Electronics > Beginners
Microcontroller, DAC and Mosfet to control TEC?
MarkF:
i'm not familiar with TEC devices. But, it appears from the discussion that you want to control the current and not the voltage?
So let me offer a modification to my previous circuit.
I stuck with the MCP4822 DAC because it has a SPI interface. Which if you use the Adafruit display with a SPI interface, it would save MCU memory by not needing both SPI and I2C libraries. (Assumption on my part. I have only done SPI on a PIC with my own lbraries.)
Ian.M:
All that to save one choke to smooth the current! :-DD
It is however worth it (with a higher resolution DAC) if you need *VERY* fine resolution for setting the current because you need to hold the temperature constant to a small fraction of a degree and don't want to have to mess around dithering the LSB of the PWM pulsewidth, as the choke requirements get ugly at lower PWM frequencies and with increasing PWM resolution.
RoGeorge:
Another thing to mention is that many laser diodes (the OP doesn't say the model of the 200mW diode) have an internal fotodiode that can sense the optical power/temp. To drive the laser diode close to its maximum power (without melting the junction), the driving circuit must work in a feedback loop with the fotodiode. There are specialized CI drivers that can properly do that. Most of the DIY blogs or videos are made by absolute beginners that don't know what they are doing.
Find the datasheet of the exact model of that laser diode, and read it. Look for application notes, too.
A Peltier cooler might help, but this will not guarantee the safety of the laser junction.
Another thing to mention is that 200mW can easily burn the retina forever. Even the reflections from the surrounding objects can do permanent damage to the eyes. Use laser protection goggles for the specified wavelength of the laser. Other goggles for other colors might not protect at all.
Apart from that, here anything over 5mW can be used only with special permit.
moonie223:
This diode does not have a photodiode or NTC thermistor, simple oclaro 120mW 635nm laser diode. Although I have schematics for a photodiode feedback driver, it's not very common to do so with galvo XY scanners. Holography, oh yeah, you need that, a full bridge tec setup, a super heavy duty table, and no trains nearby, as least as far as I can understand.
I am not looking to protect the laser junction with the TEC, just keep the diode cool. It will not be ran outside it's rated spec, and the only reason for active TEC cooling is to keep the wavelength as close to 635nm as possible as 635 diodes drift with heat, around 0.2nm/K. I have a setup running now that has all the same/similar diodes, no tecs, and it'll run for hours.
Thanks for the laser safety info, but I think I've got that mostly covered! I've got a wide frequency range of goggles since I've been messing with lasers for long as I can remember, I was buying and ripping diodes out of the casio projectors a few days after we discovered what were really in those A-140s! I'm in charge of managing a 6KW disc fiber laser too, that thing is wicked fun! Used it to cut that temporary baseplate for my projector. I might also try and borrow some liquid nitrogen, see if I can make a 638nm diode blue shift all the way to yellow! I should have a 589nm DPSS hiding somewhere still, to compare against...
And I intend to make this a legal laser projector, there are safety features that must be implemented to be legal. That's another reason I want an arduino in this project, I'll use it as the last device to control the output shutter, I might try and take a look at the galvo feedback as well, to see if I can implement a scan-fail check. I think I might need something faster than just a 328P, but I could add a few extra DACs and even run prerecorded shows off the micro instead off off ILDA software on a standalone computer. Regardless, I'm just making one of these. If anyone wants the schematic and code for what I come up with when I'm done, I'll share it freely! I do not want to sell projectors, deal with FDA, or any of that nonsense!
It sounds like a faster PWMing micro would help me forego the extra OP and get me finer resolution with a smaller filter inductor, I've never messed with the SAMD21, but it seems it should get me to 187kHz pretty easy. I need to read more to figure what sort of resolution that would get me, but it should allow an inductor nearly three times smaller, right?
Thanks again for all the help and discussion everyone!
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