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Fading/dying Blue LEDs

Discussion in 'LEDs and Optoelectronics' started by spioke, Jun 22, 2010.

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  1. spioke


    Jun 22, 2010
    Hey guys im not really a circuit type guy, but im trying to build a circuit with 2 switches and about 5 Large blue LEDs (i dont know the specs on the LEDs at all... sorry)

    The Plan:
    I want to make a circuit i can install in my car.
    The LEDs (5 wired in parallel) will be hidden in my vents, and there will be 2 switches
    1 switch will be a momentary on switch hidden in my shifter knob, so that when i push a button on the shifter, the lights will light up as long as the button is depressed.
    The second switch is an on/off 3 prong switch; when in the OFF position, the lights work according to the momentary on switch (in the shifter knob). when in the ON position, the lights will simply stay on.

    Simple circuit right?

    I built the circuit and it runs fine.
    HOWEVER, i ran it for a test to see how long the batteries would last
    and an hour later i noticed that the LEDs i was testing were pretty dim
    so i compared with new LEDs and confirmed.

    I dont know anything about the parts im using
    i just bought them at a surplus store
    and they only had the information for the regular sized LEDs
    im using the larger ones, and when i run the circuit off of 3 volts its too dim
    but running if off of 6V killed my LEDs

    Someone please help

    Thank you
  2. Resqueline


    Jul 31, 2009
    What kind of batteries were you using?
    And what about the resistors all LED pages tell you to use? No? ;)
    Some flashlights operate w/o resistor, but then at 4.5V (and they still kill the LED's, only a little slower than 1 hour).
    Typical max current for most LED's is 20mA. The current is not limited by the LED itself, that must be done externally. Consider a 3.3V LED drop to be common.
  3. spioke


    Jun 22, 2010
    i was using 4 AA batteries wired in series

    and as for resistors, its been SOOOOOO long since i have worked with any electronics that i wouldnt know where to start in picking the right resistor...

    if i were to toss a resistor into the 6V circuit what would i need to keep the brightness without murdering my bulbs?

    ps thanks for the reply!
  4. Resqueline


    Jul 31, 2009
    Poor LED's... ;)
    There are LED resistor guides & calculators all over the place.
    And you don't need "a" resistor, you need five - one for each LED..
    Anyway, here's the formula: (6V-3.3V)/0.02A=135 Ohms. This exact value is theoretical and you may have to choose between some standard values.
    If it's better to go down to 120 Ohms or up to 150 Ohms will depend on the actual forward voltage drop of the LED's (at the current they're supposed to work on).

    Oh, you won't get the brightness you were getting at first, but you'll get to keep it for a loooong time. You might opt to double the current and still get 10000(?)hours life.
    Last edited: Jun 22, 2010
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    RIP Blue LEDs

    In their short, but very brilliant career these 5 blue LEDs outshone all around them,

    Sadly for their friends and family their brilliance began to fade rapidly and now they are but a shadow of their former selves.

    When asked for comment, a spokesman said that their die was cast in a high voltage environment and with no resistance to their current path they burned brilliantly until the very fire that drove them burnt them out.
  6. Laplace


    Apr 4, 2010
    I went to the website and looked up the specs for a typical blue LED, their 5mm LED RL5-B2445. Maximum forward current is 30 mA at 25 degrees Centigrade ambient temperature. From the attached voltage/current curve, the forward voltage will be 4.1 volts at 30 mA. But look at the relative intensity curve where each doubling of the forward current only yields a 25% increase in the relative intensity. There is not much reason to have the forward current go beyond about 20 mA.

    But why would you use batteries when you could just tap into the car's electrical system? Do they still put cigarette lighters in cars where you could obtain 14.4 volts? The resistor I would use for each LED: (14.4 - 4.1)/0.030 = 343 so choose 390 ohms at 1/2 watt.

    Attached Files:

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