Doughnut Posted February 16, 2013 Share Posted February 16, 2013 I'm trying to scratch build a Feather route indicator, with two arms, in the prototypical V-formation. When a route is to be shown, a set of 5 LED s are lit, either the left or the right set of 4 LED's and the bottom-central LED. This means that the bottom-central LED is shared by the two arms, left and right, Now I would like to be able to light the left hand set and the central LED or the right hand set and the central LED, separately, but electrically i'm having problems with this. (See Diagram) All the LEDs have been wired with a common Anode, the cathode being switched. I have used 2 diodes connecting the cathode of the 4 LED sets to the central LED, to prevent all the LEDs from lighting when current flows. Unfortunately, the set of 4 LEDs light, but not the central LED, fed via the diode. The diodes are positioned facing the correct way, so this is not the problem, but it seems the diode creates a little resistance, encouraging the current to flow only to the set of 4 LEDS and by-passing the central LED ?? Could this be the case and if so is there a solution. Forgive me if I havent described this that well. Many thanks in advance for any advice. Dave K Link to post Share on other sites More sharing options...
cpman46 Posted February 16, 2013 Share Posted February 16, 2013 Dave A couple of questions. What supply voltage are you using? Your diagram appears to show the LEDs wired in parallel but I can't see a current limiting resistor or resistors? It is not good practice to wire LEDs in parallel with a single current limiting resistor since differences in each LEDs charcteristics will mean one hogs most of the current and therefore they will be of different brightness (LEDs in parallel should have individual current limiting resistors). Since the bottom central LED is used by both arms it doesn't need switching and can be left permanently illuminated, you only need to switch the set of 4 lefthand or righthand LEDs. If you are using a 12V dc supply I would suggest wiring the 4 LEDs in each arm in series via a suitable current limiting resistor to the +12V supply and switching between them using a single pole double throw SPDT toggle switch. If you need further info or a rough diagram let me know. Link to post Share on other sites More sharing options...
Edwin_m Posted February 16, 2013 Share Posted February 16, 2013 You can't leave the bottom centre LED (known as the pivot light) lit all the time, as it needs to turn off when the signal is red or if there is a "straight on" route that doesn't light either indicator. Seeing as you have the four LEDs lighting OK then you'll probably be all right to leave these - as Mike says LEDs in parallel is bad practice but you can often get away with it provided the LEDs are well matched (and presumably these are identical and from the same batch). The pivot light isn't lighting either because (as you suggest) the diode adds some more voltage drop and the electrons find it "easier" to go through one of the others, or because the supply voltage is too low to overcome the voltage drops in both the LED and the diode (you probably need at least 6V). However you are putting whatever your supply voltage is directly across the LEDs, which could cause them to fail. So if you don't fancy rebuilding the signal it should be OK to stick with your circuit but add a resistor to each of the three wires. If you're feeding with 12V I'd suggest 1k for the pivot light and about 200 ohms for each group of four lights. Increase the resistance if they are too bright or decrease for a lower supply voltage but keep one resistor about five times the value of the other two. Link to post Share on other sites More sharing options...
Doughnut Posted February 16, 2013 Author Share Posted February 16, 2013 Many thanks Mike. Yes, I am using 12v DC. I should have shown a single 1k resistor on the Positive supply wire, which I understand is not the best arrangement as you say, for leds in parellel, but it seems to work ok for now. For any future arrangement, I will follow your advice and wire in series though. Prototypical operation. I think, involves all of the leds being out when the feather is not in use and then 5 being lit. So the central bottom LED is only on when one of the sets of 4 is on. So when the left hand 4 are on so is the central bottom to make 5 and the same thing with the right hand feather. But all LEDs will be out when the feather is not in use. Link to post Share on other sites More sharing options...
cpman46 Posted February 16, 2013 Share Posted February 16, 2013 I forgot to ask what colour LEDs you are using? White LEDs have a considerably higher forward voltage drop than other LEDs and therefore 12V may not be enough to power 5 connected in series. Another thought - as you need to switch all LEDs out, one way would be to use a centre off toggle switch (on-off-on) and wire the central bottom LED of the feather between the centre contact of the switch and the 0V supply rail. With the switch in the centre off position no LEDs are lit but selecting either right or left feather will light the appropriate 4 LEDs plus the central bottom LED. Similarly using a rotary switch, using one pole and 3 ways you could achieve the same result. Link to post Share on other sites More sharing options...
Doughnut Posted February 16, 2013 Author Share Posted February 16, 2013 Yes thought as much Edwin. The LEDs are white Mike. I like the sound of your last suggestion with the on-off-on switch or rotary. Any chance of a diagram? Link to post Share on other sites More sharing options...
cpman46 Posted February 16, 2013 Share Posted February 16, 2013 Dave Will post a diagram tomorrow as I'm about to log off. Is it possible you can let me know the type number of the LEDs you're using or supplier code or other info. It will give me a better chance of creating the appropriate diagram and connections. Cheers Link to post Share on other sites More sharing options...
Doughnut Posted February 16, 2013 Author Share Posted February 16, 2013 Yes, didn't realise the time. I am using 9 No. Kingbright KPT-2012PWF-A white Ultral low profile surface mounted LEDs. Many thanks. Dave K Link to post Share on other sites More sharing options...
richard brown Posted February 17, 2013 Share Posted February 17, 2013 I know this might be mad but why not use a 2 pole swich so when you set the left feather it also light the shared one and then when you light the other it also light the shared one you will also need to isolate the shared light Richard Link to post Share on other sites More sharing options...
cpman46 Posted February 17, 2013 Share Posted February 17, 2013 Dave Back online, Sorry for the delay. Thank you for the info on the LEDs. The data sheet shows a forward voltage drop of about 3.3V at 10mA current flow. I am concerned that at that forward voltage drop per LED my suggestion of LEDs in series will not work since you only have 12V available and my circuit would effectively put 5 LEDs in series giving rise to a total Vf of 16.5V. I assume you are builiding in '00' and I wonder how many wires you could accommodate in the signal as I would still prefer to run the LEDs in some sort of series formation rather than as at present 4 in parallel. At the moment your circuit shows 4 wires, could you fit 5 or 6 at a pinch? Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted February 17, 2013 RMweb Premium Share Posted February 17, 2013 This is how I would wire it: A and B go to the switch. C goes to the series resistor. (not in signal) As stated by others you would need at least 18v DC supply (allowing for the series resistance), but there would only be 3 wires to the signal feathers. By making the bottom led common to both chains the problem of having it on for either direction is solved. Sorry for the crappy diagram but it only took a couple of minutes to knock up! Keith Link to post Share on other sites More sharing options...
Doughnut Posted February 17, 2013 Author Share Posted February 17, 2013 Keith, Is this what you mean? If so, would there not be crossover current between the two arms, where they join at the pivot light? Or would the series arrangement prevent this? I think you might be on to something there though. Dave K Link to post Share on other sites More sharing options...
AMJ Posted February 17, 2013 Share Posted February 17, 2013 Might be an idea to have micro switches on the points to provide the routing as required for the feathers. Thus have the feathers switch deppendig on the track rather than a manual switch. Link to post Share on other sites More sharing options...
Edwin_m Posted February 17, 2013 Share Posted February 17, 2013 You might be able to power five LEDs in series if you have a 15 volt AC supply available. Connect one side of the supply through a diode and a 100 ohm resistor to the positive of a 35V electrolytic capacitor and the negative of the capacitor to the other side of the supply. As long as you only draw a small current the cap will charge up to close to the peak voltage of the supply so you should get up at least 20 volts. Link to post Share on other sites More sharing options...
cpman46 Posted February 17, 2013 Share Posted February 17, 2013 Dave Is your 12V supply fixed or do you have access to a 15 to 16V ac supply? If so it would be simple enough to generate a dc supply of about 18V which would allow you to use Melmerby's circuit above and control it either with a centre off switch or microswitches operated by the turnouts as suggested by AMJ. I can generate a suitable diagram for that but if the 12V is fixed then I can generate a suitable diagram using 5 connections, both will address the switching off of the pivot LED when neither route is set. Link to post Share on other sites More sharing options...
richard brown Posted February 17, 2013 Share Posted February 17, 2013 Hi I have done a quick testhere are the pitures the first one is the 2 pole rotary swich power gose in both pole in the in as you can see I have fixed them with some wire then out put 1 goes to the left lights 2 gose to the right light the 7 and 8 get the shared light In my piture left and right are the trafic becons and the shared one is the drink machine. This dose work Richard Link to post Share on other sites More sharing options...
Doughnut Posted February 17, 2013 Author Share Posted February 17, 2013 Mike, Yes I do have a 16v ac supply. I only want to control the feather manually (Toggle or Rotary switch), so won't be needing AMJ's microswitch arrangement,(but much appreciated) I would be interested in the 12v diagram also, with 5 leads. Dave K Link to post Share on other sites More sharing options...
Brian Posted February 17, 2013 Share Posted February 17, 2013 Hi I would wire a five light double J/I like this..... How you actully switch the LEDs is up to you - centre off toggle switch, rotary switch, point operated switches etc But they need to have of some form of simple interlock so as both cant be On together. e.g a SPDT On-Off-On toggle switch or Break before Make rotary switch would provide this Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted February 17, 2013 RMweb Premium Share Posted February 17, 2013 Keith, Is this what you mean? If so, would there not be crossover current between the two arms, where they join at the pivot light? Or would the series arrangement prevent this? I think you might be on to something there though. Dave K Hi I would wire a five light double J/I like this..... How you actully switch the LEDs is up to you - centre off toggle switch, rotary switch, point operated switches etc But they need to have of some form of simple interlock so as both cant be On together. e.g a SPDT On-Off-On toggle switch or Break before Make rotary switch would provide this Exactly so. Keith Link to post Share on other sites More sharing options...
cpman46 Posted February 17, 2013 Share Posted February 17, 2013 Dave since you have a 16V ac supply Brian's circuit is the way to go. Yuo keep wiring to a minimum. If you don't want to use a bridge rectifier a cingle 1N4002 diode will achieve the same result. Use a SPDT Centre off switch or a 1 pole 3 way rotary switch to control the feather i.e. left off-right. My five wire circuit is overly complex. Keep it simple and use your 16V supply. Link to post Share on other sites More sharing options...
Doughnut Posted February 17, 2013 Author Share Posted February 17, 2013 I will go with that layout. Many thanks Mike, Brian, Keith, Richard & Edwin; you've all been very generous with your great advice. Dave K Link to post Share on other sites More sharing options...
RMweb Gold The Stationmaster Posted February 17, 2013 RMweb Gold Share Posted February 17, 2013 As a matter of interest how do the JIs drive the signal - or don't they? Link to post Share on other sites More sharing options...
Suzie Posted February 18, 2013 Share Posted February 18, 2013 White LEDs from the same batch appear to be well enough matched to wire in parallel - you soon know if there is a problem because they will have different brightness. Conventional thinking suggests that directly paralleling LEDs is a bad thing, but it is not the problem that it used to be and is done quite often nowadays If you have a 1K feeding four in parallel, you will need a 3K9 or 3K3 in series with the fifth one and it's diode to give similar brightness. Link to post Share on other sites More sharing options...
28XX Posted February 18, 2013 Share Posted February 18, 2013 For those who are want to do this or something similar, but don't have a suitable supply there are IC's that will generate a small amount of current at a higher voltage than they are supplied with, eg http://uk.farnell.com/intersil/icl7660scpaz/ic-volt-converter-7660-dip8/dp/9663711 Link to post Share on other sites More sharing options...
RMweb Gold beast66606 Posted February 18, 2013 RMweb Gold Share Posted February 18, 2013 Remember The JI should light before the main aspect changes from red. The 4 "extra lamps" in a JI are equi-distance apart but the pivot light is slightly further. Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.