CDU

Solenoid like the peco one shown have long been used to control point (or turnout) on model railways. To use them effectively they need a good pulse of electricity to operate the solenoid for a very short period of time. Power on the solenoid for too long will result in damage. The solution is to use a Capacitor Discharge Unit (or CDU). They provide a good pulse of power to operate the solenoids and importantly protect the solenoid from power being applied for too long that would cause damage.

There are many Capacitor Discharge units commercially available, including kits that make a good first DIY project. The CDU’s I have available have the addition of high power switching circuitry included in the one unit. This simplifies wiring at installation.

Capacitor Discharge Unit with High Power Switching

The solenoid is wired as shown, one side is the common, both terminals are connected (the Green wire) this goes in the centre of the Blue Connector at the top of the board shown to the left. The Red and Black wire are connected to the outside terminals of the Blue Connector. There are 6 connectors, one for each solenoid controlled.

The lower connectors are for the switches (centre common) these are very low power so any switch can be used.

Power required is 16V AC very common on most older analog controller power supplies, or 24V DC (plug packs can be supplied. The power in in on the two pin terminal on the right hand end.

There are two versions of the CDU power switching available a single CDU and a dual CDU. The dual CDU is useful for double slips or controlling a crossover with a single switch.

Price for the Single CDU is $55 with high current switching for 6 solenoids.
Price for the Double CDU is $60 with high current switching for 6 solenoids.
You will also need a 16vAC or 24vDC power supply and switches. A ON/OFF/ON momentary toggle switch is recommenced (3 pin is fine). The momentary switch is spring loaded and returns to centre (or OFF) when you let it go. Power Supply and Switches can also be supplied. 3 pin ON/OFF/ON momentary toggle switches are $3 each.

On each unit there are six sets of three pins for each Solenoid (the centre pin is common). Three pins for your momentary Switch and on the other side of the unit three pins for the solenoid.

ON/OFF/ON Momentary Toggle Switches make for simple operation. You may prefer two Push Button Momentary Switches on your control panel, just join one wire from each switch and that goes in the centre pin for that switch on the unit. As only low power is going through your switch small wire can be used and the length can be increased with no loss of power to the solenoid. You can also have multiple switches for the one solenoid if that makes operation easier to access.

If your Solenoids come with four electrical terminals (Peco) Join two on one side and these go to the centre pin on the unit. The other two are connected to the outside two screw terminals. Power in is either 16V AC (Often an output on DC train controllers) or 24V DC.

One of the CDU modules available has two CDU’s on the one board. This is useful for three way turnouts to reduce the chance of damage. You can configure three switches to set each of the three roads. Both solenoids then operate at the same time from separate CDU power. The dual CDU model can also be used to simplify crossovers, you could operate both turnouts from the one switch.

Dual CDU Module (power 2 solenoids independently at the same time)

Power supplies and switches can be supplied with the CDU modules.

CDU v7.2

Capacitor Discharge Units version 7.2 has extra circuitry on the input. I hope these can be controlled by 5V DC logic level control, but is yet to be tested. Let me know if this is something you would like.