Driving laser diodes

The thing that you might be missing is that you can blow up a laser diode i n a remarkably short time.

There have been thread here before about laser diode drivers, and the take- away message was that the active area is very small, and a microsecond's wo rth of extra current was enough to heat it to the point where atoms would s tart to move around, and end up in places that meant that it wasn't a laser diode any more.

Phil Hobbs will give you a much more authoritative story - if he has the ti me to spend on cutting and pasting it - but I think I can safely say that b eing scared shitless of blowing up the device is the right starting point.

--
Bill Sloman, Sydney

Well, the right approach would of course depend on what you want to accomplish, but oh well. ;)

Chers

Phil Hobbs

You see the thing is, Bill, the maximum current supplied to the diode cannot exceed that provided by the current limiting resistor in the drain line. When the FET switches on, with Rds being so low, the diode is effectively shorted to ground via the resistor. It can't draw any more current than the resistor will allow, so how can it fail?

Goofing around, and a ~5 watt blue laser usually means burning holes in stuff. (to the OP, I trust you ordered the safety glasses too.)

I drive lower power laser diodes from a current source...(just add an opamp to your diode-fet-R mix.) But the simplest current source is a voltage and a resistor, so as long as you burn up enough power/voltage in the resistor you should be fine.

It's not that hard to blow out a diode with electrostatics. Keep the case grounded and add reverse voltage protection perhaps.

George H.

That's not as elegant as a constant-current drive, but it should work. You can adjust Vdd to set the laser current.

You could also buy a cheap cv/cc bench supply, a good thing to have around anyhow, and play with that.

The diode itself sounds fairly dangerous. Beware reflections.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Generally laser diodes are extremely ESD sensitive, although I guess the higher power types ought to be less so.

--

John Devereux

No, the High power Blues turn into high power LEDs if you sneeze. They need a true constant current source. Current surges blow the reflective faucet off the die. Google "die4drive"...

Steve

Fitted with proper optics that compensate for the elliptical beam, a 2 watt Blue laser diode has a typical NOHD (Nominal Ocular Hazard Distance) in e xcess of 800 feet. If up-collimated more, that can reach at least 1500 Fee t. NOHD is quickly defined the power level that gives you a 50/50 chance of resolvable retinal damage for a beam strike. At these power levels, the ~2

50 mS it takes you to blink is not going to help you in the least.

At these power levels, both specular (mirror) and diffuse (scattering) refl ections can be very hazardous. 445 goes right thru the cornea and is stron gly adsorbed by hemoglobin in the retina. Be VERY careful.

Generally you need Optical Density Six (OD6)Laser Safety Goggles at these p ower levels.

These can start fires if foucused, and do wonders with burning black thermo plastics.

NOT A TOY...

Steve

This is the one I have (but rarely ever use)

adjustable/dp/4283259

But I've never used the cc functionality. And I don't even know what a "tracking" power supply is supposed to do, either.[1]

99% of the time I'll use one of my extremely big and heavy linear supplies. Being a bit of an old fart, I'm more comfortable with old technology.

1. If anyone knows, I'd be interested to hear. It might come in handy one day!

An exemplary response if I may say so. You'd doubtless be appalled, but there are one or two people here on this group who would not have taken the trouble to write all that without expecting something in return for it (cash, information, sexual favors even, who knows?) But your willingness to help for free is what Usenet discussion groups used to be

- and still should be IMV - all about. Many thanks indeed. ;->

That should work. Connect the laser, set the voltage to 3 or so, and slowly crank up the current limit.

It means that one knob sets the positive and negative voltages simultaneously. Sometimes you can put two supplies in series, for twice the voltage, and have one knob control both.

One of my (currently six!) bench supplies is an Extech 382203, 0-30 volts, 0-3 amps, with nice analog meters and off-to-the-side 5 and 12 volts. Mastech makes some nice chunky supplies too, but digital readouts.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Don't change your name to Retina Doom.

Having almost lost a retina once, I've come to appreciate them.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I built a laser photoplotter in 1996. I bought a bunch of laser diodes at the time, as I expected them to pop due to crummy circuitry. These are 5 mW US-made 670 nm (deep red) units made by NVG. Well, I've been running the very same diode for almost 20 years, now! As far as I can tell, there is no loss of optical output either, although that is hard to tell as I have run through three different types of film (as the makers of silver film keep going out of business!)

Anyway, the drive circuit for black/white films is a resistor and a 2N2222! Everybody told me this would kill the laser right away, you had to have constant current sources with terminators at the laser end of the cable.

I also have a gray scale driver, that uses a variable current source and feedback from the photodiode.

Now, maybe things are different with these monster lasers, they may run a lot closer to the limit.

Jon

Uhhh...

Those things have big fat >470uF caps on their outputs.

Use it like an ideal current source. Short the output, set the current limit, then connect the diode. Then release the short, and turn up the voltage until it limits. To turn off the diode, apply the short again, then disconnect.

Never, EVER connect a current sensitive thingy to one of those power supplies without shorting out the supply before making or breaking the circuit!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Den tirsdag den 15. marts 2016 kl. 12.30.32 UTC+1 skrev Cursitor Doom:

LM317 and a resistor?

-Lasse

And, can you be sure (without testing it) that the power supply GRACEFULLY goes from short to some lighter load without a voltage or current spike? I wouldn't TRUST some Chinese bench supply to do so without actually trying it.

Jon

IIRC, the LM317 tops out at only 1.5A; so it's a real shame it isn't suitable for this diode.

My thoughts entirely. I'd far sooner approach this manually. I've got some 1/2W cheap red laser diodes to perfect a scaleable supply.

use 2 sets of LM317+resistor in parallel adding currents

-Lasse