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Thread: Multiple LEDs: Series or Parallel

  1. #1
    Unenlightened
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    Default Multiple LEDs: Series or Parallel

    I was wondering if lights that have multiple led's such as the Gerber Trio, are wired in series or parallel? Could it be different in different brands?

    The reason I was wondering about this is in regard to reliability. If one led were to burn out, would the other two continue to function?

  2. #2
    Flashaholic*
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    Default Re: Multiple LEDs: Series or Parallel

    The more general answer, is that LEDs can fail either open or closed circuit. The LEDs that I damaged by overdriving them without adequate heat sinking, appears to just put out less light without substantially changing the electrical characteristics - no idea if this is common or not.

    If the LED fails open or closed - the challenge is the same - the circuit will die relatively quickly in most cases unless it is current managed vs DD or resistored.

    IMHO, the odds of a complete LED electrical failure are relatively low compared to other methods of a flashlight failing. In my own case, I broke my last flashlight when I dropped it on some rocks - the bulb was still fine, the plastic body had cracked, severing the thin metal ground return.

  3. #3

    Default Re: Multiple LEDs: Series or Parallel

    I would think most multiple LED lights are wired in parallel, that way if one LED dies the others just divide up the current it was using amongst those that are left. depending on the quantity of LEDs the additional current divided may or may not put harmful overdrive current on the remaining LEDs causing them to fail also. The more LEDs in the circuit the less current per LED increase would happen when one fails.

  4. #4
    Flashaholic*
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    Default Re: Multiple LEDs: Series or Parallel

    If every LED has its own resistor, then one failing open won't increase the current to the others. However, if there is a shared resistor upstream, or they are truly in parallel with just one resistor, then what Lynx Arc said will be true.

    However, if they are truly in parallel to begin with, then there is probably some current imbalance already, so several may be underdriven, while several may be overdriven. In the worst case, one wierd outlier with a very low Vf is being severly overdriven.

  5. #5

    Default Re: Multiple LEDs: Series or Parallel

    Even seperate resistors per LED won't entirely eliminate a slight voltage increase when one LED drops out. It would depends on how hard the batteries sag under load. The advantage in seperate resistors for EACH LED is protection against widely differing Vf across the collection of LEDs in parallel, including possible Vf changes brought on by heat and overdriving and failed LEDs.

  6. #6
    Flashaholic Skibane's Avatar
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    Default Re: Multiple LEDs: Series or Parallel

    As Lynx Arc implied, putting several LEDs in series has some advantages, since (1.) an equal current flows thorough each LED, and (2.) only one resistor is required (if any is required at all).

    Zetex makes a popular line of ICs that are specifically designed for this – They'll take a few volts from one or two batteries and up-convert it to whatever voltage is necessary to drive up to 8 series-connected white LEDs at full current. Naturally, the up-conversion process iteself wastes a little energy, but it's still very efficient.

  7. #7

    Default Re: Multiple LEDs: Series or Parallel

    All the *flashlights* I've seen with a few LEDs are directly in parallel, like the three in the PT Attitude which are in series with 15 ohms and the battery on the ones I have.

    OTOH, the 7 LED 4AA from Streamlite has individual resistors, 22 ohms IIRC. One per LED.

    The Eternalite has a driver per LED of course.

    The effect of failed LED varies. The 'die' (rather than just get lame as another poster pointed out) either open or short. A short in the typical parallel setup makes the other two go dark and the total battery current to about double (making the battery dead in short order), bad Karma. OTOH, an open shifts the load to the remaining (typically two) LEDs. If the drive isn't too serious, this 50% boost isn't fatal. For instance PT uses 15 ohms in the single LED Impact, so in this case the Attitude LEDs that are still good are driven to the level the single LED in the Impact has been running all along.

    In the massively parallel case like the Streamlite 7 LED, the total light output drops to 6/7 of the original, current either follows (drops by 1/7) in the case of the open or goes up (with no more light output, pure loss) by about that same amount (goes to 8/7 of original) from a short.

    So, given reasonable design, it would seem that both will survive the demise of a single LED.

    Doug Owen

  8. #8
    *Flashaholic*
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    Default Re: Multiple LEDs: Series or Parallel

    i wire both series and parellel, and all series, and all parellel, usually trying to avoid any losses with heat or conversion.
    with the tiny leds its often a total disaster :-)
    with the 8MM quad parellel it was a total disaster, parts of the parellels died, and that shoved the series current into the remaining parellels.
    with the Luxeon, and ufo leds (mondo power) its a piece of cake comparably, they dont fry at the drop of a hat.

    flashlights are likely to be lower voltages, and so more likely to be parellel only.

    most of the banked series and parellel leds in higher than flashlight things, use some sort of resister.
    mabey not enough to keep from different light output, but at least enough to try and ward off causing the whole rack to go when one does.

    are series leds like them stupid xmas lights , one goes the whole rack goes? usually it doesnt open, as much as it changes resistance, and i have seen it go both ways. but generally one overheats, starts going wackey, and the others get brighter, about then is when you realize you drove the whole rack to hard, and you stop.

    you still have to be really stupid to blow out most of this stuff, leave that to me :-) properly driven, and with proper batteries and all, the things will last. even reversed, and hit to hard for a second.

    again the Big ones are SO tough, i far prefer to deal with few big ones, vrses many tiny ones, even if they might be more efficient.

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