Power Supplies and CDUs
How They Work

This article first appeared in the July/August 2003 issue of AMRA's 'Journal'.
By Stephen Chapman.

Our mains power supplies 240 volt 50 Hertz AC power. To run our trains we need a controlled DC voltage with a maximum setting of 12 volts (or 24 volts in the case of some of the larger scales). To convert our mains voltage to use with our model railway we use a power supply.

The power supply needs to do two things.

To convert an AC current from one voltage to another we use a transformer. This transformer consists of two or more windings of wire often with a piece of metal through the centre. By getting the right ratio of turns of wire between one of the windings and another we can convert one voltage to another.

If you are going to be building your own power supply then there are two things to watch out for with regards to the transformer. First everything connected to the mains winding of the transformer is at 240 volts so you need to be qualified to work at this voltage if you are going to wire this side of your transformer. Secondly, if you wire the transformer in backwards then instead of converting 240 volts to 12 (or 24) volts, it will convert it to 4800 (or 2400) volts which can be extremely dangerous.

Another aspect of converting the voltage is that the amperage also changes. A transformer providing one amp on the twelve volt side of the circuit will only be drawing one twentieth of that on the 240 volt side. This means that it should be safe to plug all of our model railway power supplies into the same power board as each will only be drawing a very small current.

For safety on the high voltage side of the circuit we should include a switch and a fuse into one of the wires feeding our transformer. The first allows the transformer to be turned off and the second protects against the transformer drawing too much current and burning out.

That's the change of voltage taken care of. Next we need to convert the power to direct current.

To convert from AC to DC we use a rectifier. A diode only allows current flow in one direction and will block the flow of current in the other direction and so is suitable to be used to rectify the AC into DC. With AC the voltage quoted is the root mean square of the actual voltage which varies between a positive and negative value following a sine curve that looks like this:

AC voltage sine curve

If we add a single diode to the output side of our transformer then we have a circuit that looks like this:

Basic pulse power supply

The output side of our power supply now has current flowing only in one direction so we have DC current of a sort. The voltage still fluctuates up and down so we don't have a pulse DC voltage rather than a steady one. Our voltage flow now looks like this:

DC pulse voltage

With this output we now have an output voltage that is about 45 percent of that which the output winding of the transformer is supplying. Half of the voltage is lost because the diode blocks the current flow in one direction and there is also a small voltage loss due to having the diode in the circuit. If our output winding is supplying 16 volts AC then we have a maximum available voltage by this means of just over 7 volts.

While it can be useful to add pulses into our supply when operating locomotives at very slow speeds where the pulses help stop the motor from sticking, for normal use we a much steadier voltage. By using a bridge rectifier we can capture the alternate halves of the AC flow and reverse their direction on the output side of our circuit. A bridge rectifier consists of four diodes. Two of these connect either side of the transformer winding to the positive wire and the other two connect either side of the transformer winding to the negative wire. While current is flowing through one of the diodes in each of these pairs, the second stops the current from flowing directly to the other end of the transformer winding and hence prevents a short circuit. Our voltage flow now looks like this:

DC rectified voltage

With this output we now have an output voltage that is about 90 percent of that which the output winding is supplying because we are no longer blocking half of the supply. This means that our 16 volt AC now allows us a maximum voltage of just over 14 volts.

This is better than before but still not perfect. The final step is to add a capacitor across the output side of the circuit. A capacitor works the opposite of a diode in that it will allow an alternating current to pass through but will block a direct current. It does this because the two plates within the capacitor are not actually connected together electrically. What happens is that a plate will charge up when a current flows into it and will discharge itself when the incoming current stops. By placing a capacitor across the output of our bridge rectified this charging and discharging will have the effect of smoothing the current flow. This will result in an almost constant voltage like this:

DC filtered voltage

This is about as good as we can get starting from an AC supply and is good enough to run our trains. Depending on the size of the capacitor used we will get an output voltage of somewhere from 95 percent to 140 percent of the voltage supplied by the transformer (since we still have the same maximum voltage but the minimum voltage is now much higher). Of course there is nothing for nothing so the higher the voltage that is produced, the lower the current flow will be but as we are using voltages well below that of our household supply the current flow is still huge relative to what we draw from the power point. Our power supply circuit now looks like this:

Filtered power supply

The final step is to protect our power supply against short circuits. A short circuit occurs when we have a negligible load on the circuit so what we need to do is to introduce a load into the circuit. Adding a small load into the circuit will of course lower the voltage but as we have a couple more volts from our filtered supply than we actually need, this will not cause a problem. The addition of a couple of resistors will do this for us and gives us our final power supply circuit diagram.

Protected power supply

This supplies the steady DC voltage that we need to run our trains but to operate our points we don't need a steady supply with relatively high current flow, we need an intermittent pulse of power with a high voltage but relatively low current flow. This calls for a different type of circuit called a Capacitor Discharge Unit (CDU).

A CDU has the name that it does because it uses a rather large capacitor (or several smaller ones) to store power while the output is not in use and then provide it in one big power surge when the output circuit is closed. This results in the capacitor being discharges and you then have to wait a couple of seconds for it to charge up again before you can repeat the process.

The circuit actually ends up reasonably similar to our power supply circuit in the easy that it is wired but is somewhat different in the way that it is used and in the size of the capacitor (which is now used to store power until it is needed rather than to constantly filter power).

Our points will operate quite happily on either DC or AC power so we don't require to convert our power to pure DC in the way that we did for our power supply. The functioning of the capacitor to store power for us wont work with pure AC (as the capacitor will effectively pass AC straight through) so we can use our pulsed DC output and connect the capacitor across the output side of that.

Because we are constantly charging the capacitor from the same side, the capacitor will gradually build up a quite significant charge on that side. The charge will have nowhere to go until you apply power to your points at which time the capacitor will be able to discharge itself as the circuit through the point motors completes the circuit.

Discharging the capacitor and creating a momentary current flow will also create a surge back in the opposite direction called Back EMF. To protect our circuit from this we need to place a second diode across the capacitor that will stop the capacitor from being able to build up a charge on the reverse side. Finally we need a resistor to protect us against short circuits and to limit the current flow through the point motors if we forget to disconnect the power after it changes across. We can achieve this by the simple addition of a resistor into the circuit. This gives us this basic circuit for our CDU:

Basic Capacitor Discharge Unit

The biggest problem with this basic CDU circuit is that it takes a relatively long time to recharge after each use. You can get the capacitor to recharge faster by adding a transistor to the circuit and charging the capacitor through that. Here is one way that you can do it (there are others):

Fast Recharging Capacitor Discharge Unit

You will notice that I haven't provided information about the precise components that you should use to build these circuits. This is because there are a large number of minor variations that you can make to these circuits depending on your exact power requirements and this article is intended to be more about how these circuits work and not about how to build them. What I will tell you is that the components you use will need to be rated at one amp and 25 volts in order to be able to carry the typical voltages and currents expected of these circuits. For a 24 volt power supply or for a CDU that will need to operate a large number of points simultaneously you will need components rated at 50 volts (and your transformer will need an output winding giving a voltage twice as high).

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Copyright Stephen Chapman