# Zener Diode Circuit

Discussion in 'Homework Help' started by AD633, Jun 24, 2013.

Jun 22, 2013
96
1
The circuit of Figure load RL is powered by a voltage source VS which can take values ​​within a certain range of Vmin and
Vmax.Considerando queVz = 9V, Vγ = 0.7V, PZ = 0.27W, IZmin = 3mA, IL = 10mA,determine Vmax and Vmin guaranteeing IL constant current (10mA)
regardless of the value of VS.

First i determined Iz max from Pz=Vz*Iz ,giving Izmax=33,3 mA

Then for determing Vs min and Vs max i think i have to due the following meshes:

$Vsmin=R1(Izmin+Il)+9

Vsmax=R1(Izmax+Il)+9$

My problem is that i dont know R1,and for determing R1 i have to know Vs..Do i have to due a system of equations for solving this?

Thanks

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2. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
508
The value of R1 has to be either selected or specified in order to solve the problem. R1 sets the total current flowing into both the load and the zener, so you select that value to make sure you have enough current at the lowest value of VS. The current through R1 is always going to be the sum of the zener current and load current.

3. ### LDC3 Active Member

Apr 27, 2013
920
160
How did you get 33.3 mA? I don't.

Jun 22, 2013
96
1
By doing Pz=Vz*Iz
0,27 W= 9 V* Izmáx
Izmáx=30 mA

So for solving this i have to select a value for R1?The question does not specify any value for R1...

5. ### LDC3 Active Member

Apr 27, 2013
920
160
Now you have
I1max = IZmax + Il
I1min = IZmin + Il
Since R1 is not specified, then the voltage needs to be a function of R1.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784

Vγ = 0.7V

What is this?

Jun 22, 2013
96
1
Its the voltage drop across the diode when is forward biased.It does not seem to be much of a help for determing R1..

So the answer would be something like:

$

Vsmin=R1(3 mA +10 mA) +9 V

Vsmax=R1(30 mA +10 mA) +9 V$

8. ### WBahn Moderator

Mar 31, 2012
20,074
5,666
Yes. Now you should try to do a couple of things:

1) See if it now makes sense that you should be able to write these two equations down by inspection.

2) In words, describe what each piece of each equation means.

3) In words, describe what will happen if Vs is below/above this range.

Now see if you can push your understanding a bit:

4) What is the expression for the width of the allowed voltage range (the difference between the minimum and maximum voltages)?

5) What does each people of that equation represent?

6) What things does that equation NOT depend on?

7) Do the answers to Q5 ans Q6 make sense (after pondering them a bit, if necessary).