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What's the third pin for? And discerning voltage.

The Purple Death

New member
Joined
Jan 27, 2019
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6
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Hi again,
I did try searching the forums, but my search terms contained too many results.
I see on ebay various laser diodes, often these have 2 pins. Others, such as this one:
https://www.ebay.com/itm/1pc-405nm-5-6mm-5mW-20mW-Violet-Blue-Laser-Diode-TO-18/
have three.
I thought if I got 1 that I could use my tester and deduce what the third pin was for, but after doing some research I find that using a tester could damage the laser diode. What is the third pin for?
I also reached out to several sellers asking them what the voltage was for their diodes, it's been several days and I'm concerned that not even the sellers know what they are selling. Is there a way to tell?
Feel free to point me to the fine manuals.
Thanks
 

paul1598419

Well-known member
Joined
Sep 20, 2013
Messages
14,795
Points
113
These TO-18 case diodes always have three pins. The unused pin is just that. Unused. Occasionally, you will have a photo diode in there or a zener to help with ESD.
 

paul1598419

Well-known member
Joined
Sep 20, 2013
Messages
14,795
Points
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The best way is to ask. If you buy a diode from someone they should be able to tell you where the anode and cathode pins are. Now, if you don't know, you can look at the other diodes that are in this case style like the S06J where the anode is to the right and the cathode is to the left with the case pin in the down position.

I have occasionally had to test the diode to figure out the correct pinout diagram. I have done this with a good CC/CV adjustable supply set to the bare minimum current, and using my best guess, I would increase the forward voltage on the diode until I see light, or I have gone above 4 volts in the case of a 405nm diode. If I haven't seen light, it is likely that these are not the correct polarity or the case pin is an active pin.
 




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