Led Strips Help - Page 1
Forum section: electronics / beginners

author: robertlopez925
author_rank: Contributor
timestamp: August 29, 2013, 08:04:22 am
content: i am in model trains and i haft to convert my current passenger cars form 5w 18v Grain of wheats to LED i got 12DC Led strips i have already have a reflector and a capacitor... my track voltage very's between 18v-21v AC depending on where the train is what i need help with is reducing the voltage before it hits my reflector and a capacitor Circuit i would like to see it at 12v for the dimness of the light and to reduce the heat these Leds produce How can i accomplish this it hast to be small right now my reflector and a capacitor setup is about 1" by 1/4" when shrink tubed. I can fit at least 2 more in the space it can be a little bigger..... but it would be harder to fit in the small area..you probably would like some info about the leds there are 24 of them wired in series with some sort of chip in between them they are suppose to be ran at 12dc but they are the best i could findthe power supplies put out 18v-22v ac they put out a whopping 650w of power which is a lot for trains the transformer is semi smart i believe it divides the wattage between 4 channelsi would also like to add some sort of auto reseting fuse to protect the old lights from shorts what i i used wasRUEF300-NDPOLYSWITCH RUE SERIES 3.00A form digikey... as suggested by a train form i dint know if these will still work for this application

author: TonyPh
author_rank: Regular Contributor
timestamp: August 29, 2013, 08:44:17 am
content: Those LEDs are the RGB type. Is that what you wanted? I would imagine you actually wanted just some warm white or whatever single colour you needed. RGB LEDs are a lot more complicated to work with and need some sort of driver to vary the colour output. The little chip in between the LEDs is the resistor that limits the power that flows through the LED, in this case each LED acts upon each colour component of the 3 LEDs in series.If the current lights use AC then you'll need to rectify that AC to DC with diodes and a capacitor and then change the resistor value to suit the DC voltage to the forward voltage of the LED. That's going to take up some space.

author: sleemanj
author_rank: Super Contributor
timestamp: August 29, 2013, 08:48:35 am
content: Quote from: TonyPh on August 29, 2013, 08:44:17 amWhere do you see that? There are single colour LEDs in that format, just because there are 6 legs doesn't mean it's RGB.In the photo you can clearly see that this is a single colour strip, there is only a single power connection. An RGB strip would have at least 4 connections (1 common and 1 for each die individually) between segments.

author: TonyPh
author_rank: Regular Contributor
timestamp: August 29, 2013, 09:45:40 am
content: Ah you are quite right. I was blinded by the 3 resistors and made assumptions. Didn't even notice the single power rail.I would assume they are still 3x LEDs per device, just white ones rather than 3 colour, though hence the need for three resistors. Single device LED strips have usually just the one resistor per 3 LEDs. I suppose with the resistor all being 150 Ohm it makes sense that they are all the same colour.So should be fairly easy to drive with maybe an additional resistor off strip if rectified voltage is too high.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2013, 12:12:31 pm
content: Yes, each device contains three LEDs which are probably wired in series.You can just connect two LED strips in series and run them off a bridge rectifier with a small capacitor.A 3A polyswitch should provide adequate over current protection.

author: mariush
author_rank: Super Contributor
timestamp: August 29, 2013, 12:34:44 pm
content: There are 3 leds per section.Each section receives 12v.There are 3 150 ohm resistors in each section so that makes me think that each led has its own resistor, but it could also be possible that each led actually has 3 smaller leds inside each large led ( which may explain the 6 pin footprint) and you have 3 sets of 3 leds connected in series :12v - led 1, element 1 - led 2, element 1 - led 3, element 1 - 150 ohm resistor - gnd12v - led 1, element 2 - led 2, element 2 - led 3, element 2 - 150 ohm resistor - gndetcYou can test how the pins are connected using the continuity on a multimeter.Assuming each led has a voltage drop of about 3v, it would make the 150 ohm resistor reasonable :V = I x R12v - 3 x 3v = I x 150 , 4v = I x 150 => I = 20mA (which is decent value for white leds). So let's say each section uses up to 50-60mA.Anyway... since the strip or a section of the strip works with 12v DC you first need to rectify the AC voltage to DC.You can use a single diode (half wave rectifier) or a bridge rectifier (full wave rectifier) :http://en.wikipedia.org/wiki/Rectifier#Single-phase_rectifiersIf you use a single diode, as the wiki page above shows, for half the time there will be no voltage coming out, so the leds will probably flicker.If you use a bridge rectifier, the voltage will go down less often as the graph shows you.However, you will still have the voltage going between 0v dc and a peak of about 21v AC x 1.41 = ~ 30v dc.You can use a capacitor to try to smooth out the dc output and make sure that at least the output doesn't go below a certain threshold.There is a simple formula : C = 0.75 x Current / [1-2] x f x VrippleWhere current is how much current it's used, [1-2] is 1 for half wave rectification, 2 for full wave, f is the mains frequency, Vripple is how much you're willing to let the voltage go down from the peak voltage produced by rectification.So let's say we have a peak of 30v, we're willing to let the voltage go down to 10v (so our Vripple is 30v-10v = 20v), and we want to use a single section of the strip, or about 50mA = 0.05A, a mains frequency of 60 Hz and we use a single diode to rectify the DC..The formula becomes C = 0.75 x 0.05 / 1 x 60 x 20 = 0.0375 / 1200 = 0.00003125 or about 31uFSo any capacitor bigger than 31uF, let's say 33-47uF 50v capacitor, should be enough to give that section of led strips a voltage between 10 and 30v, but mostly around 10-15v.

author: robertlopez925
author_rank: Contributor
timestamp: August 29, 2013, 05:58:40 pm
content: the issue is they are getting really hot and they mount on plastic would the poly switch lower the voltage? or just act like a fuseQuote from: Hero999 on August 29, 2013, 12:12:31 pm

author: robertlopez925
author_rank: Contributor
timestamp: August 29, 2013, 06:01:59 pm
content: how much resistant's would i need and how would i accomplish thisQuote from: TonyPh on August 29, 2013, 09:45:40 am

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2013, 06:11:09 pm
content: Quote from: robertlopez925 on August 29, 2013, 05:58:40 pmA polyswitch just acts as a fuse in case of a short circuit.How have you got them wired up at the moment?A full wave rectifier and capacitor (say 100uF will do) will give between 24V and 28VDC which is too much for one strip but if you wire two in series it'll be fine as each will get between 12V and 14V.

author: robertlopez925
author_rank: Contributor
timestamp: August 29, 2013, 06:20:16 pm
content: ok so i have been working on this circuit for a while here is what i accomplished1. i have the reflector and capacitor wired in it works fine EXCEPT there is to much power feeding to the led's the circuit is solid there is no flickering or noise...i was thinking either reducing the power to 12v ac before it hits my circuit.... or make some sort of dimmer for iti was thinking of using some sort of volume Potentiometer but i think i would be to much power for that application i don't know i have some grasp of electronics but this is something new that i have never attempted before.. if you want pictures let me know

author: robertlopez925
author_rank: Contributor
timestamp: August 29, 2013, 06:22:12 pm
content: can you draw the circuit and post it?Quote from: Hero999 on August 29, 2013, 06:11:09 pm

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2013, 08:49:28 pm
content: Quote from: robertlopez925 on August 29, 2013, 06:22:12 pmYes.

author: mariush
author_rank: Super Contributor
timestamp: August 29, 2013, 09:25:22 pm
content: I don't think that's right, i'd bet those leds are in parallel.... like so.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2013, 11:27:19 pm
content: Quote from: mariush on August 29, 2013, 09:25:22 pmNo, the LED strips are rated at 12V and rectifying 18V to 21VAC will give roughly double that voltage so they should be connected in series to give about 12V per strip.

author: robertlopez925
author_rank: Contributor
timestamp: August 30, 2013, 12:50:05 am
content: i think i lost people my incoming power is 18-22v ac i need to rectify it down to 12v dc...if that helps

author: Corporate666
author_rank: Unknown
timestamp: August 30, 2013, 03:58:53 am
content: Quote from: Hero999 on August 29, 2013, 11:27:19 pmI am sure it doesn't work like that... each LED on the strip is in parallel... hence the dropping resistors on each one. That's how the strips can be cut.You would not be able to simply run two strips in series and run them at 24V, because each LED would be seeing 24V.I would guess the LED has 3 dies and those 3 dies are wired in series for a Vf of maybe 11. Then the small resistor is dropping the rest.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2013, 10:43:48 am
content: Quote from: Corporate666 on August 30, 2013, 03:58:53 amNo, there are three LEDs per package which is connected in series with a 150R resistor, either way, it doesn't make any difference, each strip is designed to work from about 12V, irrespective or how the LEDs are connected, so rectifying 18V to 21VAC will give about 24V which could be used to power two strips connected in series.

author: robertlopez925
author_rank: Contributor
timestamp: September 01, 2013, 02:44:36 am
content: i think i lost people my incoming power is 18-22v ac they are verable transformers i need power to be 12v dc... if that helps

author: Corporate666
author_rank: Unknown
timestamp: September 01, 2013, 04:55:05 am
content: Quote from: Hero999 on August 30, 2013, 10:43:48 amNo, still not correct.Each LED has 3 dies (sorry CP, dice!), connected in series - and a dropping resistor. Each of those units is connected in parallel.You can see in his picture that the strip can be cut. They have to be connected in parallel to make it able to be cut to length by the user. So there is no way to take two strips and connect them in series, because there is no voltage drop from the power connection on one end of the strip to the power connection on the other end of it. If he takes a multimeter, he will find there is continuity from end to end on both the power and ground.

author: robertlopez925
author_rank: Contributor
timestamp: September 01, 2013, 06:47:41 am
content: ok you are right they where designed to be cut to length what they are is i spool of leds i bought for this project but the sucky part was they where coated with a thick water proof cover that had to have been injected molded but the seal was not that strong and matter in fact was easy to peel off the leds are custom cut with a soft flexible circuit board every so often you will find some solder spots where they soldered two sections together to lengthen to make a 100' spool.... as is right now i have them hooked up to a diod and a compositor which makes the leds turn on but they get really hot with in 3 sec and sense they are going to be mounted on plastic i don't want to melt the plastic.... so i was thinking if i reduce the power to them they wont burn so hot they say they run on 12v DC i am running 18V AC so i need to bring down the power any ideas?Also the LEDs and 150R resistor connect to a bus feeder so there is no power drop form end to end that is why you see so many 150r on theseQuote from: Corporate666 on September 01, 2013, 04:55:05 am

author: IanB
author_rank: Super Contributor
timestamp: September 01, 2013, 07:25:52 am
content: Quote from: Corporate666 on September 01, 2013, 04:55:05 amYou can still take two equal lengths of LED strip and connect them in series electrically so that the series arrangement can be powered from 24 V rather than 12 V for a single strip. That's what people are suggesting and it would work fine.

author: IanB
author_rank: Super Contributor
timestamp: September 01, 2013, 07:35:09 am
content: Quote from: robertlopez925 on September 01, 2013, 06:47:41 amYou need to power them from 12 V DC like the instructions say, or if you have a higher voltage you need to include a suitable current limiting resistor.If you have 18 V AC and you rectify it and smooth it with a capacitor, you will get about 25 V smoothed DC. This would be about right to supply two LED modules in series giving about 12 V across each LED module.

author: Jeff1946
author_rank: Regular Contributor
timestamp: September 01, 2013, 05:56:45 pm
content: If you run the lamps per your schematic without the capacitor, you will get about 10ma current average which should be plenty. I have replaced outdoor lamps, the kind that illuminate sidewalks with similar set up and there is no flicker. I used individual Cree leds I bought from Mouser. Initially I tried different resistors and measured the voltage drop across them to get about 20 ma current.I wrote an Excel program to show the current across the leds, just select the transformer voltage, number of leds and resistor. Note it takes into account the led and diodes voltage drop which is current dependent. I am attaching the program.

author: Corporate666
author_rank: Unknown
timestamp: September 01, 2013, 11:58:49 pm
content: Duh, I just was drawing something out and I realized what you guys were saying - I win the "dumbass" award for the day.Of course if he connects them in series he could run from 24V. My mistake.OP,if they are getting as hot as you say, you are running them WAY high on voltage. These strips will get "quite warm" (and the overmolding and adhesive is prone to failing over time), but so hot that the overmolding is coming off quickly is way too hot. They will die very quick at that temperature.

author: robertlopez925
author_rank: Contributor
timestamp: September 02, 2013, 05:58:42 am
content: so how can i lower the voltage?Quote from: Corporate666 on September 01, 2013, 11:58:49 pm

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2013, 01:51:16 pm
content: Quote from: robertlopez925 on September 02, 2013, 05:58:42 amYou don't, if you connect two strips of the same size in series and wire them to the rectified 18V to 21VAC as per my diagram, each LED will have 12V.

author: Corporate666
author_rank: Unknown
timestamp: September 02, 2013, 06:42:44 pm
content: Quote from: robertlopez925 on September 02, 2013, 05:58:42 amCut the strips,Connect the + of your supply to the + connection on one end of the first strip.Connect the - of the first strip to the + of the second stripConnect the - of your supply to the - connection on the second strip.18-21V should be enough to make them run, just a bit dimmer than 24V.