IN THE INCREASING TECHNOLOGICAL age, where the latest development of Direct DCC using Bluetooth from a smartphone seems to be gaining interest on an almost daily basis, a case should be made for the older, traditional method of model railway electric traction power; straight DC. With most systems, it is a case of ‘swings and roundabouts’; in other words, all systems have their own benefits, and drawbacks too, and straight DC is simple, and relatively inexpensive.

The SR7mm Group has been around for 30 years or so, and DCC was not really developed enough, or affordable when we started, for what we were building. So, the railway was built using traditional two-rail DC power. Controllers were home-made, and mainly fixed installations, because of the number we required. Rather than use a nominal 12V, our transformers were 15V. I won’t go into too much technical detail, but because the controllers aren’t regulated, voltage falls as current draw rises, and it helps not to have a completely smooth DC output voltage. If the gradient varies, you really have to drive the trains. The only drawback of these simple controllers is that they can run very warm, or even hot; they require quite a substantial output transistor and associated heat-sinks. However, that drawback aside, we still have two of these in day-to-day use at Southwark, and also in the removable control panel at Warrior Square outside. We used to have more, but these early controllers, which are admittedly pretty robust, have been replaced with something rather more compact, and less likely to warm the room.

When the part of the layout that was outside was rebuilt inside and considerably enhanced, there was a requirement for further controllers. Coulsdon, when it was outside, had two control panels with five controllers – two of the earlier type, and three modified hand-held controllers from the late Roger Murray. All this was substantially changed once it moved inside, mainly due to the track layout having subtle changes, plus loop lines, plus the distance from one end of the shed to the other. This resulted in Coulsdon moving from five to eleven DC controllers, and this used a new design. The design came courtesy of the London, Ontario, MRC and Rob Paisley. The specific design came from this webpage: London Model Railroad Group SCR Throttle

I have built both the top two circuits on this page, and can confirm that they work very well. The one we use, in reality, is the simpler of the two, the second one. Careful selection of components means that it is very compact and doesn’t generate significant heat.

Once set-up, they have proved to be very reliable, and are only limited in output capacity by the circuit breaker you use for protection, and the capability of the transformer/ rectifier that provides the input DC supply.

Because of the number we required, I ended up using cheap PCB/Circuit design software bought from Maplin (remember them?), which I still have and use (PCB Wizard 3), to create a PCB layout. I arranged to have 20 professionally manufactured. They cost £4 each, and I still have a couple of spare PCBs, although the manufacturing files have long gone. I could, in theory, recreate the PCB design if necessary.

The rest of the components were purchased from Rapid Electronics, and are still widely available, all except the PUJT (Programmable Unijunction Transistor) 2N6027 or 2N6028; for this you would have to shop around, but they can be available via Amazon or eBay. It may seem a little excessive specifying 25A components for something which is only going to operate at 3-4A, but the reason for doing this is that they run significantly cooler given the very wide rating difference. There isn’t an awful difference in cost either for this extra margin, and it increases reliability if you have that extra in hand. If you discount the transformer – and it must be a raw DC input voltage source, not smoothed, the additional components cost around £8, so £12 in total, less transformer.

Some of our units have different voltage control potentiometers (22k against the 10k item specified, to provide altered voltage control), and we have also increased the value of the output capacitor in some cases to 3300μF. All have 3A circuit breakers. The only other thing of note is that you will need an old-fashioned multimeter to set it up; one with a moving pointer, and not a digital display.

This is only one particular example of a simple DC traction power supply, and these should not be lightly dismissed as being old fashioned. If you are modelling on a tight budget, these are eminently suitable to construction, using recovered or second-hand materials, and I consider their construction well within the capability of a modeller with basic electrical or electronic skill. There is no need for the PCB either, but if you use strip-board due consideration should be taken of the likely current loads that the copper strips will need to carry, which can easily strengthened using solid tinned copper wire.