Calculating BJT base current from the datasheet.

Discussion in 'General Electronics Chat' started by Dyslexicbloke, Jan 19, 2011.

  1. Dyslexicbloke

    Thread Starter Well-Known Member

    Sep 4, 2010
    Hi folks,
    I am about to build an H-Bridge driver for a small DC motor, I know I could probably buy one but its as much about learning as anything else.

    I have bought some Darlington power transistors BD679A and BD680, NPN and PNP respectively, that have an hFE of 750 and contain a reverse bias protection diode and base/emitter resistors within the package.

    In the past I have always found the minimum base current that would turn a transistor fully on by experimentation but I would like to learn how to calculate this from the data sheet.

    I think I understand that a BJT is a current amplifier and that hFE is its gain in specific conditions but I defiantly don’t understand how interpret the datasheet for one.

    I have looked at several posts on this subject but still don’t get it …
    If I want to use a BJT or Darlington BJT as a switch (saturated right?), in an open collector configuration, where ICE will effectively be limited by the load, how do I determine the required IB, given that the load and thus ICE may change. (Different motors for example.)

    How would I calculate the voltage drop across the transistor for a given IC? Should I be thinking in terms of forward bias threshold like a diode or ‘on resistance’ like a FET.

    If the answer to the question above is ‘forward bias’, which I suspect is the case, is that forward voltage drop then proportional to IB when the transistor is partially on.

    Any input would be much appreciated …
    Thanks, Alistair
  2. nigelwright7557

    AAC Fanatic!

    May 10, 2008
    Use the maximum current used through the motor divided by the HFE then multiply by 2 to ensure enough current into the base of the driver transistor.
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    Look at the Fairchild data sheet for these devices. From the BD679A sheet, figure 2 shows Vces vs Ic. Pick your Ic and note your corresponding Vces. Look in the upper right corner and see Ic= 250Ib. Hfe = Ic/Ib, therefor the saturated Hfe is 250.
    You will also find a graph for Vbe also.