To verify Thevenin's theorem for the given circuit

Verification of Thevenin's Theorem.

Apparatus Required

Circuit Diagram

Practical Circuit Diagrams

Theory for Thevenins Theorem

It states that in any lumped, linear network having more number of sources and elements the equivalent circuit across any branch can be replaced by an equivalent circuit consisting of Theremin's equivalent voltage source Vth in series with Theremin's equivalent resistance Rth. Where Vth is the open circuit voltage across (branch) the two terminals and Rth is the resistance seen from the same two terminals by replacing all other sources with internal resistances.

Thevenin's theorem: The values of VTh and RTh are determined as mentioned in thevenin's theorem. Once the thevenin equivalent circuit is obtained, then current through any load resistance RL connected across AB is given by, I =VTH / RTH + RL

Thevenin's theorem is applied to d.c. circuits as stated below. Any network having terminals A and B can be replaced by a single source of e.m.f. Vim, in series with a source resistance RTh

(i) The e.m.f the voltage obtained across the terminals A and B with load, if any removed i.e., it is open circuited voltage between terminals A and B.

(ii) The resistance RTh is the resistance of the network measured between the terminals A and B with load removed and sources of e.m.f replaced by their internal resistances. Ideal voltage sources are replaced with short circuits and ideal current sources are replaced with open circuits.

To find Wit the load resistor `RL' is disconnected, then VTh = [V / (R1 + R2)] x R3

To find RTH, RTH = R2 + (R1 x R3 / R1+ R3)

Thevenins Theorem is also called as Helmoltz Theorem.

Procedure

1. Connect the circuit as per fig (1)
2. Adjust the output voltage of the regulated power supply to an appropriate value (Say 20V).
3. Note down the response (current, IL) through the branch of interest i.e. AB (ammeter reading).
4. Reduce the output voltage of the regulated power supply to 0V and switch–off the supply.
5. Disconnect the circuit and connect as per the fig (2).
6. Adjust the output voltage of the regulated power supply to 20V.
7. Note down the voltage across the load terminals AB (Voltmeter reading) that gives Vth.
8. Reduce the output voltage of the regulated power supply to 0V and switch–off the supply.
9. Disconnect the circuit and connect as per the fig (3).
10. Connect the digital multimeter(DMM) across AB terminals and it should be kept in resistance mode to measure Thevenin's resistance(RTh).

Theoritical Values

Tabular results for Thevenin's theorem:

Result:

Viva Questions and Answers on Thevenins Theorem

1. The internal resistance of a source is 2 Ohms and is connected with an External Load Of 10 Ohms Resistance. What is Rth ?
2. In the above question if the voltage is 10 volts and the load is of 50 ohms What is the load current and Vth? Verify IL?
3. If the internal resistance of a source is 5 ohms and is connected with an External Load Of 25 Ohms Resistance. What is Rth?
4. In the above question if the voltage is 20V and the load is of 50 Ohms, What is the load current and IN ? Verify IL ?