Capacitor Discharge Unit
18,000u Slimline kit is available from
slimline version of CDU 18,000u is
available from Talking Electronics, to change up to 8 points at the same time.
We have called it 18,000u CDU
Slimline to identify it from our
other Capacitor Discharge Units.
THE POWER SUPPLY
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.
TYPES OF ADAPTORS
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 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.
CONNECTING THE UNIT
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.
Capacitor Discharge Unit
18,000u Slimline MkII kit is available from
This module combines two features. It is a slimline module and will accept
12v to 15v AC or 12v to 15v DC.
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 VOLTAGE REDUCER MODULE consists of a 3-terminal block
and 4 power diodes and it screws into the 3-terminal block on the
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: