The aim is for us to know the use of Thevenin's theorem and Norton's theorem in direct current circuits.
1. Thevenin's Theorem
An active circuit (with dependent or independent voltage sources and/or current sources) that is linear with 2 poles (terminals) a and b, can be replaced with a voltage VT in series with a resistor RT.
Thevenin's Theorem
- VT = voltage at ab in no-load (open circuit) = VOC
- RT = resistance at ab "seen" into the circuit with all independent sources replaced by their internal resistances. With this theorem we can quickly calculate the load current when the load is varied.
2. Norton's Theorem
An active circuit (with dependent or independent voltage sources and/or current sources) that is linear with 2 poles (terminals) a and b, can be replaced with one current source IN in parallel with one resistor with resistance RN.
Norton's Theorem
- IN = current through ab in a short circuit (short circuit) = ISC
- RN = resistance at ab "seen" into the circuit with all independent sources replaced by their internal resistances.
How to Conduct an Experiment
1. Thevenin's Theorem
In this experiment, Thevenin's theorem is used to find the current in the load R (R1, R2, or R3) on the CD branch indirectly by measuring VT, RT and R. Then the results are compared with the direct measurement of the current through the load by reading the milli Ammeter.
- Install a 20 Volt DC voltage source on AB. On the CD branch, install a mA meter in series with the load R1. Read the current through R1.
- Open the load and mA-meter, so that CD is open (open circuit). Measure the open circuit voltage of CD (same as VT) with an Electronic Voltmeter that has high input impedance. The AB source voltage must remain = 20 Volts.
- Measuring RT is the resistance that is "seen" on the left CD terminal. Open/remove the voltage source from AB. Short circuit AB. Measure the resistance on the CD terminal with an ohm meter (or bridge).
- Measure the resistance of R1
- Calculate the current through
- Compare the results of calculation (v) with the results you obtained from (i).
- Repeat steps (i) to (vi) above for prices R = R2 and R = R3
- Write the results of the experiment above in a table in a worksheet or log book.
2. Thevenin's Theorem
- Create a network as follows:
- Set the voltage equal to the VT value measured in 4.1.b.
- As RT, the N circuit is used with AB short-circuited and installed according to the picture above.
- Measure the current flowing through R1 with a mA-meter.
- Repeat the experiment for R = R2, R = R3, and R = 0 (short circuit)
- Write the results of the experiment above in a worksheet table or log book.
3. Norton's Theorem
The circuit in experiment 4.1.a above is replaced with a parallel current source IN with a resistance RN which is the same as RT.
- Find IN. Attach a 20 Volt DC voltage source to AB. Measure the short circuit current on CD (attach a mA-meter to CD).
- RN = RT can be obtained by experiment 4.1.c. but in this case we will use the N series as RN.
- Set the current source so that it produces a current of IN as obtained from experiment 4.3.a. Make a circuit as follows:
- Measure the current through the mA-meter for R = R1, R2 and RN2.
- v. Write down the results of your observations in a table in a worksheet or log book.