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Thread: using PWM for brightness control

  1. #1
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    Default using PWM for brightness control

    Hi all,

    New to the forum, and relatively new to the 445nm diode. I recently received my first and began experimenting with it. Since I had a bad experience a while back, trying to get my first 405nm diode to lase using my DIY LM317 constant-current PS (destroyed it presumably by static and/ or failure to drain caps, arrgh), I decided to be ultra-careful, and used a 6V battery and assortment of power resistors to very gingerly test my new 445. I was extremely pleased to see it lase at currents similar to reports on this forum. I'm interested in creating a plane of light, using line-generating optics, and unlike most laser display systems, want to be able to lower the light output to very low levels-- lower than the lasing threshold of this diode. I already have a nice assortment of home-made hardware and software that allows me to PWM long strings of RGB LED's (2+amps/channel), and decided to try PWM'ing the 445nm diode's battery+resistor supply, limiting current to 200-300ma. It works beautifully-- I can produce extremely low levels of brightness, and by keeping the PWM rate >100Hz, I cannot detect flicker (I do not intend to scan the output). My PWM resolution is 1000, so the shortest on time is 10usec (at PWM rate = 100).

    So now, I'd like to build a non-battery operated PS, and keep the PWM. But I'm concerned about PWMing a constant current source-- seems like that's a bit of an oxymoron. Also, I'm not interested in portability -- those tiny pointer-inspired PS's don't seem the best way to go for me. Is there a way to use PWM, and at the same time supply "safe" power to the diode? I'd like to build my own supply, but a reasonable commercial option would be OK.

  2. #2
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    Why not just use a laser diode driver with linear current control via 0 to 5v control?
    http://img62.imageshack.us/img62/3985/laser.gif

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  3. #3
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    Quote Originally Posted by Doc View Post
    Why not just use a laser diode driver with linear current control via 0 to 5v control?
    Well, I did see this before, but it's out of stock, and I'm itching. Plus I'd like to understand what's going on under the hood, if I can. But yes, I'd consider that reasonable and would get one if it were available.

  4. #4
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    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    PWM is fine for direct diodes if your driver does not feature unclamped inductors that induce spikes which will kill the diode over time. These can be checked for with a oscilloscope. Blue diodes are tough as nails, but you might now get away with it with some reds or violets.

    Since we are mainly a laser show community, the main reason we do not use PWM, is that you need to run the PWM cycle at a minimum of 2 x 256 x 1/(3.33x10-5) = 1.59 Mhz for good point by point color control in 30 KPPS graphics. Often faster then that. Which limits its use to using acousto-optic modulators in front of most lasers, or to a much faster then usual diode driver, which is not cheap, nor common. I've done PWM AOM color control 20 years ago, it works great, and the linearity to the eye is beautiful, but it is beyond the means of most amateurs to do fast enough for graphics.

    My PWM circuit was clocked at 6.4 Mhz, with the expensive Flash A/D converter updating at 500 Khz, so that is beyond many hobbyists to work with, unless they own a scope and can put up with the 20% loss in the AOMs. People who saw the RGB results were very happy with it though. I'm sure there are a few fast MHZ rate PWM chips now, but back then they were slow and I did it with digital logic.

    PWM is unsuitable for most DPSS green lasers, due to thermal delay issues in the lasing material, so we will not get into that aspect. Suffice it to say it has been discussed to death here.


    Steve

  5. #5
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    Default

    Thanks for the info, Steve. I understand why PWM wouldn't fit well for you guys. I'm just trying to extend my understanding of and ability to control LED's to LD's. DrLava's P3 seems perfect for me-- I could connect my PWM signal to the TTL modulation input and use my LED-ware, or I could use analog to control DPSS's.

    Does anyone know when/if they will be available? Alternatives?

  6. #6
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    Default Dr. Lavas Driver

    I asked him about this a couple of weeks ago and unfortunatly it looks like it will be a while possibly a couple of months. Two DYI options in the mean time this one is TTL

    http://www.photonlexicon.com/forums/...ver+Schematics

    This one is Analog

    http://www.photonlexicon.com/forums/...iver+schematic

    I am going to try the Analog one since its going to be a while and I need one soon. Once Dr. Lava has some available I will be stocking up.

    Hope this helps
    Michael

  7. #7
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    Quote Originally Posted by bruce View Post
    So now, I'd like to build a non-battery operated PS, and keep the PWM. But I'm concerned about PWMing a constant current source-- seems like that's a bit of an oxymoron. Also, I'm not interested in portability -- those tiny pointer-inspired PS's don't seem the best way to go for me. Is there a way to use PWM, and at the same time supply "safe" power to the diode? I'd like to build my own supply, but a reasonable commercial option would be OK.
    Of course this is possible. Obviously you don't want to apply the modulated, square-wave drive directly to the diode if you want to use it for shows, but if you properly filter and clamp so that you have a resonably clean DC current without spikes, there is no reason against. Commercial DPSS lasers do this as well.

  8. #8
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    [QUOTE=mmuhler;187422]I asked him about this a couple of weeks ago and unfortunatly it looks like it will be a while possibly a couple of months. [\QUOTE]



    But those links are great!-- very helpful for me to understand how to construct my own PS-- Thanks!!

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