# Maximum power to a load

1. Oct 19, 2009

### Novark

I'm doing some practice problems, and I can't seem to figure out where to even begin with this one.

I've tried solving for $$R_{T}$$ (Thevenin resistance) by removing the 9V source and adding a 1V test source across the load, and I've also tried solving for $$R{_T}$$ by leaving the 9V source in the circuit.

If someone could walk me through the steps, I would greatly appreciate it!

2. Oct 20, 2009

### Staff: Mentor

Start with either KVL or KCL equations. What is the power in RL? How does varying the value of RL affect the power equation? What's the power for a small value of RL like 1 Ohm? What about 1k Ohm?

What technique in calculus is used to maximize something?

3. Oct 20, 2009

### willem2

You can find the thevenin resistance by finding the open circuit voltage and the short circuit current.
The tevenin resistance is (open circuit voltage)/(short circuit current)

Finding these can IMO most easily be done by finding an equation for the potential of the point between the 3 resistances with KCL.

4. Nov 5, 2009

To find Rth :

short out 9v voltage supply and open RL resistor
the independent 3Vx source remains.
Introduce voltage source Vth from RL terminal
and Isc flowing into the 4ohm resistor from Vth

using KVL with mesh currents both are clockwise,
The below equation is derived :

<< complete solution deleted by berkeman >>

Last edited by a moderator: Nov 5, 2009
5. Nov 5, 2009

### Staff: Mentor

Please do not post complete solutions to homework/coursework questions. That does not help the student learn how to learn, and is against the PF Rules.

Instead, offer tutorial hints and correct mistakes.