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question about wiring leds and the resistor(s) involved! - Page 2

post #11 of 13
Thread Starter 
Firstly, thanks a million radodrill!

Secondly, I currently have 100ohm 1/4w resistors in place (for use with 5v), were I to simply add a second 100ohm 1/4w resistor in parallel to the ones already in place, would that be sufficient to switch the input current to 12v (important clarifications about this in following paragraph!)?

I actually tried the 12v input current and the LED were brighter (much to my liking), but the temperature of the resistors was ~65-70C (which is the max operating temperature of the resistor). Ideally I would like to keep that increased glow, but keep those resistors at a more manageable temperature. Would my idea achieve this goal?

Thanks in advance (rado)!

/e Just curious though, to get the equivalent of a 200ohm 1/2w resistor with 100ohm 1/4w resistors I would need 4 (a parallel of 2 series of 2)?
Edited by CooooooooL - 1/27/12 at 11:19pm
post #12 of 13
For series, resistances add; while in parallel, the reciprocals of the resistances add up.

i.e.
  • Series:
    • Equivalent Resistance:
      Req = R1 + R2+...+Rn
    • Current across individual resistor j:
      Ij = I (Note I is the current across the equivalent resistor)
  • Parallel:
    • Equivalent Resistance:
      1/Req = 1/R1 + 1/R2+...+1/Rn
    • Current across individual resistor j:
      Ij = I * Req / Rj


A pair of 100 Ohm 1/4W resistors in series would be equivalent to 200 Ohm 1/2 W.


A word of caution with LEDs, you really want to make sure you don't overvolt them. A little bit of extra voltage across them probably won't have much effect, but too much and they can easily fry; when they are overvolted they start out by burning a lot brighter than they should and often emitting the wrong color (e.g. red burns amber) and at a given voltage they'll just die.
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post #13 of 13
i think ill subscribe to this thread, it seems interesting, i might try it someday when i have to.
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