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

AI Thread Summary
To find the Thevenin equivalent circuit for a DC power supply with specified voltages and currents, the terminal voltage is 30V at 400mA and 27V at 600mA. The calculated open-circuit voltage (Voc) is 36V, while the load resistance is determined to be 15 ohms based on both current equations. However, discrepancies arise in calculating Thevenin resistance (Rth), yielding values of 75 ohms and 30 ohms from different KVL applications. The discussion emphasizes that Thevenin voltage and resistance should remain constant regardless of load changes, highlighting a misunderstanding in the assumptions made about resistance. The conversation concludes with a recognition of the complexities involved in applying Thevenin's theorem correctly.
sisyphus0321
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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: Vs=IsR1+IsR2
Vth=Isc*Rth


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 [6vload/.4A] and [9vload/.6A] and determined the solution of both to be 15ohms. I did a KVL utilizing the 30v and 400mA figures and determined that Rth is 75ohms. I then did a KVL with the other figures and received a 30ohm answer. Where have I strayed?
 
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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.
 
As you can see by now MATLABdude, Edward Lawry Norton is my arch nemesis. But I thank you for the help!
 

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