ECECS 349
Measurements
Laboratory
Experiment #8
Thevenin’s
Equivalent Circuit
OBJECTIVE:
To
verify the Thevenin’s equivalent circuit.
Figure 1a shows a
typical circuit with a source connected at the input and a load ZL
connected at the output. In some circuit analysis, it is necessary to replace
the source and the complex circuit at a given frequency with a simple complex
voltage source in series with an impedance or with a
complex current source in parallel with a complex conductance. The resulting
equivalent circuit is shown in Figure 1b for the case of voltage and impedance.
The equivalent voltage source and the impedance are known as the Thevenin
equivalent voltage and impedance. The Thevenin voltage is obtained by measuring
the open circuit voltage at the output of the circuit and the Thevenin
impedance is the impedance seen at the output of the circuit where all voltage
sources are replaced by shorts and the current sources are open circuited.
Figure 1
EXPERIMENT:
1)
Chose
a 1 kW resistor as a
load. Pick 5 additional resistors. Measure all the values with a multimeter.
Connect the 5 resistors as shown in Figure 2. Connect a DC voltage at the input
of the circuit and connect the load resistor RL at the output.
a)
Using
a multimeter, measure the voltage across and current into the Load resistor.
b)
Remove
the load resistor and using a multimeter, measure the Thevenin equivalent source
voltage and the source impedance.
c)
Find
a resistor almost equal to the Thevenin source resistance (You may use a
potentiometer) and set the voltage to the Thevenin source voltage. Connect the load resistor across this circuit
and measure the load voltage and the load current. Do they match the measured
values in 1(a)?
2)
Replace
the DC source with a sinusoidal voltage source and repeat the above measurements.
Figure
2
3)
Now
pick an inductor, two capacitors and two resistors. Measure their pertinent
values by LCR meter at 10 KHz and 1 MHz. In Figure 2, replace R1 and
R4 the two capacitors, R3 by the inductor.
a)
Connect
the load resistor of 1 KW as a load and
connect an AC source at the input. Using the oscilloscope, measure the voltage across
and current flowing into the load resistor at 10 KHz and 1 MHz.
b)
Measure
the Thevenin voltages and Thevenin source impedances at 10 KHz and 1 MHz.
QUESTIONS:
1)
In
1(b), why it was sufficient to use a multimeter?
2)
By
simple circuit analysis, calculate the Thevenin Equivalent voltage and
impedance for the circuit given in Figure 2 for the DC case. Are these results
consistent with your measured values?
3)
Using
PSPICE, simulate the DC circuit given in Figure 2 and the corresponding AC
circuit at 10 KHz and 1 MHz.
4)
Replace
the sources and the circuits by their equivalent Thevenin equivalent voltages
and impedances that you measured and simulate the circuits again. DO the
voltages across and the current through the load resistor match?
5)
Explain
the differences between the calculated, simulated and measured values and
possible sources of errors in the measurements.