help with 1st LED setup

Discussion in 'General Electronics Chat' started by lfcreds91, May 4, 2011.

  1. lfcreds91

    Thread Starter New Member

    May 4, 2011
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    i will be setting up my 1st LED base.....

    this is how it looks.
    [​IMG]

    the LED will be a 5mm spectrum or rainbow LED.......those that changes color every 0.5 sec.

    battery will be 27 V, by joining 3 9V battery together.

    resistor will be a 1/4 470 resistor.

    the problem is i dont really know the LED forward voltage/current.

    judging by the above setup, do you guys think its good to go?

    or is it gonna BLOW!!!!!!!!!
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Care to show us the rainbow LEDs datasheet or source? They are circuits unto themselves you know, so the normal rules on LEDs do not apply.
     
  3. lfcreds91

    Thread Starter New Member

    May 4, 2011
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    doesn't tell much other than this info.....

    [​IMG]
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    They mention 3V. 4X3V is 12V, so you have 27V-12V (15V).

    15V/470Ω is 32ma, which is good enough.

    However, your resistors will dissipate ½W. A general rule in electronics is over rate components by X2, so you would need a 1W resistor.

    Instead, how about putting the 4 LEDs in series for 8 LEDs. This would create a 24V drop. Then put a 100Ω resistor in series with them. That resistor would dissipate 90mw, much cooler.
     
  5. lfcreds91

    Thread Starter New Member

    May 4, 2011
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    sorry , but im not understanding your suggestion.

    is this what you are suggesting?
    [​IMG]
    so, my setup with 27V, 470 ohm is good to go right?
     
  6. lfcreds91

    Thread Starter New Member

    May 4, 2011
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    or is this what you meant?
    [​IMG]
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I do not know if the current will change when the colors are changing.
    If that is the case the pattern will show changes in current
    wich also will change the voltage on the leds,
    due to then fised resistor.

    Bertus
     
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  8. Wendy

    Moderator

    Mar 24, 2008
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    This was what I was talking about. Since these are not regular LEDs (like I said, actually circuits on chips) there is a bit of chance involved here. I've never messed with them myself.

    Odds are they will be out of sync with each other, each LED doing its own thing.
     
    lfcreds91 likes this.
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
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    These circuits want a constant voltage so just find a way to wire them in parallel with 3-4.5V each. 2-3 D cells should just about do it. By connecting them in series, you risk destroying them since the voltage drop of each is not constant, not even close. If you don't believe me, just hook one up to 3V and measure the current as it operates.

    The 3V rating is probably a minimum. The ones I've used all wanted 4V. When you consider that the blue LED in the device needs ≈3.8V to draw it's full current, the higher supply voltage makes sense. Consider this datasheet for example: http://www.etgtech.com/pdf/oldspecs/RAINBOWLED.pdf. Try to get some real specifications from the seller. When you have the right supply voltage, the device should draw a peak of ≈65mA.
     
    Last edited: May 4, 2011
  10. Audioguru

    New Member

    Dec 20, 2007
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    I agree that color-changing LEDs should never be connected in series.
    But I guess they still need something to limit their current since they are usually used in a low power solar garden light that does not have much current.
     
  11. lfcreds91

    Thread Starter New Member

    May 4, 2011
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    i'll get back to you guys once i received the specs.....

    right now cant proceed unless i know what im dealing with.
     
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