Anyway to power LM317

[Devilz Rej3ct]

This may seem odd to some but I will ask anyways. I am trying to power my driver with a single 18650 battery. I am currently using a 12 volt 2 amp wall adapter. I know the required voltage of the LM317 but what about using a TIP31 power transistor? I used to power LED strip lights off a single Arduino pin using these TIP31's. If this is not possible, then how could I do this using the LM317 or am I dreaming? Yes I could buy a driver here which I have done already from FlaminPyro but in the mean time I like tinkering and such. Basically I want to use an 18650 to power a driver using an LM317. Thanks.

 

[Pi R Squared]

I don't think you can do it. A laser driver is current regulated, not a voltage regulated power supply. You need to use 2 or 3 batteries for your LM317 driver. Laser diodes aren't LED's. If you want to use 1 battery then forget the LM317 and use a boost driver.

Alan

 

[Cyparagon]

It needs at least 3.5V higher than the load voltage. The only thing I can think of in the range of <0.2V is a length of wire. Electromagnet, maybe, but certainly not a laser diode.

 

[Devilz Rej3ct]

Okay so I can pretty much conclude that powering an Lm317 with a single 18650 battery is not going to happen. Is there another chip that can do this? I would think though I would also need a mosfet of some sort. I have tried searching schematics but haven't really found what I am looking for. Size doesn't matter really. Any ideas?

 

[GVLLD M981]

I built my own driver out of an LM317, a couple of them in fact. There are 2 ways to rig an LM317, either in voltage regulation, or current regulation configuration. I run mine from a single lithium cell with no issues, at least not for average power diodes like red. One of my projects is an adjustable diode driver I can use to test diodes, and to figure out how much current they require by adjusting my test driver until the diode reaches the lasing threshold, then I measure that on my dummy load. Works well enough. Here is a link to a set of calculators for both configurations.
http://diyaudioprojects.com/Technical/Current-Regulator/
They show the way to wire them up as well. I used it in current regulation configuration. I don't recall how much the voltage drops, but not enough that it won't drive red diodes from a single cell lithium battery.

Note: I just noticed that they have split these into 2 different pages, so I fixed the link above to the current regulator page. Here is a link to the voltage regulator configuration page:
http://diyaudioprojects.com/Technical/Voltage-Regulator/
But you want to run it in current regulation configuration. Also, for R1, I used a 25 ohm linear pot (a bit much, but it's what I had around) with a one ohm resistor in series, that way even when the pot is turned to zero, there is still one ohm, keeping the max current at 1.25A.

Since I am doing it, I got out the test driver to measure the output. My 18650 is kinda dead, showing 3.55V. But with that, my LM317 test driver puts out 3.0v no matter where I set the pot. I'll do it again and add the figures when the battery is charged. But the drop in current reg config seems to be about .5v.

 

[Cyparagon]

I run mine from a single lithium cell with no issues

Wrong.

"it lights" doesn't mean there are no issues. TRUST ME. LM317 needs 3.5V+ overhead to drive in constant current. Maybe as little as 3V if you're running low currents. Your configuration will not have current regulation, and the current will drop as the battery voltage drops, which means your driver is no better than a resistor.

Also, you cannot operate a constant current driver with no load. How is a driver supposed to send 500mA into air? It can't. But it tries by raising the voltage. Open-circuit voltage is not an appropriate metric of quantifying the dropout voltage of the regulator. As you load it down, that output voltage will drop substantially.