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Old 03-22-2008, 08:56 PM #1
m.carroll
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Default Laser diode reads 7.1 V?!?

Hey guys!

So i'm preparing the DDL driver circuit for my MXDL open can burner, and i'm testing it with 4x 1N4001 diodes as a 'dummy' load. The LM317T is set up with a 3 ohm resistor across Vout and Vadj. the battery supply is 2 RCR123a's putting out ~7.7 V (according to my multimeter). The current comes out correct at 420mA, however i get a reading of 3.4V across the diodes. What's going on?!?

EDIT: I just tried putting my laser diode in there, as i already had suspicions it was dead. Reading the voltage across the LD shows 7.1 V, which i assume is because its not letting any current through, so the lm317t rachets up the voltage to the max it can give (max batts can give). Does this 7.1V reading mean that my ld is dead? I'd like to know ASAP, so i can order another drive...but oy, 24 bucks down the drain, and not even a single red photon...damn.

(btw, i'm testing using rog8811's method of testing as described: http://www.laserpointerforums.com/fo...num=1197651171)

Thanks!


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Old 03-24-2008, 07:16 PM #2
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Default Re: Laser diode reads 7.1 V?!?

bump...

ok, the diode is behaving like an open circuit. the lm317t upps the voltage to near the battery's and charges the capacitor with that. does open-circuit = dead?

is it dead? i cant even find the 1.25V reference voltage when i plug in the laser diode. i have the 4 diodes plugged in, and i get the 1.25V reference, the correct 420mA and then the voltage across the 4 diodes is 3.4 V. i suppose that is ok, since the laser diode will determine what voltage it wants.

With the laser diode in, i get 7.1V across the diode and no current, no reference voltage...i'm not quite sure what's going on. any suggestions (i've already ordered another drive for a new LD)
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Old 03-24-2008, 07:52 PM #3
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Default Re: Laser diode reads 7.1 V?!?

Have you checked the pinout on your LD??? I think you might have connected the + or - wire to the photo diode leg. The pinout of the open can LDs is the same as SenKat's LDs:

* **-

* **+ ** photo diode (not connected to anything)
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Old 03-24-2008, 08:07 PM #4
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Default Re: Laser diode reads 7.1 V?!?

yep, the pins are correct. i'm pretty sure the diode is dead, which makes it behave like an open circuit.
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Old 03-26-2008, 08:19 PM #5
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Default Re: Laser diode reads 7.1 V?!?

I've seen a few like that too. Really high voltage even though the ma might be around where it should be. I know the driver is good, cause I replace it with another diode and that one works perfectly.
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