Mitsubishi ML520G71...Red Holy Grail or Flashlight Fail ??

[planters]

In this case we are only using the wavelength as a probe for temp. and as long as it is a 5.6mm diode then we can use any AlGaAs diode. The temp at the junction changes VERY fast as in usec. There is very little thermal inertia. You have to measure as you drive the laser. There is a whole area of supercooling diodes and it is very interesting and the 650-657nm diodes are among the most responsive.

[CDBEAM]

Pic sez it all !!! Just joking...PLEASE...keep the info/knowledge flowing !! THANX !!!.....Senior BEAM

[dave]

just a silly quetion, in case i am missing something major here ---> "what is wrong with dave's adjustable mount?"

I don't think anything is wrong with the design I have.. In fact as far as stability is concerned, I think its going to be the best as there are less parts that can slip out of alignment.

[logsquared]

So I took some time tonight to do some temp measurements. The set-up is pretty simple, not real scientific, but should give us a good baseline idea. I used my new 2X red combiner module as it was mostly assembled. I used two LM35 precision centegrade temp sensors. I read the voltage (10mv/ deg C) on my oscope. One is mounted to the baseplate and the other was mounted to the piece of brass the diode is pressed into. Also I tried with the #2 sensor on the diode as much as possible. The sensors are held in place with plastic spring type clamps. the whole module is clamped in a small vise. The diode (ML520G71.) was running at .5 A and 2.3 V with 400mw out the front. I let the set-up run for 1/2 hour as it seemed to be mostly stabilized at that point. As seen in the first scope picture the difference is aprox. 7-8mV (10mv per tick) that equals .8 deg C difference between the base and the brass holder. Next I let everything cool back down. And tried to get the sensor as much on the diode as possible. The second scope pic shows a difference of about 1.5 deg C after about 45 min. So the diode is running warmer than the mount as expected, but not by much. The room/ starting temp was 15.3 deg C. As seen after 45 min the diode was only about 17.5 deg C (probably a little more because of the sensor placement). This is good because we have 3 junctions diode to adjustable holder to mount to aluminum base and we have only a little over 2 deg total rise at .5 a current. Now consider the difference from diode to base is only ~1.5 deg! Using these numbers, we have approximately 1.3 deg C / W from the diode to the base.

Sorry Senior BEAM I think you may have over engineered

scope1.JPGscope2.JPGtemp setup.JPGtemp4.JPG

[CDBEAM]

Great work !!! So....very little heat generated !!! That means the conversion from current to photons out the front door is very efficient. OK then....but I still see a power drop from 457mW to 417mW @ .575A after 1hr. These readings were taken with the set up I have previously pictured on this thread ...with a Aluminum block mount (Laserbee II meter). This is only a 40mW drop or 8.75% power loss....Why does this occur ???? Minor temp build up inside the diode core ??? Anyway...I strongly doubt that any mount will prevent this drop....Air cooling or TEC MAY prevent this...or not !! or perhaps...who cares !!!!!! Thanx !!!....Chasing Windmills will continue via experiments with TEC !!

Senior BEAM

[planters]

Beam,
I think most of the power input to these diodes is converted to heat, but this heat is pretty efficiently conducted away by the existing mounting. I have spent a lot of time messing with cooling/stabilizing diodes and there is definitely an improvement in conversion efficiency with cooling. The magnitude of this improvement has always been greatest with the red diodes, but has been on the order of 0.25-0.5%/C. Once you move from the area of temp stabilization into active cooling you have to start looking at differential expansion, isolation/access and dumping substantial heat loads. With the LOC diodes the added (and more important ) benefit of wavelength shift makes this worth it.

What temp. is your base plate running at? Does it change during the hour as your PO drops? If it runs fairly close to ambient then put an ice cube on it and watch the PO. This should produce a large temp. drop at the diode(when compared to the small rise during normal operation) and a substantial PO increase if temp is the source of the problem.

I am pretty satisfied with my existing solution/compromise for red. However, if there is reason to suspect that these new diodes happen to have a much higher gain in efficiency or survival at reduced temps then I will look at pushing this envelope way down to see where it goes.

[CDBEAM]

Planters...I must reads up on diode efficiency in general. The ML is surly more efficient than the 445LD....a miniature " hot plate " compared to the ML red. I really have no idea on overall ML conversion efficiency .

As far as temp observations....start temp was room temp...end temp was slightly warm...very slightly....this is all I have now. Not too scientific !! I will generate more specific temps shortly...and I will experiment with cooling...as I allready have the parts. Fun with physics !!

And yes...TEC brings with it a whole host of problems/challenges !!

Thanx, BEAM

[kiyoukan]

Well as far as tecs go those are not to hard to drive.
I found some great 3W 12mm TECs.
As far as cooling these diodes i think 2 TECs stacked one on top the other
The 12mm TEC on the bottom then a larger 20mm to 40mm on top.
This will allow the first TEC to go even cooler than before allowing you to maybe even super cool the laser diode to help change the NM and keep power alive.
The trick is the second TEC must be more powerful than the first to not only take the heat away from the first TEC but also take the heat away from itself.
However we go to cold and in theory we will lose power from what i understand.
Also problems will come from going to cold in other ways.
And as far as a driver you can pick a 4 unit TEC driver for only 30$.
It is ment for driving high powered fans with a power in and around 10W total, but with some mods it can be used to drive TECs and it has a lcd screen with nice button interface and thermal probes.

[norty303]

OK then....but I still see a power drop from 457mW to 417mW @ .575A after 1hr.

Just out of interest, are these power drops being seen after driving the diode CW for 1hr?

Assuming that we are doing this for display purposes, how close to real world use is it? Personally I would say under modulation, that would be irrelevant, and you could probably drive them a whole chunk harder under modulation.
What is the real world duty cycle of a typical range of frames from circles to fans to scattered points?

IMO, these are just like speaker drivers, in as much as you can only play sine waves for so long through them safely at a certain power, but you can play music content at significantly higher powers due to the duty cycle. Fundamentally, we're just talking the same thing - heating of the diode emitter/voice coil

[steve-o]

^^ Not my shows. Full-blast balls-to-the-wall no blanking no modulation no nuttin. Just full-brightness sheets and beams in the sky ..oops , I mean on the side of a mountain .. not sky ..