Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS EECS 100/43 Lab 1 – Sources and Resistive Circuits 1. Objective In this lab, you learn how to use the basic equipment on your workbench: the breadboard, power supply and multimeter. You use the breadboard to build your circuit, the power supply to provide energy (voltage) to your circuit and the multimeter to measure voltage, current or resistance. 2. Equipment a. Breadboard b. Wire cutters c. Wires d. Resistors: one random resistor, 1k (2x) e. Various connectors (banana plugs-to-alligator clips) for connecting breadboard to power supply and for multimeter connections. f. Multimeter g. DC Power supply 3. Theory a. The Breadboard A real circuit is very different to the world of circuits on a white board or blackboard. However, you will not be responsible for understanding these differences. As a matter of fact, these differences have negligable effect on your measurements. However, you will be responsible for clean wiring. As a result, let us talk the most important item on your lab bench: the breadboard. Figure 1 shows a picture of the breadboard. Figure 2 shows how the holes in the breadboard are connected.Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS Figure 1. The breadboard Figure 2. Zoomed in view of the breadboard illustrating how the holes are connected Figure 2 is probably difficult to visualize - make sure you know how to use breadboard by the end of lab 1. Using the breadboard is not difficult, but building neat circuits is! Figure 3 shows one good wiring practice. Notice how the wires are all .sitting. on the breadboard. This will minimize debugging time. DO NOT MAKE A “HAIRY CIRCUIT” – EXTREMELY LONG WIRES THAT RISE ABOVE THE BREADBOARD.Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS Figure 3. Neat wiring on a breadboard b. Resistors and Voltage Sources Figure 4 below shows different kinds of the most common type of resistor - the carbon-film resistor. The resistors are sized differently depending on how much power they can dissipate. Higher power dissipation translates to a bigger resistor. Figure 4. Picture of different kinds of carbon-film resistors (source: http://www.compworks.faithweb.com/electronics/components/resistor101.html)Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS The color code on the resistor helps you read its value. The TA will illustrate with some examples. A real resistor circuit on the breadboard will be shown later. Let us now talk about how you will supply power to your circuit . the DC (Direct Current) power supply. A front view of this power supply is shown in figure 5. Figure 5. The digital DC power supply You have three choices of voltage: 6 V, 25 V or -25 V. For using the 6 V power supply, you connect to the 6 V + - terminals shown above. The terminal is hooked up to the ground of the power supply. c. The concept of a ground The concept of a ground is probably the hardest concept to understand in electrical engineering. You can find a very good article on this concept at http://www.ee.upenn.edu/rca/instruments/misctutorials/Ground/grd.html. This paragraph attempts to summarize some of the important concepts. According to Kirchoff's current law (KCL), current entering a circuit = current leaving as shown in figure 6 below.Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS Figure 6. KCL used to illustrate the concept of a ground. If I amps enters your circuit, I amps has to leave your circuit. Ground in a circuit usually denotes the wire that carries this return current as shown in figure 7 below. Figure 7. A simple circuit with a ground symbol The reason for choosing the word .ground. is historical. Early electrical engineers theorized the earth was electrically neutral. This established a convenient reference frame for voltage measurements . a voltage is a potential difference and the earth served as the reference. They also used the earth as a current return path to the lowest potential point of the generating system, as shown in figure 8.Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS Figure 8. The concept of an earth ground Since every circuit must have a return path for the current, a ground simply means the potential at that node in a circuit is zero. This statement will become clear when we talk more about circuits. Let us now talk about how to actually measure potential differences using the multimeter. d. The Multimeter A multimeter is a device that can measure many (multi) things: voltage, current and resistance are the quantities you will be concerned with in this lab. Figure 9 shows the multimeter used in the EECS 100 lab.Summer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS Figure 9. The multimeter configured to measure current. A multimeter configured to measure voltage is called a voltmeter; configured to measure current is called an ammeter and configured to measure resistance is called an ohmmeter. The details of configuring the meter to measure current, voltage and resistance will be covered by the TA in lab. The concept behind the measurement of voltage and current will be discussed here. It will be helpful to discuss this concept by drawing an analogy to mechanical engineering: you can think about current as water flowing through a pipe. Water can flow through a pipe because of difference in potential energy. Figure 10 illustrates how you would measure potential difference (analogous to voltage in a circuit) and the amount of water flowing through the pipe (analogous to current in a circuit). Figure 10. Parallel vs. series configurationSummer 2007 Lab 1 EE100/EE43 University of California, Berkeley Department of EECS In Figure 10 case (i), you are measuring potential difference. Hence you want to hook the meter across the two points to measure their potential difference. Thinking in a similar manner, we can see that an ammeter is hooked up in series with the circuit to measre current. Voltmeter and ammeter configurations to measure voltage and current in a circuit are shown in figure 111. Figure 11. Parallel configuration for measuring voltage and series configuration for measuring current To hook up an ohmeter
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