Kirchoffs law

[Ratchit]

sunny1982,

What do you mean by set RL to ∞ and set RL to 0?

You want to find the impedance of the circuit looking into it from RL. You do that by dividing the open circuit voltage by the short current. That is a basic definition of circuit impedance. Once you know the circuit impedance, you match it with RL for max power transfer. Changing RL from 0 to ∞, and calculating the voltage and currents at those values by implementing K's law with loop equations, gives the open circuit voltage and short current.

So are these are all the equations I should add to complete this task?
I1 = -(120*(96+7*RL))/(11112+1279*RL)
I2 = -(375*(24+RL))/(11112+1279*RL)
I3 = -9000/(11112+1279*RL)

Yes, if you cannot understand how to apply it, ask for clarification.

Ratch

 

[MikeMl]

MikeMl, ...Where in Spice is there a feature concerning the max power transfer theorem?

Not directly, but mechanizing the solution is trivial in the LTSpice variant. It lets you do iterative solutions as a function of any number of variables (PARAMs), including varying resistance of any element(s). It automatically plots Power in any circuit element. It lets you plot arbitrary math functions of any node voltage, branch current, element power, etc, including the derivative of the plotted variable. How hard is it to see where the derivative of Power in RL=0?

You never answered about having to use Thevenin to even know that there is a max power transfer theorem?

 

[Ratchit]

MikeML,

Not directly, but mechanizing the solution is trivial in the LTSpice variant. It lets you do iterative solutions as a function of any number of variables (PARAMs), including varying resistance of any element(s). It automatically plots Power in any circuit element. It lets you plot arbitrary math functions of any node voltage, branch current, element power, etc, including the derivative of the plotted variable.

Spice is great, Spice is good. But, it is really a specialized equation solver and plotter. All the things you listed above can done with a general purpose equation solver and plotter, albeit slower due to more setup work.

How hard is it to see where the derivative of Power in RL=0?

Why would anyone care what the derivative is at RL=0? I would think they would be more interested in the value of RL where the derivative of the power dissipation is 0.

You never answered about having to use Thevenin to even know that there is a max power transfer theorem?

It is a trivial problem in differential calculus to determine what the RL value in relation to the source resistance should be in order to maximize power transfer. Thevenin's theorem is not needed for this.

Ratch

 

[JoeJester]

Rachit,

I did do some calculations ... but not the current loop methods. Attached is the spread sheet where I filled out, via KVL and KCL, what values went where. Yes, I use superposition to get the Rth.

I also have a simulation program where I could have stepped the load resistance to find the maximum power ... or I could have used excel to find it.

You are correct, if the problem stated to use the current loop method, then by all means it should have been done. I did what I did to see the bottom line (RL at max power) to quick check everything. That's when I notice the abnormality in your posting ... which was something I hadn't noticed in any of your prior postings.

 

[MikeMl]

...Spice is good. But, it is really a specialized equation solver and plotter. All the things you listed above can done with a general purpose equation solver and plotter, albeit slower due to more setup work.

LTSpice lets you enter the "equations" as a mesh of circuit elements...

Why would anyone care what the derivative is at RL=0? I would think they would be more interested in the value of RL where the derivative of the power dissipation is 0.

Let me try that sentence again, this time with ():

How hard is it to see where derivative(Power(RL)) = 0?

 

[Ratchit]

MikeMl,

LTSpice lets you enter the "equations" as a mesh of circuit elements...

Yes, I have used Spice. It has a front end interface to input circuit parameters in a consistent manner.

Let me try that sentence again, this time with ():

How hard is it to see where derivative(Power(RL)) = 0?

Not hard at all. Any general purpose equation solver can easily find the current in the east loop to be -9000/(11112+1279*RL) symbolically in terms in RL. Likewise the derivative of (I^2)*RL set to zero quickly simplifies to RL = 11112/1279 = 8.688... . You have no doubt seen the plot of the power of RL and its derivative in my last post.

Ratch