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Problem with switching a laser driver (lemmy.world)

submitted 1 week ago by TDSOJohn@lemmy.world to c/askelectronics@discuss.tchncs.de

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Hello everyone, We have a problem with a circuit we built. The circuit seems to work properly with a green laser diode (130mA, lower voltage). With the red laser (higher voltage) on the other hand, the circuit delivers only 60mA max of the 130mA it should be giving the laser.

The problem seems to be the transistor's resistance because it works properly if we bypass the transitor and give +5V to the LM317.

Is it this assumption correct?

We tried a few transistors (BC241, BC236, TIP41C and TIP41) but with no luck. Is there a way to find a drop-in replacement for the transistor? MOSFET seem to have lower resistance but require an external driver with Arduino.

We need to use the Arduino PWM with these drivers, so at least 980hz switching frequency is needed.

Thanks in advance

Link to the schematics

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[–] Scafir@discuss.tchncs.de 2 points 1 week ago* (last edited 1 week ago) (1 children)

Hey! Really not my cup of tea, but here's a few comments:

what are the input outputs? Are you powering 6 lasers at the same time? Why hook it up to an arduino? How are the lasers connected? What is a good representative load for your laser (probably resistive, but maybe inductive or capacitive?)
Looking at the driving circuit, there's something fishy about them. Having a variable Vi and static ADJ pin is not what I would have expected, nor what the datasheet suggests
how about providing a fixed, appropriate voltage on the + terminal of your laser and have low side switching through a mosfet?
components seem pretty basic, you could probably simulate this throught spice to get a better idea of what's happening (you can prob do this directly in kicad). Be careful about choosing models for stuff outside the PCB like the lasers. Long cables can have significant capacitive load for example.

EDIT: I think the gist of the problem is that (1) the LM317 is powered on and off continusly, causing transients, power on delays,... (2) the adjust pin depends on the voltage on the output pin, potentially causing more resonance/weird transients. I would: stick to the suggested circuit in the datasheet of the lm317, and move the switching transistor between the lm317 and the laser.

[–] TDSOJohn@lemmy.world 2 points 1 week ago

hi, thanks for the early reply.

the relevant output is j6 which is connectect to six red lasers with separate positive terminals and a common ground to power them independently using the pwm function of arduino. They should be a mostly resistive load but they have quite high parassitic capacitance. We can't have separate grounds and switch low side since the external casing of the lasers is connected to the negative terminal of the laser diode and it is bolted to the same aluminum frame. The lm317 is working as a constant current driver and we found online that it's best to switch Vi instead of ADJ in this case link. Is it possible to use the circuit in the first schematic of the link to solve this? We are currently dropping .7V between base and emitter of the transistors.

Could otherwise placing the switching transistor after the cc driver be a solution?

The circuit seems to work perfectly with a lower voltage laser diode and doesn't seem to have problems with transients. Also thanks, will try to simulate it in spice!