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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so will these type wirewound resistors be safe for use in a laser driver? UPDATE: After spending HOURS trying to sand the surfaces down, I looked closely and found that the friggen heatsink aint perfectly flat! I could see a gap in the middle. I couldnt find something thin enough to fit in, but it sure is a gap. I pulled the laser and looked at the thermal compound pattern and could CLEARLY see a lack of it in the middle. SHEEEEEEESH! All this time, Ive been sanding with 600 grit to polish both surfaces and its the friggen heatsink. To be sure, I stuck the laser onto a small, flat heatsink and no more gap visible...
So I guess im off to find a large heatsink that is perfectly flat. Any ideas? Ive tried sanding down the heatsink. No luck. Two hours with sand paper and nothing but a huge mess! Maybe I need to take a belt sander to it? Anyone here have any advice? Matt
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
600 grit O.o thats like normal paper. Ide say buy a new one, or yes, a belt sander. You would have to climb sand paper grits thought, heavy for removal, then a bit higher grit, then more, etc. in otherwords, its time or money =)
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
If your belt sander is handy then I would for it, just be careful to not but a bow in the heatsink otherwise you are just as bad off as when you started :p
Another option would be to to take it to someone that does have a mill and have them true it up. Brake shop maybe?
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Yea, I dont think Ill have the time to fix this heatsink. I need to find a new one. Any types to lookout for? Is there a certain kind? I know the one I have was extruded. Perhaps extruded heatsinks are crap for this kind of thing? My plan is to check ebay, but I dont know how I would determine if the heatsink is flat enough. This seems to be much more complicated than I bargained for. No wonder lasers cost a fortune! They require laser precision to work at all...I dont even know if my laser is flat enough now. I applied sand paper to it. It LOOKS flat, and sits flat on other flat surfaces, but who knows? Jeeze... EDIT: Just came across this link : for lapping. Looks easy enough to do. I can do it to my laser to to be sure its also flat. What do you guys think? 12$ isnt too much of an investment... Matt
Registered Member #1628
Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95
The ideal solution would be using the sandpaper method to achieve a fairly flat surface. We use a monochromatic light and optical flats to certify flatness to just a few millionths of an inch in the calibration lab. But if you have no access to that kind of equipment lapping to an almost mirror shine should be sufficiently flat.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The key is making sure that you actually sand the block flat, not with a big bow in it (it harder to do then you think when you are talking thousands of an inch across a 2" long surface). I would expect the laser to have been flat to start with, but you never know...
As to the heat sink, large high quality heat sinks are (amazingly enough) quite expensive. I usually see these kinds of lasers mounted on those big copper heat sinks with lots a tiny fins and fans, but you don't want to know how much those cost.
If you want the laser to run 10c above ambient, and assume 3c for the thermal interface, you need to dissipate about 100w in 7c, which is .07c/w. An high quality computer waterblock + large radiator is about .03w/c You might try looking for some big fat CPU heatsinks, they seem to be the cheapest way to get large heatsinks these days. Go for one one of the heatpipe+pressed fin heatsinks. Note, you should be able to to do a bit better than the pc benchmarks since you have a much better thermal contact from the laser to the heatsink then a cpu does (at least in theory...)
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, thanks for the tips! I ordered the lapping kit and some more thermal compound. So, how does one know if something is flat enough? I am tempted to keep sanding and trying new objects for my "flat" surface, but my little laser diode is beginning to show its thermister. (Epoxied into the base). I also took off all the gold plating on the bottom. Just copper now. The kit has a super flat surface to use, so well see. I should be able to make both surfaces pretty flat and polished. Aluminum and copper seem pretty soft, so shouldnt need alot of effort. If this does not work, I will go the more elaborate heatsink route. (Id like to end the pain of spending for this laser though!). Matt
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
My usual 'test' is to take a razor blade and sit it perpendicular to the surface and check to see if there are any gaps. Check different rotations just to make sure that the heatsink isn't curved in one direction or another.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If it's badly enough bowed that the bow is visible, lapping it won't do jack. Lapping is for removing irregularities on the order of thousandths of inches.
If you can't have it milled flat by a machine shop, just get another one, and be thankful that it didn't pop the die off your laser when you tightened it down. That is, unless you put a bow into the bottom of the laser package by sanding it (why in the world did you do that? they are milled flat at the factory) and now tightening it onto a flat heatsink will break it.
If you really get into lasers, the pain of spending will never stop. Muahahaha!
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Steve McConner wrote ...
If it's badly enough bowed that the bow is visible, lapping it won't do jack. Lapping is for removing irregularities on the order of thousandths of inches.
If you can't have it milled flat by a machine shop, just get another one, and be thankful that it didn't pop the die off your laser when you tightened it down. That is, unless you put a bow into the bottom of the laser package by sanding it (why in the world did you do that? they are milled flat at the factory) and now tightening it onto a flat heatsink will break it.
If you really get into lasers, the pain of spending will never stop. Muahahaha!
Hello, I sanded the laser because the bottom was in horrible condition. It had pits and such for some reason, as well as indium foil stuck to it that just refused to scrape off. No telling what was done to these lasers. They still burn holes in boxes... There is no visible bow in either laser or heatsink, but I imagine there is one. That may be what is keeping this thing from acting right. Could it be possible that the actual laser package inside this laser package has a problem with its thermal connection too? The laser is basically a housing containing a laser package and fiber optics. Maybe somebody did something to it that ruined its internal thermal connection, and thus it wound up on ebay?
I will try using some course sandpaper to both laser and heatsink before lapping. If that doesnt work, and the laser is simply screwed, ill just have to write it off as a learned lesson about used, untested ebay lasers! Matt
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Testing an old 20watt laser diode (fiber coupled)
This seems to be much more complicated than I bargained for. No wonder lasers cost a fortune! They require laser precision to work at all...I dont even know if my laser is flat enough now. I applied sand paper to it. 
They still burn holes in boxes...