TTL-controlled Laser Diode Driver. Updated schematics.

During the course of my DIY laser cutter project I needed a laser diode driver that I could control with a CNC software, such as EMC2. I’ve already made an attempt to build one based on Linear Technology’s LT1121 voltage regulator with Enable input but the driver design that came out was not exactly successful
So, I took another stub at it …

I have to admit, it was a pretty silly move to try designing an analog circuit based on Linear Technology device and not use their own free LTSpice circuit simulation software. I was at some point deterred by the fact that it’s a Windows software but turns out it runs just fine under Wine and it was a rather easy simulation at that.
Here is the Spice model of the laser diode driver I made. I’ve been using the resulting driver pretty extensively now and it seems to be working pretty well.

This Spice model shows that the driver should start working at approx 4.8V on the Input pin. I’ve found that at such low input voltage the current across the laser diode comes up less than what’s calculated by the software. It could be that the laser diodes I used (mostly harvested from broken DVD writers) don’t have the same exact parameters as the SLD1121VS used in the model – that was the only laser diode model I could find on the Net.

I am actually using this driver with input voltage of approx +5V due to my desire to use a simple open frame +5V power supply I had available for the entire CNC cutter, stepper driver, logic, interfaces and the laser diode – all of it from the same supply voltage. It kind of works fine but at +5V input I can only get 120-125 mA currents across the laser diode. It’s just not enough for the LT1121 to bring the reference voltage up to the required +3.75V (see the datasheet). For some reason the LTSpice model does not show that. But if you raise the input voltage to +9V, you should have no problem with getting the currents you receive from the software model.

I’m going to hold off publishing the Eagle files until the boards I ordered from Olimex come and I had a chance to test them – last time I published a bad design board was rather embarrassing. So check back here in a couple of weeks – I should have both the boards for those that don’t want make their own and the Eagle files for those that do.

blast

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