Electronics help on wiring up some led's

Discussion in 'Modding' started by PrnkstrO, 7 Jul 2003.

  1. PrnkstrO

    PrnkstrO What's a Dremel?

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    I want to wire up 20 led's in parllel to light up something for my girlfriend. Im not very knowlegable when it comes to this so I thought id ask the experts.

    I found out that I need 16.5 Ohm resistor. (12V input, 2.1V leds at 30mA) Am I correct in thinking that I need 20 resistors wired up to the 20 leds or just one resistor going to all of the leds? Im pretty sure I need 20 resistors but I was just checking before I bought 19 more than needed!

    Thanks for the help!
     
    PrnkstrO, 7 Jul 2003
    #1
  2. kevbarlas

    kevbarlas What's a Dremel?

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    i think logically you would just need one at the begining because the voltage wont suddenly go back up after first LED
     
    kevbarlas, 7 Jul 2003
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  3. PrnkstrO

    PrnkstrO What's a Dremel?

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    I set it up in that fashon to try it out and after a few seconds the resistor became very hot and started to smoke.
     
    PrnkstrO, 7 Jul 2003
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  4. whypick1

    whypick1 The über-Pick

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    My guess is that you're using a typical 1/4w resistor. This is a bad thing. What you just did was well on the way to making a miniture light bulb. The resistor needs to dissipate a total of (8.9v * .03A * 20) 5.34w. I'm surprised you didn't instantly blow it up, as you're putting about 6 times as much current (maximum current a resistor can handle is expressed as (P/R)^.5) as it can handle.

    I'd consider making a series-parallel arrangement of LEDs. A 5x4 net of LEDs (5 LEDs in series, 4 parallel strings of 5) with a resistor for each string would be the best setup. In that case, a ((12 - 10.5)/.03) 50ohm resistor would be needed for each. And you can use 1/4w resistors as well, a 50 ohm resistor can handle .07A of current. Plus, if one LED fails, then all the LEDs in that string will simply stop lighting rather than blow up.
     
    whypick1, 7 Jul 2003
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  5. SteveyG

    SteveyG Electromodder

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    :eyebrow:

    How did you work that one out? A resistor can only handle its maximum power rating.
     
    SteveyG, 8 Jul 2003
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  6. PrnkstrO

    PrnkstrO What's a Dremel?

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    So what would be the best way to wire up these leds?
     
    PrnkstrO, 8 Jul 2003
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  7. whypick1

    whypick1 The über-Pick

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    Stevey: Just a little bit of Algebra, that's all.

    First, two equations must be established, V=IR (Voltage = Current * Resistance) and P=IV (Wattage = Curent * Voltage). Substitute V for IR, and we get P=I^2R. To solve for I, divide P by R (P/R = I^2) then find the square root of both sides ((P/R)^.5=I). .25/50 = .005. .005^.5 = .0707106781A or 70mA.

    Prnk: if you were to do it like I explained in my last post, it'd look like so:

    Code:
    12v - - - - - - -
    |   |   |   |
    <   <   <   <
    >   >   >   >
    <   <   <   <
    |   |   |   |
    v   v   v   v
    -   -   -   -
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    |   |   |   |
    v   v   v   v
    -   -   -   -
    |   |   |   |
    |   |   |   |
    v   v   v   v
    -   -   -   -
    |   |   |   |
    |   |   |   |
    v   v   v   v
    -   -   -   -
    |   |   |   |
    |   |   |   |
    v   v   v   v
    -   -   -   -
    |   |   |   |
GND - - - - - - -
     
    Last edited: 8 Jul 2003
    whypick1, 8 Jul 2003
    #7

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