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Thread: Multiple diode question

  1. #41
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    Wow, very impressive, that is inspiration


    Quote Originally Posted by p1t8ull View Post
    It needs a bit more work yet, but I have finally got something together. I just need to mount the PBS and mirror and also make a base/enclosure. The finished module will have a footprint of 50x85mm and be 35mm high.

    The original plan was to make 3 of these (6 diodes) and combine into one beam using mirrors/mounts for a >500mW module, but after testing the diodes and getting a bit more power than expected, I have decided to stick with these two for now, as combined they produce just about the level of power I need for the RGB projector I'm putting together at the moment.

    Thanks for all the input re. wiring multiple diodes in series, very enlightening.

    Pit.










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  2. #42
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    Quote Originally Posted by p1t8ull View Post
    Thanks Yadda and steve, spot on.
    Ramped up the voltage gradually to 6.5 volts & the current soared with it.
    Hmm. If that's how it's working, then it sounds like the current remains constant throughout the circuit, but the voltage drops evenly across each diode. That's what I'd expect with resistors from my electronics 101, but I couldn't remember if diodes worked like that too. It also means that if you had 3 of em you'd need 9 volts or so, or basically add 3v for every diode.

    A potential problem I see is if one diode does decide to short circuit, you'd suddenly have too much voltage hitting the other diode(s), taking them out too. I doubt that would happen though. It's more likely a dead diode would open the circuit I think.

    Others here with more experience with dead diodes may have more to say about that though

  3. #43
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    I don't know about LDs but I've seen alot of shorted power diodes in my time..
    a few opens too, If one diode shorts you are correct it would apply the extra V across the others for a few uS until the CC supply was able to compensate--maybe enough time to fry the others, maybe not if they can take the extra voltage. Depends I guess on the diode quality and if you're running them to the rail--balls to the wall
    Steve

  4. #44
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    I have a couple of dead LDs that are short, none of mine went open.

    Jim

  5. #45
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    Without going to the quote route, lazy today.

    I have also seen LD's short out....so series is out for me.

    and even in the same batch there is diode parameter differences from one diode to the next...
    one must buy a large batch and cherry pick..
    This will be true for either series or parallel use.
    All duals or quads I build are matched sets as close as I can get them..

    I run separate drivers ..that is the safest way , period.
    just make the drivers the same.

    Altho, I am working with an unusual arrangement at the moment, just for kicks and I was well surprised.
    In the dual 400mW set-up I just built. A new all-in-one unit..I decided to parallel both drivers as my drivers are current limited for driving only one diode and not adjustable for safety reasons. to double the needed current needed to drive two diodes in parallel. and also to run from +5 volts.
    It effectively doubled my analog frequency response too..neat. I didnt need this tho but I did have to adjust a few components in the feedback loop to get perfect square-waves out at 1 to 50khz. not by much tho.
    Also by not going to a higher voltage needed as in series, the voltage swing is smaller so the rise and fall times are quicker. like in ECL logic. So , so far parallel seems to work, but be very careful. match them first.

    I have also noticed that using the Merideth mounts sux.
    altho, nice shape and easy to throw together for an experiment they are sloppy.and the fact that I have to isolate my diodes from the base made it worse. my diodes are above ground. Ive tried all the circuits that use diodes at ground potential and found they did not get the nice fast freq response I was looking for, strange as it seems.

    Also,due to the fact they (merideth) use springs to hold the optics to the front as well as pushing the diode to the rear.. a bump will change pointing accuracy BAD.
    So, Im going back to my mounts, they never change.

    With all this trying to get the footprint down so the lasers are smaller.
    There has been some work involved, unlike when using separate lasers with there own drivers like I am doing presently.
    Last edited by marconi; 05-26-2007 at 17:22.
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  6. #46
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    Well, While it was nice...
    I went back to the single driver per diode arrangement.
    I lost a few diodes trying this experiment of using a single driver.
    Too many variables to worry about. Altho a very good driver, Temperature changes and mistakes will do you in.
    Lets just say, Dont be lazy and use separate drivers.
    There is no free lunch. Altho it can be done either series or parallel.
    Things change that you dont realize. and should one diode fail, the other is guaranteed to go for a ride too.
    OBTW, they went open circuit.

    Also, I still do not like the Meredith mounts..I did get rid of some play in the threads but the way the diodes are
    mounted and that damned spring, a good bump will make you open the box to do some adjustments.
    But, They are easier to mount and look good..just too bad about the slop.

    I do have two prototypes running this way , they seem ok, for now.
    But I didnt like doing it the single driver way.
    They will be up for sale shortly when I am thru with them.
    Each doing at least 450mW at 659nm.
    They are smaller than the dual single module route, altho not as impressive
    and definitely not as easily adjustable. but they work and take up less space.

    p1t8ull: Nice mount...some good machining there.
    I noticed the collimators that you are using.
    I have tried those and was not too pleased. they eat a lot of light. Like 18 percent.
    They do make a nice thin beam tho at twice the spot size as the Philips lenses at 5 percent loss.
    And at the spot where you have them, be sure they are easily adjustable because you will need to do that every now and then without a lot of play.
    I would like to see the mirror mounting when you are done..did you do it yet.
    Did you find matching at nearfield a problem?

    I hope they come out ok..I cant seem to figure out how to enter them properly into my post in the right order..
    I liked the old HTML way better...ah well.
    Here is a completed unit with TEC and dual driver. 475mW The cheaper way - top
    Dual expensive way to do it , Its going into an even smaller box - bottom.
    Attached Thumbnails Attached Thumbnails number1_1.sized.jpg  

    number_2.sized.jpg  

    Last edited by marconi; 06-04-2007 at 23:52.
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  7. #47
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    Damn nice!
    You must have steady hands Marconi
    What is that BS cube- about 1mm x 1mm

  8. #48
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    Exclamation


  9. #49
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    All Hail!.......................oo

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