Forums » CircuitLab Support Forum

I'm trying to figure out how to determine the value of the load resistor Rl, to maximize power to the load, and the power dissipated by the resistor. How do you make a load resistor? The VCCS value is Vx/5000. Any help would be appreciated. My circuit:

Are you saying that you want to find the value of R1 that gives the maximum power in R1 with everything else fixed?

If yes then try these steps:

Give R1 a value. 1k is OK, just don't leave it blank.

Open the DC Sweep simulation settings;

Parameter => R1.R

Sweep type => Decade

Start => 10

Stop => 100k

Points/Decade => 100

Add Expression => P(R1)

Run DC Sweep

You will see that the power dissipation peaks at around 533uW with R1 ~ 7.6k

Note that when you click in the Parameter box, the available options for parameters that you can sweep appear in grey. Just type in your choice: in this case, sweeping the resistance parameter, R, of device, R1.

For more about plotting power in devices see:

https://www.circuitlab.com/docs/expressions/

Ok, I've changed my model a little to get easier numbers. I'm trying to keep all resistors constant except RL. I ran the DC Sweep and it shows that max power is transferred to the load when RL = 10kohm. I'm trying to justify that with my calculations, though. I found the thevenin resistance to be 5kohms. But you attain max power when RL = Rth... And Isc = .0008A. So thevenin voltage = 4v. Putting that into my power equation, I get .0008 W, half of what the model shows. I don't know why my calculated thevenin resistance is half of what the model shows.

But the thevenin source resistance of 5k is only true when the transconductance of VCCS1 is zero.

With a finite transconductance in VCCS1 you have feedback (posiitve I think) which changes the effective thevenin source resistance.

Try setting k in VCCS1 to zero.

Ok, that makes sense. So if I want to solve for R source I need to include the resistance from the VCCS? How do I solve for P max on paper?

Not that simple. VCCS1 is not a simple resistance: it is a transconductance (because it is in units of Iout/Vin and Vin varies which makes Iout vary and that in turn changes the voltage driving VCCS1).

As I said, by including a dependent source (VCCS1) you have a feedback loop in your circuit so it is not a simple passive circuit to which you can apply Kirchoffs Laws in the same way you could if there was no feedback.

To be honest I can't think how you do solve it. Mainly because I haven't had to do such a problem longhand for so many years It would take me far too long to do it now. Unless you have a real burning need for an analytical solution, just run the sim and use the answers. That's what simulation is for!

That said, I get the impression that you may not actually want that dependent source in your circuit: if you are thinking of it just as a resistor then you have not understood what is really going on in the circuit.

Is this a college problem for which you do have to write down an analytical solution?

BTW: please note because you have either deleted your circuit or made it private, it is no longer visible in this thread.

What I needed to do was calculate the mesh current for the left loop with an open circuit on the right. This would allow me to solve for Vx which would give me the thevenin voltage. Once I had that, with the short circuit current, I could calculate the thevenin resistance. At max power, R thevenin equals RL.

Thank you for all your help. I definitely have a better understanding of the system now.