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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Got the laser! Good and bad news... I powered up the laser via my laser driver. Around 5 amps, I could see red light on the target box. At 15 amps the box suddenly had a large hole go through it and alot of smoke. I put it up to 20 amps and stuck a pencil in front. The pencil caught on fire and I turned the driver off.
I powered it up and down a few times burning various things. All of a sudden, current drops big time! (Around 20amps, then to 5amps). I unplugged the driver and felt the parts on it. The current sense resistors were smoking hot. (As was expected). Now I cant get more than 3 amps out of the driver. (Laser is disconnected). Sometimes the current spikes to 5amps and then back down again. Not alot can go wrong with this driver. Could current sense resistors fail somehow? (They are 30watt, 0.1ohm, film resistors).
I replaced the op-amp. Still nothing. I checked Vref, it seems fine. I dont see how the main transistors could fail seeing as they are rated for 75amps! (They werent even warm).
Now, I did not bolt the laser to the heatsink. I put alot of thermal compound on the bottom and stuck it to the heatsink and put weights on top to push it down. (I was only going to give it power for a few seconds at a time). Laser did not warm at all. Infact, it was still cool after the failure.
So, this sucks! Im left with it either being the laser or the current sense resistors. (No, I dont have spares, but am ordering them!). I doubt the laser would fail at 20amps. And seeing as the driver cant get past 3amps with the laser disconnected, fingers point to the driver. No?
As for the laser: Wasnt the one pictured. Its a 2005, so not bad. It is VERY clean! Came with what looks like indium foil on the bottom. I peeled it off as it was torn and raggedy. There looks to be a thermister connected to the laser as well. No idea why this laser was pulled? At 15amps, it instanly blew a hole straight through a cardboard box. (Unfocused, 2in away). So I guess its time to order some more parts... ??? Matt
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
HFsstc-freak wrote ...
What are u gonna use this for anyways ?
To end the war in Iraq...Nah, Im kidding. Just have ALWAYS wanted a large burning laser! No real use in mind right now. Who knows what may come later. (Any ideas anyone?). I could focus it and engrave metals I bet... Matt
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
You're testing the driver with a dummy load now, right? It's quite possible that the power resistors changed value if they were running too hot, I'm not sure how much is too much for big power resistors. Of course, three 30W power resistors in parallel should handle the (10W?) of dissipation. You should check them just to be sure of course. I think this page explains how to measure low resistances.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, Yes I am measuring now through a dummy load. I am also using my DMM on current mode, as well as tried an anolog 25amp dc ammeter. They all show the same current measure. The resistors are small. t0220 package. They needed much better heatsinking. They are both 30watt rated, but at 25C, and I can guarantee they werent kept at 25C )))
Ive never had a resistor fail like that before though...And both of them...As a refresher, my driver is a dual op-amp driver. Each op-amp has a 75amp mosfet and one 30watt 0.1ohm resistor...
I doubt the laser failed, but will find out when my new resistors get here. (Got a 100watt 0.1ohm to247 to mount to a heatsink for the high current driver...)
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
You might try using about 10ft of 20awg wire for your sense resistor, or if that still gets to hot 30ft of 14awg would probably work fine without being too inductive.
Glad to hear that its working, the one I am bidding on ends in one day, hope that I get it!
Sounds like it will be pretty awesome with a fiber connected to it.
BTW, if you are looking for a fiber I have some extras, so I could probably hook you up with something for not too much money. i would have to inventory when i get home in October, but i think i have at least one spare fiber with a SMA on one end.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
... wrote ...
You might try using about 10ft of 20awg wire for your sense resistor, or if that still gets to hot 30ft of 14awg would probably work fine without being too inductive.
Glad to hear that its working, the one I am bidding on ends in one day, hope that I get it!
Sounds like it will be pretty awesome with a fiber connected to it.
BTW, if you are looking for a fiber I have some extras, so I could probably hook you up with something for not too much money. i would have to inventory when i get home in October, but i think i have at least one spare fiber with a SMA on one end.
Hello, I ordered some power resistors. (thick film, 0.1ohm 100W) for the current limiting/sense resistor. 5volts from the supply-2volt drop/0.1ohms ought to be 30amps. Likely less. I also ordred a 1.5ohm 30watt thick film for the load resistor (to load the flatpac while in use). I ordered 2 more thick film 30watt 0.05ohm resistors for my laser driver (since the old ones likely failed somehow). Half the resistance ought to dissipate much less power ( I^2*R). Lastly, I ordered 3 more of the 75amp mosfets. (Two incase I somehow damaged the ones in my driver) and one for the current flatpac high current driver.
How much for some fiber?Maybe I deal on a new pair of goggles and some fiber? )
Time to drill the heatsink...Wish me luck, Im terrible at that!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I just remembered that coherent has the specs for their didoes listed in their website ( ), just find one of the 808nm 40w sheets to get an idea of what to expect for the diode. Looks like you would probably need a driver good for 50-60amps to get full power out of the bar (especially if it is an old one, end of life bars draw more current but usually can put out full power). Also, the power rating is with the diode casing at 25c (ie, 10k of resistance across the thermistor), so the actual power will drop considerably with having the heatsink at an elevated temperature. I didn't do a quantitative measurement of this effect, but I wouldn't be surprised to see a 50% power drop going from 70f to 120f, especially when operating around threshold.
I just sent you an e-mail with info relating to fiber and googles.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Tested at high power today. 30 amps and the laser was causing boxes to burst into flames unfocused! Spot size was about 1cm in diamter and it still caught the box on fire!
I have a problem: I am watching the temp via the installed thermister, and it appears something isnt right. Its around 9kohms at room temp. When the laser is on, the resistance quickly begins to drop thus indicating a sharp rise in temp. Now I have this laser bolted to a very large heatsink. No active cooling yet, but should temp. be rising this much? The laser diode itself feels cool to the touch, as does the large heatsink. I am wondering if it has to do with the quality of surfaces of the laser diode and the heatsink?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
It could very well be a poor thermal conductivity, how low is it dropping? 5k gets you to about 100f which is just hardly lukewarm, 3k is up to about 140f and starting to be at the edge of what I would run the diode at.
Make sure that the heatsink is 100% flat, that the surfaces are clean/dry, put on a thin layer of thermal goo (a razor blade works well to smear it, it also helps to warm everything up so its more liquid) and bolt her down as hard as you can (its a thick plate of copper in there you aren't going to hurt it).
Also, make sure you debur the holes you are using to bolt the laser down; a 1/2" normal drill bit works well for this. Just put it in a drill and make start drilling the hole until you get a little depression around the threads (doesn't need to be very deep at all, so to there isn't a bump there from the tapping process).
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Testing an old 20watt laser diode (fiber coupled)
)))
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