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[personal profile] smellsofbikes
I wanted to put a laser scribe on the CNC mill. Since we build LED drivers at work, that seemed like a good way of controlling the laser diode. This is set up to supply about 700mA or so -- a value determined by a sub-ohm sense resistor -- to power the 120mW red laser diode module.

I put together a schematic based on the LM3402 because I know it really well since I designed the official evaluation board for it. I ran the laser diode current requirements through Webench (on the right-hand side of the page) and took the schematic it produced and ran that into geda's schematic entry tool to come up with my working schematic.
lm3402 schematic

I ran that into PCB and did a layout.

I'd like to talk about that layout a little bit. This particular chip has an exposed pad on the bottom of the chip to dissipate heat from the internal switching mosfet, so the layout requires exposed ground beneath the chip. To solder this down, the best way I've found is to flow solder on the ground area so the whole area under the chip is coated, then solderwick it all back off, and add one tiny point under the chip. You put the chip down and it rocks on that point. You put the soldering iron down beside it and add a little solder, and when the chip suddenly drops down flat you know that it's down and attached, and then you can solder down the legs.

In any switching power supply you want to have the shortest traces possible on the loud and noisy nodes, most notably (in this case) the pin that switches the inductor. It's a good idea to make that just a single small copper pour with all the parts on that node as close as possible to each other so it's not spraying noise all over the place. Lots and lots of ground -- as much ground as possible, in fact, and for small power layouts you don't want thermal relief around your pads. (Which in geda/pcb means making parts with pads that have zero clearance, an experts-only move because if you don't do your layout right you'll have shorts and the program won't tell you.) I snaked the 'enable' pin around the inductor because it doesn't seem to care much about noise.

For this layout, the chip will take anything from about 7-35v as its supply, and converts that to the laser diode drive with between 80-90% efficiency.

You should have something like a 10k resistor from the enable to ground, so the chip powers up OFF rather than ON because unexpected lasers are really bad for your eyes. (In this case the controller takes care of that, but it's an extremely good idea to have it there even if the controller comes up with the IO pins grounded.) In this case the ENABLE pin is labeled DIM on the schematic.

When I run a file through a gcode processor it produces lots of Z up/down moves. I then take a look at those and use sed to change Z up moves into digital pin off moves, and Z down moves into digital pin on moves: "sed 's/Z-10.000000 F100.0/M64 P01.0/' for instance.

SO! with that done, here's a video of the result.

Need I point out that you absolutely positively must have approved laser goggles that filter the wavelength you're using? Visible lasers that are over 10mW are really bad for your eyes. Even with actual laser goggles, if I look at the burn spot for any length of time I see spots. Be careful with your eyes: you only get two and they're very delicate.

Great build and article, but ...

Date: 2013-03-25 10:30 am (UTC)
From: [personal profile] dito
I really like this project, and I am not alone : several people have share it across diy and hacker sites. The "but" is that I find the schematic too small to rebuild the driver.

Any chance to see a bigger (higher resolution) version of it ?

Thanks !



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