Help about BD140

Discussion in 'General Electronics Chat' started by shivaaa, Jan 6, 2011.

  1. shivaaa

    Thread Starter Member

    Jan 5, 2011
    I am using BD140 Transistor for some switching purpose.

    i heard that if Base supply is given the transistor wil not give any output(like NC switch).

    My question is,

    I connected supply to Emmitter, without given any supply to base, the output is comming in collector. but in Base terminal also i can read some voltage.

    My emmitter input is 6.38, I can Read 6.38 in coller also. But in Base I can read 5.67 voltage.

    why like this?
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
  3. shivaaa

    Thread Starter Member

    Jan 5, 2011
    if Base terminal is open and not connected to anything, If supply is given to emmiter, its comming out in collector just like a switch is closed.

    same time in base also there is some voltage which is just less than the given voltage to emitter. is this the normal ? any way to make base to zero?
  4. JDT

    Well-Known Member

    Feb 12, 2009
    This is due to leakage currents in the transistor. With the base open, the transistor will be "off". But the collector leakage current will be higher and the maximum collector voltage it can withstand will be reduced. Also, the transistor will conduct more current as the temperature is raised. Thermal runaway - where the leakage current is sufficient to raise the temperature and cause more leakage - and destroy the transistor - is possible.

    Normally, the transistor is never operated with the base open. In the off condition it should be connected to the emitter (usually through a resistor of a few hundred ohms).
  5. shivaaa

    Thread Starter Member

    Jan 5, 2011

    good explanation. thanks... got some idea..
  6. #12


    Nov 30, 2010
    For future, A small drawing of the transistor and its connections makes it very much easier to think at this end of the conversation.