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How Did I Fry My LPC-815 650nm?

xquercus

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I just started working with my first laser diode -- an LPC-815 650nm I bought which was pulled from a 20x DVD burner. Unfortunately, I fried it. It looks more like a red LED with an excessive voltage drop and the ability to handle quite a bit of current now. :eek:

The driver is an LM317 used as a current regulator. During the course of experimenting I tried running the LD at 100mA and 190mA. Even at 100mA, I would get smoke off a piece of electric tape placed at the focal point. Shortly after adjusting upward to 190mA, the laser stopped working. I don't recall if this happened while I had the power on or during some bout of re-wiring.

While I did have the LD powered for a number of minutes, at 100mA, the aixiz module it was mounted in was barely warm. Almost imperceptible. My impression is that at this low current, I'd be fine with a long duty cycle of several minutes. After all, I see folks recommend about 400mA on this LD for long life and builds were people often go 500mA plus with heat sinking.

So, my question is, did I kill this LD because of too much current or is some other failure mode more likely? Perhaps a voltage spike through the LD as I was rewiring? I'm measuring current by connecting my DMM in series with the high side of the LD. I may have done a disconnect and reconnect just before noticing the LD failure. Perhaps this had something to do with it? Any thoughts?
 

styropyro

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Is there a capacitor in your circuit? If so, disconnecting a reconnecting it killed the laser diode. We've all done it at least once.
 

xquercus

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Yes, I have a 1uF electrolytic in parallel with the LD. I guess the take away is to not disconnect and reconnect the LD while the cap is charged. Not sure if I did this, but it seems to be the most logical explanation.

I wonder if the cap is really necessary? Are the transients from the power source really significant after going through the current regulation?

Alternatively, perhaps a resistor in parallel with the cap could be used as a bleeder. If I put in a 10kOhm resistor in parallel with the 1uF cap:

T=CR T=0.000001 F*10000 Ohms T = 0.01s


If we assume the cap is discharged by 5T then it's 0.01s * 5 or 5/100 of a second. At 3V the 10kOhm resistor would draw .3mA. It wouldn't provide protection if I connected the LD while the circuit was powered but it would insure that the cap was discharged when the regulator was unpowered. Thoughts?
 

LtKernelPanic

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Don't feel bad I managed to kill one and maim two other 5mw 635 diodes while building my first laser. The most boneheaded one was right after I got the pen host it was going into I slid the module in the end with the wires hanging out the button hole to see how well the module would fit. I figured I mays well power it up but instead of attaching the leads from the driver still on my breadboard I connected the leads to the 8.4v battery pack. He died an instantaneous death. That was a good diode too. I forgot to unplug the battery pack one night while doing what was supposed to be a 45 minute test to see if the driver got too hot and found it happily lasing away 13 hours later. Neither it nor the driver were warm.

Anyhow as styro said everyone kills at least one diode in their life. My most recent one was a $50 ouch.
 




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