Lab 7 v6-Thevenin and Norton Equivalent Circuits

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Electrical Engineering

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Apr 3, 2024

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Cessar Lechuga Gerardo Gamez Moreno LAB 7
EECE 2105-2 Spring 2024 LAB 7 v5 Thevenin & Norton Equivalents A. OBJECTIVES Observe application of Thevenin equivalent principles Apply the least squares method to fit a linear equation to experimental data B. EQUIPMENT REQUIRED Bench top Digital multimeter Breadboard Triple Channel Power supply Miscellaneous Cables C. PARTS REQUIRED Fourteen 1/2 Watt Resistors, each of following values: 51 ohms, 240 ohms, 560 ohms, 2x1.2 kilohms, 1.8 kilohms, 2x2.2 kilohms, 2x2.4 kilohms, 3.6 kilohms, 3.9 kilohms, 13 kilohms, 18 kilohms. Hook-up wire (4/20 or #22 solid conductor) D. PRIOR TO LAB (1) Thevenin Equivalent Circuits The purpose of this lab is to experimentally demonstrate the concept of Thevenin equivalent circuits, and then apply some basic data analysis techniques to the resulting measurements. The basic concept of a Thevenin equivalent is illustrated in Figure 8-1. Consider a linear circuit 1 with a pair of output terminals, which we will call A and B, as shown at the left. Connected to these terminals is another resistance R, which we call the load. There is a simplified circuit, shown at right, which is equivalent to the circuit at the left. By equivalent, we mean that if any value R of load resistor is connected the circuit at the right, it gives the same voltage V and the same current I as if the load resistor were connected to the circuit at the left. 1 Linear circuits were discussed in the previous lab. If a circuit consists only of ideal sources and resistors, it is safe to assume it is linear. Page 1 of 10
EECE 2105-2 Spring 2024 LAB 7 v5 Thevenin & Norton Equivalents Page 2 of 10
EECE 2105-2 Spring 2024 LAB 7 v5 Thevenin & Norton Equivalents Figure 8-1 Thevenin Equivalent Circuit The circuit at the right delivers the same voltage and current as the circuit at the left, as long as both have the same value of load resistance R connected. You can verify this by doing a few test cases. For example, if R = 1000 ohms, both circuits shown in Figure 8-1 will give V= 0.222 Volts and I = 0.222 mA. If R = 2000 ohms, both circuits will give V = 0.2857 Volts and I = 0.1429 mA. The Thevenin equivalent is always a single voltage source in series with a single resistance. The value of the voltage source is called the Thevenin voltage or the open- circuit voltage. The resistance is called the Thevenin resistance or the equivalent resistance. It turns out that all linear circuits, with a few exceptions, have Thevenin equivalents. (The exceptions are cases where the equivalent resistance turns out to be infinite.) Once the Thevenin equivalent has been found for a circuit, it is much easier to determine the behavior when different loads are connected to the terminals A and B. Although we have shown the load as a resistor, it could be a complete circuit by itself or a device such as a motor or actuator. Many devices (amplifiers, filters, test instruments, etc.) can be characterized by their equivalents. A few fundamental facts that you will need for this lab are as follows: (a) The short circuit current is the current I that a circuit produces when the load R is reduced to zero ohms. (b) The open circuit voltage is the voltage V that a circuit produces when the load R is removed and the terminals A and B are left open, or, in other words, when the value of R increases to infinity. (c) The open circuit voltage divided by the short circuit current equals the Thevenin equivalent resistance. (d) Maximum Power Transfer Theorem the power dissipated in the resistor R changes as the value of R is changed. This theorem states that the power is maximum when the resistance R of the load is equal to the Thevenin equivalent resistance. Page 3 of 10
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