How to calculate the transistor base resistor value?

[AlainB]

Hi,

My parallel port output 4,8 volts. For instance, if I want to use a TIP121 for driving a stepper motor, I will put a 1K resistor betwen the base and the computer pin. Not because I know what I am doing, just beacause I saw it somewhere. If I don't put any resistor, it will work the same anyway.
Both ways could be good or bad, I don,t know.

Could you explain what is needed to calculate the right resistor values for any transistors used that way. I am looking for values for hard switching application only. If this information is needed, the transistors would be used as power stage for a 5,2 volts 1,4 amp. unipolar stepper motor

Here are 2 datasheets.

TIP121:
http://www.ortodoxism.ro/datasheets/...onics/4128.pdf

BUL45:
www.onsemi.com/pub_link/Collateral/BUL45-D.PDF


Thanks for any help.

Alain

[pastera]

The TIP121 is a darlington, so there are two diode drops in the base.

R = (Vin - 1.4)/Ib

Where
Vin = parrallel port outout (4.8v)
Ib = desired base current ( 10mA is fine for your application)

Aaron