Laser Diode - after 10 min low brightness

Hey,

I bought 3 DVD drives. So, i got 6 diodes (3 red and 3 IR).

I'm using this LM317 driver: http://laserpointerforums.com/attachments/f42/11711-diy-homemade-laser-diode-driver-lm317_components01_001.jpg but i added one more resistor of 5 Ohms. So the final resistance is 2.5 Ohms.

I tried 4 diodes (2 red and 2 IR). All of them were normal. Red were really bright. But after about 10 minutes, all diodes just reduced their brightness for about 5%. (current (220mA) and voltage (2.7V on diode) were the same as when they had 100%)

I'm using heatsink and they weren't even heating.

Any ideas ?

Thanks a lot ! and sorry for my bad english Hope you'll understand my problem.

:thanks:

Seeder98 said:

Hey,

I bought 3 DVD drives. So, i got 6 diodes (3 red and 3 IR).

I'm using this LM317 driver: http://laserpointerforums.com/attachments/f42/11711-diy-homemade-laser-diode-driver-lm317_components01_001.jpg but i added one more resistor of 5 Ohms. So the final resistance is 2.5 Ohms.

I tried 4 diodes (2 red and 2 IR). All of them were normal. Red were really bright. But after about 10 minutes, all diodes just reduced their brightness for about 5%. (current (220mA) and voltage (2.7V on diode) were the same as when they had 100%)

I'm using heatsink and they weren't even heating.

Any ideas ?

Thanks a lot ! and sorry for my bad english Hope you'll understand my problem.

:thanks:

Click to expand...

If your heatsink was not heating, it wasn't sinking the heat...and, the diodes were getting too hot?

Teej said:

If your heatsink was not heating, it wasn't sinking the heat...and, the diodes were getting too hot?

Click to expand...

Well, heatsink was heating. I mean not so much as it can burn the diode (heatsink has 45 degrees at max). And yeah, that was my theory also. For last 2 diodes i used thermal conductive adhesive heatsink glue. And it was the same

Can you tell us more about the set up? What wattage resistors did you use for the LM317? What did you power the LM317 with?

ARG said:

Can you tell us more about the set up? What wattage resistors did you use for the LM317? What did you power the LM317 with?

Click to expand...

Sure,
I'm using 12V/1.5A power supply for LM317. Resistors are 0.6W. All of the parts except laser are in box with active cooling.
I'm not using trimer/potentiometer, because it can't be set to absolute 0 Ohms.

On more thing...I'm using 1N4007 instead of 1N4001.

2.5ohms programs the driver for 500mA, not 200mA. Is the voltage across OUT and ADJ still 1.25V when the power decreases? If not, your power supply may be to blame. Or you may have inadequate heat sinking on the lm317.

Cyparagon said:

2.5ohms programs the driver for 500mA, not 200mA. Is the voltage across OUT and ADJ still 1.25V when the power decreases? If not, your power supply may be to blame. Or you may have inadequate heat sinking on the lm317.

Click to expand...

You're right !

For the first time i used only 2x10 Ohms resistor (5 Ohms). This should set driver for 250mA, but the true output measured with second diode was only around 110mA (Red laser diode from 22x DVD burner). That's the reason why i added one more 5 Ohms resistor. That increased current to 220mA.

I tested OUT - ADJ and it's still 1.18V.

For cooling LM317 i'm using old PSU fan and this heatsink http://goo.gl/OLAts2. But when i used this LM317 for the first time (i didn't have any colling system yet), i accidently heat it up (i couldn't even touch it) while using first diode, actually that was the exact same time as the first power drop happened. Do you think thats may be the problem ? because it seems fully functional except strange current values.



EDITED:
Ok, i just tested LM317 with LED.
OUT - ADJ -> 1.18V

With 2x10 Ohms (5 Ohms) there is 130mA current passing trough the LED.
With 2x10 + 1x5 in series (2.5 Ohms) there is almost 220mA current passing trough the LED.