Repairing LEDs on my computer.

Discussion in 'General Electronics Chat' started by vladthegreat1, Dec 26, 2014.

  1. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    hello everyone. I am a novice when it comes to this stuff but recently i decided to crack open a piece of my computer ( an led strip) apart because it did not work anymore. I cracked it open and found that it had 2 LEDs and 2 resistors on the pcb. Now this is SMD style... so i used a mag. glass and found out the Resistors were marked 151.... i googled and found out the 151 resistors is a 150 ohm resistor.... I am trying to understand how the circuit even works? from my undestanding it is in series... Here is a diagram of how the pcb is layed out.

    [​IMG]

    Now. I am currently in hawaii, its 10pm and wifes sleeping so im doing some research... so i do not have access to the power source... But i have a feeling the power source is 12v.. I will confirm this when I get home on sunday. But when i recreate this circuit in Virtual Breadboard, or 123d circuits.... for some reason it does not work...
    Also the LEDs have a forward voltage of 3.5, and a reverse of 5 and 30mA for the current with 100mA for the max current before damage.. sorry forgot what that was called... reverse current? forgive me...

    So my question is how does this even work... the LED strip is from a reputable company... why do they do it this way.... are the 2 resistors like this overkill? underkill? or just set up to work forever....

    The other question is, can i get away with turn this into a parallel circuit? and if i do. what kind of resistors would i need for this to work properly and for a long time...
    Possible parallel circuit?
    [​IMG]
     
  2. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    i just realized my title is bland and may not be correct per the forum rules. I cannot seem to change it now after i posted it. Admin if this title is not correct please let me know and help me change this. thank you.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    With a 12V supply, wiring the components in series as in your first diagram is the preferred solution since this is more energy efficient.

    Both circuits shown should work in theory.
     
  4. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    Ok. I even played it out on a real breadboard with through hole components and it seemed to work as well. I am just trying to figure out why. currently have a whole load of resistor and LED videos on youtube queued up to watch. trying to understand why 2 150ohm resistors are used... doing the resistor calculators online they do not give the option for a 2nd resistor. they only calculate for 1 resistor for multiple leds...

    so i guess my question is... since there are 2 150ohm resistors in this series does that mean the total resistance value is 150ohm or 300 ohm for this circuit?
     
  5. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    i guess what i need to do is really go home and find out what exactly the voltage is from the source... i think that would help me better understand the ohms law calculation..
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,432
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    Two 150Ω resistors add up to a single 300Ω resistor. They could have used two resistors for symmetry.
    Also two resistors can handle twice the wattage as the single resistor.
    Assuming that the current in the circuit is 20mA, the power dissipated by each resistor is I x I x R.
    Each 150Ω resistor has to dissipate 0.02 x 0.02 x 150 = 0.06W
    A single 300Ω resistor would have to dissipate 0.12W.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well, the current in the first circuit should be equal to
    I = (Vbat - Vled1 - Vled2)/(R1 + R2) = (12V - 7V)/300Ω ≈ 16mA
    In the second circuit
    I1 = (Vbat - Vled1)/R1
    I2 = (Vbat - Vled2)/R2
    and from this
    R1 = (Vbat - Vled1)/I1
     
  8. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    Thanks jony, and mr.chips I'm going to bed the wife is trying to take my notepad and phone a2ay. I will get back to the notes and calculations tomorrow when she's napping thanks for all this info!!
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Enjoy ;).
     
    MrChips likes this.
  10. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    haha. punctuation. you gotta love it :) im blaming the blue Hawaiians i had when i wrote that.
     
  11. bwilliams60

    Active Member

    Nov 18, 2012
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    If you're seeing blue hawaiians, that's got to be good stuff. You can change the title by going to thread tools - edit title up in the top right corner.
     
  12. vladthegreat1

    Thread Starter New Member

    Dec 26, 2014
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    Hey guys, So i am back home. I went ahead and checked the voltage that I am getting from the source. Turns out it is 12.16V... So i went on those websites where you can calculate the Resistor necessary... I think it may be possible I had my forward voltages messed up... Because Now I am getting a total resistance requirement of 280ohms... so i think thats pretty darn close. starting to make sense to me now. Thank you all. And thanks for changing the title for me.
     
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