# 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?

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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

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.