# Help finding Thevenim Equivalent - Circuit Analysis

• Xiao Xiao
In summary: An electronic component that passes a current proportional to the voltage across it is called an "emitter follower".

#### Xiao Xiao

Summary:: So I found Vth=1.19 using Nodal analysis and I'm not sure if that's right, I'm trying to find Rth now but I'm confused as to what to do, any tip that tells me at least from where to start is appreciated.

Edit: so I did 12||60 so Rth=10 ohm, is that correct? Is Vth correct too?

Yes, 10 ohms is correct for Rth, now just find the voltage from a to b. You can use either nodal analysis or loop analysis.

Xiao Xiao
phinds said:
Yes, 10 ohms is correct for Rth, now just find the voltage from a to b. You can use either nodal analysis or loop analysis.
Thank you for replying, I did Vab=Vx=Vth=1.19V using Nodal analysis.

$$\frac{30-Vth}{12}=\frac{Vth}{60}+2Vth$$

This is what I did, do you know if its correct or not?

Looks good to me EXCEPT that your 2Vth should be 2Vx

Xiao Xiao
phinds said:
Looks good to me EXCEPT that your 2Vth should be 2Vx
Thanks a lot, I appreciate it.

berkeman
Your Vth value looks good, an exact value being 25/21 V. However I disagree with the value of 10 Ohms for Rth.

Consider what happens if you place a short circuit across the output. In that event Vx is forced to zero and so the current source produces zero current and the 60 Ohm resistor, having no potential difference across it, also passes zero current. All available current will pass through the shorted output. What will that current be? Call it Iss.

Given a value for that current and the value for Vth, then Rth = Vth/Iss .

Xiao Xiao
Ha. I toally ignored that Vx is a dependent current source. Stupid.

gneill said:
Your Vth value looks good, an exact value being 25/21 V. However I disagree with the value of 10 Ohms for Rth.

Consider what happens if you place a short circuit across the output. In that event Vx is forced to zero and so the current source produces zero current and the 60 Ohm resistor, having no potential difference across it, also passes zero current. All available current will pass through the shorted output. What will that current be? Call it Iss.

Given a value for that current and the value for Vth, then Rth = Vth/Iss .
I see, thanks a lot, I'll try to solve it again.

Note that the dependent current source also has a voltage Vx across it. So the current out of the current source is 2 times the voltage across it (taking care of the sign). What kind of two terminal electronic component has the property of passing a current proportional to the voltage across itself? You could replace the current source with such a component of the proper value and then calculating Rth becomes a triviality.

Xiao Xiao

## 1. What is Thevenin Equivalent in circuit analysis?

The Thevenin Equivalent is a simplified representation of a complex circuit that contains a voltage source and a single equivalent resistance. It is used to analyze and solve complex circuits more easily.

## 2. How do I find Thevenin Equivalent in a circuit?

To find the Thevenin Equivalent, you need to follow these steps:1. Identify the load resistor2. Remove the load resistor from the circuit3. Calculate the open circuit voltage (Voc) across the load resistor terminals4. Calculate the equivalent resistance (Req) seen from the load resistor terminals5. Draw the Thevenin Equivalent circuit with the calculated values: Voc as the voltage source and Req as the equivalent resistance.

## 3. What is the difference between Thevenin Equivalent and Norton Equivalent?

Thevenin Equivalent and Norton Equivalent are both simplified representations of a complex circuit. The main difference is that Thevenin Equivalent uses a voltage source and equivalent resistance, while Norton Equivalent uses a current source and equivalent resistance. They are equivalent in terms of their behavior and can be interchanged in a circuit analysis.

## 4. Can Thevenin Equivalent be used for both DC and AC circuits?

Yes, Thevenin Equivalent can be used for both DC and AC circuits. However, the calculations for finding the equivalent voltage and resistance may differ depending on the type of circuit. For AC circuits, you may need to consider the impedance of the circuit elements in addition to the resistance.

## 5. Why is Thevenin Equivalent useful in circuit analysis?

Thevenin Equivalent is useful in circuit analysis because it simplifies complex circuits into a single voltage source and equivalent resistance, making it easier to analyze and solve. It also allows for quick and accurate predictions of circuit behavior without needing to consider the entire circuit. This can save time and effort in circuit design and troubleshooting.