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
So what are you trying to do? Are you trying to make a constant current source for the lasers or a current limited voltage source?
You are running out of voltage headroom I think Using these sort of voltage regulators as a current regulator forces a 1.25V drop across the resistors used to limit the current there. Then you have 1.5-2.5V between IN and OUT from the Darlington pair in the regulator, then 0.7 across the transistor for a total of 3.45V minimum drop before you get to the laser. That is 1.55V for the laser. It is strange that it works for the green and not the red because normally they both have a minimum of 1.8V.
Can you probe what voltage you are getting with a scope oun the laser input on J6? Profiling the laser voltage would tell you a lot.
I there are a few ways you can go with this. Since you are burning a ton off as heat anyway, just switch to a DAC controlled high side current source with a PMOS FET (+ op amp and sense resistor of course). They can do 130mA easily and have a similar amount of components.
If you want it a bit more accurate or with BJTs, you could do a current source Like described here and adjust it to your values. More components, but it works quite linearly with your DAC outputs from the arduino.
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.