Maximum power transfer and kirchoffs

[Andrew187]

Looks good.

Ok we are getting there lol I just need a clue how to work out I1 as I believe I need that for the ISC?

 

[gneill]

Ok we are getting there lol I just need a clue how to work out I1 as I believe I need that for the ISC?

Are you going to find the equivalent resistance from your diagram, or go the Isc route?

When determining the Thevenin equivalent you want to determine the Thevenin voltage Vth (the open circuit voltage, which you've done), and the Thevenin resistance. The Thevenin resistance can be found in two ways: 1) determine the short circuit current Isc for the network and divide Vth by Isc, or 2) suppress the sources and determine the equivalent resistance of the network looking back into it from the output.

In this case, I think the second method looks simpler as it's a pretty basic network with opportunities for simplification.

 

[Andrew187]

Isc=
(9+25)*i1 - 25*i2 = 15
-25*i1 + (25+7)*i2 = 0

 

[gneill]

Isc=
(9+25)*i1 - 25*i2 = 15
-25*i1 + (25+7)*i2 = 0

yes, and so,...

 

[Andrew187]

yes, and so,...

I2 = 0.81a?

 

[gneill]

I2 = 0.81a?

Yes. Carry on,... what's the Thevenin resistance?

 

[Andrew187]

Yes. Carry on,... what's the Thevenin resistance?

7.032v/0.81a=8.68 ohm ?

 

[gneill]

7.032v/0.81a=8.68 ohm ?

Yes. Carry on... draw the Thevenin equivalent circuit with the load RL attached. What value of RL maximizes the power transfer (Max Power theorem).

 

[Andrew187]

Yes. Carry on... draw the Thevenin equivalent circuit with the load RL attached. What value of RL maximizes the power transfer (Max Power theorem).

That bits not the problem I don't know how I worked out the value for I1 and how I got Isc to 0.81a

 

[gneill]

That bits not the problem I don't know how I worked out the value for I1 and how I got Isc to 0.81a

You wrote the equations for the circuit with the output shorted and solved for the current I2, which is identical to Isc... If there's some part that you didn't understand, stop and ask before moving on.

 

[Andrew187]

yes, and so,...

Isc=
(9+25)*i1 - 25*i2 = 15
-25*i1 + (25+7)*i2 = 0

This Is the bit I didn't understand i took this out of my notes I know here that the value of I2=0.293a, but I didn't quite understand how I worked out I1.

 

[gneill]

Isc=
(9+25)*i1 - 25*i2 = 15
-25*i1 + (25+7)*i2 = 0

This Is the bit I didn't understand i took this out of my notes I know here that the value of I2=0.293a, but I didn't quite understand how I worked out I1.

Can you be more specific about what you don't understand? You have written two equations in two unknowns, which can be solved using whatever technique you're familiar with. What part is stumping you?

1. Writing the equations from the circuit.
2. Solving two equations for two unknowns.

 

[Andrew187]

Can you be more specific about what you don't understand? You have written two equations in two unknowns, which can be solved using whatever technique you're familiar with. What part is stumping you?

1. Writing the equations from the circuit.
2. Solving two equations for two unknowns.

Writing it shouldn't be a problem but solving the two equations from the two unknowns is a problem I know the answer but don't know how I solved it.

 

[gneill]

Writing it shouldn't be a problem but solving the two equations from the two unknowns is a problem I know the answer but don't know how I solved it.

Then it sounds like you need to review your algebra and simultaneous equation solving. This is a fundamental skill that you must have in order to solve problems in any technical discipline. A web search on "solving simultaneous equations" will turn up some on-line tutorials.

 

[Andrew187]

Then it sounds like you need to review your algebra and simultaneous equation solving. This is a fundamental skill that you must have in order to solve problems in any technical discipline. A web search on "solving simultaneous equations" will turn up some on-line tutorials.

Sorry what I was meant to say was solving the equation isn't really the problem because I can sit till the cow's come home calculating the unknowns I eventually get there but the bit I struggle with is writing the equations and drawing the correct diagram, because when I try to look at notes and books its always done differrent to the way in which I want to achieve. So This is where I am at, I have found the value of RL I also know the unknowns but I 'm not sure what format and what diagrams to use bearing in mind I have used kirchhoffs and thevenins equivalent I think.

 

[gneill]

Sorry what I was meant to say was solving the equation isn't really the problem because I can sit till the cow's come home calculating the unknowns I eventually get there but the bit I struggle with is writing the equations and drawing the correct diagram, because when I try to look at notes and books its always done differrent to the way in which I want to achieve. So This is where I am at, I have found the value of RL I also know the unknowns but I 'm not sure what format and what diagrams to use bearing in mind I have used kirchhoffs and thevenins equivalent I think.

If I may be frank with you, Andrew187, I get the impression from the way this thread has progressed that you don't really have an adequate grasp on any of the techniques or concepts required to solve the problem, or even a solid understanding of the problem itself (what steps are required to determine which result). You should probably look for some tutorials to firm up your fundamentals.

 

[hussain bani]

Thevenine req

i have tried to short both bateries for finding rth but i can't understand how to solve to find rth... can someone please help me finding rth..

 

[The Electrician]

i have tried to short both bateries for finding rth but i can't understand how to solve to find rth... can someone please help me finding rth..

You should have started a new thread with your question. By asking a new question at the end of an already existing thread, you have "hijacked" someone else's thread.

Hopefully, a moderator will split this off to a new thread.