Hi, I am trying to determine the operating specs for operating a transistor base (as a switch) from a voltage divider.
What I am trying to figure out is the appropriate values of R1 and R2, given a desired Base current and source voltage/source current available (I know the source voltage, and how much current I can pull from it).
If I eliminate the "bottom" (R2) resistor in the voltage divider and only use one resistor, I can easily determine the current that flows through that resistor (E/R), and therefore the current that flows through the base ((E-0.7)/R), which is just computing the value of a Base resistor.
Right now I am getting hung up on what effect the "bottom" resistor has on the base current. I know that without the transistor, I can figure out the voltage at the center point as a ratio of the resistances, and the current through both resistors, but what current is available from the center point to drive a transistor base?
The closest I have come is by calculating a "resistance" for the transistor Base/Emitter junction by dividing 0.7V by the desired base current, and treating the transistor as a resistance in parallel with R1, but I'm not very confident with that solution.
BTW, here is an ascii of what I have: