how to find out base current

Discussion in 'Homework Help' started by jonisonvespa, Mar 25, 2012.

  1. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    hi,
    been a way from electronics for a long long time, im trying to build a circuit to switch on relays and leds, cant remember how to work out the base current of a tranny resistor?, can someone please remind me also in the speck sheet what value is the base current? my circuit, (i cant seem to post a link of the sheet) its a sm sot-23

    ok had another long look am i right in saying Ic is the collector current if so in the speck sheet it says 100 im asuming this is 100ma? so you use ohms law to work out the resistor? using a 5v to the base of tranny

    am i correct in saying 50 ohm resistor


    thank you
     
    Last edited: Mar 25, 2012
  2. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    The general rule of thumb is to make the base current 1/10 of the collector current.
    So, if your load requires 20mA, set the base current to 2mA.
     
    Last edited: Mar 25, 2012
  3. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Yes and also in some european transistors the base current is 1/20 of collector current,for example BC547,BC548 .....

    Good Luck
     
  4. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    hi mr chips
    ok i think ive established the collector current for sot-23 is 100ma are you saying 1/10 of this ? so 10ma

    so the resistor required is 5/.01 500 ohm resistor?


    thank you
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    Looks good.
     
  6. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    thanks very much thats got me started
     
  7. Shanjaq

    Member

    Dec 6, 2010
    12
    0
    Base Current to Collector Current would be the DC Current Gain(hFE) of that particular transistor? That's a dynamic property, which usually has a graph in the datasheet, or a few quoted values, like the PN2369A has an hFE=40 @ Ic=10mA and Vce=1.0V. That's an amplification ratio of 1:40 Base vs Collector current.
     
Loading...