Why use a CDU?
Why a CDU?
A capacitor discharge unit (CDU) is a device used to throw point motors with a solenoid in, protecting it from “burning”. We sell Peco point motors that operate with a solenoid.
What is a solenoid?
A solenoid has a moveable ferromagnetic core with a helical coil around it. Once a current passes through the coil, a temporary (electro) magnet is created in the core and it is attracted to one side of the coil. Once the current is switched off, the core is no longer magnetized and becomes stationary. This current to the solenoid is provided by a suitable power supply.
Solenoid
A switch is installed between the power supply and the solenoid to switch the power on and off. The problem is that if the current is switched on for too long, the wires will get too hot and the solenoid will “burn out”. The solution to this problem is to apply the current just long enough for the core to move to the desired position.
Which switch to use?
With a normal on-off switch, you are supplying continuous current when switching it on. One solution is to install a 3-terminal spring loaded on-off-on toggle switch. The switch is activated by pressing the toggle to one side. When the toggle is released, it will move back to a centre position, cutting off the current.
DTSP Toggle Switch
Another possible solution is to use two momentary contact push button switches (normally open). The switch is pressed down to close the circuit and the electricity will flow while being held down. Once the switch is released, the circuit is open and electricity will no longer flow.
Momentary switches (normally open)
In both cases above the possibility still exist that the switch will be pressed for too long, resulting in a “burnt out” solenoid.
CDU, a perfect solution.
This is where the capacitor discharge unit (CDU) comes in as a perfect solution for the problem. It is installed between the power supply and the switches activating the point motors. The CDU has a capacitor(s) that charges and stores electricity for a short period of time. Once the switch is pushed, electricity is released in a short pulse. The short pulse of electricity is enough for the solenoid core to move to the desired position.
Using a CDU with a solenoid point motor will avoid the current passing through for too long, even if the switch is held down too long. Once the switch is released, the capacitors recharge again.
We sell a capacitor discharge unit (CDU) that can do this, with instructions included. If the wiring is done correctly, you will need only one CDU for a small to medium size layout. Larger layouts will need more than one CDU. There is no need for a CDU per point motor.
On some layouts, a single button(s) can be used to power two solenoids simultaneously, like a cross over between to mainlines. A CDU will ensure that there is enough power to activate both solenoids simultaneously, if installed correctly.
When buying a CDU, make sure that it will meet the needs of your layout. There are different models in the market.
Hints on using a CDU
Use a power supply with the specifications as indicated by the manufacturer.
Use thicker wires from the CDU to the point motor, preferable 1mm or more in diameter.
Keep the wires from the CDU to the point motor as short as possible.
Keep the points’ moving parts clean and free from any debris. You can lubricate the moving parts with dry graphite powder.
Thank you Niel Wilson and Henry van Wyk for their inputs with compiling this document.
