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Old 09-25-2009, 08:39 PM #1
Ryo Ryo is offline
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Default IR Laser Diode replacement & Test Load build

Here's the situation. I a red laser/light combo unit. I want to replace the red LD with a IR LD. It is integrated together to a switch so I can't just replace the driver board with something else unless I custom a board.

I was hoping to replace the red LD (<1mW output) with a IR LD with 5mW output (don't care on the actual output power). I measured the current to the red laser diode to be 27mA, which is the high side of the IR laser diode specs. So I proceeded to connect the IR LD and noticed it turned on briefly when I applied power. I tried it a couple more times, and it did the same thing. Next thing I tried a red open can red LD in the path and powered it up and it worked.. so then I tried the IR laser diode and it didn't turn on this time and the current draw was out of the roof. I figured I fried it.. checked it with a multimeter and the diode was shorted.. yup.. it's fried.

I then decided to put my long can diode in again and check the current draw and it's going all the way up to 400mA and kept on rising!!!

Now I'm at a loss.. I always thought the driver board is a current limiter. Why would it give only 27mA to the original LD and tons of current to a Open Can LD and IR LD. I currently don't know what the original red LD is since it's epoxy over. I always assumed the black epoxy is just to keep the wires from breaking off the pin since this is what I do on my other laser builds.

Any suggestions on what this driver circuit might be?

Also what type of test load would emulate a IR LD? I build up red and bluray test loads.. just don't know how many diodes I should put in series for a simulated test load of the IR LD. EDIT: I believe I read it was 3 diodes (0.7V)

Here's the datasheet for my IR LD:
850nm laser diode

Thanks guys.

Oh I just tried measuring the diode breakdown voltage on a Fluke MM and I get a voltage values in forward and reverse polarity... does that mean the cathode and anode are wired together, meaning LD and PD are wired together?


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Last edited by Ryo; 09-25-2009 at 10:26 PM.
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Old 09-29-2009, 01:08 AM #2
Ryo Ryo is offline
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Default Re: IR Laser Diode replacement & Test Load build

Hmm.. after a bit of debugging, I figured out why my diode died. It was getting full 3.3V without current limiting as noticed by my large current ramping. The current limiting board must be in the module itself, but it's so small I couldn't tell it was built in. In fact it looks like a regular LD module with wires sticking out of the epoxy. Reason why I know that there's a current limiter in the module is due to the fact I supplied 3.3V directly to the wires to see if the current draw would ramp up and damage the diode. No such thing happened.. Interesting.
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