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Old 09-03-2008, 11:22 PM #1
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Default Power Supply question

Hello all!

I've been checking out this forum for a while and I thank you for all the ideas! I had a Plextor DVD writer die on me, so I scrapped out the laser diode and am ready to have some fun with it.

I've built the LM317 current-limiting power supply posted elsewhere on this forum and it doesn't regulate the current like I expected... It's powered by four 'AA' batteries, it's hard-set for 103 milliamps output, and here's some tests I've performed: If I connect a 1-ohm resistor to the output, I get 103 milliamps on my ammeter. If I connect a 10-ohm resistor to the output, I get 103 milliamps. If I hook up my laser diode, however, I get about 40 milliamps on the ammeter and the laser diode doesn't do anything spectacular.

Shouldn't I get 103 milliamps when the laser diode is hooked up to this power supply?

Thanks in advance for the help, I appreciate it!


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Old 09-04-2008, 12:43 AM #2
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Default Re: Power Supply question

How is your circuit built?
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Old 09-04-2008, 01:10 AM #3
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Default Re: Power Supply question

Likely you're not giving the LM317 enough voltage to regulate. For red diode, I think the LM317 may need more like 7V. The reason the resistors have the correct current is because they only have a 1V or less drop of voltage, so the LM317 has plenty excess voltage to regulate.

Also, you have the batteries connected in series, right? End to end, not in parallel (all positive ends connected to one wire, all negative to the other)? If in series, try adding another battery or two, see if that fixes the issue.
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Old 09-04-2008, 02:26 AM #4
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Default Re: Power Supply question

Thanks for the replies!

rkcstr was right, I wasn't giving it enough voltage. *I connected the regulator to my model helicopter battery (11.1-volt LiPo) and it now gives a constant current whether the laser diode or 1-ohm resistor is hooked up to it. *Bionic Badger, I'd post a link to the current-limited regulator I used, but I can't post links since I haven't yet written enough posts. *It's in this forum, Daedal authored the post I referenced for my own power supply.

I'm sending the laser diode 130mA (??? watts power consumption) with this current-limited LM317 power supply, and I used to give it 108mA at 3.2 volts (0.3456 watts power consumption) using a voltage-regulated power supply. *I got more lasing power with the voltage-regulated power supply, but I imagine my laser diode was on borrowed time. *How much current should I give my laser diode? *It's scrapped out of a Plextor PX-708A DVD-RW drive, and it's mounted in a machined aluminum block to keep it cool (it hasn't even warmed up yet).
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Old 09-04-2008, 04:38 AM #5
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Default Re: Power Supply question

Not sure how much those can handle, though you'll probably want to burn it out anyway for a more powerful diode anyway hehe. Just make sure you have enough cooling to keep it alive. Keep a record of how many mA you manage to push through it.
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Old 09-04-2008, 03:27 PM #6
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Default Re: Power Supply question

Thanks for the reply! I'd go ahead and cook it, but this is the only diode I have at the moment. When I get another diode to replace this one, I'll go ahead and push it to its limits, heh! Although I don't know, since my wife's getting pretty mad at all the magazines we have that have pictures laser-cut out of them... It's fun what you can do when the laser's focused to a pinpoint! Well, fun for me, not my wife.

I scoured the Internet for specs on the diode that Plextor uses, but came up with nil. I went to Roithner Laser's website to make comparisons between the power requirements of different laser diodes within the 50-100mW range, but the power requirements range from about 85mA all the way up to 400mA! I really have no idea of the optical power output of my laser.

Is there a way to test the optical output of my laser diode based on performance references, since I don't have an actual laser power tester? For example, if I point it at a strip of electric tape, it will just start to burn through after about five seconds. And if I point it at a piece of newspaper that's colored black with a white back, the white side will start to darken after about 45 seconds with the laser pointed at the black side. The beam diameter is a little over 1/8 inch... How much optical power is required to do that?
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