What is a CDU?

A CDU is a Capacitor Discharge Unit. It is an electrical component that can be included into a circuit and stores up an electrical charge, it releases the current in a powerful short sharp burst on the demand of an operator (you) by pressing a button or flipping a switch.

The most common application for a CDU in model railway terms is to operate solenoid point motors. A solenoid point motor works by inserting the point motor's drawbar (usually through the baseboard from underneath) into the factory fitted hole in the turnout. The drawbar is slid from left to right and vice-versa by the point motor and the points move, that bit most modellers know already. But what you may not know is that the drawbar's movement is caused by two electromagnets, one on one side of the motor and one on the other. The magnets are temporarily energised by a short sharp burst of current, and that is where a CDU comes in because the usual amount of current used on model railways may not be enough to create sufficient magnetism to move the points, especially if you have multiple points operating from the same switch or points that are a long distance from the power source.

A CDU can be fed by DC or AC power and can deliver an output up to four times greater than the original input. The normal way for modellers to use a CDU though is to connect it to a 16v AC supply (such as the auxiliary port on the back of many controllers) and release the charge by using a passing contact switch. Depending on the capacity of the CDU, this will create enough output to power up to four sets of points simultaneously.