NINJA Posted June 25, 2020 Share Posted June 25, 2020 I want to install some LED's on my hidden sidings trackwork to show if that particular length of track is energised. Do all LED's have to have resistors when worked of 12 V DC? Thanks. Link to post Share on other sites More sharing options...
Yardman Posted June 25, 2020 Share Posted June 25, 2020 Yes, Some LED's have resistors built in and are known as 12volt LED's , 5volt LED's etc. but a standard LED needs a series resistor. A 1k or 2k resistor is a good ball park figure to start with. If you are just putting them acrross the track you will also need a series Diode to prevent reverse voltage from destroying your LED's. I take it you are on analogue control? The LED's will only work when the track polarity is the right direction. You can just use a lamp. 1 Link to post Share on other sites More sharing options...
NINJA Posted June 25, 2020 Author Share Posted June 25, 2020 Thanks Yardman Using a lamp may be the simple solution. Link to post Share on other sites More sharing options...
PatB Posted June 26, 2020 Share Posted June 26, 2020 (edited) A pair of LEDs wired back to back (you can use a single resistor) will give you light whatever the polarity. Use different colours and you get to see which direction your track is energised too. Will reverse voltages really destroy LEDs? They are, after all, diodes in and of themselves. Edited June 26, 2020 by PatB Link to post Share on other sites More sharing options...
RMweb Gold Roy Langridge Posted June 26, 2020 RMweb Gold Share Posted June 26, 2020 2 hours ago, PatB said: Will reverse voltages really destroy LEDs? They are, after all, diodes in and of themselves. Yes, if the reverse voltage exceeds what is called the breakdown voltage. Roy Link to post Share on other sites More sharing options...
PatB Posted June 26, 2020 Share Posted June 26, 2020 3 hours ago, Roy Langridge said: Yes, if the reverse voltage exceeds what is called the breakdown voltage. Roy True, but my empirical experience suggests that either manufacturers' quoted reverse breakdown voltages seem to be quite conservative and/or the necessary current limiting resistor provides some protection. Assuming maximum 12V track voltage, I'd be inclined to risk it for the sake of reduced component count. Link to post Share on other sites More sharing options...
Il Grifone Posted June 26, 2020 Share Posted June 26, 2020 7 minutes ago, PatB said: True, but my empirical experience suggests that either manufacturers' quoted reverse breakdown voltages seem to be quite conservative and/or the necessary current limiting resistor provides some protection. Assuming maximum 12V track voltage, I'd be inclined to risk it for the sake of reduced component count. It rather depends on how difficult it would be to replace a failed LED. Suitable diodes cost next to nothing. Link to post Share on other sites More sharing options...
PatB Posted June 26, 2020 Share Posted June 26, 2020 Just now, Il Grifone said: It rather depends on how difficult it would be to replace a failed LED. Suitable diodes cost next to nothing. Also true. I just often seem to not have any to hand when I need one and I don't fancy a 60km round trip to the shop, or a weeks wait for an online order. It's a reflection on my engineering, and former bush-mechanic, background that I tend towards the "what can I get away with? " approach, rather than the "what would be ideal? ". Link to post Share on other sites More sharing options...
RMweb Gold Roy Langridge Posted June 26, 2020 RMweb Gold Share Posted June 26, 2020 52 minutes ago, PatB said: Also true. I just often seem to not have any to hand when I need one and I don't fancy a 60km round trip to the shop, or a weeks wait for an online order. It's a reflection on my engineering, and former bush-mechanic, background that I tend towards the "what can I get away with? " approach, rather than the "what would be ideal? ". Whereas as I come from being trained as a electronics engineer on Military equipment, as everything is done "to spec" . A bog standard "get them off ebay" type led will typically have a reverse voltage of 5V. 12V is obviously way above that and I would not take the risk, they have been known to blow even with a resistor limited the current to 1 milliamp. If you want to avoid having a diode to protect against reverse voltage, I would get 12V reverse voltage LEDs to start with. Roy 1 Link to post Share on other sites More sharing options...
Il Grifone Posted June 26, 2020 Share Posted June 26, 2020 It would be cheaper to order the LEDs and 1N4007 diodes at the same time. The latter cost about £1 for 100 so not expensive and have lots of uses around the layout - ensuring the right headlamp comes on for instance. Their rating of1A means they are quite robust enough for general use. I have been involved with military equipment - a PITA! Link to post Share on other sites More sharing options...
Grovenor Posted June 26, 2020 Share Posted June 26, 2020 Get two wire bi-colour LEDs then they work on either polarity and also show you the polarity. Just put in series with resistors as above 1K will be OK. https://www.rapidonline.com/kingbright-l-57egw-5mm-red-green-led-bi-colour-55-0172 1 Link to post Share on other sites More sharing options...
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