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Duty Cycle calculations

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I have two laser pointers that I will be setting up some photos with and lining the beams may take up to 20 minutes or so. I searched the forum and found some limited info on a post 4 years ago, but rather than resurrect it, I have some additional questions I would like to ask your help with.

Pointer 1: 445nm M140 2W in copper module w/ 3 element glass lense (DTR's)
58gm tapered/extended copper heat sink (Survival Lasers)
Fixed current 1.8A driver (survival lasers)
C6 aluminum host
powered by 7.4V li-ion (2x 18650s)

I will be mounting it to a tripod to keep its position steady in a room 68°F
Does anyone else have a similar build? This is my first one. Did I do ok?
I can't find a data sheet on this diode and I'm not quite sure how Tj = Ta + P / Rja works :-(

Tj (max temp of diode) = 50°C ??
Ta (ambient temp) = 20°C
P (heat from diode) = Is their data on this or do I need to use an IR thermomenter on the Heat sink?
Rja (thermal conductivity of heat sink) = according to the older post: http://laserpointerforums.com/f44/how-determine-duty-cycle-63807.html
I understand that mass (58gm for me) has little contribution for thermal resistance so do I need to measure the diameters (more than one since it is tapered or take an average) and use some (unknown to me at this moment) equation for thermal conductivity? The thermal conductivity coefficient for copper is 401 W/(m k) at 25°C according to: Thermal Conductivity of some common Materials and Gases

Any help would be appreciated.

My second build is for my nephew and I would like to be able to tell him how long on/off his will be.
Oclaro 250mW 638nm Diode In Copper Module (DTR's)
anodized aluminum heat sink (Survival Laser)
1.2A driver (Survival Laser)
C6 aluminum host
powered by 3.7V (1 x 18650)

I haven't built the second one yet but the parts are on the way. I haven't lit my 445nm pointer for more than 20 sec until I can get more info.

Any help, warnings, suggestions, or praise will be much appreciated,
:thanks:
 
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Yes I have a similar build, you did ok. Forget about calculating the duty cycle, there are too many variables. How perfectly does the module fit the heat sink? Did you use any thermal paste between the module and heat sink? Or between the heat sink and host? The C6 isn't so good at dissipating heat from the heat sink but with the extended heat sink it's better because the heat sink should make contact with the pill. Yes use an IR thermometer if you want to be sure. Remember if the host is warm then the heat sink is warmer and the module is hot and the diode is hotter. Just stick with 30 seconds max on that thing and you will be ok.

For your second build I am not sure but I think 1.2A will fry that diode, you better check with DTR on that.

Alan
 

10fenny

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I got a bunch of c6 hosts from DX so right now I have four running builds with various spectrums. They get warm very quick, so stick to 30 secs.
3.14 is right.. too many variables.
I have one 650nm putting out about 230mW and I've blown the diode twice for not exercising duty cycle. I got ten 300-400mW diodes for cheap. I enjoy soldering diodes.
 
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Thanks for the encouragement. I emailed DTR with your stated concerns. Thanks for warning me. I've seen that crazy scientist avatar of yours on lots of posts and you seem to be very helpful. I just use my first rep point even though I'm still not exactly sure what they even do.
...but it's the thought that counts and you have my sentiments,
;-)
Thanks. Just so you know about the rep, it doesn't do anything until you have 50 or more posts and rep power of at least one. Also avoid double posting or people will get on your case about that. Learn to use the edit and multiquote buttons.

Alan
 

10fenny

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Well actually when I first started building... I thought I kept getting the diodes too hot. I figured I had everything correct, but my completed circuit would not shine. I kept trying and trying with less and less heat. Well I figured out I was wiring an obscure driver wrong. Once I got it working I was pretty good at soldering. I was even un-soldering diodes and transfering them. I am a full time mechanic so I'm constantly being pushed into a gray area for me.
Just start with cheap diodes. I still get semi nervous when I'm soldering a 60-100$ diode, but I try and not let it get to me.
Good luck!!
 




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