# 4.2.4: Procedure

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1. Consider the circuit of Figure 11.3.1 using E = 10 volts, R1 = 3.3 k, R2 = 6.8 k, R3 = 4.7 k and R4 ($$R_{Load}$$) = 8.2 k. This circuit may be analyzed using standard series-parallel techniques. Determine the voltage across the load, R4, and record it in Table 11.5.1. Repeat the process using 2.2 k for R4.

2. Build the circuit of Figure 11.3.1 using the values specified in step one, with $$R_{Load}$$ = 8.2 k. Measure the load voltage and record it in Table 11.5.1. Repeat this with a 2.2 k load resistance. Determine and record the deviations. Do not deconstruct the circuit.

3. Determine the theoretical Thévenin voltage of the circuit of Figure 11.3.1 by finding the open circuit output voltage. That is, replace the load with an open and calculate the voltage produced between the two open terminals. Record this voltage in Table 11.5.2.

4. To calculate the theoretical Thévenin resistance, first remove the load and then replace the source with its internal resistance (ideally, a short). Finally, determine the combination series-parallel resistance as seen from the where the load used to be. Record this resistance in Table 11.5.2.

5. The experimental Thévenin voltage maybe determined by measuring the open circuit output voltage. Simply remove the load from the circuit of step one and then replace it with a voltmeter. Record this value in Table 11.5.2.

6. There are two methods to measure the experimental Thévenin resistance. For the first method, using the circuit of step one, replace the source with a short. Then replace the load with the ohmmeter. The Thévenin resistance may now be measured directly. Record this value in Table 11.5.2.

7. In powered circuits, ohmmeters are not effective while power is applied. An alternate method relies on measuring the effect of the load resistance. Return the voltage source to the circuit, replacing the short from step six. For the load, insert either the decade box or the potentiometer. Adjust this device until the load voltage is half of the open circuit voltage measured in step five and record in Table 11.5.2 under “Method 2”. At this point, the load and the Thévenin resistance form a simple series loop as seen in Figure 11.3.2. This means that they “see” the same current. If the load exhibits one half of the Thévenin voltage then the other half must be dropped across the Thévenin resistance, in other words $$V_{RL} = V_{RTH}$$. Consequently, the resistances have the same voltage and current, and therefore must have the same resistance according to Ohm’s law.

8. Consider the Thévenin equivalent of Figure 11.3.2 using the theoretical $$E_{TH}$$ and $$R_{TH}$$ from Table 11.5.2 along with 8.2 k for the load (RL). Calculate the load voltage and record it in Table 11.5.3. Repeat the process for a 2.2 k load.

9. Build the circuit of Figure 11.3.2 using the measured $$E_{TH}$$ and $$R_{TH}$$ from Table 11.5.2 along with 8.2 k for the load (RL). Measure the load voltage and record it in Table 11.5.3. Also determine and record the deviation.

10. Repeat step nine using a 2.2 k load.

This page titled 4.2.4: Procedure is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by James M. Fiore.