Finding Thevenin Voltage w/ Voltage Sources & Zener Diode

[sampsoninc916]

Homework Statement

http://l.bitcasa.com/-QwLBhUK

Here is the problem statement

Homework Equations

Vth = V1*R2/(R1+R2)
Rth = R1*R2/(R1+R2)
Vth - Rth*Id - Vz = 0

The Attempt at a Solution

http://l.bitcasa.com/4ycq3BMN

Here is my attempt at a solution.

I am having trouble finding the Thevenin Voltage for a circuit with a Zener Diode with two voltage sources. This attempt at a solution proved to be no help, because as you may have seen in the picture you downloaded, it kept on telling me it was wrong. Once I find the Thevenin voltage, I will be able to find the current through the Zener Diode with the formula Vth - Rth*Id - Vz = 0. May somebody please help me?

 

[gneill]

Your calculation of Vth2 is not correct. You're using superposition to determine the net Thevenin voltage, which is fine, but you have the contribution of V2 being developed across the wrong resistance. Note that for the Thevenin voltage determination R3 will be open circuited (the zener removed) so that no current flows through it.

 

[sampsoninc916]

http://l.bitcasa.com/7wz_SYeM I've tried solving for it again, but to no avail. Must be something wrong with the computer system as I may have been on the problem for too long.

 

[gneill]

Your Vth looks good now! But you've "broken" Rth. R3 is still in series with the parallel combination of R1 and R2.

 

[sampsoninc916]

http://l.bitcasa.com/h1JexqbW

Refreshed the page, solved the equation with different variables using the same methods above, including Vth - Rth*Id - Vz, and it still did not work and it gave me a wrong answer. It's practically mocking me. Something is wrong with the computer system this homework is running on. What am I doing wrong?

 

[gneill]

Yeah, now you've broken both Vth and Rth. Why'd you change your Vth calculation?

Redo your math for Rth, too. The formula is right but the result is not.