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CDU – 2A

Capacitor Discharge Unit.

Project Credit:  Paul Harman and Jeff Hughes

The Capacitor Discharge Unit is intended to be used in conjunction with the accessory decoders, but can be used anywhere that you have solenoid point motors to provide a high current pulse from a low current power supply.

While many commercially supplied units will offer some improvement to a DC supply this design is purely for AC power supplies and therefore should not be used on DC. It can be used from any AC supply such as a mains transformer or DCC track supply.

The G4 bulb will fit in the supplied 0.1″ header block across any pair of diagonal holes. The bulb is used as a PTC current limiter to limit the charging current to approximately 2A, while maximizing charge rate when the capacitor voltage is nearing the target voltage.

There is no need to use fat wires for the AC input since it is only 2A max, but the output can supply much more current so 2.5mm^2 wire will be more suitable.

Output voltage will be approximately twice the peak input voltage. If using DCC track power as the input the output voltage will be approximately twice the track voltage. If using 16V AC from a transformer expect more like 50V on the output. This CDU can not be supplied with DC it is an AC only device.

If using industrial frequency AC supply from a mains transformer, there is no need to use the fast diodes and 1N4001 can be used instead if desired. Fast diodes are only required if using DCC track power to supply the CDU.


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Component list is as follows:-

C1,C2 22,000uF 35V radial aluminum electrolytic
D1,D2 UF4001
Terminal blocks for 2.5mm^2 wire
Socket suitable for G4 bulb
G4 20W 12v bulb
Stripboard 11 strips by 20 holes or larger
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