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A Capacitor Discharge Unit for twin-coil points motors Got a model railway? If it is not just a simple loop of track it is bound to have one, two or maybe dozens of sets of points. That means you need at least one Capacitor Discharge Unit (CDU) to power them. Most layouts can make do with just one CDU but this unit is so cheap you might want to have several. T his Capacitor Discharge Unit, or CDU, is designed to drive the twin-coil snap-action points motors which are widely used on the majority of model railway layouts. These have the virtue of being cheap and easy to install under each set of points. In action, if one coil (more correctly a solenoid) is energised, the points move across to favour one direction for the on-coming loco. If the other coil is energised, the points move across in the other direction. Many rail enthusiasts energise these point motors by simply connecting the two coils to a 15V (or thereabouts) DC or AC supply via momentary contact pushbuttons. Briefly pushing one or other of the buttons operates the points. Simple. The big disadvantage of that method is that if you press the button for too long or the button becomes jammed by something or someone leaning on, the respective coil will burn out. Why? Because its resistance is only about 4.7Ω and it is wound with many turns of fine wire which simply cannot withstand the resultant dissipation of 40 watts or more for more than a second or two. This is where the CDU comes in. It has a large capacitor which is charged from the 15V supply and then when one or other of the pushbuttons is pressed to energise one of the coils, it delivers a brief pulse to do the job and no damage can result if the pushbutton is pressed for longer than need be. Now this CDU is being presented as a companion unit to the Automatic Points Controller in this issue but it can be used independently on any lay68  Silicon Chip This twin-coil points motor can be actuated manually (via the lever) or electrically. This simple project is designed to make the latter as foolproof as possible. out where points are being employed. The CDU is housed on a small PCB which can be located in a convenient position underneath the layout. The circuit The circuit is shown in Fig.1. It consists of a small NPN power transistor, two 2200µF 25V capacitors and not much else. It works like this. Whenever the circuit is connected to the 15V supply (which may be DC or AC) current flows via diode D1 to the collector of NPN transistor Q1. Q1 is biased on by the 1kΩ resistor between its base and collector. While ever Q1 is turned on, it acts to charge the two 2200µF capacitors. Once they are charged, the current through Q1 is quite low, due to the by Jeff Monegal leakage of the capacitors themselves and the current through LED1 which indicates that the unit is active. When one of the pushbuttons is pressed, the capacitor charge is dumped via diode D3 to the respective solenoid coil, energising the points motor in one direction or the other. D3 can easily withstand the brief pulse of current which is likely to be no more than 3A peak. Diodes D2 & D3 act to suppress any back-EMF spikes which could possibly occur if the pushbuttons have contact bounce. Normally of course, the pulse current will die away quickly while you hold the button down for a second or two and no back EMF spike should be generated If you keep the pushbutton depressed for longer, no damage can result since the base of Q1 is effectively siliconchip.com.au K A 1k 0.5W B C Q1 TIP41 D3 1N4004 E A 1k 2x 2200F 0V D2 +V D3 4004 2200F 2200F .U.D.C A  D2 1N4004 MWJ A CAPACITOR DISCHARGE UNIT grounded via the respective solenoid coil, keeping Q1 turned off. Once the pushbutton is released, Q1 is biassed on again via the 1kΩ base resistor and the capacitors are quickly recharged, ready for the next points operation. Note that this CDU can power multiple sets of points. Each twin-coil points motor is wired to the CDU via a 3-way ribbon cable and two pushbuttons. PCB assembly The CDU circuit components fit on a small PCB measuring 69 x 41mm. Assembly is straightforward but remember that all components, except the two resistors, are polarised and must be installed as shown on the overlay diagram in Fig.2. LED1 K SCINORTCELE YELTAO CAPACITOR DISCHARGE POINT MOTOR DRIVER TIP41 LED SC OUTPUT Q1 TIP41 1k 0.5W TWIN COIL POINT MOTOR K 2013 4004 K A LED CDU OUT K D1 INPUT 0V 1N4004 1k 12-15V AC or DC K 4004 A 0V 12-15V D1 1N4004 K A C B C Fig.1 (left): the circuit diagram of the capacitor discharge unit shows it is basically a couple of capacitors and a switching transistor. Above (Fig.2) is PCB component overlay. It’s simple enough – but watch component polarity! E After double checking that you have all components in the correct position and the correct way round you can apply a DC power supply of around 12-15V DC or AC to the power in terminals. The project is polarityprotected by diode D1 so if you connect the supply the wrong way nothing will happen. But if all is well, the LED will come on shortly after power is connected. Using a twin-coil snap-action points motor and some hookup wire, join the centre terminal of the points motor to either output terminal. Using another length of hookup wire with one end connected to the other output terminal touch the free end onto either of the other two terminals of the points motor. The motor should snap in one direction or the other. At the same time the LED should go out but then come back on within a few seconds. Try again with the other points motor terminal but this time leave the hookup wire connected. There should be very little (a few mA) load on the power supply. Because the transistor is held off while ever the points motor is connected across the output, no current should flow. When the hookup wire is removed current should briefly flow again to charge up the capacitors, SC ready for the next application. Parts List – Model Railway Capacitor Discharge Unit 1 PCB measuring 69 x 41mm 3 1N4004 power diodes 1 TIP41 NPN power transistor 1 5mm LED (any colour) 2 PCB-mount 2-way connectors 1 1kΩ 1/2W carbon resistor 1 1kΩ 1/4W carbon resistor 2 2200µF 25V electrolytic capacitors Currently the PCBs for this project can be purchased at the Silicon Chip website for $15.00 ($13.50 for magazine subscribers), directly from here: http://www.siliconchip.com.au/ Shop/8/1940 Here’s what the Capacitor Discharge Unit looks like when. assembled. The LED can be mounted remotely if it’s more convenient – otherwise, it’s a cinch to put together! siliconchip.com.au All enquires for this project should be directed to the designer, Jeff Monegal. He can be contacted via email only. (jeffmon<at>optusnet.com.au) All emails will be replied to but please allow up to 48 hours for a reply. March 2013  69