I am looking to design a simple constant current circuit to draw a precise load from a variable voltage rail. The voltage rail (V+) should be 10V-20V in normal conditions, but it is possible for V+ to be floating. The control signal (CTRL) is to be driven from an MCU on 5V rail. I am looking for 150mA from V+. Note that I am sizing resistors such that R1 and R3 will drop majority of power when V+ is 10V. The transistor would then handle the additional power as V+ goes to 20V. (So T1 will be rated for >1.65W to handle up to 11V at 150mA).
I understand that normally, R2=0ohms so I'd have a constant 4.3V at the emitter (assuming Vbe of 0.7V). That provides the easy formula of R1 = 4.3V / 0.15A.
My concern with R2=0 is that if V+ is ground or floating, there's nothing to limit the base current except for the MCU's output driver. When I add R2 to solve this concern, I'm not sure how to set the current in the circuit as it will be dependent upon base current, which I assume is going to be based on transitor's Beta.
Is there a way to create a constant current while protecting the base in my situation?
