Thevenin Equivalent: Solve DC Power Supply 30V/400mA, 27V/600mA

[sisyphus0321]

Homework Statement

Find the Thev Eq circuit for a DC power supply that has a 30-volt terminal voltage when delivering 400mA and a 27-volt terminal voltage when delivering 600mA.

Homework Equations

KVL: V_s=I_sR1+I_sR2
V_th=I_sc*R_th

The Attempt at a Solution

Assuming that each 3volt step is a 200mA change, than Voc would be 36v. I determined the load resistance by checking both current EQs [6v_load/.4A] and [9v_load/.6A] and determined the solution of both to be 15ohms. I did a KVL utilizing the 30v and 400mA figures and determined that R_th is 75ohms. I then did a KVL with the other figures and received a 30ohm answer. Where have I strayed?

 

[MATLABdude]

The Thevenin voltage and Thevenin resistance are fixed for a particular black box and don't change when you hook up different loads to it. The load resistance most certainly changes, but you've basically assumed that they're constant (how else would the first circuit only draw 400 mA while the second draws 600 mA?) while the Thevenin Resistance is changing.

 

[sisyphus0321]

As you can see by now MATLABdude, Edward Lawry Norton is my arch nemesis. But I thank you for the help!