Can someone help me with SMD's?

Discussion in 'Electronics Resources' started by gurvir44, Sep 3, 2011.

  1. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
    Hey guys i am new here and i needed help on SMD resistors. I just got the hang of wiring less (3mm and 5mm) with resistors and i need to use smd's (0603) for a project. Now i used a 1/4, 1/2, and 1W resistor with 12V to light up a smd but they all burnt out. Can anyone help me on what to use, or if i have to get special smd resistors? resistor size is not a concern.
    Thanks!
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    did you mean LED's?

    What resistor values did you use? Can you draw a diagram of what you did or explain in detail?
     
  3. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
    Ok well I used a led calculator, put in the values I got from the manufsctre I bought the smds in and got that I need to use a 1/2 watt resistor. I am using a 12v battery. I hooked up + to battery to + to smd. Connected - of battery to resistor then resistor to - smd and then bright flash and burnt out. I tried it with a 1/4w and 1w resistor, all at 1 ohm
    Thanks for your reply
    Edit: yes and I ment LED's, sorry auto correct.
     
  4. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You have revealed the problem: internet LED calculators. They only work if you know basically how to do the job without using the calculator. We get a dozen a week having trouble with LEDs. Tell us ALL the values and we can probably help.
     
  5. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
    Ok the smd values

    Wavelength: 464-473nm
    Lens Color: Water Clear
    View Angle: 120 Degrees
    Forward Voltage: 2.8-3.4v
    Current: 20mA
    Size: 1.6mm x .8mm x .58mm
    Luminosity: 140mcd

    I need to connect two of them with 12v power. Once I get the hang of it, my next project needs 8 of them side by side in a line.
    I only have 1/4w, 1/2w and 1w resistor so If I need to order some I will
     
  6. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You need to use up about 20% (or more) of the 12 volts for regulation because the LED voltage wanders with temperature. Try 3 in a row with 180 to 330 ohms and measure the REAL voltage the LEDs use. Then you can zero in on the right value.

    Personally, I don't run LEDs at absolute maximum current, which is 20 ma in this case. They last a lot longer (and still look good) if you slow them down to 18 ma or even 10 ma.
     
  7. Tealc

    Member

    Jun 30, 2011
    140
    10
    If you are using 1 ohm as detailed above then you are putting far too much current through the LED. For two LEDs in series you'll need to get some larger resistance 1/4w resistors such as 270-330ohm depending on the exact Forward voltage of the LEDs.
     
  8. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
    Ok i understand now that using 1 ohm for all my led's is not good, i just ordered a variety of resistors for my led needs. But i still havnt figured out my SMD problem, what do i need to use in order to power it from 12 volts?
    Thanks for the information about 270 ohm and 330 ohm!
     
  9. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Look at my post #6. Do the experiment and find the answer.
     
  10. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
    Ok i will try as soon as my resistors come in the mail. Thanks!
     
  11. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Same resistors as #12 indicated. 0603 series comes in 1/4W versions too.
     
  12. gurvir44

    Thread Starter New Member

    Sep 3, 2011
    6
    0
  13. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    My favorite is www.mouser.com, but judging from the location you didn't post, I don't know if you are in a country they ship to.

    There is also Jameco, Digikey, and several others.
     
  14. Tealc

    Member

    Jun 30, 2011
    140
    10
    Regular resistors work just fine but of course take up a lot more room in your project.

    If you assume a typical voltage drop lies somewhere in the centre of the specified voltage drop range you can just calculate what single resistor you need and get that one. A small deviation in Vf won't make a massive difference.

    For single LED and resistor on 12v source.

    12 - 2.8 / 0.02 = 460
    12 - 3.1 / 0.02 = 445 <---- get nearest standard highest of 470 ohm.
    12 - 3.4/ 0.02 = 430

    For two LEDs in series with a single resistor on 12v source.

    12 - (2.8*2) / 0.02 = 320
    12 - (3.1*2) / 0.02 = 290 <----- get nearest higher value of 300 ohm
    12 - (3.4*2) / 0.02 = 260

    Even if the Vf is the lowest specified value and you use the 300 ohm you still are only feeding 21mA or so through them. The problem may arise if you are feeding more than 12v through the LEDs, say for example if your 12v source is 13.4v when fully charged (25mA). In this case you'd need to make sure you go higher resistance to compensate.

    I usually get my stuff from eBay. A lot of component suppliers do tend to hammer you with postage. Here in the UK there are a few places you can get resistor kits or just a few of each value and pay small postage charges.
     
Loading...