zener diode problem

Discussion in 'Homework Help' started by onlyvinod56, Jun 6, 2009.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Hi,

    iam not able get the solution for this. of course it seems simple, but i dont have dat much idea over this.

    The circuit of figure uses two Zener diodes each rated at 15v, 200mA. If the
    circuit is connected to a 45volt unregulated supply, determine
    (i) the regulated o/p vg (ii) the value of series . . . . resistance R.


    My analysis:
    -> As the load resistance is parellel to the zeners (zener voltage = 15+15 = 30V) i found the load current = 30/200 A (0.15A).

    ->The current through the zeners is 200ma (0.2A)

    -> so, the current through the series resistor is 0.2+0.15 = 0.35A

    -> As the zeners are providing a potential diffrnce of 30V, 15V should drop at the series resistance.

    -> so the series resistor value is R = 15/0.35 = 42.85 OHM

    Is my analysis correct?
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Why? Why? Why?
     
  3. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    heeee:D

    ya i know dats the mistake as it is the rating of the zener. but i didnt find another way....
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You can start by making some assumptions:

    1. Unregulated 45V supply can change by +/- 10%.

    2. Minimum current for zener to give good regulation = 15mA
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You don't need the load current to calculate R.

    R=(45-30)/Izenermax

    This is to protect the zener in case the load is removed.

    If calculate R with the load current included and the load is removed, a current higher than the zener can handle will flow and destroy the zeners.
     
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