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ArcticMyst Security by Avery

What am I doing wrong with my diode?

flammable

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Jun 13, 2023
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Posting for my friend. My laser was super bright at first but then dims. I thought it was the cooling since the copper mount im using (dtr copper blank) is not pressed and has no thermal paste but i checked the current draw and it was actually decreasing instead of increasing (0.172 amps right when I turn it on to 0.169 in around 20 seconds which makes the beam way dimmer) I am using a basic lm317 driver with a 0.5 ohm resistor across the adj and Vout. The diode im using is the m140 m type and i set the bench maximum current to 1.5 and voltage to 6 if that even matters. I have 2 aluminum hs on the mosfet since it was getting really hot before. What should I do?
 
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Dec 12, 2012
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A current draw of 172mA dropping to 169mA should have no noticeable effect on brightness.
If its running off of a bench ps then battery voltage dropping isn't an issue. It is possible that the lm317 is overheating and entering thermal protection, but at 0.17A thats unlikely.
6 volts may not be enough for an 450nm with lm317. The lm317 needs I think 2.5 V minimum, so right now that only leaves 3.5V for the diode.
 

WizardG

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Yeah, if the current draw using a '317 and a 0.5 ohm resistor is only ~170mA you need more input voltage. That regulator is not in regulation. Try this: disconnect the laser diode and short your driver. Measure the short circuit current (keep on time short, your regulator will heat up quickly) and with that resistor you should see 1.2/0.5=~2+ amps. Replace the resistor with a 1 ohm and turn the bench supply to 8 volts and you'll see much better output from your laser. But be sure to keep the on time short until you have that diode properly heat sinked or you're gonna have other issues. Can you show us your schematic?
 




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