Help finding Thevenim Equivalent - Circuit Analysis

[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?

 

[phinds]

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]

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?

 

[phinds]

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

 

[Xiao Xiao]

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

Thanks a lot, I appreciate it.

 

[gneill]

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 .

 

[phinds]

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

 

[Xiao Xiao]

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.

 

[The Electrician]

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.