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10-26-2009, 11:55 PM #1
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CollegeLaserAmateur
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How to use a LM317T with my first DIY laser

Hey guys, this is my first forum post and my first attempt to build a laser, so i'm trying to learn as much as possible before i start putting this together. I do remember a lot about circuits from AP Physics in High School last year...

here's my parts:

350-400mW blue laser diode
blue laser driver (runs at no more than 180mA with 3V)
LM317T
resistors
capacitors
momentary switch

I've seen complicated diagrams of how to properly in-cooperate the LM317T with the resistors, but they are hard to understand and they use example numbers that are not relevant to me. Here's what I'm trying to do:

Use 6V, and bump it down to 3V for my circuit. I'm trying to use the simple v=ir and c=q/v equations to find what resistance and capacitance i need but i'm foggy on the exact ways to use capacitors in parallel or series with resistors, and then the LM317T gets thrown into the problem and makes matters worse. Can someone help me out here?

So:

Power = .38W
Current = .18A
Voltage = ? P=IV -- 2.11 volts???
Resistance = ? V=IR -- 12 ohms???
Capacitance = ? I read that capacitors are a good safety net so i don't surge my diode when starting it up (completing the circuit with the momentary switch)...

I think I'm in too deep with everything, how could i just make it as simple as possible? With: 3V, 22 ohm resistor, a switch, driver, and diode?

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10-27-2009, 12:03 AM #2
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Re: How to use a LM317T with my first DIY laser

Hey Buddy, You have a PM to read..................rob
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10-27-2009, 12:08 AM #3
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Ramsey_innovations
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Re: How to use a LM317T with my first DIY laser

Ok, good start. But first a questions. What diode do you have, we need more info than 350-400mw, it sounds like and ebay purchase, no? What capacitor do you have and whats the blu laser driver thats 180ma @3v. I would use the lm317 as a driver if you don't mind powering with 9-12v. To figure out the ma you divide the referance voltage of the lm317 (which is 1.25v) by the resistance value to equal the ma delivered. The lm317 will then deliver the required voltage to drive the diode. Capacitor should be tantalum and have a 16v rating and be at least 47micro henries.

10-27-2009, 12:10 AM #4
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Ramsey_innovations
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Re: How to use a LM317T with my first DIY laser

ALWAYS SHORT THE CAPACITOR BEFORE CONNECTING THE DIODE!!!!!!!!!!!!

You may also want to add a 1N4001 diode for reverse polarity protection!
You can thank Rog for this.
http://www.rog8811.com/laserdriver.htm

10-27-2009, 12:27 AM #5
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Re: How to use a LM317T with my first DIY laser

This is a very good compilation of information, including good diagrams of the LM317 driver and how to test and set current with a test load as well: (from rog8811)
It can be done - Laser driver
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10-27-2009, 01:31 AM #6
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Re: How to use a LM317T with my first DIY laser

Ramsey_innovations -
don't hate me for this, but i bought it from laserdiy.com (not a good site in my opinion, don't ever buy from there)... i got it before i found this forum, if it ever gets to me, still in the mail i think, i will get pictures up or something. But i now know where to buy from... For the diode, i know it's a blue laser, don't even know the wavelength. The driver, i have no idea, i don't even know what a driver is made or the physics behind it's purpose. But i'm willing to learn if you could point me in the right direction. using the lm317 as the driver is a good idea, but i don't know how to do that, it would help me out a lot because i can get my hands on lm317 easily.

(Radio Shack is where i get all of my extra supplies)

I have a 47uF capacitor, i'll use that in the way described the way Rog8811 says to.

Last edited by CollegeLaserAmateur; 10-27-2009 at 05:06 AM.

10-27-2009, 02:39 AM #7
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Re: How to use a LM317T with my first DIY laser

Quote:
 Originally Posted by CollegeLaserAmateur Power = .38W Current = .18A Voltage = ? P=IV -- 2.11 volts??? Resistance = ? V=IR -- 12 ohms??? Capacitance = ? I read that capacitors are a good safety net so i don't surge my diode when starting it up (completing the circuit with the momentary switch)...
You've misunderstood here. Laser diodes are for the most part run at a constant current, however the voltage across it is (roughly) independent of the current, and will remain constant(ish) across a range of currents, for blu rays this is between 4.5 and 6v. Therefore the power in is 5v *0.15A = 0.75W in. At 150mA, you'd get maybe 150mW out, so efficiency is around 150/750. The remaining 600mW is lost as heat. Ohm's law is also mostly redundant here because the voltage remains so similar across a range of currents - this is also why we use constant-current drivers, because a small increase in voltage would lead to a huge increase in current, and blow the diode.

10-27-2009, 04:55 AM #8
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CollegeLaserAmateur
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Re: How to use a LM317T with my first DIY laser

Power lost to heat... that's one thing i let slip from my mind since physics class...

So for a typical laser, would you assume 20% efficiency? or is that just for diodes/lasers in the blue wavelength?

So does total resistance become obsolete or is this still somewhat useful? should i always test a driver before i add the diode? what do i test for? because i'll need to get a volt-meter from radio shack then... right?

10-27-2009, 05:51 AM #9
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Re: How to use a LM317T with my first DIY laser

Here is a pictorial layout of the LM317 layout I modified for BluRay (125ma) from rog8811:

The PHR-803 diode you most likely purchased, will run best at this current.

The center terminal on the regulator is the output of the LM317.

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10-27-2009, 06:41 AM #10
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Re: How to use a LM317T with my first DIY laser

Test the driver using a dummy load (look it up, there are a million posts on the topic). 20% efficiency is probably about right for a violet diode since it runs at about 5V and produces about a mW for every mA at that (with variations).

10-27-2009, 10:02 AM #11
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Re: How to use a LM317T with my first DIY laser

Quote:
 Originally Posted by CollegeLaserAmateur i bought it from laserdiy.com
Laser DIY is way over priced, best to stick with sites run by fellow LPF members

Quote:
 Originally Posted by CollegeLaserAmateur For the diode, i know it's a blue laser, don't even know the wavelength
The diode you purchased in most likely going to be a PHR 803T as stated earlier. It is a 100mW diode and like all other VIOLET diodes it will produce 405nm light
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09-20-2010, 07:36 AM #12
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Re: How to use a LM317T with my first DIY laser

how can i put an lm317t in a circuit with a swithch, battery and 808nm diode?

09-20-2010, 11:51 AM #13
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Re: How to use a LM317T with my first DIY laser

Not to jump on you too hard mikey395 but the answer to your question is posted independently in posts 4,5 and 9. Just follow the instructions. Had you read the thread completely you would not have needed to post I think.

If you're still unsure then there is another link to similar setups, (even a video), in the guide in my sig.

M
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09-21-2010, 09:11 AM #14
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Re: How to use a LM317T with my first DIY laser

i usnderstand
thanks morgan

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