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Thread: what resistor for 445 diodes in parallel?

  1. #1
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    Default what resistor for 445 diodes in parallel?

    im sure its on the forum somewhere but i cant find it.

    when running more than one 445 diode off a driver what resistor should i use?

    cheers
    Eat Sleep Lase Repeat

  2. #2
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    Andy dont just use a resistor use ohms law!

    Rob
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  3. #3
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    is it really the law, if i dont use it will i get arrested?

    Eat Sleep Lase Repeat

  4. #4
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    I use 1/2 watt 1ohm. Its easy to measure the current through a 1 ohm resistor!

  5. #5
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    Given the massive tolerences of those 445s I would be more comfortable with a bit more than 1Ώ cushion between them and 1A @ 1V is 1W so a half watter will be toast
    http://img62.imageshack.us/img62/3985/laser.gif

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  6. #6
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    your right. If driving at 1amp need 1 watt. 3 is the most I have paralleled. If I remember right, I drove them with 2amps total. I know for fact i checked the current to each diode and they were within a few mA.


    V=I X R

    P=I X V or P = I^2 X R
    Last edited by logsquared; 02-16-2011 at 13:10.

  7. #7
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    Yeah, if the diodes a fairly well matched there will be no problem. LDs behave like zeners so if you have two that are wildly apart and you haven't got enough 'flex' between them, you could end up with one crowbarring all of the current while the other twiddles it's thumbs.

    Analogy of the day: Two eggs need to be clamped to a surface with one clamp, if the clamp's face is rigid you better hope that the eggs are exactly the same size < or should that be egg-sactly

    You could fit a spring loaded spreader to the clamp, but again, the springs need to be soft enough to give before the yolk squirts out
    Last edited by Doc; 02-16-2011 at 13:37.
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  8. #8
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    Quote Originally Posted by Doc View Post
    Analogy of the day: Two eggs need to be clamped to a surface with one clamp, if the clamp's face is rigid you better hope that the eggs are exactly the same size < or should that be egg-sactly
    Lamb.

    Or should I say "steak and..."

    - There is no such word as "can't" -
    - 60% of the time it works every time -

  9. #9
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    I used to use water analogies but they dried up, tried bread for a while but that got stale so I just stick to dairy products now. W * E + H = O

    Mmmm omelette.
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    Doc's website

    The Health and Safety Act 1971

    Recklessly interfering with Darwinís natural selection process, thereby extending the life cycle of dim-witted ignorami; thus perpetuating and magnifying the danger to us all, by enabling them to breed and walk amongst us, our children and loved ones.





  10. #10
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    I would have used more than 1 ohms with these 445's, simply due to they higher Vfwd spec and variance. But then again, they are rugged little buggers.
    Quote Originally Posted by andy_con View Post
    is it really the law, if i dont use it will i get arrested?

    That is the beauty of ohms law, you can't break it! You can ignore it, but it still exists and is the law.
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