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DVD-R Laser Diode

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dan

Tue Jul 25 2006, 06:28PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I have recently extracted the DVD read/write laser diode from a 'dead' Toshiba (model No. SD-R5272) DVD-RW drive. I am using my PCâ€™s USB port with a 25ohm current limiting resistor to power it. With 85ma input the laser diode appears to be brighter then my 15mW HeNe laser when compared side by side. However the diodes beam isnâ€™t collimated so the output is probably around 30-50mW but without a quantitative means of measurement this comparison is useless for judging power since human eyes are more or less receptive to different wavelengths.

I have since mounted this diode on a heat sink since it was heating up a little bit without it. I have heard these little diodes can take up to 150ma and put out 100mW but without a datasheet I probably wonâ€™t push it past 100ma. I have searched for the markings printed on the diode (AH402D0) but have turned up no results.

On another note I First tried using a 50ohm resistor because using ohms law 100ma = 4.95V/50Ohms. But I wasnâ€™t taking the diodes voltage drop into account which is 2.18V so the actual current was (4.95V-2.18V)/50 = 55ma. This seemed like the correct way to calculate the current limiting resistor. However now that Iâ€™m using the 25ohm resistor using the previous calculation (4.91V-2.45V)/25 = 110ma but Iâ€™m only measuring 85ma. What is going on here? All the values in the calculations are the measured values. Maybe my meter is dodgy? Or most likely itâ€™s my maths.

Well anyway heres some pics:

...

Sun Jul 30 2006, 02:41AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Laser diodes don't always drop a constant voltage, which is probably what is causing problems with your maths...

You should be able to get a colminating lens out of the dvd-r drive, just try diferent lenses in the optical assembly until you find one that gives a good beam (it is usually the first one in front of the diode). What good is a laser if it isn't colminated?

And most importantly don't stare into the beam with your remaining eye

dan

Sun Jul 30 2006, 05:54PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I am aware that laser diodes don't have a constant voltage drop. Thatâ€™s why I measured it twice for each resistance change. But my maths still didn't line up. V drop tends to increase with increasing current so I guess if I plot several data points (Limiter Resistance Vs. Current) I should be able to predict what resistor I need for a specific current. Temperature also affects voltage drop but by keeping the diode on a big heatsink it should keep it near ambient and at a constant temperature.

I should probably just use a 5v voltage regulator wired as a current regulator. That way it will compensate for voltage drop automatically and all I need to do is use ohms law to figure out what resistance I need. Too bad it won't be USB powered anymore. (I could probably use a DC-DC converter to up the voltage a bit so the regulator has something to play with.)

The optical assembly uses a mirco lens which is very hard to mount, adjust and align. I'll probably just buy a $1.99 laser pointer an use the collimating lens assembly from that. Just swap out the crappy 1mW diode with the DVD-R one then adjust the lens for the new diode. Probably loose a few mW from the crappy optics but it will be quite an improvement on the original.

...

Sun Jul 30 2006, 06:31PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

If all that you want to do is give it a constant 100ma then just find a resistor that gives 100ma of current draw... You don't need to calculate it

Keep in mind that resistors aren't exactly the stated value (could be as much as 20% off) and that the 5v in usb ports isn't exactly regulated either. Just put a meter in series with the laser and add resistors until you get the disired current... If you really want to figure it out you could graph the current/resistance and get an idea of what you need...

Sulaiman

Mon Jul 31 2006, 12:24AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have designed and built a couple of laserdiode drivers
and I've killed more laserdiodes than I've designed circuits

You have to be VERY careful with transients in feedback circuits
it's incredibly easy to turn a good laserdiode into a pathetic led or short.

I strongly recommend the simple apprach - a series resistor and a battery.
Don't try to get the maximum possible output (too tempting)
There's not a lot of visible difference betwee 1/2 and full power anyway.

Since I know you will ignore this advice
please post here when you kill your diode for others to avoid the same mistake

dan

Mon Jul 31 2006, 02:35AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

Well I went with a 100 ohm pot.. it allows me to slowly adjust the current to the desired value. I have a 15 ohm resistor in series which limits current to 110ma if something should go awry with the pot. I keep it adjusted at 100ma which gives a fairly bright beam.

There is a quite noticeable difference from 85ma to 100ma but nothing from 100ma to 110ma. So Iâ€™ll just keep it at 100ma just in case. This should give some head room for voltage fluctuations on the USB port and as the diode heats up. There is also a 1n4004 diode in anti parallel with the laser diode to prevent me from blowing the laser diode if I connect it the wrong way. (Trust me i'm probably dumb enough to do it.)

I managed to construct a collimation lens mount out of a 1/8'' NPT plug and a 1/8'' NPT to some other type of thread adapter. It's rather crude but it works. Once I get it adjusted I'll lock everything in place with crazy glue.

P.S. I know all to well how sensitive these diodes are. I *used* to have a 2w fiber coupled laser diode at an unknown IR wavelength. Thatâ€™s until a 12v wire from the computer PSU I was using shorted against the case of the diode. It turned into a really expensive IR LEDâ€¦ ouchâ€¦ I felt really stupid after that one.

But luckily I got it cheap off ebay. It was a misspelled listing (â€œlsaer diodeâ€™) so I got it for $2 plus $10 to ship. (Blowing things seems to be a reoccurring theme with me for some reason. Just my karma I guess.)

Even with the crappy optic mount the beam is still faily tight after 100feet (5cm dia. still needs some fine adjustment). Even with the spot 5cm in diameter it still fairly bright (looks like a high intensity LED a foot or less away from the wall). Compared to a cheap $1.99 laser pointer and my 15mW HeNE it kicks ass.

.

Hereâ€™s some pics:

Steve Conner

Mon Jul 31 2006, 10:30AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The reason your calculations are off is that you forgot to take into account the burden resistor inside the meter that it uses to measure the current.

Conundrum

Wed Aug 09 2006, 05:49PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

One interesting approach is to parallel a 1K or 470 ohm resistor across the LED, this tends to absorb the worst of the spikes while allowing the feedback circuits to work if present. (thank Sam Gs laser faq for this one)

I did this to my 60mW visible diode and so far no damage from static or random glitches.

I've had this diode etching black plastic with 149mA although this might be that it is an old diode and probably lost emission..

Regards, -A

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