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

In summary, the Thevenin equivalent circuit for a DC power supply with a 30-volt terminal voltage at 400mA and a 27-volt terminal voltage at 600mA can be found by using KVL and determining the load resistance to be 15 ohms. However, the Thevenin voltage and resistance are fixed and do not change with different loads.
  • #1
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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  • #2
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.
 
  • #3
As you can see by now MATLABdude, Edward Lawry Norton is my arch nemesis. But I thank you for the help!
 

1. What is a Thevenin Equivalent circuit?

A Thevenin Equivalent circuit is a simplified model of a complex circuit that is used to analyze the behavior of a circuit at a specific output without having to consider the entire circuit. It consists of a voltage source in series with a single resistor, representing the equivalent voltage and resistance of the original circuit.

2. How do I solve for the Thevenin Equivalent of a DC power supply?

To solve for the Thevenin Equivalent of a DC power supply, you will need to follow these steps:

  1. Disconnect all loads from the circuit.
  2. Determine the open-circuit voltage (Voc) by measuring the voltage across the positive and negative terminals of the power supply.
  3. Determine the short-circuit current (Isc) by connecting a short circuit between the positive and negative terminals of the power supply and measuring the current.
  4. Calculate the Thevenin voltage (Vth) by subtracting the voltage drop across the internal resistance of the power supply (Voc - Isc*Rint).
  5. Calculate the Thevenin resistance (Rth) by dividing the Thevenin voltage by the short-circuit current (Vth/Isc).
  6. The Thevenin Equivalent circuit will have a voltage source of Vth and a resistor of Rth in series.

3. What is the significance of the Thevenin Equivalent in circuit analysis?

The Thevenin Equivalent is significant because it allows us to simplify complex circuits and analyze their behavior at a specific output without having to consider the entire circuit. This can save time and effort in circuit design and troubleshooting.

4. Can the Thevenin Equivalent be used for AC circuits?

Yes, the Thevenin Equivalent can be used for AC circuits as well. The steps for solving for the Thevenin Equivalent in an AC circuit are the same as in a DC circuit, except that instead of measuring open-circuit voltage and short-circuit current, you will need to measure the voltage and current across a load resistor.

5. How can the Thevenin Equivalent be used in practical applications?

The Thevenin Equivalent can be used in practical applications such as circuit design, analysis, and troubleshooting. It allows us to simplify complex circuits and determine the behavior of a circuit at a specific output, making it easier to design and troubleshoot circuits. It is also used in the design of voltage dividers and in determining the maximum power transfer in a circuit.

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