Lab 6: Electrical Engineering Technology References: 1. Resistor (electronic) color code: http://en.wikipedia.org/wiki/electronic_color_code 2. Resistor color code tutorial: http://www.michaels-electronics-lessons.com/resistor-color-code.html 3. Ohm s Law tutorial: http://www.physicsclassroom.com/class/circuits/u9l3c.cfm Materials: resistors: (5) 1 kω, (4) 2 kω, 2.2 kω, 3 kω, 3.9 kω digital multimeter (DMM) power supply w/ leads breadboard, jumper wires Introduction: Electrical Engineers and Electrical Engineering Technologists use a variety of complex tools every day. However, the most often used and most fundamental tools are resistors, breadboards, power supplies, digital multimeters, and of course, Ohm s Law. Imagine a carpenter with no hammer, or a trucker without a truck! These are all tools without which electrical engineering technologists would be unable to complete their duties. Consequently, it is vitally important for you to thoroughly understand and be comfortable using these tools. Objectives: To understand the operation and application of resistor color code. To understand the operation and application of a DMM. To understand configuration and use of an electronics breadboard. To understand the basic concept of Ohm s law. Instructions: Follow the procedures below and record all work and answers in your lab notebook. Show your notebook to the instructor when finished to receive completion credit for this lab. Procedure: Part 1: Resistor Identification for the not-so-colorblind In order to mark resistors according to their resistances, a specialized color code is used. Resistor color codes use either a 4-band or 5-band system. In this class, we will use 4-band resistors, meaning that each resistor is marked with four colored bands which tell the nominal resistance value and tolerance range of the resistor. As shown in figure 1, resistors are read left-to-right when the wide gap is to the right, with the first three bands denoting the nominal resistor value and the fourth band at the right of the gap denoting the tolerance percentage. Note that a tolerance is included because it is not feasible to create resistors which have the exact value of the nominal resistance. The manufacturing cost of creating these resistors would be exceedingly high. Consequently, resistors, like other electronic components, are made to fall within a specified tolerance. Generally speaking, smaller tolerance values (i.e. more accuracy) result in higher component costs. This shows us that it is important to select parts for a design, such as resistors, which are made as close as possible to the required design tolerance; not more, not less. For 4-band resistors, the first 2 bands are the first 2 digits of the resistance value. The third band represents the multiplier, and the fourth represents the tolerance range. The multiplier band represents what power of 10 the first 2 digits are multiplied by. 1
Figure 1 Resistor color code for 4-band resistors For example, the resistor pictured at the top of the chart in figure 1 has the color code, from left to right, Brown- Black-Red-Gold. From the chart we can read that the first band, Brown, represents a 1 and the second band, Black, represents a 0. Thus, the first 2 digits of the resistance value are 10. Now, the third band is red. From the chart, this represents a multiplier of 10 2 or 100. So the nominal resistance value of this resistor is: R = 10 100 = 1000 Ω or 1.0 kω Note the use of the Greek letter omega, Ω, to represent resistance. Finally, the fourth band, Gold, represents the tolerance of the resistor. From the chart, a gold band means a resistance of ±5% of the nominal resistance value. So the actual value of the resistor is: R = 100 Ω ± 5% = 1000Ω ± 50Ω = [950Ω, 1050Ω] Thus, the actual measured resistance of this resistor should be between 950Ω and 1050Ω. EXERCISE 1: Reading Color Codes Now it s your turn. Using the chart in figure 1, give the nominal resistance and tolerance values for the resistors with the color codes given below. 1. Brown-Black-Orange-Silver 2. Yellow-Violet-Orange-Gold 3. Orange-White-Red-Gold 4. Blue-Green-Gray-Gold 5. Red-Red-Red-(None) 6. [instructor given] Note that the preceding discussion of resistor color code applies to conventional through-hole resistors. Newer and more commonly used surface mount resistors use a numeric marking scheme in lieu of color bands (see reference 1). 2
Part 2: DaMM! Meters are an important tool in anything dealing with electronics. They have evolved from the old-style analog display to modern digital displays and typically include settings to measure various electronic properties. The Fluke digital multimeter (DMM) is used in this lab to measure voltage, current, and resistance. It features automatic scaling for voltage and resistance, so that measurement ranges do not need to be set manually. Consequently, it is important when taking measurements to record not just the reading but the unit as well as any multiplier (k, M, etc.). The Fluke requires connections to both its + and - inputs. Figure 2 shows the knob settings needed to measure resistance, voltage, and current, with the black (negative) and red (positive) leads positioned to measure voltage or resistance. Resistor values in Ω are measured before inserting the resistor in a circuit. After a resistor is inserted in a circuit, you can measure the voltage drop across the resistor, but not its resistance. When measuring voltage drop, the leads from the multimeter are put in parallel with the circuit. That means that you do not have to take any part of the circuit apart to measure these values. Resistance Current Voltage Measuring current requires a different procedure. The red (positive) lead of the Fluke must be moved to either the port marked A or the port marked maμa depending upon the range at which the measurement must be taken, i.e. greater than or lesser than 1.0 Amp. Also, current must be measured in series. This means that you must insert the multimeter BETWEEN two components in a leg of the circuit through which the current is to be measured. Therefore, you will break the circuit and reconnect it using the multimeter to join the 2 circuit elements. EXERCISE 2: Measuring Resistance Figure 2 Fluke Digital Multimeter Select 5 differently marked resistors from the bins. (Make note of their locations or use the color codes to return them to the correct bins when you are finished!) Translate the color code; then measure the resistance of the resistor using the DMM. Refer to figure 2 to find the correct setting. Note that holding the resistor in your hand can change its measurement, so hold the resistor with one hand (on one end) and then place the Fluke probes on either side of the resistor. Resistor Color Code Calculated Resistance Measured Resistance 3
Part 3: Make me a samich! ( The Breadboard ) Did you notice how difficult it is to hold the resistor while taking the previous measurement? Imagine if the circuit contained many components that needed to make contact. It quickly gets impossible to hold the circuit in your hands! The breadboard is a crucial piece of lab equipment for the Electrical Engineer or Engineering Technologist. It allows for the construction of reasonably complex circuits without the need for soldier joints or other complex/permanent means of holding the circuit together. Looking at your breadboard, verify that your breadboard is blank (nothing plugged in it). Refer to figure 3 for the following discussion. The center of the breadboard contains two major columns each with 63 rows of 5 horizontal tiepoints (a-e and f-j). These sets of 5 tiepoints are connected together within the breadboard. The central separation gap is typically used to place dual in-line package (DIP) components such as integrated circuits (ICs). When such a component is inserted over the vertical channel straddling e/f, the remaining 4 horizontal tiepoints on each side are available for wiring or test instrument connections. The left and right edges of the breadboard each contain two vertical power busses, also internally connected for the length of the breadboard. These power busses, usually marked with blue & red lines, typically connect to Ground and +V from the power supply, respectively. NOTE: The red and blue power busses are not bridged between left and right sides of the breadboard (see markings). You must connect these with jumper wires if power or ground is desired on both sides of the breadboard (highly recommended)! Figure 4 shows the proper way to jumper the power and ground lines from left to right sides of the breadboard. Figure 3 Breadboard connection layout Figure 4 Jumpering power and ground Using the Power Supply Adjustable power supplies such as the ones we use in lab provide one or more outputs with at least one output being adjustable in terms of output voltage and maximum current. For this lab, the output voltage will be set to +5.0V. Additionally, to avoid damage, the power supply should be set to a maximum output current of +0.25A. To do this, first set the power supply to a voltage output of +5.0V and an output current of 0.0A. Then connect leads to the black (-) and red (+) terminals, respectively, but DO NOT connect these to your circuit yet. Touch the alligator clip ends of the leads together to cause a short circuit across the power supply. Since our power supplies are short-circuit protected, this will not cause damage to the equipment, but will instead allow the maximum current to be specified using the current knob on the power supply. Turn this knob until the reading is just at but no greater than 0.25A. This will prevent damage to the equipment in the event of a mistake. Ask for help if in doubt. 4
EXERCISE 3: Making Circuits Using jumper wires as needed, create the following circuits on the breadboard, and then show them to the instructor for credit. (Hint: you can create all of them on the breadboard at the same time, if you are creative and neat.) Note: these are fundamental circuit arrangements and are studied in detail in other courses. a) Single Resistor b) Series c) Parallel Completed: Completed: Completed: Part 4: Oh-my Oh-my Ohm s Law is one of the most fundamental laws governing electrical circuits, and one of the most applied relationships in this field of study. As such, it is imperative that any engineer or technologist working with electronics obtain a thorough understanding of this concept. Ohm s Law is most commonly written: V = I R where: V is the voltage across an element in volts, I is the current flowing through the element in amps, and R is the equivalent resistance of the element in ohms. From this relationship, it can be observed that increasing the resistance of an element while maintaining a constant voltage drop across it will result in a decreased current through the element. Conversely, increasing the resistance of Figure 5 Ohm s Law an element while maintaining a constant current will result in an increased voltage drop across the element. This relationship is humorously depicted in the cartoon of figure 5. 5
EXERCISE 4: Applying Ohm s Law Using the equation for Ohm s Law above and for each of the following resistance values, calculate the expected current based on the actual resistor resistance and a source of 5V. Then for each resistor, assemble the circuit and use the DMM to measure and record the voltage across and current through the resistor. Remember to connect the red DMM lead to VΩ port for voltage measurements and the maμa port for current measurements. Finally, verify accuracy of your calculations. Desired Resistance 1kΩ Color Code Actual Resistance (Ω) Calculated Current (A) Measured Voltage (V) Measured Current (ma) Measured Current (A) 2000Ω 2200Ω 3x10 3 Ω 3.9kΩ Observation: Carefully note the process you just performed. You learned about a concept (Ohm s law), applied scientific principles (mathematics) to the concept to predict outcomes, and then applied experimental principles (circuits & instrumentation) to validate and verify the concept. This is a powerful and proven learning paradigm that will be used over and over again during your studies at CalU as well as during your future career! Complete the Ohm s Law tutorial in reference 3. Part 5: Further Study 6