dasatcopthorne Posted October 8, 2012 Share Posted October 8, 2012 Hi Guys. Have got one of the above LED lights to fit on layout. They supply a resistor 1K for 12 volt DC opperation. I'd like to pick up the DCC track voltage from our NCE equipment (18ac) to light these. Is there anyone on here that can advise how to do so please? Cheers Dave Link to post Share on other sites More sharing options...
RMweb Premium newbryford Posted October 8, 2012 RMweb Premium Share Posted October 8, 2012 It'll work from the DCC supply - maybe use a larger resistor 1.5K or so. Red LED's should be fairly tolerant of the reverse voltage of the AC nature of the DCC supply, but I would be tempted to put a 1N4001 diode or similar in reverse parallel with the LED to protect it over the longer term. I learned the hard way with white LED's........ (I needed two white LED's so I wired them in reverse parallel with one series resistor) Cheers, Mick Link to post Share on other sites More sharing options...
Phil S Posted October 9, 2012 Share Posted October 9, 2012 Surely, not in 'reverse parallel', but simply in series with the LED and suitable resistor - with a 50% duty cycle, this might not need changing - but you'll need to judge this yourself on whether the LED is too bright for you, or not. Simply made by soldering the resistor to one leg, and diode to the other (getting its polarity correct, or no light) I typically aim for about 5mA, as modern LEDs are quite bright. If you have several buffer stops, then half connected one way, and the other half connected the other way, will ensure equal current taken form each half cycle of the dcc (or ac, if running off a 16Vac feed .... I use Buffer Stop lamps, and lamposts etc as Monitoring devices to show that a local power supply is present: whether dcc on track, dcc on accessory bus, dc power to nearny modules, or 16Vac to accessory modules, Link to post Share on other sites More sharing options...
RMweb Premium newbryford Posted October 9, 2012 RMweb Premium Share Posted October 9, 2012 Surely, not in 'reverse parallel', but simply in series with the LED and suitable resistor -- with a 50% duty cycle, this might not need changing The D in LED is diode, so it doesn't need another in series - all it will do is reduce the voltage slightly. It's not about duty cycle, it's about the negative voltage across the LED. The reverse parallel diode is to allow the alternate cycle to bypass the LED, thus protecting it from potential damage. Look in the spec sheets for "reverse voltage" - typically very low for LED's. Exceed that and you're asking for trouble. Normal semiconductor diodes (1N4001 or similar) have much higher reverse voltage capability. Cheers, Mick Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted October 9, 2012 RMweb Premium Share Posted October 9, 2012 The D in LED is diode, so it doesn't need another in series - all it will do is reduce the voltage slightly. It's not about duty cycle, it's about the negative voltage across the LED. The reverse parallel diode is to allow the alternate cycle to bypass the LED, thus protecting it from potential damage. Look in the spec sheets for "reverse voltage" - typically very low for LED's. Exceed that and you're asking for trouble. Normal semiconductor diodes (1N4001 or similar) have much higher reverse voltage capability. Cheers, Mick But the normal way of connecting LEDs to AC is with a series diode, which has a very high reverse voltage rating (and resistance) and will prevent the LED experiencing any reverse voltage. Keith Link to post Share on other sites More sharing options...
RMweb Premium newbryford Posted October 9, 2012 RMweb Premium Share Posted October 9, 2012 But the normal way of connecting LEDs to AC is with a series diode, which has a very high reverse voltage rating (and resistance) and will prevent the LED experiencing any reverse voltage. Keith True - I was always taught to use a reverse parallel diode! Reverse parallel works well for two LED's across AC and no need for a series diode. Cheers, Mick Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted October 9, 2012 RMweb Premium Share Posted October 9, 2012 Reverse parallel works well for two LED's across AC and no need for a series diode. Cheers, Mick If you have two LEDs to light off an AC supply that's the way to do it, but with one, most (maybe not all) circuits I have seen simply use a series diode and resistor. The problem can be if you get the polarity of the diode wrong, with parallel the LED will possibly die, with series it just wont light, mind you LEDs are generally not as fragile, in my experience, as sometimes suggested. EDIT: as long as there is a limiting resistor! Keith Link to post Share on other sites More sharing options...
Phil S Posted October 9, 2012 Share Posted October 9, 2012 I would suggest a basic review of electrical theory, and energy conservation was due! LED's, particularly white, but including others, tend to have a relatively low reverse breakdown voltage - often about 5V, and therefore less than the voltage typically being applied to the circuit. (Note: Combined IC+LED devices such as flashing LEDs may accept 12V in the forward direction, on their own, but only 0.5V in the reverse direction!!) The use of a series diode provides additional protection against this breakdown, and could either be such as a 1N4001, or a 1N4148 which is a signal diode, and more appropriate for the fast(er)-switching of the dcc waveform compared to 'mains-derived' 50/60Hz. A single-diode in series with the LEDs will protect them ALL against reverse-voltage, from either accidental misconnection on dc, or ac (as dcc) use. An inverse-parallel diode across the LED simply wastes power from the controller, and means the resistor rating would have to be >50% greater than with the series-diode circuit, because current would be flowing continuously, in both half-cycles... in fact more current in the unwanted half cycle!!! [because the Diode would only drop about 0.5-0.7V instead of 2-5 V across the LED]. A STRING OF LEDS would need an inverse diode across the back of EACH LED !!! On ac[/dcc] The EYE responds to the PEAK brightness of the pulsed LED, and therefore a higher resistor value can be chosen for the required perceived visual effect - and this would also save power consumption and simultaneously permit lower-rating resistors to be used. When you have MANY illuminated buffer stops, and/or coach lighting; the 'quiescent current' through the track without a train moving increases measurably, and could reach >1 Amp quite easily. TO AVOID 'imbalance' problems between each half-cycle (which in an extreme case could affect braking-detection or Rail-Com feedback) - it is best to 'alternate' the buffers stop lights, so that an equal number are on each half-cycle. [Having both polarities monitored is also a useful diagnostic aid] ALSO for Rail-Com (now part of the dcc standard, even if not yet implemented by all), it is advised to have a MINIMUM OF 2 DIODE DROPS so as not to lose the return-signal ... the inverse-parallel diode would not achieve this, although the resistor will have an effect. On the related subject of Coach Lighting: bulb lighting in coaches is best avoided, as they are heavy current consumers, and also have no inbuilt diode-drop for RailCom® A simple method of assembly is therefore to solder the resistor to 1 leg of the LED, and the series diode to the other. In this way, current only flows 50% of the time - which means only 50% of the heating effect in the LED (and resistor) - which allows a higher current to be used, IF NEEDED, during the ON time [This is commonly used in remotes and strobed-LED displays, with a 'higher-than-rating' current for a small fraction of the time ... sometimes this even saves the resisitor and its power consumption - but is not appropriate in this application] [ideally perhaps, full wave rectification is preferred for coach lighting, as this gives full dc/dcc compatibility - but for dcc-only use, I often light half of each coach off each polarity, with a single (protective) diode drop for a series of 3-5 LEDs (depending on colour) and series resistor. Hopefully you are now encouraged to change to wiring LEDs with a series protective DIode as well as the series protective resisitor 8-) Link to post Share on other sites More sharing options...
Tase Posted October 9, 2012 Share Posted October 9, 2012 Cant help but think that the responses to the OP have now become somewhat technical ! It was a simple question which seems to have been answered in post #2? I dunno I'm confused :s !! There are guys, like me, out here that just want simple answers to questions - can it be done and if so how! Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted October 9, 2012 RMweb Premium Share Posted October 9, 2012 I would suggest a basic review of electrical theory, and energy conservation was due! LED's, particularly white, but including others, tend to have a relatively low reverse breakdown voltage - often about 5V, and therefore less than the voltage typically being applied to the circuit. (Note: Combined IC+LED devices such as flashing LEDs may accept 12V in the forward direction, on their own, but only 0.5V in the reverse direction!!) The use of a series diode provides additional protection against this breakdown, and could either be such as a 1N4001, or a 1N4148 which is a signal diode, and more appropriate for the fast(er)-switching of the dcc waveform compared to 'mains-derived' 50/60Hz. A single-diode in series with the LEDs will protect them ALL against reverse-voltage, from either accidental misconnection on dc, or ac (as dcc) use. An inverse-parallel diode across the LED simply wastes power from the controller, and means the resistor rating would have to be >50% greater than with the series-diode circuit, because current would be flowing continuously, in both half-cycles... in fact more current in the unwanted half cycle!!! [because the Diode would only drop about 0.5-0.7V instead of 2-5 V across the LED]. A STRING OF LEDS would need an inverse diode across the back of EACH LED !!! On ac[/dcc] The EYE responds to the PEAK brightness of the pulsed LED, and therefore a higher resistor value can be chosen for the required perceived visual effect - and this would also save power consumption and simultaneously permit lower-rating resistors to be used. When you have MANY illuminated buffer stops, and/or coach lighting; the 'quiescent current' through the track without a train moving increases measurably, and could reach >1 Amp quite easily. TO AVOID 'imbalance' problems between each half-cycle (which in an extreme case could affect braking-detection or Rail-Com feedback) - it is best to 'alternate' the buffers stop lights, so that an equal number are on each half-cycle. [Having both polarities monitored is also a useful diagnostic aid] ALSO for Rail-Com (now part of the dcc standard, even if not yet implemented by all), it is advised to have a MINIMUM OF 2 DIODE DROPS so as not to lose the return-signal ... the inverse-parallel diode would not achieve this, although the resistor will have an effect. On the related subject of Coach Lighting: bulb lighting in coaches is best avoided, as they are heavy current consumers, and also have no inbuilt diode-drop for RailCom® A simple method of assembly is therefore to solder the resistor to 1 leg of the LED, and the series diode to the other. In this way, current only flows 50% of the time - which means only 50% of the heating effect in the LED (and resistor) - which allows a higher current to be used, IF NEEDED, during the ON time [This is commonly used in remotes and strobed-LED displays, with a 'higher-than-rating' current for a small fraction of the time ... sometimes this even saves the resisitor and its power consumption - but is not appropriate in this application] [ideally perhaps, full wave rectification is preferred for coach lighting, as this gives full dc/dcc compatibility - but for dcc-only use, I often light half of each coach off each polarity, with a single (protective) diode drop for a series of 3-5 LEDs (depending on colour) and series resistor. Hopefully you are now encouraged to change to wiring LEDs with a series protective DIode as well as the series protective resisitor 8-) A rather over complex explanation of what I said was the norm IMHO! Keith Link to post Share on other sites More sharing options...
RAF96 Posted October 10, 2012 Share Posted October 10, 2012 A picture would be worth the thousand words above. One showing reverse parallel and the other a series diode. I have used Maplins side looking leds each with a 1K resistor as buffer stop lamps for years on DCC without failure but it pays to read the spec sheets and build in additional protection if so desired. Rob Link to post Share on other sites More sharing options...
Blacque Jacque Posted October 10, 2012 Share Posted October 10, 2012 A picture would be worth the thousand words above. One showing reverse parallel and the other a series diode. I have used Maplins side looking leds each with a 1K resistor as buffer stop lamps for years on DCC without failure but it pays to read the spec sheets and build in additional protection if so desired. Rob This page shows a typical ac operated led, about 2/3 down the page: http://web.onetel.net.uk/~uncletony/lighting.htm Link to post Share on other sites More sharing options...
JZ Posted October 11, 2012 Share Posted October 11, 2012 I played safe and used a bridge rectifier and a voltage regulator mounted on vero board with a suitable heatsink. Link to post Share on other sites More sharing options...
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