# Solving a simple circuit

#### bipin

Joined Mar 21, 2004
80
Hi All,

Its quite a long time since I have visited to this site. So much of changes so much of improvement.. and ofcourse a lot of new pals

I am attaching one simple electronics circuit, can some one help me out in solving the current through R2 and R3. Assume that Zener voltage is 4.596Volt.
I am unable to solve this issue because of the dynamic resistance constraints of zener.
I am also attaching one Pspice simulation, but wanted to know how can I calculate it if the zener voltage is 5 Volt?

Thanks for droping in...

BPN

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#### Salgat

Joined Dec 23, 2006
221
I've been going through the old posts on this forum and have seen that you were very active and quite helpful back in 04, nice to see you

#### recca02

Joined Apr 2, 2007
1,214
hi bipin,
nice to see you back though i see u for first time (i hope u understand the irony )
well i am a jerk when it comes to solving ckts having comp other than R,L,C
but taking zener voltage as 4.595 and using kirchoff's voltage law the answer comes in no time.
what i dont know is, is 4.595 zener breakdown voltage? can voltage across zener exceed it. also can the above example be worked out with the help of
V/r characteristics of zener in case 4.6 v drop across zener is not known.
sorry for my ignorance but i dont have any experience solving ckts with zener.

#### n9352527

Joined Oct 14, 2005
1,198
If we assume that the zener voltage is sharp, i.e. the zener acts as open if the voltage across it less than Vz, and conducts otherwise so capping the voltage at Vz, then as recca said it is easy to calculate the currents using Kirchoff's law. We can peg the Vz at any chosen value, like 5V that you required.

If we take into account the IV characteristic of the zener, then there is no easy way to calculate the current, or the Vz. Vz depends on Iz, they are both governed by IV characteristic curve.

Two possible approaches are, use the IV curve to estimate the Vz, or use iterations to calculate Vz. The first approach is somewhat easier, but not that accurate. The second one is more accurate, but tedious.

#### bipin

Joined Mar 21, 2004
80
I think I could not explain my constraint clearly.

Lets assueme that the zener is having an ideal charecteristics, and its body resistance is zero. so VI curve becomes vertical with no zero slope.

If R2 was not there in the circuit, I can easily reduce 5 volt from the supply voltage(20V) and find out the current through R1 & R2. So simply it becomes 15/3k = 5 mA.

Now there comes R2, which is in parallel with R3& Zener. Because of this I am unable to take the equivalent resistance of R2 parallel with R3+Zener.

Is there a simple solution for this problem? dont get confused about the 4.596Volt of zener. It was selected by Pspice according to the VI Charecteristics of the particular part.

Hope I could explain my simple problem.

Thanks once again.

#### recca02

Joined Apr 2, 2007
1,214
i dont get it if the v/i line is vertical it has slope = infinity which means res = infinity does it not.

#### n9352527

Joined Oct 14, 2005
1,198
Then you just treat the zener as a simple voltage source. You can use the loops to calculate the current through each resistor.

#### wegener

Joined Apr 25, 2007
2
why dont you just use p - spice?? it has all the tool needed..