Capacitor Discharge Unit 18,000u Slimline kit is available from
Talking Electronics for $15.00 plus $6.50 postage. 
Click HERE for details.
This module is also available fully built and
tested for $19.00 plus $6.50 postage


A slimline version of CDU 18,000u is available from Talking Electronics, to change up to 8 points at the same time.
Read the article
CDU 18,000u before buying this module.
You need 15v AC from a train transformer or from an AC plug pack.  But AC plug packs are very hard to find on eBay and that's why you need to source your power supply before buying this module. 
This module has the same capacity (4 x 4700u electrolytics - the same storage capacity as
CDU 18,000u) but only one input option: 15v AC. 

Here is the circuit for CDU 18,000u

We have called it 18,000u CDU Slimline to identify it from our other Capacitor Discharge Units.

The CDU requires an input voltage and this is called THE SUPPLY.
One of the advantages of a CDU is the ability for it to deliver a high current to a point motor to make sure it operates correctly.
A point motor is a solenoid and it has a resistance of less than 5 ohms. This means the current it will draw on 12v is nearly 2.5 amps.
If it is left activated for more than a second, it will get very hot as it is consuming about 30 watts. The CDU prevents this from happening. It only activates a point motor for half a second.
The other advantage of the CDU is it delivers this current from a supply that can be rated at 100mA to 3 amps. If the supply is only capable of 1 amp, it cannot be used reliably to operate the point and a 100mA supply cannot operate the point motor directly. 
This project allows a wide range of supplies to be used. 
The circuit will accept either an alternating voltage (called AC) or a "battery voltage" (called DC - Direct Current).
It effectively multiplies (increases) the current by charging the capacitors over a long period of time at about 100mA and then provides a current of about 2.5 amps over a shorter period of time.
Since the current required by the CDU is very small, any type of supply from 100mA to 1 amp can be used.
The electrolytics charge-up and store the energy. It is then released in a short burst and this changes the "point."
Almost all the adaptors you will have in your possession will be  the type called DC. These CANNOT BE USED. You need an AC type of adapter.
 You may have a train transformer with an AC output for this project.
The project monitors the voltage on the capacitors and prevents it rising above 25.5v by using zener diodes that "remove" the fist 23v and the next 3.2v is used to illuminate a LED and the charging current goes to this LED. This allows a voltage up to 30v to be supplied and the circuit stops the charging when the capacitors reach 26.5v. The voltage drop across the two base-emitter junctions of the Darlington transistor equal 1.4v and when this is subtracted from 26.5v, the result is about 25v. This is theoretically the maximum voltage across the electrolytic after being fully charged.

This project needs a supply from 12v AC to 16v AC and a current of about 100mA.
This means a small AC power supply can be used and some are rated at 200mA and 500mA.

Here are photos of the completed module:

The top view of the 18,000u CDU Slimlinee

The front view of the 18,000u CDU Slimline

Another front view of the 18,000u CDU Slimline
The module has a voltage doubling circuit on the input and the AC waveform is effectively doubled and converted to pulses of DC and passed to the rest of circuit via the second diode. Current flows at a rate determined by the ability of the supply. As the capacitors charge, the current reduces until it is less than 50mA and this current is passed through the LED to illuminate it.
Recharge time depends on the supply voltage and will be only a few seconds.

The high-current output diode allows a high current to flow to the solenoid and while the switch is pressed, the circuit is prevented from charging the electrolytics because the output is connected to the base of the transistor. When the switch is released, the output is allowed to go high via the three 100R resistors because the diode is reverse biased. This allows the capacitors to be charged.

Assembly of the PC board is straightforward. Three 100R 1/4watt resistors are wired in series to form a 300R resistor.
All the other components are clearly marked on the board and you must pay attention to make sure they are around the correct way. 

$15.00 plus $6.50 postage. 
Click HERE for details

3 - 100R
1 - 100u 25v electrolytic
4 - 4,700u 25v - 35v. see text
1 - 5v1 400mW zener diode
2 - 9v1 400mW zener diodes
2 - 1N 4004  1-amp diodes
1 - 1N5404 3-amp diode
1 - 3mm white or blue LED
1 - BD 679 Darlington transistor
2 - 2-screw terminal blocks
1 - 20cm fine solder

1 - CDU 18,000u Slimline PC Board

Disconnect the wires going to the point motor, from the transformer, and connect them to the input of the CDU.
Connect the output of the CDU to two switches as shown in the diagram below. Connect the other pins of the switches to the point motor.  It is now ready.
When the power is applied, the white LED will come on after a few seconds.
The LED can be placed on the main control panel of the layout to indicate the condition of the unit. The LED will light to indicate when the unit is ready. When a point is operated, the LED will extinguish, then come back on when the capacitor charges. If it remains extinguished, it indicates a fault is present and the solenoid may be still in circuit. No other points can be operated until this is fixed, but at least the solenoids will not be damaged!

Note: Train transformers have an output of 15v to 16v AC and this is ideal for this project.

with 12v to 15v AC or 12v to 15v DC.

Capacitor Discharge Unit 18,000u Slimline MkII kit is available from
Talking Electronics for $16.00 plus $6.50 postage. 
Click HERE for details.
This module is also available fully built and
tested for $19.50 plus $6.50 postage


This module combines two features. It is a slimline module and will accept  12v to 15v AC or 12v to 15v DC.

A clear view of the completed CDU 18,000u Slimline MkII

The two diagrams show how to connect the DC and AC to the module.
The DC wires MUST be around the correct way.
The AC wires can be
around either way.





If you are sure the voltage is between 12v and 15v but not sure if it is AC or DC, connect the two wires to the middle and lower screw terminal (the wires can be around either way) and if the white LED illuminates after 5-8 seconds, the voltage is AC and the CDU is ready for use.

If it does not illuminate, the voltage is DC and now you have to be careful.
If you have a multimeter and can prove the red wire is positive, it is taken to the top screw terminal and the black wire to the right screw terminal.
If you do not know the red lead is positive, you have to do the following:
Take a power diode and screw the lead of the diode into the top terminal so that the white stripe on the body of the diode is the lead that goes into the terminal.
Now connect the red lead to the other lead of the diode and the black wire to the lower terminal. If the LED does not illuminate, reverse the wire. The LED will now illuminate. You can leave the diode or remove it. It has been used a piece of "test gear" to prevent damaging the module and is no longer needed.
Never reverse the leads and connect them directly to the module as the surface mount chip on the underside of the board will BLOW UP.

The DC input voltage for CDU 18,000u Slimline MkII must be 16v MAXIMUM and each module comes with a VOLTAGE REDUCER module so you can adjust (set) the voltage on the module to 15.5v
The AC voltage is not critical. It can be from 12v AC to 15v AC or even as high as 18v AC
as the circuit will convert this to 27v and the on-board zener regulator will reduce this to 26v across the capacitors because the excess voltage will be sent to the LED to illuminate it very brightly. .

IF YOU HAVE A VOLTAGE between 16.5v DC and 18.5v DC, you can make or buy the following device called a VOLTAGE REDUCE (it is included with the fully assembled module).


The VOLTAGE REDUCER MODULE consists of a 3-terminal block and 4 power diodes and it screws into the 3-terminal block on the module.
This is how the Voltage Reducer Module works:  Each diode reduces the incoming voltage by 0.75v, making a total of 3v.
When you get CDU 18,000u Slimline  MkII, check the voltage of your power supply (before touching the module) and make sure it is less than 18v. The voltage reducer module only works for voltages: 16v, 17v and 18v DC.
If it is 18v DC, connect the VOLTAGE REDUCER to CDU 18,000u Slimline MkII as shown in the following image:

If the input voltage is 17v DC, you can remove one of the diodes by soldering a link across one of them (to short it out), as shown in the following image:

If  the input voltage is 16v you can remove 2 diodes and if it is 15v, you do not need the VOLTAGE REDUCER MODULE.

If you want to deliver 15.5v DC to CDU 18,000u Slimline - MkII with two plug packs, here is the circuit:

An easy way to get 15.5v for CDU 18,000u Slimline - MkII
 is with two plug packs - 5v and 12v.  
You can also use three 5v plug packs and you will find many of these
in your parts-bin from old mobile phones !!!!