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Thread: Questions on abuse - Rohm vs. Opnext

  1. #1
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    Default Questions on abuse - Rohm vs. Opnext

    Looking for opinions regarding 'over spec' tolerances of the Rohm rld65pzb5 vs. the Opnext hl6545mg, both of which I have.

    While re-doing some connections on my Rohm diode'd, plastic box laser recently, I finally took a current reading. 306mA! Way over spec, and much more than expected. The circuit is just 3.4V (2 E2 liths) through a 2.7 Ohm resistor, and a 100uF cap across the diode. Now, according to a power plot created by Dr Lava over on CPF, this thing might be putting out as much as 170mW at that current.

    Even pumping that much juice though, I'm not sure my usage patterns would neccessarily qualify as abuse. Neither this red laser, nor my greenie (Atlasnova also on e2 liths) see more than 15 seconds per shot, usually much less, followed by a much longer off cycle. They also don't get used more than a couple/few times per month. I doubt either one will see more than 10 - 15 hours use over the course of their existence.

    Now I come to the meat of the matter. My next project will either use another Aixiz module, but with a glass lens instead of plastic, or possibly a Meredith w/ glass. Either way, I'm hoping for the beam to be slightly thicker, and with better collimation than the first one. With this project, I'd like to see how far I can project a fairly tight dot... like from one mountain peak to the next while in the Catskills, Upstate NY. The idea is to use the Opnext for this, because mA for mA, it gives more output than the Rohm. However, I get the impression from The Doctor's (here on PL) reporting, that the Opnext's are a bit less forgiving of overdrive.

    Now FINALLY, the real question. Given my very light usage patterns, does it matter? Can I just go ahead and crank up the Opnext because it'll see so little average use?

    Sorry for the long post to ask a simple question.
    Alas, poor diode. I fried him well.

  2. #2
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    Abuse. That's the way to do it!

    Some Opnext's are more forgiving than others. I think the HL6545MG is probably more forgiving than most, on a par with the Rohm.

    I've slowly established the true safe CC drive for a Rohm diode, current depends on the diode (they vary), and the temperature. The output power should be limited to 115 mW for longer than 1000 hours. This rule works for most diodes no matter how they vary. You can push them to make more, as you say around 170+ mW seems about right. I know people were saying 200+, but that's assuming 658 nm, which turns out to be a bad assumption, it's more likely 665 or even longer unless you have some TEC cooling.

    Two things affect lifetime, one is a roughly exponential fall in lifetime with a linear rise in power over the safe limit. The other is heat, which causes a slow degradation that eventually leads to immediate facet damage that destroys the OC mirror. Nicolai Pusch sells modules rated for 125 mW, using Rohm diodes. These are rated for 645 or 650 nm, so he has a small TEC in there. This would slow down the heat decline, allowing decent lifetime at a few extra mW as well as brightening them by wavelength shortening. The main question is how much shortening of life do you get for an uncooled diode, for each 10 mW you want out of it. I've not graphed this, but as a rough guide, I've seen 125 mW run for as short as 28 days full uptime, 135 mW for 8 days, and 150 mW run for only 3 days. The Opnext diodes you have will do better, but not much. They'll put out more if you want to risk short burts though. You'll get several hours or a couple of days at outputs that would kill any of the Rohm diodes instantly. Just make sure you use a good slow start. Forget modulation by the driver, because you'll need very gradual rise/fall times on the power supply if you want to stress them that hard.

    Estimated top whack:
    Opnext HL6545MG, 200 mW.
    Rohm RLD65PZB5, 180 mW.
    Both assume a 3 day lifetime at 35°C case temperature.
    If anyone can get better than this, please post details. Take care to verify the output with a thermopile meter or other method that is not wavelength dependent. A basic TEC based meter will do, if you can reliably calibrate it with a Lasercheck and a stable known wavelength at 100 mW or so.


    Edit: Take care with the Rohm diodes, when assessing power. Turn up slowly from around 100 mW. At some point almost all will mode hop to a lesser output, and you can turn the current up further to get the same output you had before, before you can go on to get more. If you want longest life, chose the highest output you can get before that mode hop fallback happens. Also, if you count how many times you see such hops on the way up from 100 to 170 mW or so, you can get a guide as to diode quality. The threshold of that first hop is especially useful, the higher, the better.
    Last edited by The_Doctor; 05-23-2007 at 10:47.

  3. #3
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    Thanks, Doctor! Very informative post. I believe I'll try the Opnext for this project.

    What current have you found to give approximately 200mW? Is that around the 266mA you mentioned in another thread, assuming that be after wavelength corrected estimates? I saw P1tbull's charts, but I'm chalking those numbers up to the possibly erroneous assumption of 658nm (although he did say they looked slightly orange-y, so I'm confused about that).

    If it's likely to live through 24 - 48 hours of being highly overdriven, I'm willing to go with that. It will never see that total amount of use. The only possible issue, and you've previously mentioned this as well, is the mode hopping. With the goal of trying to project a reasonably tight dot at very long distances, I'm sure that hopping won't help. Unfortunately, I presently have no way to check for hopping, as I don't feel my green tinted BLPS laser glasses (OD4 @ 694nm) are protective enough at shorter wavelengths to look closely at the dot on a white wall. I currently cannot afford good red blocking glasses for 635 - 670nmm, although the blue tinted Wicked's might be a possibility.

    My possible workaround is to spray a small, flat metal surface with high temperature, matte black paint (possibly engine paint), since it will reflect less than a white surface yet hopefully withstand the heat. I ask anyone here to step in and say "no" to that, if they think the MPE using my present glasses will still be too high. Additionally, all suggestions are welcome as to how to closely, yet safely examine a projected dot for TEM modes.
    Last edited by PNjunction; 05-24-2007 at 08:49.
    Alas, poor diode. I fried him well.

  4. #4
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    Don't trust the current because it varies too much with each diode. A rough guide is that current will be around 1.5 ma for every 1 mW out, but the mW's are critical so I just measure those. So long as the current source is stable, that's all you need to do.

    I didn't see Pit's observations although he sent me a diode to test. I ran it at a lower maximum drive because I didn't want to duplicate his test exactly, which would be pointless given only one diode, it made sense to get some different results for slightly different conditions, especially as I know that the full power will be short lived. As it was, it died in three days, surprising me, I thought I was treating it gently, and it died while nothing was happening, during quiet night hours. I think it was a strong crimson, not orangeish (vermillion). I think that looking orangeish in this case wouldn't have been due to a shorter wavelength, but to a loss of spectral purity, as the diode is strongly pushed, the linewidth broadens, that's one way the extra input energy can be converted, and the result is a visibly bleary pastel shade, a slight loss of sharp specularity. A red diode will appear slightly more orange as well as slightly flat in intensity when that happens, even though there's more power there. If you ever see that effect, back off on the drive current immediately, because lifetime will be in hours, a day or so at most but usually a few hours.

    Don't worry about goggles, any observation you need to make will be safe, and you'd not spot mode hops easily except by looking at the scattered light spilled from the lens onto the surface the laser is resting on. That light will be dim enough that you'll need to turn the room lighting down to see it.

    One way to examine a dot is to deliberately decollimate. Ideally the light should not focus through a point before diverging, it should just diverge. Take the lens a bit closer to the diode that you need for collimation. If the spot is a couple of inches wide you can probably look at it directly with no problem. An alternative is to spread the collimated beam across a matt black surface, which can be a cheap thermoplastic, it won't burn or melt, as the energy will be spread across an ellipse a couple of inches long, and most reflected light will be leaving at the same shallow angle the incident light arrives at, but you'll be watching perpendicular to the surface, so you'll not see anything really bright. The simplest way is to watch the scattered light though, you could project the beam across a foot or two to a wall, but have a matchstick stuck in blutak or clay so it blocks the beam, then you can stare at the residual scatter all day at close range with no injury. The scatter pattern will be changed by rotation to some extent if you turn the lens, but the bulk of it is formed by the diode, and a mode hop will change it as an obvious step change in the pattern shape and brightness. You won't see clear TEM modes in a single diode output, only a change in the distribution of light in the blob of light, and the scattered light from the lens is the easiest way to see that it happens.

  5. #5
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    Hi PN,

    Quote Originally Posted by PNjunction View Post
    What current have you found to give approximately 200mW? Is that around the 266mA you mentioned in another thread
    I have been running a pair of them for months in a small RGB projector at 240mA and get bang on 200mW out of each diode with a good quality single glass AR coated collimator. Had plenty of hrs use (designing shows) and they're still happy
    The red in the RGB projector in the center of my avatar is the 400mW in question.

    Quote Originally Posted by PNjunction View Post
    I saw P1tbull's charts, but I'm chalking those numbers up to the possibly erroneous assumption of 658nm (although he did say they looked slightly orange-y, so I'm confused about that)
    I have no accurate way to measure the wavelength, hence the visual comparison with another known 660nm diode. My LC was always set at 660nm, and when I took the readings in that chart, the diodes were cooled by TEC at the time so you would expect to see a few nm's drop.

    Quote Originally Posted by PNjunction View Post
    If it's likely to live through 24 - 48 hours of being highly overdriven, I'm willing to go with that. It will never see that total amount of use
    I drove a pair at over 320mA (294mW) for 24 hours with passive cooling before they died. Never tried on/off cycles but I'll bet they would last a good while if operated in that fashion. Another test saw some others run continuously for 3-4 days @ 260mA which ties up with the Doc's findings.
    I think 240mA is about as much as these things will take at a pretty continuous push, and should last a very long time when operated at that current in an entertainment projector, with a good driver.

    HTH

    BTW, currently building a >3watt red with 18 of the Opnext diodes, I'll post some pics in the next few weeks as it comes together.
    Last edited by p1t8ull; 05-24-2007 at 23:51. Reason: woke up and remebered a few things... :-) bold type is corrected or added text
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

  6. #6
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    Ambitious!

    Let me guess:
    Three in triangle by three again, via beam cube. Repeat three times, then combine each of the three clusters of six with mirrors as for the original triples?

    Btw, I got more news on Rohm's, both good and bad. Bad first: I saw a diode powered at only 115 mW out die today while I was slowly waking up in bed. (I leave one on at all times as a nightlight, best way to monitor one, having it there all the time). This last one only lasted 18 days! It's now absolutely clear that a Rohm diode must NOT EVER be expected to last the lifetime it is specifed for by its makers unless you're willing to run them at 100 mW or less (why else do you think that those who know, those who bought them by the MULTI-THOUSAND were selling them cheap on eBay, in BULK?) Florian Rotter said he had trouble with life expectancy, and I know he's not alone.

    On the other hand, there really IS good news. If you're willing to accept projector lamp lifetimes (likely wild variation between 50 and 500 hours), you can push them HARD. I decided to risk a live fast, die young strategy on one today, and I'm running it off a battery to guarantee a clean doubt-free supply. So far it's been putting out 218.5 mW according to a Scientech head and a multimeter and a scale factor calibrated with a stable DPSS which agrees closely with the factor specifed by Scientech, and >220 mW according to a Lasercheck set to 660 nm. I got one reading of 234 mW shortly after setting it up. Subsequent measurements were made over ten feet away. The beam is 5x3mm at that distance, about 2.5x3 at aperture. This is larger than it would be at modest drive, so the active area has increased. The diode has run at that rate for over 8 hours.

    A slightly unfocussed beam, as still as I could hold it, is shining up to a branch of a sunlit lime tree in the first picture, the spot is about 50 feet above ground, about 70 feet distance from the camera. The second is a comparison in a room with daylight coming in through a part-closed doorway, and a 100W incandescent pendant lamp (dimmed to 40W). The fog is light, visibility is clear. The camera is set to small aperture and fast shutter to make sure it isn't oversaturated by either beam. The green beam is a LambdaPro UG 100 mW laser, probably putting out around 85 mW of green. Not measured it for a while though, but it's behaved itself since I last did. The third picture is right after the best of three first measures of power with the Lasercheck set for 660 nm. The fog is very light, the camera flash was used. It's over 8 hours since I set this up, and it's still good, so a Rohm diode will get you through a serious all-night rave at this power, I think. The pictures are GIF's, they fuck less with the sharpness and intensity of the image than JPG's do, even rendering gradients nearly as well given good colour count and dither settings, but are bigger in bytecount.

    My design will not be ready for some time, I can't get all the metal stock I need. I also know my design intended for easy diode replacement is vital now. I've totally abandoned any visions of long life, I'll sell systems with a limited service after the sale, covering a few replacement diodes, probably. It's the only way to compete with Marconi's lasers until I can get diodes that live for a long time at >200 mW output. So long as people don't mind changing diodes like projector lamps, I think I'll be able to out-spec any multimode diode system with these. The TEC in that picture is NOT running, but that TEC idea is part of the main design, I intend to run these at >225 mW per diode at shorter wavelengths if possible. It depends on whether the Rohm diodes can reliably give me a minimum of three days, or hopefully 100 hours, at that output.
    Last edited by The_Doctor; 05-25-2007 at 22:36.

  7. #7
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    Well, Doctor... once again, your replies are too good for my own good. Seriously though, your excellent answers are highly appreciated! Thanks as well, P1t... great info!

    Regarding "my own good", I mean that thanks to the Doc's explanation, and pictures, I'm looking to see if a small TEC can run off 7.2V battery power, and trying to dream up a way to stuff one into the next project... along with two or three AA size Li-ion batts, LD driver board, LD/lens module, heatsink(s), and a 40mm fan, all into a 5" X 2.5" X 2" plastic box (might have to get the bigger one though). The intended purpose is to turn on the TEC for 5 minutes prior to firing the laser, and chill the diode to ensure the lowest wavelength possible.

    However, if this is to work, I'll need to do something similar to what you have shown above, Doc... a small heatsink that sits flat on the TEC, and contains a tight fitting hole to house the LD/lens module. Mind if I ask where you got yours, and the diameter of the hole? I need something just like it, but to fit the 12mm Aixiz module.

    Also need a 40mm fan that runs on much less than 12V. IIRC, they do exist.

    Assuming I can find the right combination of parts, then make it all fit together and work, there's still a concern about condensation. Since this project is meant to be used outdoors in whatever varying climates mother nature has to offer, is there a possibility that water will form all over the cooled module and heatsink, possibly dripping onto the electronics, shorting things out, etc? Anyone have experience with using TEC's outdoors during the summer (heat, humidity)?
    Last edited by PNjunction; 05-27-2007 at 09:49.
    Alas, poor diode. I fried him well.

  8. #8
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    Hehe... the vision of the finished product cracks me up. Heatsink and fan sticking out, a very dangerous looking, red LED tipped switch with flip-up safety cover, air intake holes, etc. If anyone's wondering why I'm doing this for a stupid, plastic box laser... I want to massively out-do and out-hi tech the first one, and I really enjoy making things that look ridiculous, yet are functionally impressive. So far, nobody's told me I can't.

    Here's the switch. How could one NOT want to make something ridiculous with this as inspiration?!


    BTW, if I can ask one more question... does a TEC need a driver, or can it run direct drive from a battery, maybe with just a resistor if needed?
    Alas, poor diode. I fried him well.

  9. #9
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    Hi PN.,

    Yes, I have experience with condensation down here in the Florida tropics.

    I have found to not go below 13c or 55f ..so I make all my TEC/peltiers to work at 15c or 59f. Only enough to keep the diode/s cool.
    Anything else is inviting trouble..unless you are very good at sealing the compartment for the diodes/optics I wouldnt do it.


    No, You shouldnt use just a battery..it would need to be controlled.
    I have just built-up a very simple TE controller which I wish to share as soon as I have it complete..It is a ramping type not a on-off kind of thing.
    Not super accurate but will hold the temp within a degrees which is perfect for lasershow use.
    It uses just one op-amp and a FET and runs from 12v for now..Im driving two 6v peltiers at the moment. then I will slim it down to 5volt to run one 6v peltier..when its done..hopefully soon.
    But, Ive got other projects to get out of the way first.
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  10. #10
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    Thanks, Marconi. Figured fantasy would meet reality at some point. But, fantasy isn't quite ready to give up the fight yet. (LOL... gotta' love the devil smiley)

    More "makeshift active cooling" questions. Still looking to use batteries if possible. Can an adjustable, constant current board run a TEC off a Li-ion rechargeable or two?

    Would that be possible with your eventual TE controller, Marconi? If so, would it be small enough to fit in a crowded box?

    Already have the "sealing" solution worked out... lol.

    Thanks, everyone for putting up with my half-retarded ideas and questions. If it's any consolation, I am learning.
    Alas, poor diode. I fried him well.

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