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Help! diode driver not working.


New member
Jul 30, 2020
Hello, I tried to make a driver circuit for a 5 watt laser using an lm317 voltage regulator. the diode is supposed to consume 5 volts at 5 amps. (I know it can since I have seen it create a much brighter output with unregulated batteries). the problem is the diode only consumes 40 milliamps (measured with mulitmeter in series) running on usb power. is there a way I can force the laser diode to consume more current or build a better yet still simple driver circuit? The schematic is in this video at 2:00 into the video.
Thank you for your time!


Well-known member
Jul 10, 2015
LM317 won't do 5A

LM338 will do 5A, you need 0.25 ohms between your left and center pin and your resistors have to be rated to dissipate the heat....if 3 x 18650 in series and you want 5V @ 5A then you will have 1.2v@5A ( 6W ) in heat for the resistors to disipate and the rest dissipated in the regulator so you must heat sink it, it will get hot.

USB only puts out 5V @ 0.5 - 1.0A and you need more Vin than going out, even with a low drop out LM338 you need 7V at least but 2 x 18650 will sag and what laser diode are you driving @ 5A 7A75 or 44 ? If so better to aim for 4A and you can use 2 X 18650 and 3 X 5W rated 1 ohm resistors in parallel will give you about 4.0A output. ( 0.33 ohms divided into 1.25 )

1.2 is your constant ( actually 1.2 to 1.25 ) for the zener so you divide your resistors into 1.2 - 1.25 to get your current when configured as a current regulator.

Put your resistors between your ADJ and Vout ( left and center pins ) and +batt on your right pin, you connect your laser diode + to the left ( ADJ ) pin. Batt ground to laser diode ground and of course switch one side of the battery.

NOTE* The case tab and center pin are tied together so if your case is tied to batt you need to isolate if you sink to the same case.

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