Capacitor Discharge Unit

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If the answers are mostly yes then a Capacitor Discharge Unit (CDU), diode matrix and a point motor memory indicator could be just what you want.

"Oh dear" I hear you say. "What’s all this? Hasn’t it been done to death already? Books have been written on this"

Well. . . .Yes. But people seem to have lost the books or don’t know where to find them. Also I think that the following ideas haven’t been presented before as an integrated solution. What I would like to do is give you a couple of projects that will accomplish all or, if you require, some of the above.

The project is a little like an onion in that it has several layers. FIG THREE gives an overview of the onion. FIG ONE, FIG TWOand FIG FOUR show the different layers. Namely:

The Capacitor Discharge Unit uses the energy stored in a large capacitor to give a crisp, decisive operation to one or more point motors. (I am generally speaking about Peco point motors here but the principle applies to any type). The CDU is far more effective than trying to operate point motors from the 16V AC auxiliary usually found on model railroad power supplies. This will operate a single point motor but if you want reliable operation of two or more then a CDU is your man.

b. Cheap push buttons can be used. The discharge current will NOT burn them out.

CDUs can be purchased ready made but they are simple to build and if you want to incorporate all the facilities of this project then you will have to build one which is powered from a common A/C supply as described in FIG THREE. Conversely, if you only wish to proceed as far as a diode matrix and CDU, then simply construct a CDU as shown in FIG TWO.

I built the two DC power supplies and the CDU on a section of matrix (Vero) board. The transformer was a 40Volt/Amp, multi-tapped type commonly found in electronic supply shops. Farnell have a suitable type in their catalogue. The transformer and matrix board was enclosed in a metal ‘instrument’ box.

Suppose you have a main line with three or more sidings leading off it then it would be nice to just have a single push button to select the routing to the desired siding. (This is often called selective routing. It has the advantage of reducing the number of push buttons on the control panel and makes operation more intuitive). This means that some push buttons will have to operate two or more point motors. Hence the need for a CDU. To steer the CDU output to the correct halves of the point motors a small diode matrix is required.

Have a look at FIG ONE. This shows a typical layout. The point motor coils are described as Normal (N) for through working or reversed (R) for diverging working. Underneath it is a small matrix (this is just a collection of intersecting lines that help you lay out a wiring solution). The matrix shows what point motors have to be operated. What push buttons have to be operated and what electrical connections are required to achieve the required operation/s. To prevent electrical conflicts some connections have to be wired via a diode (1N4004 is recommended). Don’t skimp here or the CDU will destroy them). The matrix approach allows solutions to be quickly found for large or small arrangements.

For a small arrangement, such as FIG ONE, which has only five diodes, a tag strip used for mounting electronic components on would probably be sufficient. Draw up the component layout. Label parts as you go. Pay attention to the markings on the diodes. If you get one reversed things will go wrong. Keep your wiring neat. Use multi-strand hook-up wire. Do NOT use ribbon cable. It has too much resistance. Keep the common ‘return’ wire from the centre taps of the point motors as short as possible. If the layout is larger you might find it easier to use small size 12 way electrical terminal strips. It is neater, doesn’t require soldering and is easy to trouble shoot.

Now if this all you want out of the system then go no further but if you would like a memory indicator and a relay operation to work concurrently with the above then read on.

To do this we have to tap some of the brief ‘pulse’ from the CDU and have it operate a logic chip in such a way that there is a permanent ‘memory’ of the event. Said logic chip could operate LEDS to indicate which way the turnout has been set or, and more usefully, the logic chip/s should cause a relay to operate. This would then give independent contacts that can be used for live frog switching, signalling indications or any other useful function.

This can be achieved by using a couple of 74LS28, R-S bi-stable devices (NOR gate latches).

Have a look at FIG THREE. This is a block diagram showing how the various bits hang together. Note that all power supplies are derived from the one multi-tap transformer. The 0 Volt common must be a true common otherwise you will get spurious voltages appearing on the IC inputs and the arrangement won’t work. i.e. Strap all the zero points together. This also explains why you must use the Positive output of the CDU.

FIG FOUR gives the circuit for the logic operations using the 74LS28 IC. Operation is straightforward. The ‘pulse’ used to operate the Point Motor coils is rectified and then applied to an R-S bi-stable latch. This NOR gate latch is set or reset, depending on the nature of the input signal, and this will cause the relevant LED to light up or, if you wish, operate a 555/556 timer IC to further operate relays or other ,higher powered devices.

If your layout arrangements already use a diode matrix this may have to be duplicated at the inputs to the logic ICs. 1N4148 diodes will be just fine as very little power goes through them. Alternatively, if you can readily hook up to the wires leading to relevant point motor coils then you can dispense with this second matrix completely. The logic outputs from the 74LS28 NOR gate latches go to an IC 556 configured as a Schmitt trigger. The 556 can handle up to about 200mA and will operate a sizeable relay. Make sure you have a 1N4004 diode connected, as shown, across the relay coils or the back EMF from the coil will destroy the 556.

I built six circuits on a piece of matrix (Vero) board 110mm by 200mm. I.e. three ICs followed by three 556 ICs followed by six relays. This gave me plenty of room to mount all components. The various connections were made using the Vero board tracks and wire ‘jumpers’ as required. I don’t have a layout for you as there is considerable scope for varying the basic design to suit your own ends but I didn’t find designing the various connections too difficult. The layout doesn’t seem to be critical. Determine where the various components are to go. Don’t try to jam components together on the layout. Identify where you will have to cut tracks. A Dremel tool is excellent for this. (WEAR EYE PROTECTION). Have a test fit for one circuit to ensure it will all fit and copy for the remaining circuits.

If you don’t wish to use the relay option then you only need to make up the R-S latch portion of the circuit and extend the outputs to LEDs as required. Obviously a smaller piece of Vero board would then suffice. If you wanted to use Bi-Colour LEDs you will need to obtain COMMON ANODE LEDS. (See www.ledidea.com). I leave it to the reader to decide.

Make sure your soldering techniques are good. You should use a 25 Watt soldering iron. Note that it easy to miss a wire or badly solder one. I missed a lead from one of the capacitors and it took me all day to track it down. Examine your work carefully. Use carriers (sockets) to mount your ICs so that you can easily trouble shoot. If you want to impress people (and make sure it is doing what you expect) you can connect the various outputs of the 74LS28 and the 556, to some LEDs, which will wink on and off as the circuitry performs its logical functions.

The diode input matrix, as mentioned earlier is optional. I assembled one on a large (12 pin) ribbon cable PC socket. A bit fiddly but I only had to connect a few straps and five diodes. Works perfectly.

I used small Matsushita relays with a standard 14 pin configuration. These mount directly onto the Vero (matrix) board or you could use an IC socket. They have two separate change-over (C/O) contacts and draw about 60 mA. I extended one set of relay contacts to screw terminals soldered to the matrix board. The other set I extended to a 20 pin socket. This will provide a ribbon cable connection to a LED display at some later date. Test to make sure that you understand the logical operations and know what relay contacts change over and install to suit your requirements.

The power supplies and the CDU were made up on another piece of matrix board, which I mounted adjacent to the transformer with all contained in a metal box. There is nothing clever about any of these circuits. They are taken from data books and work quite well. I used a 4,700MF 40Volt working electrolytic in the CDU. Don’t skimp here either. The extra grunt from the large electrolytic is useful but, and THIS IS IMPORTANT, don’t use too low a working voltage and don’t make a habit of shorting out the capacitor. This will damage it. Remember that the capacitor associated with a half wave rectifier will charge up to a voltage equal to 1.41 times the applied AC voltage. Large capacitors can self destruct with enough oomph to damage adjacent components and alter your pulse rate.

This was not some esoteric experiment. It was built in response to a specific problem. I had six turnouts involved in my station throat and I was having problems with power feeding through to the final turnouts. Thus it was all worthwhile to me and was fun to build and see it all come together as expected.

Once again many, many thanks to:

Roger John Henry

Feel free to email with comments or questions.

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This website cannot be held responsible for any diagrams, mistakes or electrical work carried out using these pages. Electricity is Dangerous! We strongly recommend you to use a qualified electrician.


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