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Modifying RBG LED Ribbon for 5V Input

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Mike44

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Hi guys! Yes, another newbie here to ask questions - but have been hanging around here as a guest reading over some threads for a while. Very helpful forum and a ton of great information!

Anyways, I figured it was time for a question of my own. I currently have a little project that I'd like to do involving RGB LED Ribbon lighting but couldn't find too much info on what I need to do. Right now I'm in the 'theoretical' stage! This is the ribbon I am talking about:

**broken link removed**

**broken link removed**

These strips are rated for a 12V input. However, I currently only have a 5V input (via a USB cable from a computer) which I would like to be able to use to power it. I e-mailed the retailer and he said these may not even light up with 5V, but said that I should not try to change anything with them and instead get a 12V input source. Well, that kind of answer is unacceptable - especially on this kind of forum!! lol. Anything can be done! My best guess is that the ratings of the SMD resistors are the cause for why it will not light up (or be very dim) at 5V. They are probably rated to deliver a safe amount of power to the LED's with a 12V input.

So my thinking is that I could just desolder and resolder on new SMD resistors with the appropriate rating for a 5V input. The only problem is that I'm not exactly sure of the power ratings of the SMD LED's, so I can't calculate the sizes of the resistors to use. I think this info sheet resembles these LED's the most: https://www.electro-tech-online.com/custompdfs/2008/11/PLCC6RGBCT.pdf Actually, to be honest, I'm not really sure how I would go about to calculate the resistor sizes to use for this type of application!!!

I'm not sure if you guys need any more info on the number of LED's on the strip as well? I would probably be using about 2 feet of the ribbon, which equals 18 SMD RGB LED's (1.3 LED's per inch). This is the technical page from one of the stores: Ribbon LED Strips (scroll down the RGB ribbon).

Okay, well I've tried to be as detailed as I can! Any help on determining resistor size for a 5V input (instead of 12V) would be greatly appreciated!! Or any (constructive) criticism as well! If you guys don't think that this would be a good idea, I would also be up to making a PCB and soldering on regular RGB LED's as well.

Thanks, Mike
 
I've played with those ribbons too, it's not possible to light them with lower voltage than about 10,5v because the leds are wired in series (three blue leds = 3*3,5v = 10,5v). You would have to unsolder the leds and make a new ribbon where the leds are connected in parallel all with their own current limiting resistor, which would mean that the current requirement would be three fold. A RGB-led has as the name suggests three leds in it. So that would equal a total of 9 leds (for one section of the ribbon) with all a current requirement of 20mA = 180mA total, which is more than the allowed amount of current that can be drain from the USB (100mA)

So in short, no it can't be done without an external powersupply. And if you want to take the power from the USB-port you could only light one RGB-led.
 
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Oooh, I see what you're saying. So I guess each section has their own set of 3 LED's wired in series, but then each section is wired up in parallel? Yeah, so I guess 5V wouldn't be enough then.

Okay, so then maybe instead of using the ribbon, what if I just made my own LED strip on regular PCB? I don't need it to be flexible or anything, but I thought of using the ribbon for convienience sake of having it pre-made already. But if I made my own, that way, I could ensure that each LED was wired in parallel. I would think I could use the 5V supply then?? I would control it with an RGB controller - this is the one that I was thinking of using: Make an RGB controller.. I spoke with the creator of it and confirmed that I could modify it so that it uses a 5V input instead of 12V by just removing the 1A rectifier diode (and linking the two pads) as well as the three pin voltage regulator and link the two outer pins of it together. Just as long as the control chip recieves 5V he said it should be fine.

As for the LED's though, would it be better to use the regular RGB LED's then instead of the SMD RGB LED's? I guess I'm thinking about ease of soldering them - probably easier to solder component legs as opposed to SMD pads?

Okay, well thanks again for your help guys!!

Mike
 
Yes, if you make your own circuit with all the leds in parallel with their own resistor it would work. The datasheet states that the red led's forward voltage is 2,3v so you could wire two red leds in series for 5v, but this won't work for green or blue as their forward voltage is 3,6v. If you wire two red leds in series they would only have 0,4v headroom which can be a bit small, but if the supply is stable enough there shouldn't be any flickering. To calculate the resistors use the oh-so-known formula: R = (Vs - n*Vf)/0.02A where Vs is the supply voltage, n is the number of leds wired in series and Vf if the led's forward voltage drop. This assumes 20mA of current which is a good value.

As for the surface mount RGB-leds, I don't see why you couldn't use them, just use a soldering iron with a fine tip, the pads aren't too small to be soldered by hand :)

I have to say that it should be clear that you cannot use the USB-port to drive 18 RGB-leds which means that you are going to need a powersupply anyway, so why not just buy a 12v supply? Then again if you plan this to be some kind of computer-lightning you could always just take the power straight from the computer's powersupply, and as it supplies both 5v and 12v I see no real reason to use the 5v-rails.

EDIT: Oh noticed just that the 2,3v forward drop is the nominal value, not the worst case scenario, which is 2,6v, so it is better not to put two red leds in series (for 5v)
 
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Thanks for detailed reply hantto! Yeah, I think I'll design my own PCB for the LED's in order to put them in parallel. I was taking another look at a schematic for one of the them, and there is definately no way to alter those to wire them up in parallel. Well, there might be some way, but the way I see it, it would just be easier to make my own PCB with the correct pattern.

But I totally never thought about running them all at once (which I definately will be doing), so it's probably not the best idea to use only 5V!!!! Darn, oh well. The reason I wanted to have 5V was because this is going to be a computer keyboard lighting mod (I have a Logitech G15 gaming keyboard which has existing blue LED's to light up the keys, but I want to switch them so that I can different colours). I didn't want to have any extra wires going into the board, so I was thinking of taking the 5V power supply off of the USB to power the LED's, but it would definately be too much for the keyboard AND the 18 LED's combined. Thanks for pointing that out.

Anyways, this other guy did a similar mod with some pretty good results (Fusion Mods - A Modding Community), but I hate the controller that he used which is why I'm going with the one I mentioned above. I also didn't like how he had an additional cable going into his keyboard for the 12V power source, which was why I was hoping just to use the 5V. Ahh well, it won't be that bad anyways, I'll just wire it up for 12V instead and just bring in another line beside the USB cable. That second wire is just a minor detail anyways. It's the colour that really matters!!

Alright, well thanks a lot again for your help!

Mike
 
Hmm, I know I replied to this a few days ago, but it still says that my post is being moderated since I have a low post count? Any chance of getting the reply up?

Well, if not though, that's okay. Basically all I said was thanks Hantto!! Big help! I actually realized after I posted previously that there wasn't enough current in the USB to power all the LEDs!!!! So I think I'll just end up using the 12V power supply and have a second line running to it. Oh well, it won't be that bad after all, it'll be the lights that you notice right away :)

I'll let you know how it goes though!

Thanks!!

Mike
 
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Hi,

Cheat! Double the 5v with a tlc555 low power timer and feed the leds with reduced duty cycle pulses to reduce current consumption ( leds will light but more dimly )

joecioppi
 
I sell the **broken link removed** and I am certain it cannot be run on less then 10v.
I have seen however devices that can pull 12v from a standard USB port. I think this would be the best route to go down if you have to use the computer cable.
However we have had similar problems where we are unable to get power near the ribbon and a mains cable was too big. I found the best way is to run the small cables that connect to the power supply the distance to the mains supply.
 
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Hey, thanks guys! Neat tip with the current reducer idea! It turned out that what I did was just tap off the 12V supply from the computer and fed that into my keyboard to run my LEDs.

Thanks for all your help!

Mike
 
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