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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
HFsstc-freak wrote ...
You forgot to leave him a neg.
Yes, I am making sure I get my money back. Once its in my account again, I let the world know. At least he did give me a refund...Perhaps all of us with ebay accoutns ought to harass him some? Matt
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I should point out, there is a 'real' power supply for these lasers that has been floating around ebay ( was the most recent listing) that is just slightly out of my price range (if I could get a single channel one for $75 I would be sold, but alas I don't have a need for 4 of my diodes running at once...) but would be perfect for driving these diodes. It even looks like it has a manual (although 4 pages seems a little short to be the full manual, you might have to fight with newport to get the real one), but should be pretty much plug and lase...
I really would like to get arround to building the buck converter based design I have been thinking about, but I honestly don't think I will have time to devote to it until next summer. Although I might end up having to make some time over winter break to work on it. I really think that it should be strait forward to get it working; because I am not worried about step response bandwidth and whatnot, I just want to have a little feedback to keep the current stable with temperature over the period of a few seconds. Famous last words I know...
BTW, I am trying to get one of those modules you posted; 3 days left and no bids! (has ebay always removed BIN after fhe first bid?) I am hoping that if there is anyone looking for one they will try for the complete packaged version (that has a big fat tec integrated) that is sitting with no bids at $150 at the moment. I already have an uuber cold plate so the bare fiber coupled diode is more useful for me...
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Alright, got my money back from the seller! I will leave feedback in a little bit, but now I am all about building a driver for it. I can test it out asap with my lower power laser driver, but up to a little under 20amps for only a few seconds before it overheats. I will begin ordering power resistors to drop the voltage and limit current on my 40amp flatpac.
So now, whats a good way to look for spikes with an o-scope? When I scoped the output, I just saw a pretty steady line at 5 volts. I turned the psu on and off several times while looking at the trace on the scope. Nothing that looked like spikes to me? There also seems to be a slight delay from turn on till output.
Seems pretty simple. A load resistor to keep the supply loaded. Another resistor to limit current via ohms law, and a fast high current diode across the laser diode. How does this sound?Ima need to bolt the laser down to a large heatsink I have been saving for a good occasion. Matt
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
In our lab, we loaded experimental drivers with a few diodes in series (to simulate a laser diode without the bill when it gets wrecked) and used a current shunt in series with that, connected to a DSO in single shot mode, to examine the turn-on and turn-off behaviour. This is one of the few situations where a DSO really wins over an analog scope, capturing single transient events. If you set up the triggering and persistence right, it will see and remember everything.
We flipped the thing on and off hundreds of times, pulled the plug out of the wall while driving the laser at full current, and so on, to check that the current never spiked or reversed.
If you have a big enough thermal and compliance budget, you can design a driver circuit where serious spikes are mathematically impossible. Put a resistor in series with the laser and a reverse diode across it, Ohm's law does the rest.
You can also use transzorbs. I once had to design a laser mount for an intrinsically safe application that would protect the laser diode against a direct short to mains. The real purpose was to limit the laser's output power such that it couldn't ignite flammable gas, but it ended up protecting it from destruction as a side effect. I used a couple of resistors, a transzorb and a fuse. I still have the DSO plots somewhere of laser current and optical output when the mount was hit with 240V AC
I don't know of any laser that was ever destroyed by any of our designs, and we worked with little telecom DFBs, $10k optical amplifiers, and so on. I think a big 40 amp diode would be pretty hard to blow with current spikes in the forward direction, so I'd just connect a Schottky from an old PC power supply across it in the reverse direction and then go to town. Leave the diode permanently installed right on the pins to save your ass the day you plug it into the driver backwards.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Steve McConner wrote ...
In our lab, we loaded experimental drivers with a few diodes in series (to simulate a laser diode without the bill when it gets wrecked) and used a current shunt in series with that, connected to a DSO in single shot mode, to examine the turn-on and turn-off behaviour. This is one of the few situations where a DSO really wins over an analog scope, capturing single transient events. If you set up the triggering and persistence right, it will see and remember everything.
We flipped the thing on and off hundreds of times, pulled the plug out of the wall while driving the laser at full current, and so on, to check that the current never spiked or reversed.
If you have a big enough thermal and compliance budget, you can design a driver circuit where serious spikes are mathematically impossible. Put a resistor in series with the laser and a reverse diode across it, Ohm's law does the rest.
You can also use transzorbs. I once had to design a laser mount for an intrinsically safe application that would protect the laser diode against a direct short to mains. The real purpose was to limit the laser's output power such that it couldn't ignite flammable gas, but it ended up protecting it from destruction as a side effect. I used a couple of resistors, a transzorb and a fuse. I still have the DSO plots somewhere of laser current and optical output when the mount was hit with 240V AC
I don't know of any laser that was ever destroyed by any of our designs, and we worked with little telecom DFBs, $10k optical amplifiers, and so on. I think a big 40 amp diode would be pretty hard to blow with current spikes in the forward direction, so I'd just connect a Schottky from an old PC power supply across it in the reverse direction and then go to town. Leave the diode permanently installed right on the pins to save your ass the day you plug it into the driver backwards.
Thanks for the tips! I really appreciate you guys helping out! I dont have any old PSUs to get a schottky from, but would any super fast diode work? I have some ultrafast diodes rated for 15amps. Voltage drop on them is 0.4volts or so. Also, for the resistors, what do you think about these? I would use one of them for the current limiting resistor and one of these for the smps loading resistor.
I also plan to add a mosfet to act as a switch to turn on the laser after the smps has turned on for a few seconds. (As was suggested earlier).
I had the same feeling about spikes being less troublesome for such a large diode. Its rated for 56amps! I wont be able to give it more than about 35amps right now anyways. I may get another identical flatpac and parallel them for more current IF this laser turns out to be good. (Ill drive it to 15amps or so with my driver) matt
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I would agree, you will need a several hundred amp spike to do any noticeable damage to the die. I have a stack of 50% fill 38 emitter bars (ie, 38 laser chips with a 150uM active region) that I have run at 90a (as much as my 120lb linear monster could muster) for a few minutes producing over 70W (the limit of my thermal power meter head), with the block mounted on a cold plate at (IIRC) 12c, with no noticeable power depredation (although I wouldn't have been able to detect <5% fluctuation with that setup). That is probably the same diode that is in your package, so it should give you an idea of what to expect.
I haven't tried powering a diode up backward yet, but so long as you remember that the case is + you should be safe... A normal fast diode should do a great job of protecting it so long as you don't vaporize the diode (which with the currents we are talking is a very real potability...)
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so the general consensus is that TVS or transzorbs ect. would just be a waste, no? So my real problem will be getting rid of heat. I have a large heatsink weighing several pounds. I will mount a fan to it and see how it does. I will mount everything to this while I am at it. (Power resistors and whatnot). Laser ought be here tomorrow! Cross your fingers, say some prayers (for the faithful) and pour me a beer!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Sounds good, just try to minimize inductance in the rig. No big fat wirewound resistor please!
BTW, the cooler you can get the diode the better; optical output drops considerably at elevated temperatures. Not to mention that whole diode-life-time-halved-every-10c-increase-in-
operating-temp thing.
One thing that will probably help with your uuber heatsink is to bolt the laser right in the middle of the heatsink so that it dissipates evenly, and some fans would certainly help.
If you are interested, I have looked into water cooling rigs and for about $250 you can put together a rig that will keep the diode at 25c with a good 100w or so running through the diode. (still need to try working the system, but the idea was to make a complete laser driver with psu and cooling to go with my bars)
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Testing an old 20watt laser diode (fiber coupled)
