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Thread: Multiple diode question

  1. #1
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    Default Multiple diode question

    Not being all that sharp with electronics...

    I'm not sure if this would work, but does anyone have an opinion on connection up several 200ma diodes in series to a single ACC driver?

    I'm guessing not, as the voltage would rise and fry them?

    Cheers

    Pit.
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

  2. #2
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    I'm not sure what an ACC driver is, but bare diodes are VERY sensitive to any voltage (including static discharge) other than their design parameters. Why series and not parallel?

    Steve

  3. #3
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    ACC = Automatic Current Control.

    Googling for "laser diodes in series" brings up enough references (including one in a PDF by Wavelength Electronics, who are one of the most established general suppliers of laser diode drivers) to suggest that it's a common practise.

    Advantages are lower waste energy, and easier control. It has to be automatic current control, ideally, so that the current never rises dangerously if one diode fails by going short-circuit.

    The over-voltage problem won't exist, because the voltage across a laser diode is determined by diode's forward voltage, not that of the supply, so only the supply current affects output. If a series chain is switched on, and the voltage has risen to close to the entire voltage drop for the chain, there is not yet any current flow, and as the voltage rises with applied current, the balance across each diode is maintained. If one diode starts conducting before the others, it won't see a further voltage rise, so a controlled current prevents any unsafe voltage rise, unless it be induced by some spike from a bad supply.

    Same thing exists with LED's, and those are often driven in long series chains. Personally, I'd limit the number of laser diodes per chain, purely on a cost basis. Don't keep all the eggs in one basket, and such... Basing the driver count on the square root of the diode count seems about right to me.
    Last edited by The_Doctor; 02-23-2007 at 15:18.

  4. #4
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    Cool, thanks for the info. It all makes a bit more sense now

    Pit.
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

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    Oh, constant-current mode, i see...

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    Question

    There's a lot a greek up there... but could you blank them separatly?
    Love, peace, and grease,

    allthat... aka: aaron@pangolin

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    Quote Originally Posted by allthatwhichis View Post
    There's a lot a greek up there... but could you blank them separatly?
    What's so hard to understand? I might be wrong in some point (talking about one diode wanting to conduct early in a series chain, for a start, is improperly put, considering that NO current flows till the total voltage reaches the total voltage drop), but everything else is clear.

    Of course you can't blank diodes in a series chain separately. Why would you want to anyway? The point of series diodes is to run them from a single driver, and you modulate that.

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    If your diodes are expensive, I would recommend against running it in series with a single driver... Conceptually it would work well, right up until you start "tuning" them for max power... Near that point, you'll note that the vdrop will make the math a hair shaky when balancing your current. This raises a high potential to pop the first diode in the chain. The "sketch" factor increases exponentially with the number of diodes.

    If you're not planning on running the diode at max power, perhaps at 80%, then I see no problems with it, aside from figuring out the current for the lasing threshold (which may not be totally sane when modulation is used).

    If asked, I would try it if I knew the individual specs on each diode I was driving but I'd certainly use seperate drivers if the diodes were unknowns or if I only had a general spec from the manufacturer. YMMV

  9. #9
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    Quote Originally Posted by The_Doctor View Post
    What's so hard to understand? I might be wrong in some point (talking about one diode wanting to conduct early in a series chain, for a start, is improperly put, considering that NO current flows till the total voltage reaches the total voltage drop), but everything else is clear.

    Of course you can't blank diodes in a series chain separately. Why would you want to anyway? The point of series diodes is to run them from a single driver, and you modulate that.
    When it comes to electronics... at least to the degree here, I'm a dumbass... mainly ignorant. Most of the conversations about "making" lasers or drivers and such are over my head... I just like to draw attention to it now and then. Most of the people I hang around make me feel very intelligent relativly... It is moments like this that I feel a little stupid; that feels good every once in a while.

  10. #10
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    Allthat: I should have pointed out originally that these are all the same diodes simply being optically combined into one beam, so modulating them simultaneously is a good thing.

    All the diodes are new (so I have the full spec) 130mw DVD jobbies that aren't toooo expensive - so I'm not scared of sacrificing a few in the quest for power so I'll connect up 6 in series with one driver and push them to around 90% and see how they run and modulate. I can always add more drivers if the smoke escapes.

    Thanks for the thoughts, I'm going to start milling some aluminium and I'll let you know what happens in the next couple of weeks...


    Pit.
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

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