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ENGR201 Circuits I Fall 2001 Basic DC Measurements BACKGROUND OBJECTIVES: The goals of this laboratory exercise are to learn how to: A. use some basic pieces of laboratory equipment, including 1. a circuit breadboard, 2. a dc power supply, and 3. a digital multimeter; (DMM) B. use a breadboard to construct circuits from a circuit schematic and measure electrical quantities of resistance, voltage, and current. I. THE BREADBOARD A breadboard is a device used to facilitate the building and testing (called prototyping) of circuits. The breadboard accepts pre-stripped wires, making it easy to make connections between various electrical components, power supplies, and meters. Breadboards allow circuits to be tested and modified easily, since no permanent connections are required. Figure 1 shows a picture of typical breadboard that also has terminals for connecting to a power supply (V1, V2, V3, and ground). One way to think of a breadboard is a collection of nodes with multiple points (tie points) where components may be connected. A node (also called a bus) is an area of a circuit where the voltage anywhere in the area is the same. Several nodes have been highlighted on Figure I. Figure 2 shows several resistors connected to two power sources. Resistor R1 and R2 are in series, while R3 and R4 are connected in parallel. Note that one end of each of the resistors R2, R3, R4, and R5 are all tied to the same node (one row of the breadboard). The schematic corresponding to the connection is shown in Figure 3. Use the most sensitive range of a DMM to measure the resistance between any two points on the same node. What reading is displayed? ________________________________________ Use the least sensitive range of a DMM to measure the resistance between any two points on different nodes. What reading is displayed? ________________________________________ ENGR201 Basic DC Measurements Page: 2 Figure 1 A node with 40 tie points Nodes with 5 tie points Nodes with 10 tie points ENGR201 Basic DC Measurements Page: 3 Figure 2 C A R2 R1 R5 B R3 D R4 E Figure 3 R1 A R2 B V1 R5 C D R3 R4 E V3 ENGR201 Basic DC Measurements Page: 4 II. RESISTANCE COLOR CODE Many, but not all, resistors have their nominal value specified by means of four (4) color-coded bands positioned towards one end of the resistor. Figure 4 shows how to interpret the color code of a resistor. Figure 4 - Resistor Color Code Example: Yellow/Violet/Red/Gold 47x102 = 4.7 K +5% Multiplier x 10N Color Black Brown Red Orange Yellow Value 0 1 2 3 4 N Color Green Blue Violet Gray White Tolerance Value 5 6 7 8 9 Tolerance Gold + 5% Silver + 10% III. DC POWER SUPPLY A dc power supply is used, to supply dc power at a “constant” voltage to electrical and electronic circuits. The dc power supply used for this lab is an ac-to-dc converter that has three nominal voltage levels: 5V @1A, 12V @300mA. The three outputs share a common reference, and the supply is connected to the breadboard so that +5V is available as V1, +12V as V2, and –12V as V3. IV. THE BEL MERIT DX405 DIGITAL MULTIMETER (DMM) The DX405 DMM is a digital (computer-like) device capable of making the following measurements: 1. 2. 3. 4. 5. dc voltage from 200mV to 1000v ac voltage from 200mV to 750V, RMS dc current from 200A to 10A ac current from 200A to 10A , RMS resistance from 200 to 20M The DMM can also measure capacitance and frequency and be used to perform continuity and diode tests. Figure 5 shows a picture of the DX405. Figure 6 illustrates the proper connections for measuring resistance, voltage, and current measurements. ENGR201 Basic DC Measurements Page: 5 Voltage measurements are performed by placing the DMM probes in parallel with the element under test current measurements are performed by placing the DMM probes in series with the element under test resistance measurements are performed by de-energizing the circuit and placing DMM probes in parallel with the element under test and Figure 5 – DX405 Digital Multimeter Figure 6 - Connections for Measuring Resistance, Voltage and Current Resistance V//Hz RX DMM COMMON Voltage V//Hz DMM COMMON - VX + + (Note different connection.) Current mA DMM Note, circuit must be “broken” to measure IX COMMON + - IX ENGR201 Basic DC Measurements Page: 6 Series and Parallel Connections series – two components and only two components share a common node; the two elements, therefore, have the same current. I2 = I1 I1 parallel – two (or more) components are connected between the same two nodes; the two elements, therefore, have the same voltage. A + + V1 V2 = V1 - - B Connect two resistors in series as shown: How do the actual values of R1 and R2 compare with the nominal values: R1 = 1k What is the total resistance measured across the series connection? Add a third resistor, R3 = 3.3k in series and measure the total resistance. What conclusion can you make about series connected resistors? R2 = 2.2k ENGR201 Basic DC Measurements Page: 7 Connect the circuit shown and measure: VAB _______________ VBC _______________ VCD _______________ VAC _______________ VBD _______________ What can you conclude about the way voltage is distributed across series-connected resistors? How is the voltage across series connected resistors related to the amount of resistance? Connect two, the three, then four 100k resistors in parallel, each time measuring the total resistance: 100k || 100k = __________ 100k || 100k || 100k = __________ 100k || 100k || 100k || 100k = __________ What conclusion can you make about the relationship between R total, the value of R, and the number of resistors connected in parallel? ENGR201 Basic DC Measurements Page: 8 Connect the following size resistors in parallel and measure Rtotal : 1k || 10k = __________ 1k || 47k = __________ 1k || 100k = __________ What conclusion can you make about connecting a relatively small resistor in parallel with a large resistor?