Bharathwaj Muthuswamy EECS 100 Summer [email protected] Lab IntroductionIntroduction1. ObjectiveIn this lab, you learn how to use the basic equipment on your workbench – thebreadboard, 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 tomeasure voltage, current or resistance.2. Constructing CircuitsA 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 offact, these differences have negligable effect on your measurements. However, you willbe responsible for clean wiring. As a result, let us talk the most important item on yourlab bench – the breadboard. Figure 1. shows a picture of the breadboard. Figure 2 shows how the holes in thebreadboard are connected. Figure 1. The breadboard Figure 2. Zoomed in view of the breadboard illustrating how the holes are connectedFigure 2 is probably difficult to visualize - make sure you know how to use breadboardby the end of lab 1.Using the breadboard is not difficult, but building neat circuits is! Figure 3 shows onegood wiring practice. Notice how the wires are all “sitting” on the breadboard. This willminimize debugging time. DO NOT MAKE A “HAIRY CIRCUIT” - EXTREMELYLONG WIRES THAT RISE ABOVE THE BREADBOARD. Figure 3. Neat wiring on a breadboardResistors and Voltage SourcesFigure 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 candissipate. 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)The color code on the resistor helps you read its value – the TA will illustrate with someexamples. A real resistor circuit on the breadboard will be shown in section 6. Let usnow 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 supplyYou have three choices of voltage – 6 V, 25 V or -25 V. For using the 6 V powersupply, you connect to the 6 V + - terminals shown above. The – terminal is hooked upto the ground of the power supply.The concept of a ground is probably the hardest concept to understand in electricalengineering. You can find a very good article on this concept athttp://www.ee.upenn.edu/rca/instruments/misctutorials/Ground/grd.html. This paragraphattempts to summarize some of the important concepts. According to Kirchoff's currentlaw (KCL), current entering a circuit = current leaving as shown in figure 6 below.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 infigure 7 below. Figure 7. A simple circuit with a ground symbolThe reason for choosing the word “ground” is historical. Early electrical engineerstheorized the earth was electrically neutral. This established a convenient referenceframe for voltage measurements – a voltage is a potential difference and the earth servedas the reference. They also used the earth as a current return path to the lowest potentialpoint of the generating system, as shown in figure 8.Figure 8. The concept of an earth groundSince every circuit must have a return path for the current, a ground simply means thepotential at that node in a circuit is zero. This statement will become clear when we talkmore about circuits. Let us now talk about how to actually measure potential differencesusing the multimeter.4. The MultimeterA multimeter is a device that can measure many (multi) things – voltage, current andresistance are the quantities you will be concerned with in this lab. Figure 9 shows themultimeter used in the EECS 100 lab. Figure 9. The multimeter configured to measure current.A multimeter configured to measure voltage is called a voltmeter; configured to measurecurrent 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 becovered by the TA in lab. The concept behind the measurement of current and voltagewill be discussed here.It will be helpful to discuss this concept by drawing an analogy to mechanicalengineering: you can think about current as water flowing through a pipe. Water canflow through a pipe because of difference in potential energy – water can flow from thetank on the 1stfloor of an apartment complex to the basement. Figure 10 illustrates twopossibilities to set up a potential meter that measures this potential difference.Figure 10. Series vs. parallel configuration of the potential meterIf you think about it, case (ii) in figure 10 makes sense for measuring voltage. You aremeasuring potential difference - hence you want to hook the meter across the two pointsto measure their potential difference. Thinking in a similar manner, we can see that anammeter is hooked up in series with the circuit to measre current. Voltmeter andammeter configurations to measure voltage and current in a circuit are shown in figure111. Figure 11. Parallel configuration for measuring voltage and series configuration for measuring currentTo hook up an ohmeter to measure resistance, you connect it across the resistor. You willhave a chance to practice these techniques in the experiment, coming up next...1 You may have heard electrical engineers talk about “voltage across” and “current through” – now yousee why. NEVER SAY “voltage through” and “current across”.5. The ExperimentPLEASE NOTE: 1. YOU GET 4 POINTS (FOR THE PRELAB) IF YOU SHOW UP FOR LAB 2. FORM GROUPS 3. WIRE NEATLY 4. CLEAN UP YOUR STATION AFTER YOUR DONE 5. IF YOU HAVE ANY QUESTIONS ON THE BREADBOARD, POWER SUPPLY OR MULTIMETER - ASK THE TA ASAP!TASK 1:Your TA
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