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Old 09-27-2008, 09:18 PM #1
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Default LM317 not working?

A while back, I left my driver on for a while and the LM317 got really hot and it burned my finger

And when I hooked a multimeter and LED to it, I turn the pot and the voltage and mA both go up at the same time

I thought the voltage should always stay the same?And it also outputs 8volts with no LED attachedd


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Old 09-27-2008, 09:45 PM #2
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Default Re: LM317 not working?

All it does is maintain 1.25V between the output and the adjust pin. Test if it's doing that, and it should be working fine.
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Old 09-27-2008, 09:56 PM #3
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Default Re: LM317 not working?

How would I test that?
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Old 09-28-2008, 12:07 AM #4
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Default Re: LM317 not working?

Just test the voltage between the output and the adjustment pins on the LM317. *To find the pins for it, consult the datasheet on National's website. *That's all the LM317 is designed to do: maintain 1.25V between those two pins. *Everything else is up to the designer. There are some nice examples in the datasheet as well as an explanation of the operation of the device. It's a pretty good thing to learn.
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Old 09-28-2008, 03:56 PM #5
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Default Re: LM317 not working?

Why not do the full test then you will be sure that you will not blow an LD when you connect it.....

Instructions here.... http://www.laserpointerforums.com/fo...num=1197651171

Also the LM317 will shut itself down if it gets too hot so it should, as BB says, be OK.

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Old 09-28-2008, 09:06 PM #6
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Default Re: LM317 not working?

i also thought the voltage coming out from the driver would be constant, but it changes when i adjust the pot
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Old 09-28-2008, 10:57 PM #7
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Default Re: LM317 not working?

Sure it does, but the way that LM317 works is that it is only interested in keeping the 1.25V between the two pins. *The maximum voltage it can output is 2-3V below the input voltage. *Whether you get that isn't guaranteed (unless you change the circuit), but as long as the regulator's output voltage is above the threshold voltage needed to cause the laser diode to pass current, that's all that matters. *If you want the input-output margin to be less (for example to use a lower input voltage), you'll have to find a different regulator.

That's the thing with electrical circuits that is hard to grasp at first: *only go by what you know beforehand and work your way back to other parameters, not what you think you know or expect. This is especially true for semi-conductors, as they're designed to do &quot;weird&quot; (i.e. non-linear) things not generally possible in passive circuits. For example, you expected the voltage to be constant for the regulator, but that's not what the regulator does unless you set it up to be that way. *The only thing the LM317 really does is keep that 1.25V between those two pins, and the maximum voltage it can provide is 2-3V or so below the input voltage. *That's it. *If you want a specific voltage, you can rig it up to do that, but then the current it would provide might not be what you expect too. There are some interesting circuits in the datasheet, which show other applications and circuit topologies.
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