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
Thanks for the detailed answer! We're doing a constant current source. The circuit you have linked would be perfect for future designs, but could a PMOS-FET be a drop-in replacement if we were to reuse the current PCBs we have? Do you have any suggestions on a specific model.
I think it is possible, but would require a ton of trace cutting and wire bodging, even the higher current traces which is never great.
An interconnect PCB (or one per channel) might be your best bet if your current PCB is too complex to reorder and resolder. Getting it done at a cheap board house like Aisler, oshpark, jlcpcb, etc...
Design it to solder to the required pads with castellated half holes, then you can solder the new parts on top of the interconnect board, and that is also easier if you have a proto batch of 5-10.