Jon Fitness Posted June 13, 2014 Share Posted June 13, 2014 I know, it's a question asked many times and usually leads to er..robust discussions, but this is a little bit different. I have built a large number of 7mm scale semaphore signals over the last few years, nearly all of them illuminated with 3mm warm white LEDs. They are wired in parallell when there is more than one fitted, earthed to the brass structure and the positive tail at the base of the signal has a 1k ohm 0.25 watt resistor tagged on. 99% have been, and continue to be, reliably lit on 12vdc. Recently I have been trialling some surface mount warm white LEDs (neater and smaller)and as far as I know (no complaints from customers..yet) they work just as well, but this afternoon I came across a little problem. I have installed a SMD LED into a lamp case on a banner repeater and wired it up using the same method as I have always done.. I tested it on my usual 3v battery pack as I went along and it was fine, if a little bright. I completed the wiring and added the 1k 0.25w resistor as usual and then tested it with a 9v battery as usual. It worked fine for about 5 seconds, flickered and faded out. Had I blown the LED? Before I condemned it I tested it again, same thing but it faded after 2 seconds. I bypassed the resistor and tried it on 3v and all was fine. Had I damaged the resistor? I tried a big fat 1k 1w resistor, same results..Every resistor I used, I tested on the meter afterwards and they still showed the correct value. I left the LED lit without a resistor using the 3v battery pack for 2 hours and it was still fine, so, apart from not using a PWM dimmer for my LEDs what am I doing wrong? The SMD LEDs are from Rapid, part no 72-4980, 3528 Warm white 2900K 2100mcd. The technical spec sheet for these is here Any ideas? Cheers Jon F. Link to post Share on other sites More sharing options...
Crosland Posted June 13, 2014 Share Posted June 13, 2014 "3V battery pack"? More detail please. What sort of batteries? Fresh batteries recently? Don't blindly assume AA batteries are 1.5V. Unless you REALLY, REALLY know what you are doing, NEVER, EVER connect LEDs without a resistor. Andrew Link to post Share on other sites More sharing options...
Jon Fitness Posted June 13, 2014 Author Share Posted June 13, 2014 "3V battery pack"? More detail please. What sort of batteries? Fresh batteries recently? Its a couple of 6 month old C batteries in a holder.....tested on the meter they are showing 2.8volts.. and the PP3 is of a similar age and showing 8.4v on the meter. JF Link to post Share on other sites More sharing options...
RMweb Gold SHMD Posted June 13, 2014 RMweb Gold Share Posted June 13, 2014 Sounds like the 9v battery is dying. Can you measure it when powering the LED after the light has gone out? Kev. Link to post Share on other sites More sharing options...
Baby Deltic Posted June 13, 2014 Share Posted June 13, 2014 9V / 1000 ohms is 9mA. The maximum pulsed forward current is given as 100mA over a duty cycle of 0.1s. The absolute maximum DC current is given as 30mA with an absolute maximum voltage of 3.2V. 3V with no resistance will give potentially 3A I would have thought. Maybe the LED has an overcurrent / constant current circuit built in? It might also have over-voltage protection built in if ts a SMD LED. Link to post Share on other sites More sharing options...
Baby Deltic Posted June 13, 2014 Share Posted June 13, 2014 Max forward voltage 3.2V if 20mA, therefore V/I = 3.2 / 0.020 = 160 ohm resistor at 3.2V. 3V = 150R. Me thinks.... It states that the maximum power dissipation of the package is 120mW. 3V x 0.02A = 60mW. Well within the safe limit with 150R resistor. Link to post Share on other sites More sharing options...
Jon Fitness Posted June 13, 2014 Author Share Posted June 13, 2014 Sounds like the 9v battery is dying. Can you measure it when powering the LED after the light has gone out? Kev. I will do but I tried a little 12vdc plug in transformer as well with similar results.! JF Link to post Share on other sites More sharing options...
Jon Fitness Posted June 13, 2014 Author Share Posted June 13, 2014 Maybe the LED has an overcurrent / constant current circuit built in? It might also have over-voltage protection built in if ts a SMD LED. To be honest, the electronics of it are a bit of a mystery to me but your comment about the over voltage protection sounds useful. So if I "forgot" the resistor and "accidently" flashed 9v across it with my PP3, it would survive by breaking the circuit rather than going "phut"? Also would I get the same effect if I used one of those little pulse width modulation dimmer kits from MERG? JF Link to post Share on other sites More sharing options...
Baby Deltic Posted June 13, 2014 Share Posted June 13, 2014 To be honest, the electronics of it are a bit of a mystery to me but your comment about the over voltage protection sounds useful. So if I "forgot" the resistor and "accidently" flashed 9v across it with my PP3, it would survive by breaking the circuit rather than going "phut"? Also would I get the same effect if I used one of those little pulse width modulation dimmer kits from MERG? JF The PWM would allow you to use more current and maybe get more intensity because its intermittent. A duty cycle of 0.1s AT 100mA is given in the data sheet at 3.2V for absolute maximum. I would use 3V witha 150R resistor. 9V with a 300R resistor will give a forward voltage of 3V across the LED at 20mA, but it will give off 120mW of power which is the absolute maximum. I would use a 3V power source. http://www.daycounter.com/Calculators/LED-Tutorial-Calculator.phtml Link to post Share on other sites More sharing options...
Jon Fitness Posted June 14, 2014 Author Share Posted June 14, 2014 The PWM would allow you to use more current and maybe get more intensity because its intermittent. A duty cycle of 0.1s AT 100mA is given in the data sheet at 3.2V for absolute maximum. I would use 3V witha 150R resistor. 9V with a 300R resistor will give a forward voltage of 3V across the LED at 20mA, but it will give off 120mW of power which is the absolute maximum. I would use a 3V power source. http://www.daycounter.com/Calculators/LED-Tutorial-Calculator.phtml Thanks for that. I need to be able to dim the LEDs really as they are way too bright. I maybe need to source a less bright version! It's just that these particular LEDs are just the right physical size for the job and the connections fixed to customers signals really need to be cabable of "layout" voltage.. Cheers JF Link to post Share on other sites More sharing options...
Suzie Posted June 14, 2014 Share Posted June 14, 2014 It is a common fault on white and blue LEDs and a quality issue. It is one of the bonds failing inside the LED. I have not had this problem with any Rapid LEDs myself, but a few in cheap consumer goodies from China. It is worth speaking to Rapid, they are concerned about quality. If possible use the Kingbright LEDs, especially when failure is a bit of a pain to remedy. The TruOpto brand can be a bit of a mixed bag (literally) so always check the colour is right before installing, I had some yellow ones where 90% of the bag were orange - the mixture made some of the position lights look a bit odd. Link to post Share on other sites More sharing options...
Jon Fitness Posted June 14, 2014 Author Share Posted June 14, 2014 Finished the signal with the dodgy LED and tried the wiring again. Its now gone back to a 1k 0.25w resistor and I've left it on test. this time the LED lit and stayed steady for 30mins before starting to flicker a bit. I put a meter before and after the resistor. A steady 8.8volts from this battery but the voltage beyond the resistor was fluctuating between 2.7 and 2.95volts as the flickering occurred. It's still lit as I look at it now after 1hour on test but flickering occasionally. Thanks for all the input folks. I'll probably have to tell the customer to apply a 3v battery pack! JF Link to post Share on other sites More sharing options...
Crosland Posted June 15, 2014 Share Posted June 15, 2014 Max forward voltage 3.2V if 20mA, therefore V/I = 3.2 / 0.020 = 160 ohm resistor at 3.2V. 3V = 150R. Me thinks.... It states that the maximum power dissipation of the package is 120mW. 3V x 0.02A = 60mW. Well within the safe limit with 150R resistor. You can't use ohms law to work out the resistance of an LED, they are not resistors and do not behave like resistors. If you are trying to calculate he value of current limit resistor then you have got it just as wrong! Andrew Link to post Share on other sites More sharing options...
RMweb Premium melmerby Posted June 15, 2014 RMweb Premium Share Posted June 15, 2014 You can't use ohms law to work out the resistance of an LED, they are not resistors and do not behave like resistors. If you are trying to calculate he value of current limit resistor then you have got it just as wrong! Andrew IIRC They act more like Zener diodes, with a voltage plateau maintained over a large range of current. Keith Link to post Share on other sites More sharing options...
Baby Deltic Posted June 15, 2014 Share Posted June 15, 2014 You can't use ohms law to work out the resistance of an LED, they are not resistors and do not behave like resistors. If you are trying to calculate he value of current limit resistor then you have got it just as wrong! Andrew Sorry, you are right. What I should have done, me thinks, is say 9V supply to give 3V across LED at 20mA, Therefore 9V - 3V = 6V. To get 20mA we need 6V / 20mA = 300R resistor. 20mA x 6V = 120mW on resistor 3V x 20mA = 60mW on LED. I think that's right, anyways. Link to post Share on other sites More sharing options...
Geoff Cook Posted June 15, 2014 Share Posted June 15, 2014 John I think you could be better of by decreasing the current rather than increasing it which seems to be what is being suggested, increase the resistor size, to 2k or 3.3k or 3k by putting 3 x 1k in series Geoff Link to post Share on other sites More sharing options...
Baby Deltic Posted June 15, 2014 Share Posted June 15, 2014 You could always use a reasonably high value pot and adjust it until you get what you want, then measure off the resistance and substitute a normal resistor. The data sheet gives typical values of 20mA at a forward voltage of 3.2V. Start with the pot at maximum resistance and work down. As Geoff mentions, maybe a couple of 'K' to get the current right down. Link to post Share on other sites More sharing options...
Jon Fitness Posted June 15, 2014 Author Share Posted June 15, 2014 John I think you could be better of by decreasing the current rather than increasing it which seems to be what is being suggested, increase the resistor size, to 2k or 3.3k or 3k by putting 3 x 1k in series Geoff I've been up to 3k as suggested with a similar result (flicker/die!). I think my next test will be with the smaller value resistors or pot as suggested. It's still fine on a straight 3v (or 2.8 as it's dropped down to after 2 hours on test) without any resistor. I'm wondering if maybe I've damaged to LED during the soldering process. I was as quick as I could be with a 13w iron and fine bit honest!! Thank you all for your continued input. JF Link to post Share on other sites More sharing options...
RMweb Gold SHMD Posted June 15, 2014 RMweb Gold Share Posted June 15, 2014 Try doing everything you have done, so far, but to a new one BEFORE you solder it to a Signal Post. Kev. Link to post Share on other sites More sharing options...
Crosland Posted June 15, 2014 Share Posted June 15, 2014 I've been up to 3k as suggested with a similar result (flicker/die!). I think my next test will be with the smaller value resistors or pot as suggested. It's still fine on a straight 3v (or 2.8 as it's dropped down to after 2 hours on test) without any resistor. It sounds like you are saying two C cells have dropped from 3V to 2,8V after only two hours driving an LED? Please listen the advice and STOP testing without a resistor. Andrew Link to post Share on other sites More sharing options...
Baby Deltic Posted June 15, 2014 Share Posted June 15, 2014 Chances are the substrate within the LED has begun to break down as a result of too much voltage/current and is therefore sucking more energy out of the batteries. Without a current limiting resistor it could literally become a dead short if it breaks down in a short circuit condition. You would certainly get smoke and the Gallium Arsenide stink if that happened. Link to post Share on other sites More sharing options...
DCB Posted June 16, 2014 Share Posted June 16, 2014 It sounds like you are saying two C cells have dropped from 3V to 2,8V after only two hours driving an LED? Please listen the advice and STOP testing without a resistor. Andrew I am surprised you got as much 3 volts from two C cells, they were non rechargable presumably, Freshly charged recharables seldom reach 1.5 volts, 1.48 is good and your figure of 2.8 or 2X 1.4 is quite typical. However you can measure battery voltage accurately whereas you and the test meter have to guess transformer output which is part of a waveform dropping to zero and reaching high peaks, even the average of this can be 19 volts from "12 volt" supplies, and even when the meter shows 12 volts the peaks will be way above 12 volts. Comparing like for like I would assume 14 volts for any 12 volt transformer when calculating resistors, and the actual voltage with recharged / new cells when using batteries. What you should not do is use rechargable batteries, where there is the remotest chance of a short, they discharge very rapidly and can start fires, they certainly melted and burned the wiring on my class 37. I ,. Link to post Share on other sites More sharing options...
Jon Fitness Posted June 16, 2014 Author Share Posted June 16, 2014 OK thanks folks. LED replaced and all is working as it should now thanks. I can only presume it was a dodgy/damaged LED. Ta JF Link to post Share on other sites More sharing options...
Baby Deltic Posted June 16, 2014 Share Posted June 16, 2014 OK thanks folks. LED replaced and all is working as it should now thanks. I can only presume it was a dodgy/damaged LED. Ta JF What power source/resistor size did you settle on in the end? Link to post Share on other sites More sharing options...
Jon Fitness Posted June 17, 2014 Author Share Posted June 17, 2014 What power source/resistor size did you settle on in the end? My usual 1k resistor, tested with a PP3 then "layout voltage" from a 12v loco controller. JF Link to post Share on other sites More sharing options...
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