eBook, Vol 3, Chapter 4 :The Transistor as a switch.

Discussion in 'General Electronics Chat' started by dandeeny, Jul 28, 2014.

  1. dandeeny

    Thread Starter New Member

    Jul 28, 2014
    Re the second diagram of this eBook Section; there is no reference in the text to the resistor which is included between the mechanical switch in the base bias circuit located between base and supply battery positive. What is this resistor's purpose, how should its value be ascertained and what would happen if it wasn't there ? (these questions should reveal just how limited my general electronics understanding is :confused: Hope someone can help.

  2. MaxHeadRoom


    Jul 18, 2013
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    Your answer appears in the first two lines of that same page.

    A relatively small flow of electrons (current). That is the purpose of the resistor. It is called biasing. Too large a flow would damage the transistor, so it is set and limited by the resistor.

    To calculate the resistor note that
    the voltage drop across the resistor = supply minus the base voltage.

    Resistance required = this voltage divided by base current.

    The base current is obviously the switched collector current divided by the transistor gain (usually 50 -100) for switching transistors.
    dandeeny likes this.