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Questions about LM317/LT317 Laser Driver


New member
Jun 13, 2016
To preface this I have done a lot of research for the past few weeks on this stuff and these are just questions I haven't been able to get answered and was hoping you guys could.

I have a LT317 regulator which shares a data sheet with the LM317 so am I right in assuming they are about the same and can be used as such? Anyway I tried recently to set up a breadboard version of the current regulator without attaching the laser diode just to see how it works. For starters I was a little confused by the pinouts because the datasheet says its adj, out, in but schematics tend to read as in, adj, out. This is just to make the schematics more readable I suppose? I wired it up the way the data sheet says the pins are so hopefully thats correct. I used a 9v battery as the input because its what I had and then attached a 10 ohm resistor to set the current (I put one end of the resistor on the output then ran it to an empty row of the breadboard the had a wire from the adj to the same row). Theoretically this should get me 125mA of current correct?

As the 1.25v from the LT317 / .125A nominal current = 10 ohms

However when I tried to read the current with a multimeter I was getting current all over the place. Nothing constant and nothing near 125 mA. I tried attaching LEDs just to see if anything changed. All that happened was that they got fried after a bit. In the mA setting the meter read anywhere from .6mA to 40mA if I remember correctly. I didn't take any pictures but I can try if that would be worth it to help you guys see it better. This is probably just me not knowing how to correctly wire this up or know how to use a multimeter but any advice or answers the my questions would be really helpful.

If there is any more information I can supply just let me know what I can do.


Well-known member
Sep 12, 2007
Schematics typically have the pin wherever is convenient on the drawing. This is indeed to make the schematic easier to read and often has no relation to the physical pin locations on the IC. 555 schematic for instance:

Yes, your calculations are correct. 10ohm should result in ~125mA. Several problems you need to address:

1. Most standard (5mm) LEDs are rated for 20mA at most, not 125mA.
2. A CC source cannot operate without a load. Put an adequate load on the output. Perhaps search the forums for "test load"
3. A 9V battery is only rated for low currents, and may be unable to supply the 125mA you are asking of it... especially if it is the carbon zinc or "heavy duty" type.
4. Where are your multimeter leads when taking this measurement?

If these suggestions do not help, a picture will be necessary.