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Thread: Driver circuit w/ TTL blanking for bare laser diode

  1. #1
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    Default Driver circuit w/ TTL blanking for bare laser diode

    I took a laser diode from a DVD player and currently I am using 3v (from batteries) and a MOSFET from radio shack to drive it. The diode an battery is in series with the drain of the MOSTFET. The TTL out from my laser show driver hooks to the gate of the MOSFET. And, I have to say that it does all work.

    However, I am not sure that is the best way to do it and I want to eventually use a power supply (not batteries) so I would like a better driver. I have heard about the dangers of power spikes so thats why I am using batteries for now.

    Anyway, does anyone have a schematic for this? I have seen drivers on SAMs FAQ but they don't have TTL modulation. So, I am asking for help here.

    Thanks,
    Gary

    PS Right now I drive the laser diode straight with the 3v from the battery. It uses 250ma. Is there a way to tell how what the laser diode is actually rated for as far as voltage or current? I don't want to be over or under driving it.

  2. #2
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    250 ma!!!!

    I'm amazed you haven't blown the diode yet. Normally those things run at just under 100 ma. I've personally cranked one up to 135 ma, and Fred (Astroguy) has has some of his up to 200 ma. But he's had a few failures too.

    I use the "Sam's laser diode test circuit" from the laser faq to drive my DVD burner diodes. (Yeah, it's a cumbersome circuit, but I figured I wanted to be extra safe!) I haven't tried modulating the thing though, and now that I think about it, it really wasn't designed with modulation in mind. :?

    My guess is that you could put a power mosfet in series with the output of the driver and use that to switch the diode on and off. I'd suggest a series resistor though, to prevent a current rush when it switches on. Might need to play around with the values a bit to make it work. Oh yeah, and make sure you don't have *ANY* capacitors downstream of the final voltage regulator if you decide to use the mosfet idea. Reason is that when the mosfet switches on, the cap could discharge through the diode and you'd get a current spike that could kill the diode.

    Let us know how it works out!

    Adam

  3. #3
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    Really, under 100mA? My 8mw uses 50ma. Aren't the DVD burner laser diodes > 100mw? 100mA seems small for something of that power. But, I really don't have anything to compare it to since I am really all new to this.

    Anyway, I appreciate your response and I'll take a look at Sam's driver.

  4. #4
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    Quote Originally Posted by carmangary
    Really, under 100mA? My 8mw uses 50ma. Aren't the DVD burner laser diodes > 100mw? 100mA seems small for something of that power. But, I really don't have anything to compare it to since I am really all new to this.

    Anyway, I appreciate your response and I'll take a look at Sam's driver.
    I have run the DVD diodes at 180ma with great success. I would not go over this without knowing the specs of the diode. I have tried lots of diodes with no specs to work with, I have had them put out as much as 100mw at only 80ma and some take as much as 250ma to get 100mw.
    I have also blown more than I would like to think about ops:
    In short with no specs you fire em up and take your chances. :lol:
    "Gravity its not just a good idea its the law"

  5. #5
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    Do you have a feel for what the typical voltage drop across the laser diode is for the ones you have used? Or does it vary?

  6. #6
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    Quote Originally Posted by carmangary
    Do you have a feel for what the typical voltage drop across the laser diode is for the ones you have used? Or does it vary?
    It varies from one to the other, Most of them have no markings so I can`t tell if they are from same manufactcuer or not.
    "Gravity its not just a good idea its the law"

  7. #7
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    Quote Originally Posted by carmangary
    Really, under 100mA?
    Yeah, just under 100 ma. I've read that you can pulse them at a higher current level than you can run them CW, but even so I wouln't go above 200 ma under any circumstances. Like Astroguy posted - it's easy to burn them up. (Also, you're going to need better heat sinking if you want to run up around 200 ma.)

    Not sure exactly how much power they make when they're burning disks, but I didn't think it was anywhere near 100 mw. Remember that there is a focusing lens in the path, and in any case they don't want to start a fire - just make a little discoloration in the dye layer. So my gues is that the power needed is a lot less than 100 mw.

    Considering that at 60 mw you can light a match (if you use a lens), I'd say that you don't need near that much power to write to a DVD-R. Also remember that the ratio of light output to drive current is not linear. You might need 60 ma before you get it to threshold, but between 60ma and 100 ma you might see the power change from 1 mw to 65 mw or more!

    Don't worry too much about voltage drop - it will vary among diodes. All you need to do is build a rock-solid current regulator and then make sure you don't go too high w/ the current.

    Adam

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    Again, thanks. I am glad I asked the question here before I went and burned out my diode. I wish I had some kind of detector to know how much power it really has. What's the cheapest one you can buy that actually works?

  9. #9
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    The Coherent Lasercheck wand is the preferred meter for hobbyists. Edmund Scientific sells them for $300. Or you might get lucky and find one on E-bay for half that. They're good for measuring up to 1 watt of power. Wavelength-selectable from IR up to near UV.

    Sam's laser faq has some info on how to assemble a cheap home-brew power meter, but it won't have the accuracy that the Lasercheck does. (Calibrating it is pretty hard unless you've got a known standard...) Still, it's better than nothing.

    It all boils down to your budget I guess. But the lasercheck is a very handy thing to have. I resisted the purchase for a long time, but once I finally bought it I was *very* happy. (I also realized that I should have bought it a long time ago.)

    Adam

  10. #10
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    Default I found my answer

    This website has a driver with blanking. Excellent!

    http://www.die4laser.com/dvd-rec/Die4Drive.htm

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