Transistor as a switch

Discussion in 'Homework Help' started by jirakst, Jun 14, 2014.

  1. jirakst

    Thread Starter New Member

    Jun 14, 2014
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    0
    Hello,

    I've got a lot of homework and can't solve following example:

    Transistor KFY34 has to switch a load with rated voltage 24V and resistance Rs=60Ω.
    a)Solve size o the Ib after switching load.
    b)Solve size of current and voltages in the circuit Ib = f(t), Ic= f(t), Ube = f(t) and Uce = f(t) after switching on and switching off.
    c)Decide if additional cooling is needed.
    KFY34: NPN, h21E = 35 až 125, UCB0 = 70V, UCER = 50V, ICmax = 500 mA, UCES ≈ 1,5V, PC = 800mW

    Thanks for any guidance in advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,989
    3,226
    We don't do homework for you but we will help. You need to show you work and what you don't understand. Post the circuit schematic to start.
     
  3. jirakst

    Thread Starter New Member

    Jun 14, 2014
    3
    0
    Thank you for your post.
    I uploaded schematic here: http://s7.postimg.org/6fsrlhzzv/obr_zek.jpg
    but problem of mine is I don't understand it at all. Quite frustrated about it for I can't find a solution anywhere.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    A transistor switch doesn't normally have an emitter resistor. Why does your circuit?
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    767
    To google : transistor as a switch, you will get the conception.
     
  6. jirakst

    Thread Starter New Member

    Jun 14, 2014
    3
    0
    Oh yes, the emitter resistor is used to set the operating point of the transistor when connected as a amplifier. But do I actually need the base resistor or it has to be used with other then low-power sources (e.g. solar cell)?
    How will I determinate the base current?
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,989
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    For a BJT switch the base current should be at least 1/10th of the maximum collector ON current to insure the transistor is fully turned on (saturated).
     
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