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CDU Declassified

Image of MARTIN LOVELLMARTIN LOVELL looks at Capacitor Discharge Units and their use.


Firstly; what is a CDU?

A Capacitor Discharge Unit or CDU is a device for powering solenoid type point motors (for example the Gaugemaster PM1 or the PM10, giving them a large 'kick' when the switch is activated. Generally, they are used to give a point motor a bigger burst of power to overcome sticky points, or for operating 2 or 3 points simultaneously.

The Gaugemaster CDU

How Do They Work?

The capacitors, (the large, usually battery shaped, things on the device) store electrical charge. When the point operating switch is operated they release all of their charge instantly, therefore operating the solenoid which moves the point blades to the desired position.

It is also useful for preventing point motors burning out. Without a CDU, if the switch develops a fault causing the motor to be permanently operated, they will quickly burn out. Whilst the CDU releases its single pulse of power, it is not recharging its capacitors, therefore there is no output to the solenoids after the initial pulse.

Unless you have a large number of points or a larger than average layout, only one of these units is required. More can be added if you wish to have separate sets of point motors in fiddle yards, separate stations and the like, however, they must be installed on separate circuits.

Once the solenoid has been operated, the CDU will then recharge, ready to operate the next point motor. This is almost instantly, less than a second in real time.

How Is It Powered?

The CDU can be powered from anything between 16 and 24 volts AC inclusive, so will normally work off of the accessory output of a standard analogue controller if one is present, or a dedicated transformer. For DCC users, who wish to operate their points by switches rather than decoders, a separate transformer will also be needed.

Both the GMC-WM1 Wall Mounted transformer and GMC-M3 are suitable transformers to separately power the CDU.

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How Are They Wired Up?

The wiring is very straightforward. There are 4 terminals on the CDU, 2 In, and 2 Out. 2 wires from the power supply go to the 'IN' terminals, and then one of the 'OUT' pair serves the common of the point motors, and the other serves the commons of the point switches.

Wiring diagram for two singularly controlled motors.

Wiring diagram for two motors controlled from a single switch.

Not Just For Those Using Switches.

A CDU is particularly useful for stud and probe point operation, as this involves only a brief contact of the probe on a stud to operate the solenoid, which, without a CDU may not be enough to throw the point fully. If you are looking at building a control panel with either switches or stud and probe, have a look at James' article on Points and Panels.

A CDU can of course be retro fitted to an existing system. Simply take the two wires out of the power supply and transfer them to the 'OUT' terminals of the 'CDU', then add 2 wires from the IN terminals of the CDU to the now vacant connections to your power supply - simple!

So there we are - A brief overview of a very useful addition to your layout, Happy Modelling.

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