Simple voltage regulator

Discussion in 'Homework Help' started by Ledwardz, Aug 23, 2011.

  1. Ledwardz

    Thread Starter Member

    Oct 31, 2010
    hey, i am a bit stuck with this voltage regulator just a few things i need cleared up if any one could take a look it would be much appreciated.

    first off sorry for the rubbish photo the only thing i have on me is my phone....
    the question asks me to calculate vo which is the regulated ouput voltage over the resistor on the far side.
    the Vce junction of the transistor
    and the current in the 300 ohm resistor on the left.

    okay so the transistor is the conrtrol element acting like a variable resistor over the CE junction depending on the value of VBE. The zener acts as my reference voltage of 15v (which i am told in the question). so i figure that Vo is = to Vz - Vbe = 15- 0.65 = 14.35 V

    I then went on to guess that because the transistor is in parallel with the 300 ohm resistor its VC = 25v but because it acts like a variable resistor there is a voltage drop over it which i assume is 25 - 14.35 = 10.65V ???? but what about the Vbe ??

    okay moving on there is 25v accross the zener and resistor. 15 volts over the zener so 10 over the resistor ??? or is the 0.65 taken out of this? so should it be 9.35 V over the resistor giving a current of 9.35/ 300 ?????

    Thanks for any help, Lee.
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    It's seems to me that you understanding this simply circuit quite well.
    Vo = Vz - Vbe
    And voltage across resistor is equal
    Vr = Vin - Vz
  3. Ledwardz

    Thread Starter Member

    Oct 31, 2010

    okay cheers for the reply

    so vo is correct and Vr = 10 v

    what about vce?
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    VCE= VC-VE = 25-Vo =25-14.35=10.65 as you have stated.

    VBE is not part of this path. You have already accounted for VBE in finding Vo.
  5. Ledwardz

    Thread Starter Member

    Oct 31, 2010
    okay, Thanks for your help. Lee