Current Source with two settings

[jbord39]

Hey all,

I am trying to create a type of current source to switch between 10 mA and 1.4 A at some frequency. I have an idea to use a BJT transistor as a variable resistor in a LM317 (or whatever the high current version is) but am not sure how to do it. Any suggestions? This circuit is to pulse a laser diode. Also, does this circuit have a name?

Thanks,

John

 

[yungman]

I make a lot of assumptions. i assume the laser diode is just a diode that has 1V drop when turn on. You have to tell me more info if I am not correct on the circuit. here is the quick I have for the first pass. I use an enhence mode MOSFET instead because I don't want to deal with the large base current and low beta that cause inaccuracy. basically I put the diode at +5V and turn it on by pulling current. I assume the turn on voltage is 1V so I use 400 ohm as R4 to pull 10mA when the transistor is off. When the transistor is on, it pull 1.4A so the 10mA just become insignificant. The circuit is a current sink control by the op-amp. The input should be 0V for turning the current sink off, 1.4V for pulling 1.4A. The accuracy depend on your requirement. You better put a heat sink on the MOSFET and the R3 because it will generate heat. R2 is to isolate the input capacitance of the MOSFET from the output of the op-amp ( that might cause instability) and C1 is to create a zero to guarante stability of the op-amp.