDU

This page is a follow on from the diode matrix page where you may want to switch several points at once to select a route.
To operate more than 3 or 4 points with the available propriatory CDU (Capacitor Discharge Unit) will be almost impossible and the unit will run out of steam with more points than this being operated in unison.
What is required is a "Beefed up" CDU that can supply enough energy to move all the points at the same time.
Note that unlike the proprietary units that only supply a pulse of power then turn off, this unit supplies a large "kick" then continues to supply current as long as it is connected to the point motor. This makes it an ideal companion for the home made uncoupler featured in these technical topics pages.
The components and wiring for such a device is shown below.
cpu wiring
Circuit for a "Beefed up" CDU

The circuit is quite simple and the components list is given below for ordering from "Maplins"
AC mains is fed into the 240 volts side of the transformer and 18 volts AC is available at the output. This is fed to a "Bridge Rectifier" made up from 4 diodes.
Note that the 18 volts has a limiting lamp in in the circuit and both filaments (in parallel) of a Car "Stop and tail" lamp is ideal for this function. The idea being that should a short circuit develop in your wiring or if some one keeps the power on to the points then the lamp lights and gives you a warning. At the same time it also prevents damage to components of your CDU and your point motors.
The output from the Bridge Rectifier which is now +ve and -ve is connected to an electrolitic capacitor of at least 10,000 Microfarad and I have used 2 of these in parallel to operate up to 8 points at once.
Componant list:-

Transformer....... 12VA Maplin code no WB11M Note This is a 9 volt Transformer with 2 windings that you connect in series to give 18 volts.
Lamp................. Standard car 12v Stop and Tail Lamp (available from any car accessory shop or local garage)
Diodes.............. 1N4004 Maplin code QL76H (you could use a ready made bridge for this item if you wish)
Capacitor.......... 10,000 Microfarad electrolitic Maplin code LE04E (connect 2 in parallel if you want to operate more than about 6 points together.) Note:- (Maplins seem to have ceased this item. Try Rapid Components an on line supplier who still have them in stock)

John Essex
Heywood Model Railway Group


This page updated 18th May 2012