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Old 10-19-2012, 03:43 PM #1
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Default Did I goof?

I started with lasers as a hobby about a month ago, and have been lurking in the forums for quite some time since then. I had successfully built the same exact driver in this post: DIY Homemade laser diode driver, albeit with a 10K Ohm POT and 3-4 10 Ohm resistors (went with what I could find at Radio Shack). I built it out on one of those solderless breadboards, and used a pair of 3.7v 14550 Lithium Ion batteries as the power source (7.5v at the output) and had the pot adjusted to output 300, then 400mA, all verified with a digital multimeter.
I had tested the driver with one of those high-intensity LED's before I took the diode from a DVD burner and fired it for a second or two (still in its sled) just to see what happened. It looked dim through the laser safety goggles I picked up online, but watching it through the display on my iPad, it was pretty bright.

After that all worked out, I removed the components from the solderless breadboard and soldered them in to an actual breadboard. I tested all of the connections and the output from the driver with my multimeter again and confirmed the numbers were the same. I installed my test LED to confirm that it worked, and then the DVD burner diode. Both ended up failing... my best guess is that I forgot to discharge the capacitor before reconnecting the diodes, I couldn't find any shorts. I went back to using the solderless breadboard - new test LED, new dvd-burner diode, worked fine.
Online I ordered a 5-pack of LPC-826 diodes and 1 x 12x30mm housings from Cajunlasers.com, and then visited Wayne Electronics in NJ to get a few 100ohm pots, and a handful of 3.5 and 4 ohm resistors (so I didn't need to hook up 3 or 4 10 ohm resistors in parallel to get my desired mA output). I reworked the (solderless) driver again with the new pot and resistors, verified the V and mA, and waited for my diodes and housings to arrive.

The diodes were packed in a small pink plastic bag which was in a clear 1"x1" plastic box, while wearing my anti-static strap I took out a diode, placed it into the housing, then pressed it in to the housing using the rear portion of the housing (inverted it) - lastly I took a small metal sheath with the same circumference as the diode and pressed it in so it appeared recessed (got that from another post here I believe). I then soldered the positive and negative pins to 24 gauge insulated copper wire, and went a step further with the positive pin by cutting the end off of the plastic ink tube of a ball point pen, and sliding that over the exposed bit of pin, wire, and solder. I put the whole piece together, and then drilled a hole through the aluminum heat sink from a CPU and slid the module through (used a small amount of Arctic Silver Ceramique thermal paste) - it was a good snug fit and allowed me to adjust the focus of the beam.

I tested the driver one more time with my multimeter and the LED, discharged the capacitor, hooked up the diode, and fired it. I adjusted the beam a few times, the highest mA I fed it was 500mA. I burned a few pieces of electrical tape for kicks, and was satisfied that it worked. I then decided to mount and prepare my remaining diodes in the modules to save some time later on. This is when the problem began... I prepped another diode (they were still in that bag), set the driver to 300mA, discharged the capacitor, attached the diode and did a quick test fire of the laser (didn't use any heatsink besides the assembled housing). I immediately noticed that the laser was VERY dim. I re-checked all the connections, verified the voltage and current, adjusted the current to 250mA, then 300mA, then 350mA, and finally 400mA (each time discharging the capacitor before re-trying. I then tried the original diode which I had used for a few minutes the day before - same thing. Increasing the current made no difference.

I replaced every single component on the board (resistors, capacitors, LM317T, silicon diode, pot, wires), used different pins, tested it, and then tried diode #3, same thing. Then I tried diode #4.... same thing. I checked the voltage and current on the original diode (when everything was working) and saw ~2.8v at the diode and the current was the same. I checked one of the diodes that wasn't working the same way and got the same numbers. One thing I didn't notice when everything was working (but did after the fact) is that the LM317T got *very* hot so I attached another heatsink to it.

Now I'm trying to figure out what went wrong before I burn any more money on diodes... Voltage too high? Could I have damaged the diodes by leaving them in the bag they came in? Maybe the solderless breadboard is messed up? I'm completely flummoxed. I feel like I've jinxed myself here! I could go ahead and purchase a pre-made driver but a lot of the fun for me is building as much of this from scratch as I can...

Any ideas?



Last edited by unholy79; 10-19-2012 at 03:44 PM.
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Old 10-19-2012, 06:14 PM #2
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Default Re: Did I goof?

Pictures would help.

You're setting the current the hard way. The easy way is to use an ammeter in series with the battery. It's a linear driver, so the current from the battery is the same as the current to the driver.
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Old 10-19-2012, 06:23 PM #3
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Default Re: Did I goof?

The driver is visually/functionally identical to the photo in the post I referenced...



...with exception to the capacitor/pot/resistor used...

I'll snap some photos when I get home.
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Old 10-20-2012, 07:01 AM #4
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Default Re: Did I goof?

Hook up a (presumably) dead diode and turn the circuit on. Measure input voltage, output voltage, input current, and voltage between OUT and ADJ pins on the lm317.
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Old 10-20-2012, 01:35 PM #5
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Default Re: Did I goof?

Figured it out, it actually wasn't the driver or the diode, it was me overdoing it with the soldering iron. I played a hunch and soldered my last remaining diode at a lower power setting on the iron and made quick with the connection... diode fired up fine. I was simply applying too much heat to the pins when I was soldering. Talk about a pricey learning curve! Time to raid the DVD burner pile again until I can afford to buy some more diodes. I'm thinking maybe a low temp solder or actual connector may be a smarter option for me... Perhaps re-appropriating one of those cpu/case fan cables with the tiny jacks instead of soldering might be a better option for me.
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Old 10-20-2012, 01:40 PM #6
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Default Re: Did I goof?

If you want more problems down the line then use
a connector between the Driver and LD...

You can easily control the heat of a solder joint by
the time you leave the soldering iron tip on the
joint.
Using a good quality Rosin solder flux will decrease
the joint heating time.


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Last edited by lasersbee; 10-20-2012 at 01:42 PM.
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