Connect with us

LED's in PCB project - using different LED's

Discussion in 'LEDs and Optoelectronics' started by dynamis_dk, Jan 27, 2010.

Scroll to continue with content
  1. dynamis_dk

    dynamis_dk

    5
    0
    Jan 27, 2010
    Firstly hi everyone - another electro-noob joining in :)

    I'm making a PCB to backlight some temprature dials in a car to replace the poor lighting effect you get just replacing the bulbs.

    I've worked out i need 15 LED's to light each section of the dial so my plan was to have 5x sets of 3 LED's in series each with a resistor. Problem has come to light when i've looked up the voltage requirements for each LED. The blue LED's (13 of the 15) are all 3.0-3.2 but the remaining 2 LED's are red which use 2.0-2.3v.

    Would i be best off redisigning the PCB layout to have the 2 Red LED's running on there own with a bigger resistor, or running all the LED's at 2v, 3 in each group with a resistor in each group??

    If it helps i can upload an image of the PCB i've worked on so far.

    I've never done this before other than swapping LED's over in existing hardware (i.e. keyboard LED's, siwtch LED's in cars etc) so be nice with me :) Haven't done anything electronics theory based since school
     
  2. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Heya,

    I've attached a schematic which is based on what I think you're asking.
    I'm assuming you're working with +12Vdc... and I'm not sure what kind of forward current the LEDs you're using should be run at, but those resistor values will give you ~20mA. Depending on the LED, it may rise to ~25mA when everything has reached a thermal stasis. All depends on the LEDs, power consumption, etc. You can monitor current and adjust the resistors as needed. I'd use 1/4W 1% metal film (if through hole) resistors, but you can probably get away w/ 1/8W 5% if you want to go super cheap.

    Correct, you will need to drop more voltage across the resistor on the red/red/blue leg.

    Things to keep in mind though: LEDs, by nature, should be driven with a constant current since their forward voltages are so varied (even between different lots of the same LED.) This can't always be feasible, such as when all you have is the 12Vdc system in the car. But you can try to mitigate it...

    The voltage requirements (referred to as forward voltage) for a certain forward current of the LEDs do change based on a couple of factors.

    1.) Junction temperature: The warmer the LED gets, the Vf will start to drop a little, which equates to a rise in forward current, (in this constant voltage example.)
    2.) Individual die construction: LEDs usually come in Vf bins, some chemistries require less Vf, some more. This is most apparent in different colors (in this case red vs. blue.) But different individual die constructions will normally have a small difference in Vf even in the same LED between lots.

    Hope this isn't too much information, it probably won't be critical. Just make sure you stay under the LEDs max rated current and junction temperature, and they should last forever.
     

    Attached Files:

    • LEDs.zip
      File size:
      36.2 KB
      Views:
      273
    Last edited: Jan 27, 2010
  3. dynamis_dk

    dynamis_dk

    5
    0
    Jan 27, 2010
    I get the general idea of what your saying. I'm not on a set budget so i'm happy to spend a little more to get better components if it helps in the bigger picture.

    The schematic you attached is pertty muc spot on (from my understanding) with the exception that it would be 3 lines of blue led, and 2 lines of 2 blue led/1 red led.

    I've attached a jpg of the PCB design as it stands, i'm pretty limited as it needs to be within thats shape. I don't know what the real items should look like to i've used the Circle and Squares to show as spots for the led's - its there position which is vital to the lighting working so its a bit all over the place.

    also note - first time pcb designer :p
     

    Attached Files:

  4. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Aaah, ok. Makes more sense now.

    The squares on your board would be anode, circles cathode. These are... 3mm or 5mm through-hole type LEDs? The other pads are for the resistors. (For some reason I had surface mount LEDs in mind, like the Nichia NESB064.)

    The prudent thing to do is plan for the lowest Vf possible, based on the data sheet.
    Blue = 3.0Vdc
    Red = 2.0 Vdc

    So for the 3 blues in series: 3.0 + 3.0 + 3.0 = 9.0Vdc You need to drop the other 3.0V across the resistor. I'm still going to assume 20mA... so 3V / 20mA = 150 Ohms.

    2 blue + 1 red in series: 3.0 + 3.0 + 2.0 = 8.0Vdc ... 4V / 20mA = 200 Ohms.

    Substitute whatever current you're planning on running these at.

    Power dissipated in the resistors:
    150 Ohms... 20mA * 3V = 0.06W (Use an 1/8W or 1/4W resistor)
    200 Ohms... 20mA * 4V = 0.08W (Use a 1/4W resistor)

    1% tolerance metal film resistors should dial in your current pretty close. (Again, all depends on the nuances of the LEDs AND the +12Vdc tolerance. Most supplies will have a +/- 5% or so tolerance, not sure about in the car though.)

    If you wanted to get really fancy, you could probably use LM317 (or similar) regulators set up to regulate the current in each leg, but that may be overkill. :)
     
  5. dynamis_dk

    dynamis_dk

    5
    0
    Jan 27, 2010
    Car supplies are a little funny, power generated (as i understand) its normally 11.5-14v.

    I'd have to get a multimeter out and see if the car has some kinda of built in limiter for the lighting circuit already, failing that my backup plan was to assume the car would always give 14v and calculate the resistor figures from there. Am pretty sure it would give me lower light levels overall bit i'd prefer that then presuming 12v and popping LED's all day long :)

    You're asumption on the design is right, squares on your board would be anode, circles cathode. LED's are all going to be 3mm through hole type - looked in using SMD's but i think i'd be causing myself issues as i'm not a experienced soldering guys.

    What exactly would the LM317 (or similar) regulators involved?? would this be per LED?
     
  6. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Using the LM317 should be pretty easy... one for each leg. (They should be pretty cheap in a low power package.) Use the LM317LZ-AP for instance. Should be something like $0.50 a pop. They will do 100mA max, something like 40V max input.

    You'd have to re-do your board, obviously, but it would definitely help with the huge tolerance of the 12V in the car. LEDs don't respond well to large increases in Vf (even half a volt will usually shoot the forward current sky high.)

    See attached schematic. (If I'm thinking straight, that's all you'd need.) Shouldn't need to worry about filtering the inputs on the LM317's.

    You should be able to find 1% metal film resistors in 62 Ohms pretty easily, maybe not from The Shack, but they are out there. A run of the mill 5% carbon film should be OK if you can't find the 1% guys. I'd go higher with the resistance than lower, if you can't find 62 Ohms.

    62 Ohms will run the LM317 at ~20mA. (1.25V / 20mA = 62.5) Again, no idea if you're running the LEDs at 20mA, 30mA, etc... alter formulas to taste, minding max If of the LED.

    Once you get this finished you should post pics! I can imagine it will look really nice.

    Edit: Oh, one more thing... I'm not sure what kind of clearance you have in the Z dimension (from the PCB to the dial's face.) Using the through-hole 3mm LEDs, those are semi-tall. If they just fit behind the dials, you're going to end up with spotting. I'm not sure if you've got that all sorted out already, just thought I'd bring it up in case. :)
     

    Attached Files:

    Last edited: Jan 28, 2010
  7. dynamis_dk

    dynamis_dk

    5
    0
    Jan 27, 2010
    Afraid i've not done much more this week due to working on a UV exposure box (see attached).

    Is there something i could do to regulate the somewhat variable input from the car so its would go: 12-14v --> regulator --> 12v regulated --> PCB

    Failing that, do it take it, it would mean one LM317 type per run of 3 led's??

    Any consideration for heat using these??

    I've got about an inch to play with between the PCB and the dial front, its gonna allow me to leave a bit of the legs showing so i can bend the directions to tweak the led light spot (in theory). I did a test run and used some SMD led's but the throw was too wide to give the effect i wanted. On the plus side SMD's arn't as bad to solder as i was expecting!!
     

    Attached Files:

  8. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Hehe, smd components are the way to go for a lot of reasons. Though only you can decide what type of light output works for you in this case.

    As for regulating the 12V, that is possible, but you may have to find a low drop out (LDO) regulator since the voltage overhead on the 317's is a volt or two. If the car's supply goes below 12V, it's another issue.

    Best bet if going that way would be to regulate a 10 or 10.5V output anyhow, why waste watts when you don't need the whole 12v for the LED strings. Then you'd set the current limiting resistors based on that voltage. Keep in mind you'd need a regulator rated for a higher current than the ones from my previous post. 200mA plus. The LEDs will draw about 100mA in total. This has the benefit of not needing a board re-design, but now that you're regulating voltage you may need to worry about the filtering caps on the input/output if the supply is a good distance from the regulators, to keep a nice regulated level output.

    Even then, it's always best to run LEDs with a regulated current.

    At these low power levels, heat shouldn't be an issue.
     
    Last edited: Feb 1, 2010
  9. dynamis_dk

    dynamis_dk

    5
    0
    Jan 27, 2010
    I think over the weekend then i might strip the dash parts out and pop a multimeter on there and see what kind of range i'm dealing with. Its just dawned on me that i've never checked and it might be that the feeds to the lighing might be regulated already (fingers crossed).
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-