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Low-cost model train controller Throw away that primitive rheostat controller. This low-cost unit offers much improved running for your model trains and features simulated inertia as well. By GREG SWAIN Most model train sets come supplied with a simple rheostat controller but this must be the worst type of throttle you can have. OK, so they 're cheap but that's about all they have going for them. On the debit side, they result in poor low-speed running characteristics, jack rabbit starts and a model that frequently slows (and even stalls) on curves and gradients. Why does the rheostat controller cause these problems? It's all to do with the fact that this type of controller simply consists of a variable resistor in series with the supply voltage to the track. It's job is to control the armature current of the motor. This in turn controls the torque produced by the motor and thus the speed of the model. This scheme works fine at high running speeds because the control resistance is quite small. It's mainly the back EMF produced by the motor that determines armature current in this situation, and so the speed will be virtually independent of load variations (due to gradients and curves, etc). It's at the low speed settings that we strike problems. The reason for this is simply that, to get the train to run at a lower speed, the resistance in series with the supply is increased. At very low speeds, the rheostat con- +12·1 8 V ~ - - - - - - - - - - - - - - - - - 1 . . . . . - - - - , B 01 BC337 1N4001 SPEED C VR1 5k 0 EOc VIEWED FROM BELOW BRAKE ~ BCE S1a S1b Constant voltage source 4700 FORWARD SZa FOR S2b 4700 TO TRACK BRAKE LEVEL VR3 1k REV SIMPLE TRAIN CONTROLLER WITH INERTIA Fig.t: the circuit is based on Darlington transistor pair Qt & Q2. These form an emitter follower which buffers the output of speed control potentiometer VRt. VR2 & the 4700µF capacitor provide throttle inertia while VR3 and the 470Q resistor set the braking inertia. Q3 provides overload protection by removing the drive to Qt when the voltage across the tQ resistors exceeds 0.6V. 42 SILICON CHIP troller behaves as a constant current source and this swamps out the otherwise beneficial effect of reduced back EMF as the motor slows. Let's take a closer look at this situation. Normally, when a motor slows down, its back EMF falls and the armature current rises, thus increasing the torque. However, because our rheostat controller is now behaving as a constant current source, it prevents the armature current from rising in response to this reduced back EMF. This reduces the torque of the motor just when we most need it and leads to the poor low-speed control characteristics mentioned earlier. Another problem with the rheostat controller is its poor voltage regulation. If the motor attempts to draw additional current in response to an increased load, the voltage across the resistance increases and so the track voltage falls. This reduced track voltage adds further to the low-speed running problems encountered with rheostat-type controllers. Finally, it's impossible to start a model train smoothly with a rheostat controller. That's because a motor requires a much larger armature current to start than it does to keep running. So what happ ens as the throttle is advanced? At some point, the motor suddenly starts and, once started, it quickly gathers speed. The result is a jack-rabbit start which hardly makes for realistic control. The answer to these problems is to use a controller that behaves as a constant voltage source at any given control setting. And that's precisely what this circuit does. It's really nothing more than a variable voltage power supply with a low output impedance. Because the controller has a low output impedance , the current through the armature now varies according to the back EMF and this leads to much improved torque at low run- Most of the parts are mounted on a small PC board which can be hidden under the layout. The controls can be mounted in a small plastic case to give a walkaround throttle or they can be mounted on the main control console. A substantial heatsink should be fitted to transistor Q2 (at the back of the board). ning speeds. This in turn leads to much improved control characteristics , eliminating the tendency for the model to slow down and stall on curves and gradients. A constant voltage source also overcomes the problem of jack rabbit starts. Unlike the previous situation, the armature can now draw significant current at low throttle settings (ie, at low track voltage) and so starting is much smoother and more realistic. Inertia & braking As a bonus , this new controller includes a couple of features to make your model behave just like the real thing. When you open the throttle on a real train, it doesn 't speed up immediately. Instead, it gradually builds up speed to match the new throttle setting. Similarly, when the brakes are applied, the train does not come to a "brick-wall " stop but slows down gradually. So , to make things more realistic , this low cost controller includes simulated inertia circuitry so that the track voltage builds up slowly when the throttle is wound up and drops slowly when the brake is applied. A couple of preset pots allow you to independently adjust the amount of inertia for throttle and brake to suit your layout. Finally, the controller includes output short circuit protection. This is necessary because short circuits can occur quite frequently in a model train layout; eg, if a loco becomes derailed. It's also easy to accidentally short circuit the track when you modify your layout. To overcome this problem, the con- troller automatically current limits when the track is short circuited and lights a LED to indicate the overload condition. How it works Now take a look at Fig.1 which shows all the circuit details. It's uses just three trarisistors plus a few other components. The input ·to the controller is unsmoothed DC of15-18Vand this voltage is tapped off by VR1 which is the throttle control. This voltage is then applied to the base of transistor Q1 via S1a, D1 , the inertia trimpot (VRZ) and the series 5.6kQ resistor. The voltage change on the base of Q1 is not instantaneous when VR1 is adjusted to a new setting, however. That's because it takes time for the 4700µF capacitor to charge up to the throttle voltage via D1 and VRZ. Instead, depending on the setting of VRZ , the train will build up speed gradually until the capacitor is full y NOVEMBER1990 43 less than 0.6V and so Q3 is off and has no effect on the circuit operation. However, if a short circuit occurs, the output current shoots up until there is 0.6V across the two H1 resistors. At this point, Q3 turns on and reduces the drive to the output stage, thus limiting the output current to about 1.2A. It also turns on LED 1 to indicate the overload condition. At the output, double-pole switch SZ is used to provide forward/reverse switching. It simply switches the supply polarity to the track. Diode DZ is included to protect the transistors from any spikes which may be generated by the loco motor or by track switching. Power for the circuit can be derived from any 12-18V unsmoothed TO TRACK Fig.2: install the parts on the PC board as shown here. Take care with component polarity and note that the metal tab of Q2 goes towards the edge of the board (see Fig.1 for transistor & LED pinouts). charged. Diode Dl prevents the 4700µF capacitor from discharging through VRl when the throttle setting is reduced. So VRZ and the 4700µF capacitor PARTS LIST 1 PC board (available from Electronic Toy Services) 1 5kQ linear potentiometer 2 1kQ PC-mounting trimpots 2 DPDT miniature toggle switches 1 heatsink (see text) 1 TO-220 mounting kit (mica washer plus insulating bush) Semiconductors 2 BC337 NPN transistors (01,03) 1 TIP41 NPN transistor (02) 2 1N4001 silicon diodes (D1 ,D2) 1 red LED (LED 1) Capacitors 1 4700µF 25VW PC electrolytic 1 47µF 25VW PC electrolytic Resistors (0.25W, 5%) 1 5.6kQ 1 470Q 21Q, 1W provide the simulated inertia feature for the throttle. Similarly, brake switch Slb and VR3, in conjunction with the 4700µF capacitor, provide the braking feature. When brake switch Slb is closed, the 4700µF capacitor slowly discharges via the 470Q resistor and VR3. Thus , the voltage on the base of Ql gradually reduces and so the train slows to a stop. Note that Sla switches out the throttle control (VRl) when the brake is applied. That's done for two reasons: (1) to eliminate the need to reduce VRl's setting to zero in order to stop the train; and (2) so that the train will gradually build up speed to its previous setting when the brake is released (assuming that VRl is not touched). VR3 sets the level of braking inertia. Transistors Ql and Q2 form a Darlington output stage and this stage is forward biassed as soon as the voltage on Ql 's base reaches 1.3V. These two transistors together function as a compound emitter follower, with QZ supplying current to the load via two parallel lQ resistors. Q3, LED 1 and the two parallel lQ resistors provide the overload protection feature. Nmmally, the voltage across the two lQ resistors will be MICA WASHER BUSH NUT \ \ :~~~ :s-1 scrw ~ DEVICE ' FINNED HEATSINK Fig.3: mounting details for the TIP41 transistor. Smear all mating surfaces with heatsink compound before bolting the assembly together, then use your multimeter to confirm that the metal tab of the transistor & the heatsink are correctly isolated. TO CONTROLLER Fig.4: if you don't already have a suitable power supply, this simple circuit will do the job. Use a power transformer with a 12V secondary that's rated at 60VA or more & take care with the mains wiring. RESISTOR COLOUR CODES D D D D 44 No. 1 1 2 SILICON CHIP Value 4-Band Code (5%) 5-Band Code (1%) 5.6kQ 470Q 1Q green blue red gold yellow violet brown gold brown black gold gold green blue black brown brown yellow violet black black brown brown black black silver brown DC supply. Most model enthusiasts will already have a suitable supply but if you don't, the circuit shown in Fig.4 will do the job. Construction There are a couple of choices when it comes to building this unit. Many modelling enthusiasts will prefer to retain their existing control console and so will bury the PC board under the layout. Others may want to fit VRl and the two switches into a small case to provide a walkaround throttle. This would then be linked to the PC board via a multi-way cable. Because each individual's requirements will vary, we'll simply show you how to assemble the PCB. Fig.2 shows all the details. You don't have to follow any particular order when installing the parts on the board but it's generally easier if you mount the smaller components first. Many of the components are polarised so be sure to orient them exactly as shown in Fig.2. These include the transistors, diodes, LED and electrolytic capacitors. Q2 is installed with its metal tab towards the edge of the board. Check the resistor values with your digital multimeter before installing them on the .board. Alternatively, refer to the accompanying table to read off their values from the colour codes. We mounted the LED directly on the board but it could also be mounted in some other location and connected by flying leads if that's more convenient. Rainbow cable can be used to wire up LED 1, S1 and VRl but use medium duty hook-up wire for the connections to S2, the track and the power supply. The prototype used trimpots to preset the throttle and braking inertia but you can substitute a couple of full-size potentiometers if you wish. These could be mounted on the front panel and linked to the PC board via flying leads. Heatsinking Because it can be required to dissipate quite a lot of power, a substantial heatsink must be fitted to Q2 (TIP32). A commercial finned heatsink with a rating of 2°C per watt would be satisfactory or you could bolt it to a sheet of aluminium (about 200 square cm should be OK). In either case, it's advisable to iso- To keep it cool, the metal tab of the TIP41 transistor (Q2) should be bolted to a substantial heatsink. Use a mica washer & insulating bush to electrically isolate the transistor from the heatsink as shown in Fig.3. late the metal tab of Q2 from the heatsink using a mica washer and insulating bush to prevent accidental short circuits (see Fig.3 ). However, you can bolt the transistor directly to the heatsink provided you make absolutely sure that the heatsink touches nothing else. If you elect to mount the board in a metal case, the case itself can be used for heatsinking. Be sure to isolate the tab of Q2 from the case though, otherwise the supply will be shorted out. Note: the metal tab of the transistor is connected to its collector. Testing To test the unit, connect up a power supply and check that the track voltage slowly increases (or decreases) to a new value each time the throttle (VRl) is varied. You can do this by monitoring the output across DZ. If this checks out, check that LED 1 lights if you momentarily short-circuit the output. It should go out again when the short-circuit is removed. If you strike problems, first check the voltage across the 4700µF capacitor. No voltage here? - check that Dl is oriented correctly, that Sla is closed and that Slb is open. Ql and Q2 can be checked by measuring their baseemitter voltages. In each case, you should get a reading of about 0.6V (assuming that there's at least 1.2V on Ql's base to start with). IfLED 1 stays on, check DZ , the wiring to S2 and for track shorts. Finally, check that the output swaps polarity each time the forward/reverse switch (S2) is operated. Naturally, you should always make sure that the loco has come to a complete stop before operating this switch. Flicking this switch while the model is still moving will only lead to a derailment and could even damage the gearing. ~ Where to buy the kit A kit of parts for this project is available from Electronic Toy Services, PO Box 491 , Noarlunga Centre , South Australia 5168 (Shop 2/111, Glynville Drive, Hackham West, SA). This kit includes the PC board, all on-board components , the throttle control pot. and the two switches, but does not include a mains transformer or case. The price is $19.95 plus $2.50 p&p. Payment may be made by cheque or by phoning (08) 382 8919 and quoting a credit card number. Note: copyright of the PCB artwork associated with this project is retained by Electronic Toy services. NOVEMBER 1990 45