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Thread: Laser grounds making me mad

  1. #1
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    Default Laser grounds making me mad

    Has anyone else run into this problem? I have an aluminum plate with my green and laser mounted to it. My green laser case is connected to the positive lead. The red laser case is connected to the negative lead. I am using a single power supply to power each laser driver. So, the red laser is shorting out my green laser!

    I can probably insulate the red laser from the plate but I didn't want to have to do that. Or, I can use a seperate power supply for each laser driver but I am having a hard time thinking through what the electrical implcations would be... Would that even work?

    The green laser is a 50mw Aixiz and the red laser is a home built from a DVD burner diode from Dr. Lava. Why would the Aixiz have the case positive? That really throws a wrench into things.

  2. #2
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    You will have to insulate the laser.

    simple as that.

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  3. #3
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    Yes this is a common problem that we all suffer with. Actually the red diode is the one breaking convention as laser diodes have traditionally been positive earth (i.e. anode to case) for reasons best known to diode manufacturers. Now the recent trend is to make all these DVD diodes cathode to case again for some unkown reason - though im sure there must be a GOOD reson to do this as all DVD diodes I have come across have this format.
    As Dave says insulate the red. You can buy sheets of thermally conductive matting (like the stuff to220 insulation is cut from) that will provide electrical insulation but let you benefit from a common ally base.

    Whats easier adding another power supply or putting a thin layer of insulation between one of the lasers and keeping the convenience of a single psu? No contest really.

    I have a similar problem in that I am building a projector with 2 combined reds and want them to stay cool and be mounted on the same base plane but have only a driver circuit that controls the cathode - the grounded case - of the diodes. I have just accepted that insulation is the only way to go.


    Rob

  4. #4
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    I will use the insulation you suggest but not with the red. I will use it with the green since it is the only thing that doesn't have a grounded case. Now I just need to figure out where to buy some 3mm nylon screws. Grrrr.

  5. #5
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    Quote Originally Posted by carmangary View Post
    I will use the insulation you suggest but not with the red. I will use it with the green since it is the only thing that doesn't have a grounded case. Now I just need to figure out where to buy some 3mm nylon screws. Grrrr.
    Or nylon bushings. M3 is a standard for fitting transistors and IC's in TO220 packages, so you can use steel hardware which will stay aligned better, longer. There is silicone sheet that is thinner than the usual stuff for transistors, and slightly rougher as it's a glass fibre weave lightly loaded with silicone rubber. I can't remember who sells it (mine lasted years so far, still got lots left), but you can have very good thermal contact with it, as well as electrical isolation.

  6. #6
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    Quote Originally Posted by stanwax View Post
    Now the recent trend is to make all these DVD diodes cathode to case again for some unkown reason - though im sure there must be a GOOD reson to do this as all DVD diodes I have come across have this format.
    PC hardware and most domestic metal cased gadgets have negative grounds. So do car systems, mostly. It's cheaper and less risky to design for that than to try to stick to an earlier convention whose place is limited to the inside of a diode can. I think the new negative ground laser diodes are a good move. Some isolate both terminals from the case, if they don't have a photodiode. Only one I had like that though, was Opnext HL6526FM.

  7. #7
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    the reasons for cathode ground makes sense and I agree entirley its a good move - just wonder the reason anode ground was adopted in the first place as was clearly an unconventional way to do it that we have all just gotten used to.

    I also have made use of the M3 insulators they are one of those items I horde when I strip dead stuff down before discarding.

    Rob

  8. #8
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    Quote Originally Posted by stanwax View Post
    the reasons for cathode ground makes sense and I agree entirley its a good move - just wonder the reason anode ground was adopted in the first place as was clearly an unconventional way to do it that we have all just gotten used to.
    I'm not sure why it was, never used to have any idea, just thought is was a convention designed to pervert a rival's bid to sell. Someone here posted about it though, saying it had to do with the way the diode chips were mounted, positive side to the metal mounting post. I don't know why it had to be that way, or even if it did, as it doesn't seem to be necessary now.
    I also have made use of the M3 insulators they are one of those items I horde when I strip dead stuff down before discarding.
    Me too. I used to have lots from that, anything good quality stayed. And usually got used.

  9. #9
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    Ok, so what are M3 insulators?

    I ended up ordering a 3"x5"x.004" sheet of mica from McMaster-Carr today, along with some plastic M3 screws. A quick pass on the milling machine to remove .004" from my base and I should be back in business. I should be making pictures with 3 colors by this time next week.

    Building custom hardware is certainly a pain in the butt!

  10. #10
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    M3 you know already, just shorthand for 3mm hardware for round holes... So the insulators will be small bushings to fit between nut/washer(s) and bolt head. If you search for transistor insulators you'll find them. They allow far better clamping force than nylon screws, because you can use stainless steel ones. Nylon resists clamping better than pulling or twisting.

    Mica is good, but that could really be a pain. Cutting it neatly is hard, avoiding flaking and even severe damage to it is harder still. And I didn't know getting a large sheet of it was even possible. Not sure what the price of mica is, but a square foot of silicone loaded glass fibre weave is cheap. And so much easier to use, I'd not use otherwise unless I was wanting mica for very high temperatures or some other specific reason I've not run into yet.

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