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Old 06-17-2014, 07:56 AM #1
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Default LM317 question

I am working on my first build. I recently bought a 2w m140 laser from DTR. Great package was totally impressed. I wanted to build a LM317 driver. I have been working for a few days and today during my research discovered that the LM317 is supposed to only be rated to 1.5amp. Is the true? I built my driver and used the math (v/I=r). 1.25 / 1.6= .78ohm. I want to leave room not to push the diode to the recommended 1.8a max. I included a 100 ohm pot, a .78 ohm resistor, a 1n4001 rectifier diode to protect from reverse power hook up, a 10 microfarad 16 volt cap to help protect against power spikes. Having hard time testing this unit. don't want to destroy LD. Used 6 1n4001 diodes and a 1.3 ohm resistor as dummy load. But I feel like dummy because I can't get a reading on my DMM. I can show that I am running about 7.8 volt but cant get amp reading. Any ideas? Or any advise on a premade driver? almost ready to give up and buy one. But if I go that pre made route which type?


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Old 06-17-2014, 12:01 PM #2
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Default Re: LM317 question

The LM317 is indeed limited to 1.5 amps, at least by specification. What happens if you push it further depends on the exact model you have, but most simply will not output much current above 1.5A.

This is because the LM317 is actually intended to be a voltage regulator, and the 1.5 amp limit is a short-circuit protection in that application. Also, it has a thermal protection system built in and will lower the output current/voltage when it starts overheating.
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Old 06-17-2014, 02:07 PM #3
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Default

If you can't measure the current you must be using the test load wrong.
Refer to this thread for help to use test loads:
Test Load Diagram's for all laser diodes!
At these high currents a pre-made driver may be a better option because the lm317 gets so darn hot!
Lazeerer has a wide selection of drivers.
For your purposes the xdrive will work well, here's a link:
FS: X-Drive V6 3A Buck Driver "Sale and Info"
Just PM lazeerer and he'll hook you up!
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Old 06-17-2014, 03:47 PM #4
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Default Re: LM317 question

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Originally Posted by Benm View Post
The LM317 is indeed limited to 1.5 amps, at least by specification. What happens if you push it further depends on the exact model you have, but most simply will not output much current above 1.5A.

This is because the LM317 is actually intended to be a voltage regulator, and the 1.5 amp limit is a short-circuit protection in that application. Also, it has a thermal protection system built in and will lower the output current/voltage when it starts overheating.
So then if I put a straight jumper between the vout and adjust pins I should be limited to 1.5amp? and not hit the 1.8amp rating on the diode. and use 2 18650 batteries as planed instead of 9v tester? Could this be why I am not getting readings? because I am trying to push to high of an amp through the LM317?
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Old 06-17-2014, 04:29 PM #5
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Default

NEVER short the vout and adj pins as it will create a voltage spike and kill the diode.
The lm317 will not limit is own current to 1.5 amps.
Despite the data sheet 1.5a limit, I have run my lm317 at currents over 1.7a.
So you will need a resistor to set the current, no exceptions.
And 2 li-ion batteries would not work as a power source.
The lm317 drops at least 4 volts on its own, and that is why it gets hot at higher currents.
A 10 and above volt power supply will be able to power a 445 diode.
Let me know if you need any more help, I can still remember getting frustrated over my first lm317 driver.
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Old 06-17-2014, 04:36 PM #6
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Default Re: LM317 question

Use the LM1085 regulator instead. It can go up to 3A and is basically the same as the LM317 in specs.
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Old 06-18-2014, 01:15 AM #7
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Default Re: LM317 question

Indeed, use a regulator that is rated for the job.

The LM317 was NOT designed to be a laser diode driver, keep that in mind. In can be used as a constant current source up to 1.5 amps with a rather large voltage drop between input and output, but that is about it.

Same goes for the LM1085, but that is rated for a higher current and could be used in ddl type drivers up to 3 amps or so.

I understand why people would want to build 317 based drivers because of their simplicity and ruggedness, but if you look at their shortcomings more closely, they really are not an ideal choice. It is just a simple design with commonly availabl components that could be trusted back when the most demanding laser diodes were dvd writer 200 mW-ish red ones. It simply does not scale well into multiwatt applications.
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