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Help! diode driver not working.

Jeremytheimer

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Jul 30, 2020
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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!
 



RedCowboy

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Jul 10, 2015
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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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