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Thread: Series LED help please

  1. #1
    Flashaholic* Streak's Avatar
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    Default Series LED help please

    I have been testing some blue 5mm LED's from a new manufacturer.
    I placed 3 or 4 LED's in series and ran them off a small 33A 12v battery.
    Sometimes all three lit up sometimes one of them did not want to light up but flickered a bit.
    When the one that flickered was tested in isolation across a 3v lithium coin battery it seemed dead.
    What is going on here? Being in series, Vf should not be a huge problem. I tried direct voltage as well as through a 1K resistor, same problem.
    I have tried this with 3 different batches of LED's and each batch has this problem.
    Same problem with greens but the whites are OK.

    Could it be a bad batch of LED's or static
    damage?

    By the way, the final product requires that the LED's are mounted in series and run off this liitle 12v battery sometimes with a resistor in series.

    Thanks for the help.

  2. #2
    Flashaholic
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    Default Re: Series LED help please

    Well, if you run it without the series resistor first, you might be blowing the LEDs in that case.

  3. #3
    Flashaholic*
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    Default Re: Series LED help please

    If you ran the LEDs without a resistor, its a good chance that you exceeded the max current of the device and destoyed it; its not going to work again

    Keep adding up the forward voltages of the LEDs until it is just under 12v, then use Ohms Law to calculate the series dropping resistor to dissipate the excess voltage

    For example, if you had 3 3.5v LEDs, that would total up 10.5v - you would need to "get rid off" 1.5v, assuming the LEDs are 20ma - the equation would be:

    I=E/R
    R=E/I
    R=(12-10.5)/.020
    R=1.5/.020
    R=75 Ohms

    1/4 Watt should work just fine

    Most light bulbs are "forgiving" and will take a incredible amount of overload for a short time before they "blow out", LEDs are not as forgiving and will not take the excess current

    When an LED fails - it can either be "open" or "short"

    Also remember, that LEDs are semiconductor devices and only work when the current flow thru the device is in the proper direction, not like light bulbs that work either way - so you could have connected them "backwards" as well; and this could have damaged the LEDs as well

    Always use a current meter when designing with LEDs, since the LEDs would be in series - monitor the current thru the string and keep the current under 20ma - assuming you are using T1-3/4 (5mm) Parts

  4. #4

    Default Re: Series LED help please

    Originally posted by Streak:
    Thanks Mike/Entropy.
    What I find strange is that this happened when I used 4 LED's in series. 4 Leds would have a total Vf greater than 12v so no resistor should be necessary.
    3 would light up and one in the middle would not. Pull one working led out of circuit and all of a sudden the remaning 3 work.
    All a bit confusing!!
    <font size="2" face="Verdana, Arial">Here's my guess....
    InGaN LEDs that have experienced quantum well damage due to static electricity or overcurrent can exhibit the following strange behaviors:

    o They will act as a low value resister for
    small currents and not light up. So in a series
    string they will be dark for low currrents while
    the other LEDs in the string will light.

    o At higher currents they may light - though not
    as brightly as a non-damaged device.

    o At very high currents (60mA - like when you
    connected the LED to a 3V lithium cell) the
    quantum well damage can get vaporized allowing
    the device to work normally again for a time.
    Generally though this LED would not be as
    efficient and might be prone to becoming dark
    again.

    I've observed these behaviors in several vendor's
    LEDs, and talked with a few LED guys about them.
    The LED guys suggested the failure mechanisms I
    mentioned. I had to return a batch of LEDs due
    to static damage causing symptoms like this.

  5. #5
    Flashaholic* Streak's Avatar
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    Default Re: Series LED help please

    Thanks PHP. I figured something like this. Just wanted to understand the facts before getting back to the manufacturer.

    http://www.worst4x4.freeservers.com

  6. #6
    Flashaholic* Streak's Avatar
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    Default Re: Series LED help please

    Thanks Mike/Entropy.
    What I find strange is that this happened when I used 4 LED's in series. 4 Leds would have a total Vf greater than 12v so no resistor should be necessary.
    3 would light up and one in the middle would not. Pull one working led out of circuit and all of a sudden the remaning 3 work.
    All a bit confusing!!

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