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Design by BARRY GRIEGER Part 3: Power Station & Command Station Wiring Last month we covered the description of the Command Station which is really the encoder portion of the system. This month we describe the Power Station which takes the signal from the Command Station and feeds it to the track. Design by BARRY GREIGER I N ESSENCE, the Command Sta- tion is just a big power supply and it is modulated with the block signals from the Command Station (encoder). Fig.1 shows the circuit. It uses a 30V centre-tapped trans­former to drive a 10A (or 35A) bridge rectifier BR1. This feeds an 8000µF 75VW chassis mount electrolytic capacitor (C1). By the way, we specified the capacitor voltage at 80V, not because the voltage is high but because the capacitor needs to be physically large in order to give it an Run your model railway with Command 54  Silicon Chip Fig.1: the power supply is essentially an adjustable 3-terminal regulator which is modulated by the 5V signal from the encoder board. adequate ripple current rating. Ripple current is the AC current which flows through the capacitor as a result of it filtering the raw (un­ smoothed) DC down to smooth DC. As a rule of thumb, the ripple current in the filter capacitors of a DC supply such as this is roughly equal to the DC current drain. So if we require 5A DC we need filter capacitors with a total ripple current rating of at least 5A. The transformer is rated at 100VA and so it should easily be able to de­ liver up to 5A DC to the track. The DC voltage across C1 is about 21-23V DC, depending on the actual value of the 240VAC mains supply and the loading caused by the various locomotives on the track. Now, to get back on track with this article (pun intended), the smoothed DC from capacitor C1 is fed to an LM338K adjustable 3-terminal regu­ We used this large plastic instrument case to accom­modate both the Command Station (encoder) and Power Station components. The front panel carries the various indicator LEDs. Control March 1998  55 Fig.2: component layout for the Power Station PC board. Take care with parts orientation. lator. This is controlled by a circuit consisting of two NPN transistors (Q4 & Q5) and the associated resistors. The two transistors act to modulate the output of the ad­justable regulator, switching it between 11V DC and 16.9V DC, with the 5.9V difference Below: a large finned heatsink is required for the LM338K adjustable regulator. Note that this heatsink must be fully isolated from the heatsink using an insulating washer and TO-3 mounting kit. 56  Silicon Chip Fig.3: actual size artwork for the Power Station PC board. being the serial data stream from the Command Station (encoder). Q4 & Q5 work in the following way: When the signal from the Command Station is low, transistor Q4 is off and therefore Q5 is turned fully on, by dint of the 10kΩ resistor R3. With Q5 fully on, the voltage drop between its collector and emitter will be around 100mV or less and so the voltage de­ livered by the LM338K will be largely determined by trimpot VR1 and the resistor network attached to the ADJ termi­nal. During the setup procedure, VR1 is set so that the output from REG1 is 11V. When the signal from the Command Station is high, Q4 is turned on and Q5 is turned off. With Q5 turned off, trimpot VR1 is effectively out of cir­ cuit, since no current flows through. In this condition, REG1’s output is close to 16.9V, as set by the ratio of resistors R5 & R6. The average voltage from REG1 is effectively about half way between 11V and 16.9V and therefore lies around 14V DC. The light emitting diode LED7 func­ Fig.4: this diagram shows the wiring details for the Command Station and Power Station circuitry. Take care with the mains wiring and sleeve all exposed mains connections with heatshrink tubing. tions as a track power indicator while the 1kΩ 1W resistor R7 is there merely to ensure a minimum load current on REG1. The 30V zener diode (ZD1) across the output is to help prevent commutator hash from locomotive motors from upsetting the regulator in any way. Construction In presenting this project we are conscious that each model railway enthusiast will have his (or her) pre­ ferred way of in­stalling the system within the layout. If it is a small lay­ out, only one power station is likely to be required. On the other hand, if it is a large layout, more power stations March 1998  57 This inside view shows the almost completed prototype, without the wiring from the Command Station module (lower right) to the various hand controllers (see Fig.4 for details). will be re­quired. The Command Sta­ tion presented last month has outputs to drive four power stations. Each power station would be con­ nected to a portion of the layout and each track section would be isolated from the others by insulators in the tracks. When a locomotive crosses between sections, the relevant power station outputs would be connected together via the locomotive’s wheels and chassis. However, this would not represent a fault condition since each power station would put out the same track voltage. All of this amounts to a restatement of the operating con­cepts outlined in the first article in this series, in the Janu­ ary 1998 issue. While all that may be clear and obvious, it does present a problem of presentation in the magazine. Do we present a version which would be applicable to a small layout or a large layout? Our answer is 58  Silicon Chip to present it in a way suited to a small layout; ie, with the Command Station driving one Power Station. From this we then proceed to the logical decision to in­stall the Power Station and Command Station in one case, as our photos show. This leads to a much more professional approach than is used by many model railway hobbyists. In particular, many mod­ ellers are very casual about power supply construction and wiring. They often have a largish power transformer and the rest of the circuitry laid out on a piece of timber or sheet metal. Often there is no cover, with the whole assembly sitting on the floor underneath the layout. You don’t need to be a rocket scientist to work out that this is an accident waiting to happen but it is quite common­place. So rather than present this project with the Command Sta­tion and Power Station is separate cases, we are pre­ senting them both in the one large case. This is neat, safe and cheaper in the long run. We selected a large plastic instru­ ment case from Altronics (Cat H-0490). It measures 355mm wide, 122mm high and 250mm deep. It has plenty of room inside for all the power supply (Power Station) components and the encoder PC board (Command Station). Actually, there’s probably enough room in this case for two Power Sta­ tions, if you wanted to do it that way. The front panel is bare except for seven LEDs. Four of these come from the encoder board and indicate that the main clock is working (see circuit description last month). The other three LEDs are the track power indicator (LED7) referred to above and indicators for the +12V and +5V rails. To be frank, only the track power indicator (LED7) is really necessary but a few LEDs do add interest, don’t they? On the aluminium back panel, there is an IEC mains power socket, Fig.5: the mounting details for the LM338 3-terminal regulator. the on/off switch, the finned heat­ sink for the 3-terminal regulator and a pair of binding post terminals for connections to the track layout. There are also two access holes for cables to the throttle panel and to other Power Stations, if they are required. Inside the case there is a steel base­ plate which mounts the power trans­ former, the bridge rectifier and other hardware and it also makes it easy to properly earth these components. The small components of the power supply shown in Fig.1 are mounted on a PC board which measures 65 x 48mm (code 09103981). It is shown in Fig.2. You can see that we have made provision for an onboard filter capac­ itor but this is not used in the version we are presenting here. The capacitor would only be used if a smaller ver­ sion of the circuit without the large 8000µF capacitor was envisaged. So in effect, the PC board layout of Fig.2 shows only two transistors (Q4 & Q5), the trimpot VR1 and a few resistors. It will only take a few minutes to solder all the necessary compon­ents and the PC pins or stakes onto the board. When the board is assembled, you will need to start work on the case. You will need to drill seven holes in the front panel to accommodate the LED bezels. On the rear panel, there will be cutouts for the on/off rocker switch and the fused IEC power socket and various holes for the heatsink, binding post termi­nals, earth solder lug and so on. The steel baseplate will need to be drilled to take the power transformer, bridge rectifier, main filter capacitor (8000µF) and the two PC boards. The mounting positions of these compo­ nents are not critical and you can work out where you want to put them by reference to the photos. You will also need a hole in each corner of the baseplate for a screw into an integral pillar on the case bottom section. Fig.4 shows how all the connec­ tions are made between the two PC boards and the rest of the power supply components. We have specified a fused IEC power socket which makes the wiring easier to and safer, into the bargain. All the mains wiring should be run in 250VAC-rated hookup wire and all exposed connections should be sheathed in heatshrink tubing. The Earth wire from the IEC socket should be connected to an adjacent solder lug, as shown in Fig.4. All the connections to the Com­ mand Station (encoder) board and the Power Station board can be run in light-duty hookup wire. Do not make the hookup between the encoder and signal input to the Power Station board until you have made the first voltage check. Note that we have in­ cluded a series 56Ω 5W wirewound resistor in the 23V power connection to the encoder board. This reduces the power dissipation in the on-board reg­ ulators, without otherwise affecting the performance. When all the wiring is complete, check your work carefully and apply power. You should then be able to measure about 23V DC across the main filter capacitor C1. Check the outputs of the 12V and 5V regulators on the encoder board. Next, set trimpot VR1 on the Com­ mand Station board so that the output of REG1 is 11V. Then connect the signal input of the Command Station board to the +5V rail on the encoder board. The output of REG1 should now be close to +16.2V. Disconnect the +5V rail from the signal input and connect it instead to the S1 output on the encoder board connector strip. You are now ready to make up handheld throttles. We’ll discuss that next month. By the way, some readers have expressed concern about the ZN409CE encoder ICs. These will be available soon from Jaycar Electronics SC (Cat ZK-8827) at $29.95 each. Parts List For Power Station 1 instrument case, 355mm x 250mm x 122mm (Altronics H-0490) 1 steel baseplate (Altronics H-0492) 1 aluminium back panel to suit case 1 250VAC rocker switch (DSE Cat P-7700) 1 IEC chassis-mount socket with fuse holder (Altronics P-8324) 1 IEC mains cord (Altronics P 8410) 1 20mm 500mA fuse to suit IEC socket 1 100VA 15V-0-15V power transformer (Altronics M-2170) 1 8000µF 75VW chassis mount electrolytic 1 single-sided finned heatsink, 110mm x 33mm x 72mm (Altronics H 0560) 1 TO-3 insulating kit (Altronics H 7200) 1 red binding post 1 black binding post 1 16-pin DIL IDC line plug 1 1m length 16-way IDC ribbon cable 1 grommet to fit 12.7mm mounting hole 1 PC board, 65 x 48mm (09103981) 7 panel mount 5mm LED bezels 6 PC stakes 3 solder lugs 6 M3 screws 10mm long 11 M3 screws 15mm long 17 M3 nuts 2 M4 screws 10mm long 1 M4 screw 20mm long 1 1kΩ horizontal mount cermet trimpot Semiconductors 1 bridge rectifier 400V, 10 or 35A, (BR1) 1 LM338K adjustable positive regulator (REG1) 1 30V 5W zener diode (ZD1) 2 PN100 NPN transistors (Q4,Q5) 1 5mm red LED (LED7) Resistors (1% or 5%, 0.25W) 2 10kΩ 1 1kΩ 1 2.2kΩ 1 820Ω 1 1.5kΩ 1 120Ω 1 1kΩ 1W 1 56Ω 5W wirewound March 1998  59