I've been thinking about the various circuits I see posted utilizing LM317 as a current driver for laser diodes. All looks great until I look at that capacitor sitting in parallel across the laser diode.
I see the potential for disaster there should battery / switch connection transients occur, charging the capacitor and then allowing it to discharge through the laser diode without any current limit.
Doing some research in SAM's laser FAQ, I found this little warning:
Isn't this in effect what we are doing since there is not any resistance limiting current flow from a charged capacitor through our diode?
( http://stonetek.org/sales/pix/LD_Driver_Schematic.jpg )
(Just asking here and hoping those with a deeper understanding can explain this.)
I see the potential for disaster there should battery / switch connection transients occur, charging the capacitor and then allowing it to discharge through the laser diode without any current limit.
Doing some research in SAM's laser FAQ, I found this little warning:
( http://www.repairfaq.org/sam/laserdps.htm under Battery Power for Laser Diodes )CAUTION: There must NOT be any filter capacitance in the power supply after the current limiting resistor. This is to minimize the chance that a bad connection to the diode will result in excessive current should such a capacitor charge to a much higher voltage and then discharge through the diode without current limiting.
Isn't this in effect what we are doing since there is not any resistance limiting current flow from a charged capacitor through our diode?
( http://stonetek.org/sales/pix/LD_Driver_Schematic.jpg )
(Just asking here and hoping those with a deeper understanding can explain this.)