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Train controller for model railway layouts This easy-to-build Train Controller will give full, re­alistic control of your model trains. One control provides full reverse to full forward speed. The circuit provides inertia and a brake switch and has automatic overload protection. By RICK WALTERS The big virtue of this new Train Controller is its single knob control. The one throttle knob gives full reverse speed when it is fully anticlockwise and full forward speed when it is fully clockwise. And when the knob is centred, the train is stopped. This simple throttle control does away with the need for a forward/ 66  Silicon Chip reverse switch or a relay and thus reduces the possibili­ty of derailments which can damage expensive model rolling stock. This is especially the case if derailed rolling stock falls to the floor! What is the problem with a forward/ reverse switch or relay? Surely they are simple enough and are reliable? Well, yes they are but it is amazing how many people driving model trains oper­ate the forward/reverse switch by mistake; it is quite easily done. And if the train is going at a fair pace, throwing it into reverse often just derails everything, which doesn’t do a lot for realistic operation (to say nothing of the possibility of damage). With this new Train Controller though, if you have the train going forward and decide to throw it into reverse by rotating the throttle knob quickly to full anticlockwise, there is no drama. The train slows down smoothly by virtue of the built-in inertia, comes to a stop and then accelerates equally smoothly in the other direction. Oh, and there is another virtue in not having a forward/reverse switch. For one reason or another, many peo- Fig.1: the circuit is essentially a combination of two complementary emitter followers controlled by the throttle poten­tiometer VR1. Overload protection is provided by Q3 and Q4. These monitor the track current through the two 0.47Ω resistors. The complementary design does away with the need to include a for­ward/reverse switch. ple have trouble wiring them up correctly! Other features of the controller are preset trimpots for maximum forward and maximum reverse speed and a trimpot for adjusting the degree of braking; you can have it really swift or more leisurely. Actually, if the brake is applied to stop the train without rotating the control knob to the centre position, the train will stop as you would expect it to. But if the brake is then switched off, the train will gradually pull away and accelerate until it reaches the previous speed set on the control knob. Finally, although this is an “unseen” feature, the Train Controller has automatic overload protection. So if a loco de­rails or someone inadvertently (or deliberately) shorts out the track, the Train Controller will take care of the overload and once the short is removed, normal operation will be instantly restored. We’ve built our prototype into a plastic case, as shown in the photos but we assume that many modelling enthusiasts will build the controller underneath their layout and will make their own control panel. Circuit operation The complete circuit of the Train Controller is shown in Fig.1. It is virtually two speed control circuits in one. For forward speed operation, transistor Q1 feeds voltage to the track while for the reverse operation, transistor Q2 does the work. It is this scheme which allows us to do away with the forward/re­verse switch. This controller works by feeding pure DC to the track. It does not use pulsed DC or unsmoothed DC. While these other forms can give more reliable loco operation when the track or the loco wheels are dirty, pure DC results in the quietest operation of the loco motor. For some modellers this is a most important point. A transformer with a centre-tapped 18V winding (ie, 9V a side) feeds a bridge rectifier (BR1) and two 4700µF 25VW capaci­tors to provide balanced supply rails of ±12V (nominal). As shown, the +12V rail feeds the collector of NPN Darlington power transistor Q1, while the -12V rail feeds the collector of PNP Darlington power transistor Q2. Trimpot VR2 is connected across the +12V rail to provide the maximum forward speed setting while VR3 is connected across the -12V rail. The wipers of these two trimpots then feed each end of the throttle potentio– meter, VR1. Now let us see what happens when the throttle knob is rotated clockwise from its centre setting. Let’s also consider that switch S1 is set to the “Run” position. As we rotate the throt­ tle control clockwise, the voltage picked off by the wiper will rise accordingly and it will charge the 4700µF capacitor via the 470Ω series resistor. After a short delay, caused by the charging of the 4700µF capacitor, the voltage at the base of transistor Q1 will be high enough to turn it on. From there on, as Q1’s base voltage rises, it will act like an emitter follower, reproducing the voltage fed to its base at the emitter, less the base-emitter voltage of about 1.3V. So if the base voltage to Q1 is +6.7V for argument’s sake, the voltage across the track will be close to +5.4V. If a loco is connected across the track, it April 1997  67 Fig.2: the component overlay for the Train Controller. Secure the mains wiring with cable ties so that the leads cannot move if one comes adrift. The mains terminal block is secured using a nylon screw and nut and all exposed mains terminals are covered with heatshrink tubing. 68  Silicon Chip will be run­ning in the forward direction. If the throttle control is now rotated in the reverse direction, the 4700µF capacitor is discharged via the 470Ω resistor and the wiper of VR1. As the voltage across the 4700µF capacitor goes below ground, the voltage at the base of transistor Q2 will be sufficient to turn it on, while the same voltage applied to the base of Q1 will turn it off. Q2 now acts like an emitter follower, reproducing the negative volt–ages at its base, at the emitter, less the base-emitter voltage of about 1.3V. So if the base voltage is -6.7V under the same argument, the voltage across the track will be close to -5.4V and the loco will be running in the reverse direc­tion. Braking When the brake switch is turned on, the 4700µF capacitor is discharged through the 470Ω resistor and the brake trimpot VR4. The time it takes to discharge the capacitor and hence the time it takes for the train to come to a stop is determined by the setting of VR4. When the brake is switched off, the 4700µF ca­pacitor will slowly charge up again to the voltage on the wiper of VR1 and the train will eventually resume the speed set before the brake was applied. The two Darlington power transistors (Q1 & Q2) are mounted on a U-shaped heatsink, as shown here. Note that Q2 requires an insulating washer & bush (see Fig.3 below). Short circuit protection One of the features of the circuit is short circuit protec­ tion and this is provided by transistors Q3 and Q4. Q3 monitors the current through the 0.47Ω emitter resistor associated with Q1. If the emitter current of Q1 rises above about 1.3A, the resulting voltage across the 0.47Ω resistor will be sufficient to bias Q3 on. This will cause Q3 to shunt base current away from Q1, throttling it back. If the emitter current tends to rise further, Q3 will turn on harder, shunting even more base current away from Q1 and throttling it back further. A similar process applies to Q2 and Q4. Q4 monitors the emitter current of Q2 via the associated 0.47Ω resistor. We have not included a warning device to indicate an over­load as it should obvious when the train has stalled that someth­ing is wrong. Don’t ignore the short as the conducting transistor will get very hot and the heatsink Fig.3: details of the heatsink mounting for Q1 & Q2. Note that Q2 must be electrically isolated from the heatsink. temperature will rise rapidly. In other words, the protection feature is really only intended to cope with short term overloads. fiers to develop positive and negative DC rails. We’ll talk more about these options later. Power supply options Building the controller The circuit of Fig.1 shows that two possible power trans­former connections can be used. The first option is for a centre-tapped transformer, as described above. The second option is to use a single-winding 12V transformer. Whichever transformer is used, the circuit is unchanged. When the single winding trans­former is used, the bridge rectifier acts like separate positive and negative halfwave recti- The Train Controller is housed in a plastic case measuring 203 x 68 x 158mm. The components are mounted on a PC board meas­uring 89 x 120mm and coded 06104971. Fig.2 shows the wiring details for the Train Controller. Begin construction by carefully checking the PC board for shorted tracks or breaks. Repair any defects before proceeding further. Mount the parts on the PC board April 1997  69 Fig.4: this is the full-size etching pattern for the PC board. Check your board carefully for etching defects by comparing it with this pattern and fix any problems before installing the parts. exactly as shown, taking care to ensure that all polarised parts are correctly connected. The two Darlington power transistors Q1 & Q2 are mounted on a common U-shaped heatsink. Q1, the BDV65B, is mounted directly on the heatsink while Q2, the BDV64B, is mounted using a mica insulating washer. By not using an insulating washer we get improved heat dissipation for Q1. Note that since the heatsink is electrically connected to the collector of Q1, it will be “live” at +12V or whatever is the value of the positive supply rail. Both transistors should be installed with thermal compound applied to their mounting surfaces. Fig.3 shows how the heatsink is effectively sandwiched between the transistors and the PC board. When you have installed both transistors on the heatsink, use your multi–meter (switched to a high Ohms range) to check that the transistor col- lectors are isolated from each other. You can solder all the external connections directly to the PC board or you can connect to solder stakes if you prefer. Use different coloured hook-up wire for the various off-board connec­tions. It makes it a lot easier to troubleshoot the unit if it does not work when you first fire it up. The transformer is screwed directly to the base of the case and one mounting foot is earthed back to the Earth wire of the mains power cord. As discussed previously, you have two options for the power transformer. If you only have a small layout and will be using one loco at a time, a transformer with a single 9V to 15V 1A secondary winding can be used but if you intend to have a larger layout, it is worthwhile going for the larger centre-tapped transformer. You could also use a ±12V DC power supply to feed the con­troller. If you do this you can fit 470µF capacitors instead of the more expensive 4700µF units specified. The PC board overlay allows for both sizes of capacitor. Note that whichever supply option is used, the inertia capacitor must be 4700µF. The front panel has only the main throttle control and brake switch mounted on it. Hence you will only need to drill two holes for these components before they can be wired. On the back panel, you will need to drill holes for the two-way insulated terminal block for the output leads, the mains switch and the cordgrip grommet for the power cord. We used a snap PARTS LIST 1 PC board, code 09104971, 120 x 89mm 1 mains transformer 18V CT 60VA, Altronics M-2165 or equivalent 1 plastic case, 203 x 68 x 158mm 1 3-core mains flex with 3-pin plug 1 cordgrip grommet to suit mains flex 1 SPDT switch (S1) 1 240VAC SPST snap-fitting rocker switch (S2) 1 large knob to suit VR1 1 U-shaped heatsink, DSE type H-3401 or equivalent 1 BDV64B mounting kit 2 2-way mains terminal blocks 70  Silicon Chip 1 3mm x 10mm nylon screw & nut (to secure mains terminal block) 4 6PK x 6mm screws 3 3mm x 10mm bolts 3 3mm nuts 3 3mm shakeproof washers 1 6A bridge rectifier (BR1) Semiconductors 1 BDV65B NPN Darlington transistor (Q1) 1 BDV64B PNP Darlington transistor (Q2) 1 BC548 or BC338 NPN transistor (Q3) 1 BC558 or BC328 PNP transistor (Q4) Resistors (0.25W, 1%) 2 4.7kΩ 2 470Ω 2 1.5kΩ 2 0.47Ω 5W wirewound Capacitors 1 4700µF 50WV PC electrolytic 2 4700µF 25WV PC electrolytic 1 .0068µF 3kV ceramic Potentiometers 2 10kΩ trimpots (VR2,VR3) 1 5kΩ linear potentiometer (VR1) 1 1kΩ trimpot (VR4) The Train Controller is built into a standard plastic instrument case. Make sure that the mains cord is firmly anchored and that the mains wiring is correctly installed. fitting power switch which requires a rectangular cutout. This can be easily made in the plastic panel by drilling a suit­able hole and then filing it out to the desired size. The 3-core mains flex is passed through the cordgrip grom­met which anchors it. The Active wire is terminated directly to one side of the mains on/off switch (S2) while the Neutral wire is terminated to a 2-way terminal block. The Active wire from the other side of the mains switch is also terminated at the terminal block. This block, which is secured using a nylon bolt, also terminates the primary wires from the transformer. Note that the .0068µF 3kV suppression capacitor is wired directly across the mains switch S2. All connections to this switch should be fitted with heatshrink sleeving to prevent any chance of accidental contact. When all the wiring is complete, go over your work thor­oughly and crosscheck it with the circuit and wiring diagrams of Figs.1 & 2. Testing Apply power and check the positive and negative supply rails. They should be roughly the same (absolute value) and will typically be about ±15V for a nominal 18V centre-tapped trans­ former, with no load connected to the output. This will drop when loaded. Now rotate VR1 fully clockwise and check that the output voltage gradually rises towards the positive supply rail. We would expect a maximum value of about +13V, again with no load. You can tweak this value to whatever value you finally decide upon by adjusting trimpot VR2. Similarly, rotate VR1 fully anticlockwise and check that the output voltage builds gradually to the value of the negative supply rail. We would expect a value of around -13V, with no load. Again, you can set the maximum negative value by adjusting trimpot VR3. There will be some interaction between these two trimpots but a couple of tweaks should get them just right. VR4 can be set at any time to give a realistic braking distance. With these checks done, it is time to run a train. Connect the Train Control to your layout (or a loop of track) and confirm that you can control a locomotive smoothly. When VR1 is at its centre setting, the loco should slowly come to a stop. If you want to remove the inertia feature you can omit the 4700µF electrolytic capacitor connected to S1. Alternatively, if you want to reduce the inertia effect then make the capacitor smaller (1000-2200µF). The engine will now come up to speed quicker and brake quicker. SC April 1997  71