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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so I have ordered some stuff to make an adjustable supply. Got some heatsinked power resistors, 25watts 0.1ohms. I will solder it all up as soon as it all comes in. I have a High heat load 3watt laser diode that has all the bells and whistles built in. (TEC, thermister, photo diode). Now, will the TEC need to run while the laser is powered on? What Im getting at is, is the tec in between the laser diode and the package? (If so, heat would not transfer properly if not running, no?). I checked Coherent's site and could not find those sort of details. I have a MASSIVE heatsink I plan to mount everything on. (laser and driver). The diode ought not get hot at all if the TEC does not need to be run. Matt
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, the TEC is in between the laser diode and the package. Yes, the TEC probably needs to be powered when running the laser.
If it didn't, Coherent could have saved money by leaving it out altogether. But then again, maybe it's only there to stabilize the optical performance: it could be that the laser won't suffer any damage with the TEC off, but thermal expansion will throw it out of tune, like Carbon-Rod suggested. Hence, to get the specified peformance, you're supposed to use the thermistor with a TEC controller, to stabilize the laser temperature at whatever the datasheet says it should be.
I used to design all sorts of laser and TEC drivers in my last job, both linear and switchmode, with and without computer control, and it was a fiddly business to get them all safe and spike-free.
I did one for DFB lasers that are actually tuned by changing the temperature. It had a tuning knob like a radio, with a LCD readout in nanometres, and equations inside that converted from wavelength to diode current and TEC temperature. The idea was to supply a constant 1mW at whatever wavelength the user dialed up. The TEC on that had to be stable to something like 0.01'C.
My favourite laser driver circuit was a transistor current source stabilized by a current sense resistor and op-amp. It's simple, and inherently a current source. I wouldn't use a voltage regulator and ballast resistor, that's ghetto
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Dr. Conner wrote ...
Yes, the TEC is in between the laser diode and the package. Yes, the TEC probably needs to be powered when running the laser.
If it didn't, Coherent could have saved money by leaving it out altogether. But then again, maybe it's only there to stabilize the optical performance: it could be that the laser won't suffer any damage with the TEC off, but thermal expansion will throw it out of tune, like Carbon-Rod suggested. Hence, to get the specified peformance, you're supposed to use the thermistor with a TEC controller, to stabilize the laser temperature at whatever the datasheet says it should be.
I used to design all sorts of laser and TEC drivers in my last job, both linear and switchmode, with and without computer control, and it was a fiddly business to get them all safe and spike-free.
I did one for DFB lasers that are actually tuned by changing the temperature. It had a tuning knob like a radio, with a LCD readout in nanometres, and equations inside that converted from wavelength to diode current and TEC temperature. The idea was to supply a constant 1mW at whatever wavelength the user dialed up. The TEC on that had to be stable to something like 0.01'C.
My favourite laser driver circuit was a transistor current source stabilized by a current sense resistor and op-amp. It's simple, and inherently a current source. I wouldn't use a voltage regulator and ballast resistor, that's ghetto
Yea I was afraid that the TEC would need to be run...I dont care about temp. tunning right now, just having fun with 3watts. Since it is an 808nm laser, green light may come later. So right now, I just want to keep from killing the laser from over temperature. I wonder if Coherent would reply If I emailed them about it? It is a recent laser by them...Ghetto?Hahaha! I know its ghetto....It wastes alot of power and creates heat, but SEEMS to be a safe way to run the laser. A start for now, right? I guess I will also plan to keep the TEC running until Coherent tells me otherwise. (TEC requires 8volts, 5amps!) Geeze!
Another question: DO I really need indium foil instead of heatsink compound? EDIT: Here is a link to a site that has a driver using an op-amp and some transistors. Is this about what you were refering to?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Matt
Yes, that link is more or less the circuit I used, although I improved it to make both the control voltage and the output referenced to ground. In that version, the control voltage is referred to the positive supply. And the guy managed to make it look ghetto anyway
You shouldn't need indium foil, no. If the laser is ready packaged with a TEC, the surface area of the package should be big enough to get rid of the heat with ordinary compound.
If you do run the TEC, be careful not to freeze the laser diode before you turn it on, or worse still, put the TEC into thermal runaway and roast it along with the diode. (don't ask me how I know this!) At least power it through a variable supply of some sort, and use a DMM to keep an eye on the thermistor resistance. Practically every laser I've ever seen likes to run at 25'C which is a resistance of 10k ohms.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Dr. Conner wrote ...
Hey Matt
Yes, that link is more or less the circuit I used, although I improved it to make both the control voltage and the output referenced to ground. In that version, the control voltage is referred to the positive supply. And the guy managed to make it look ghetto anyway
You shouldn't need indium foil, no. If the laser is ready packaged with a TEC, the surface area of the package should be big enough to get rid of the heat with ordinary compound.
If you do run the TEC, be careful not to freeze the laser diode before you turn it on, or worse still, put the TEC into thermal runaway and roast it along with the diode. (don't ask me how I know this!) At least power it through a variable supply of some sort, and use a DMM to keep an eye on the thermistor resistance. Practically every laser I've ever seen likes to run at 25'C which is a resistance of 10k ohms.
Hello, thanks for the reply! As for the voltage reference, he said he used a LM723....Could I use an LM350 instead? Im pretty new to this stuff, how does this circuit control the current through the diode? In other words, how do I make this variable? Is the voltage reference varied or the 2.5 ohm resistors varied? Im so used to building tesla coil circuits and drivers...This circuit looks crazy simple, but I cant make out whats going on. I see two OP-AMPS driving some transistors with feedback...(Any great websites out there that could lay down the basics as to what this thing is doing?) I lived in the ghetto once...(Im a college student)....really dont want to make this ghetto Hahaha....Still waiting on Coherent about the TEC thing...Id love to not have to run it as it draws 40 watts! EEEK! Thanks for all your help!
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so I went with this circuit instead, but replaced the lm317s with lm350s. I will also be replacing the rheostat with a 12watt 2ohm one (since Im pulling more current). I scoped the output from the voltage regulation part and is looks funny. How ought it look? I figured a solid line? It looks more of a sinewave/squarewave thing with the voltage swinging both positive and negative... Matt
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so I blew up my laser driver when I accidentally reveresed from my 12volt 17amp battery. BOOM!!!!!!!! Blew the LM350 in half! I am waiting on new stuff to arrive to rebuild. I also ordered a FLATPAC rated 5 volts 40amps. I have heard good things of these as far as lasers go and it was on sale. Apparently, one can use an op-amp to control the current. For now, it will replace the giant battery.
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SIMPLE Laser PSU
