Thevenin and Norton Equivalent Circuits - Lab Assignment 2 | ECE 53, Lab Reports of Electrical and Electronics Engineering

Download Thevenin and Norton Equivalent Circuits - Lab Assignment 2 | ECE 53 and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! ECE 53A: Fundamentals of Electrical Engineering I Laboratory Assignment #2: Thévenin and Norton Equivalent Circuits Spring 2008 General Guidelines: - Record data and observations carefully for each lab measurement and experiment. - You must obtain Lab. Assistant’s signature on each page of your lab data before leaving the lab. Signed pages must be included in the report. - Make sure you understand the experiment procedure before executing it. You must obtain enough data to complete the various parts of the procedure. - Request Lab Assistant’s help to verify your circuit before turning on the power supplies and generators. - Please operate the equipment in a reasonable manner. Avoid power supply short circuits. Report failures to the Lab. Assistant. Parts: Equipment: -Resistors as needed Breadboard Digital Multimeter (DMM) Signal Generator Oscilloscope Objective: The objective of this session is to practically verify the existence of Thévenin and Norton equivalents for simple circuits through laboratory measurements. Background: Ideal and practical sources Voltage and current sources used in laboratory are not ideal. Ideal voltage sources have zero series resistance and ideal current sources have an infinite parallel input resistance. The absence of any resistors implies that there is no internal power dissipation. In turn when we look at the ideal voltage source (Thevenin circuit without a resistor), we note that if we short circuit the output, the predicted output current is infinitely large, a very non-practical result. In the Norton case (current source without a resistor), the open circuit output voltage will be infinitely large. We then have a circuit that will have a voltage breakdown if we don’t provide a load at the output. Practical sources have some finite resistance associated with them. The function generators in the laboratory have internal resistances of 50 or 600 Ohms, depending on the model. Thevenin and Norton theorems allow us to model the function generator used in lab as follows: For both circuits we have added loads that draw the same load current, IL, but the loads need not be otherwise specified at this time. For many applications these circuits can represent complicated circuits that are effectively in a black box that we cannot open and investigate. In this case we cannot distinguish between a Norton and a Thevenin circuit by studying its current-voltage output characteristic. The two circuits above will have identical output current and voltage regardless of the load type: V V I R I I R out T L T N L T Here to satisfy the equivalence of terms: V TI N R N , R TR N Voc/Isc  where VOC is the open-circuit voltage and ISC is the short circuit current as shown in Fig.1. Figure 1. Norton and Thévenin equivalent circuits Measuring Thevenin and Norton Equivalent Circuit Parameters: Because both circuits are assumed to be linear, measuring two operating points on the Voutput vs. IL relationship allows us to measure the circuit parameters. Convenient points are the open circuit voltage and the short circuit current output. These work from the theoretical point of view but not necessarily from an experimental point of view. Never try to measure the short circuit current of a regulated voltage supply or the open circuit volt-age of a regulated current supply. Prelab: 1- Why shouldn’t you do a direct measurement of the short circuit current of a regulated voltage supply, or the open circuit voltage of a regulated current supply? (Hint: What happens when you short a voltage source?) 2- Shown in Fig.2 is a plot of the I-V characteristics of the accompanying circuit (Black Box). Figure 2. Black box and I-V characteristic a) Evaluate the parameters IN, VT , and RT of the black box. b) Draw the Thevenin and Norton equivalent circuits for the black box. The plot in Fig. 2 is called the load line. Potting the load line to measure the Thevenin and Norton equivalent circuits is referred to as the load line method. This method allows the characterization of the equivalent circuits without having to physically apply a short or an open at the load. For example, if one needs to determine the Norton equivalent circuit of a voltage source, shorting the