# Finding Thevenin Voltage w/ Voltage Sources & Zener Diode

• sampsoninc916
In summary, the problem is that the Vth and Rth variables are not being correctly calculated, and this is causing the problem with getting the correct answer for the Thevenin voltage.
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?

Last edited by a moderator:
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.

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.

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

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?

Last edited by a moderator:
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.

## 1. What is the Thevenin voltage?

The Thevenin voltage is the voltage across two terminals of a circuit when all voltage sources and current sources are replaced by their internal resistances.

## 2. How do you find the Thevenin voltage with voltage sources and a Zener diode in the circuit?

To find the Thevenin voltage in a circuit with voltage sources and a Zener diode, you first need to remove the Zener diode and any other non-essential components from the circuit. Then, calculate the equivalent resistance across the two terminals of the circuit. Finally, apply the voltage divider formula using the equivalent resistance and the total voltage of the circuit to find the Thevenin voltage.

## 3. Can the Thevenin voltage be negative?

Yes, the Thevenin voltage can be negative. This occurs when the internal resistance of the voltage sources is greater than the external resistance of the circuit, causing the voltage to drop across the internal resistance.

## 4. What is the purpose of using a Zener diode in a circuit?

A Zener diode is used in a circuit to regulate the voltage. When the voltage across a Zener diode reaches a certain threshold, it begins to conduct in the reverse direction, effectively maintaining a constant voltage output.

## 5. Can the Thevenin voltage change in a circuit?

Yes, the Thevenin voltage can change in a circuit if the internal resistance of the voltage sources or the external resistance of the circuit changes. It can also change if additional voltage sources or components are added or removed from the circuit.

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