Mini Capacitor Discharge Unit

As I mentioned in the last post, I took my part-built 009 society challenge layout to show on the demo stand at EXPO-NG. This layout has solenoid point motors, and I recently posted about my large power-pack, now fitted with two transformers (for two controllers) and a big capacitor discharge unit (CDU) which will power up to 6 points at once. I also have a mini power-pack with a single transformer, much more suitable for micro layouts, but without a CDU it couldn't power the point motors on the challenge layout. I didn't really want to take the larger, heavier power-pack, so could I fit a CDU inside?

I had a vague idea what was needed, but a quick google turned up a useful site that confirmed the circuit diagram. Essentially a diode creates a DC supply from the 16 V AC input, a capacitor stores energy to kick the point motor over when required, and a resistor prevents the solenoids burning out if a switch is left "on" by limiting current flow. My sketch of the diagram is shown below, along with my implementation using a choc-block screw terminal.

What's more I found a diode, a 2200 uF capacitor, and a pack of resistors all in stock, so I gave it a go. The completed home-made CDU is shown below, about to be squeezed into the box. I didn't know where to start with the resistor, so put in a 1 kohm, it worked but took >30 seconds to recharge before it would work again - not ideal! I went down to 100 ohms which was better, but then I thought I could do some maths. I measured the voltage at the capacitor - about 22 V - and reckoned up to 1 A should be safe for the coil, at least for a few seconds. V=IR means a resistor of 22 ohm is the smallest I should use, so I found one and fitted that. The recharge time is now under a second, so perfectly adequate for a layout with just three points.

So the mini power-pack not only provides a single controller from one winding of the transformer, but the other winding now powers the track cleaner, the CDU (22 V DC pulsed), and a 16 V AC supply for any other accessories. The 5-pin DIN socket is for the controller, and the 6-pin DIN socket takes track power, CDU, and 16 V AC to the layout. Also seen in the box is a switch to power on/off the track cleaner, and a thermal cut-out protecting each of the transformer windings.

The track cleaner is rarely used so I could remove it, which would provide space for a "proper" CDU (larger, faster recharging, better point motor protection). However I'm happy with the way the simple CDU works for one point at a time, and with an acceptable recharge time, at least for small layouts.