ELECTRONICS AND 16mm scale model railways by Dave Bradwell (Northern Dave)
With modern components and techniques we can do anything that the prototype can - we even smoke a bit. Many have seen the Binnie "Persephone" with electronic Chuff; I now hear that he is working on whistle and Westinghouse pump sounds to add to the confusion. Fortunately we are spared the boredom of track circuiting on the lines that we model, but other problems appear such as providing sufficient power for locomotives fitted with big motors from junk shops, or operating point motors under arduous conditions. Many other weird and wonderful gadgets are possible right up to the latest electronic engineers' toy - the microprocessor. Here we have a complete computer on a board about 12" x 6". That should give your imagination a bit of scope - perfect for exhibitions !
This week's device is therefore the silicon bridge rectifier. Now, there is nothing new about bridge rectifiers: they have been fitted to model controllers for years but the device to which I am referring is smaller, cheaper, and more efficient than it's predecessor, the selenium rectifier (that big thing in your power unit with all those thin plates). A quick browse through your favourite electronics magazine will reveal quite a few in the adverts at about £1 each. We are most likely to use the rectifier in its classic role to convert alternating current (A.C.) at between 12 and 16 volts into direct current (D.C.) to feed to the track via a suitable controller.
Choosing a suitable device from the array advertised is really quite simple. All will withstand the voltage we use, being rated at between 50 and 600 volts. This just leaves us to choose the one which will carry the current we require.
I suggest that you choose something fairly meaty such as 6 amps; this should allow for the occasional accident, but it does not make the component completely idiot-proof. Send off your cheque for a couple (at least - they are always useful). When the package arrives you will be really disappointed as this great product of modern technology appears as just a small block of black plastic with 4 wires sticking out. There is also a hole for mounting the thing on an aluminum plate which thus dissipates the small amount of heat generated. A bit of experiment helps here - don't let the device exceed about 100ºC in temperature or you may destroy it in a non-spectacular way which is very disappointing! This leaves us with only four wires to connect up. The two marked "+" and "-” are the d.c. output to the controller whilst the remaining pair, which may or may not be marked with a "~" , go to the 16 volt a.c. output of the transformer.
A word about soldering: take reasonable care to avoid over-heating the device as it is possible to cause damage in this way (but unlikely!). Equipped with a suitable transformer and rectifier as described you will be ready for the ultimate in controllers which I hope to describe next time.
Don's garden line has an unusual application of the bridge rectifier. As already described this line has a return loop - tricky in 2-rail. Using the conventional switching technique would have meant miles of wire up to the control panel and back' but by restricting running round the loop to one direction we were able to use the bridge rectifier to keep things moving and save wire. A section of track on the loop is isolated (saw through both rails at each end) from the rest and connected to the d.c. output of the rectifier (+ and - terminals). The a.c. terminals go to the rails before the loop points, as in the sketch. The rest of the layout is electrified normally.
It can be seen that, no matter which way trains run on the main line, the rectifier will ensure one-way traffic on the isolated section. To negotiate the loop, run round clockwise until the engine is on the special section, then reverse the controller and change the point - without stopping if the section is long enough. This operation will be easier if the controller has a reversing switch separate from the speed control.
