bertiedog Posted July 16, 2011 Share Posted July 16, 2011 After a discussion in another RM topic on the old Codar controllers, a modified circuit for one was published, but contained a LM 317T regulator to supply the Codar type circuit. Frankly this is not needed as the LM 317T Regulator IC can handle a plain DC control circuit and the circuit is shown below. For those modellers who are after a very cheap and simple DC speed controller this design is very hard to beat. It is feed by 16 volts DC, so needs a 9 volt transformer feeding 9 VAC to a bridge rectifier, which will give about 16 VDC . You can feed any voltage up to about 20 Volts input (it will take more).,but it must be DC., not AC 12 volts DC can be feed in but full power will not be obtained from the output. A 13.5 VDC CB radio power unit could be used, and recommended if not experienced with mains wiring. The power source should be rated at about 3 amps.approx., (gives a margin), a 36 watt rated transformer.will do feeding a bridge rectifier. If you feel uncertain about wiring up a mains transformer, then I suggest using a CB radio Power unit, (lots on Ebay). Most wall mart type 9-12 volt power units do not deliver enough power, but could be used, but you will not get full power output, which with the LM317 is 1.5 amps. The regulator itself can stand shorts etc, shutting down on overloads.It is pretty bullet proof. Rather than use a PC board I suggest using a 15 amp chocolate bar electric strip connector arranged as in the diagram. Where a group of wires are shown they all go into the hole together.(this is why a 15 amp one is used). There is some soldering, most modellers have soldering irons etc., but in this case a lot is supported by the chocky strip itself. The output is not 0 volt to 12 volt DC, but is 1.25 volt to 12 volt DC, so if 0 volt is required to run coreless motors etc, then put two 3 amp diodes(1N4003) in series in the + output lead. This can be done on the strip or in the lead to the reversing switch, (not shown in diagram.) The values of the capacitors is not critical, except the 0.1uf. The electrolytic capacitors can be any higher value, it barely makes any difference. They should be rated at 25 VDC or more. Make sure they are the right way round, + to + etc. The variable resistance should be 1000 ohms, any higher and the control range is cramped at the upper end. If more than 16 volts is applied as input, then RA (the variable should be increase to 1.5 K ohms approx.) The wires are connected to the centre terminal and one of the outer terminals, so that Zero is at the anti clock end, swap if wrong way round! Note that the layout design diagram shows the back view of the LM317. The bolt hole in the regulator can have a simple aluminium plate about 50mm x 50mm (2 x2) bolted firmly to it. There is now need to add heat sink compound, just make sure the hole is de-burred etc., you can add a drop of superglue to aid heat getting to the heat sink. The heat sink does not need to be bolted to anything, just in air in the case. If it has a mains transformer as a component rather than a power unit feeding it, then case in a proper instrument case, with earthed case, proper mains grommets and restraints etc, and proper mounting inside on insulators etc.with transformer in separated compartment inside. If built to work with a DC supply, external like a CB power unit, then the box can be wood etc., or plastic. or the whole unit will fit a handheld box, or could be panel mounted. RA can be mounted a few inches from the regulator at most. Make sure the cables used throughout are able to take at least 1.5 amps. Any standard centre off reversing switch can be used, two pole.type( i will add a diagram). Cost should be very low, under a fiver, Ebay do the bits and pieces, or Maplin, although a bit more expensive. The LM317T(plastic pack) is about 99p for 5, (I have just bought a pack),.the most expensive bit will be the 1000 ohm potentiometer (the variable resistance), and the switch, but again check Ebay, a lot of suppliers do free postage on electronics. At 1.5 amp output it will suit most users for single locos, double headed will not damage it, just a shut down. With the diodes added then all coreless motors will work fine. All round a general purpose DC controller. The design is not copyright, it is a generic circuit design based on freely published data sheets from the chip manufacturers.. Link to post Share on other sites More sharing options...
meil Posted July 17, 2011 Share Posted July 17, 2011 The problem with the circuit as shown is the residual 1.25v of the output. I suggest you add a couple of diodes to the output. This will also provide reverse current protection to the circuit. Link to post Share on other sites More sharing options...
bertiedog Posted July 17, 2011 Author Share Posted July 17, 2011 The problem with the circuit as shown is the residual 1.25v of the output. I suggest you add a couple of diodes to the output. This will also provide reverse current protection to the circuit. It is already in the post, two diodes in series, suggested as 1N4003, as these are beyond the rating by 100%, and cost the same as lower ratings. They are not required with older less efficient motors, most of which need about 2/3 volts to move off under load. An extra to consider is a LED lamp across the output, with a 470 ohm resistor to limit it for 12 volts. This will indicate from a fairly low setting,and go out on a short condition on medium to high output. it is added before the reversing switch. It should be connect before the limiting diodes. A different coloured LED can be added to show the power is connected to the input if you like. Stephen. Link to post Share on other sites More sharing options...
bertiedog Posted July 17, 2011 Author Share Posted July 17, 2011 The circuit with diodes and Leds(optional). The diodes are 1n4003, 3amp, over rated, but they are the same price as 2 amp type. The LEDs will glow at a fairly low setting. If the input is above 12 volts, then change to 700 ohms or more for the input LED only. Stephen. Link to post Share on other sites More sharing options...
bertiedog Posted July 17, 2011 Author Share Posted July 17, 2011 Wiring the reversing switch, centre off type 2 pole. Feed power in to the centre wipers, and as the switch moves it feeds it one way or the other way round, simple? Stephen. Link to post Share on other sites More sharing options...
Gordon H Posted July 20, 2011 Share Posted July 20, 2011 The diodes are 1n4003, 3amp, over rated, but they are the same price as 2 amp type. For 3A rating I would suggest one of the 1N540x range, such as the 50V 1N5400. The 1N4003 is only rated at 1A, like all the 1N400x range. Link to post Share on other sites More sharing options...
bertiedog Posted July 20, 2011 Author Share Posted July 20, 2011 For 3A rating I would suggest one of the 1N540x range, such as the 50V 1N5400. The 1N4003 is only rated at 1A, like all the 1N400x range. Yes it should be the 1n5400, sorry did not notice the mistake, any higher rating will do. Stephen. Link to post Share on other sites More sharing options...
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