So what I really need to know is, how to calculate the max value of the series resistor that I can wire to the base of transistor, to still allow maximum current through collector-emitter?

We can make some appoximations & assumptions to simplify the calculations. You need to allow for tolerances anyway, so there is no exact answer. ( A lot of engineering has to do with knowing what's critical and what can be assumed, ignored, or approximated.

)

We do need to know the approximate collector current. Or the "worst case" or maximum for your application. (You don't need the transistor's maximum current rating for the

* calculation*... But, you need to make sure your application doesn't exceed the device's maximum rating.)

You shouldn't be thinking in "maximum" values... You need to find an approximate value that will work with the actual current in your circuit.

First, I'll assume this is a switching application, and that the emitter is grounded.

Next, I'll assume the transistor beta (current gain) is around 100. The data-sheet will give you the

*minimum* value (and sometimes a "typical" value). But since this is a switching application we dont need to know the exact value. So, we can design a circuit that works with betas between 20 & 50 and we can be sure it will always work. (Assuming a low beta insures that we alway have enough base current to saturate the transistor.)

**The base-emitter voltage is around 1V with the transistor turned-on.** The exact voltage (usually) isn't important, since (in most applications) most of the voltage is dropped across the resistor... A rather large-percentage change in B-E voltage won't change the voltage across the resistor that much.

**In fact, you can assume ***zero* B-E voltage, and most of the time it will work, since we are assuming a low beta and providing plenty of base current.

**Now, you can calculate a resistor value that gives you a base current that's about 1/50th (to 1/20th) of your collector current.** Example - If you need 1 Amp, and you have 5V

*into* the resistor (4V

*across* the resistor). Ohms' Law says you need a ~200 Ohm resistor.