DC Fundamentals Series Resistive Circuits Exercise 3: Voltage in a Series Resistive Circuit EXERCISE OBJECTIVE When you have completed this exercise, you will be able to determine the voltage in a series resistive circuit by using a formula. You will verify your results with a multimeter. DISCUSSION V = I x R through the resistor. Each voltage drop is indicated by V RN V R1 = 1 ma x 4 k = 4 Vdc V R2 = 1 ma x 6 k = 6 Vdc FACET by Lab-Volt 137
Series Resistive Circuits DC Fundamentals The sum of all voltage drops in the series string equals the applied voltage. V A = V R1 + V R2 10 Vdc = 4 Vdc + 6 Vdc When you measure a voltage drop, a reference point must be established. points A and B are positive (+) with respect to the circuit common. Circuit points B and C are negative with respect to common. 138 FACET by Lab-Volt
DC Fundamentals Series Resistive Circuits For either case (circuit common at A or C), the magnitude of the voltage is equal to the source voltage. circuit common? a. Point A is positive and point B is negative. b. Points A and B are positive. c. Points A and B are negative. In the procedure that follows, you will use the circuit shown. Unless stated otherwise, make all voltage measurements with respect to circuit common. Therefore, all voltages will be positive. PROCEDURE Locate the SERIES CIRCUIT circuit block, and connect the circuit shown. FACET by Lab-Volt 139
Series Resistive Circuits DC Fundamentals Measure the applied voltage. V A = Vdc (Recall Value 1) Measure the voltage drops of R1, R2, and R3. Input your voltage values with proper polarity indications. Remember that the negative terminal of the voltage source is circuit common. V R1 = Vdc (Recall Value 2) V R2 = Vdc (Recall Value 3) 140 FACET by Lab-Volt
DC Fundamentals Series Resistive Circuits V R3 = Vdc (Recall Value 4) In your series circuit, what is the relationship between the source voltage and the sum of the string voltage drops? Refer to your data recalled below. a. The source voltage and each voltage drop are added to determine total voltage. b. The source voltage equals the sum of the voltage drops. c. The source voltage and each voltage drop are subtracted to determine total voltage. MEASURED SOURCE MEASURED DROPS V A = Vdc (Step 1, Recall Value 1) V R1 = Vdc (Step 3, Recall Value 2) V R2 = Vdc (Step 3, Recall Value 3) V R3 = Vdc (Step 3, Recall Value 4) Again measure the voltage drop of R1. This time, place the BLACK lead of your multimeter at the top of R1. Input your voltage reading, including the sign (+ or ) of the voltage. V R1 = Vdc (Recall Value 5) Based on the last meter polarity indication, which statement applies to your circuit? a. The voltage across R1 has doubled because Vdc (Step 3, Recall Value 2) and Vdc (Step 5, Recall Value 5) indicate different magnitudes of voltage drop. b. The voltage across R1 is zero because Vdc (Step 3, Recall Value 2) and Vdc (Step 5, Recall Value 5) cancel each other when added. c. The sign of the voltage drop is negative due to the multimeter connection. The magnitude of the voltage drop does not change. FACET by Lab-Volt 141
Series Resistive Circuits DC Fundamentals Place the BLACK multimeter lead at the junction of R2 and R3 to establish this point as the With respect to the established circuit common point, what is the expected polarity (+ or ) at the junction of R1 and R2? With respect to the established circuit common point, what is the expected polarity (+ or ) at the top of R1? 142 FACET by Lab-Volt
DC Fundamentals Series Resistive Circuits With respect to the established circuit common point, what is the expected polarity (+ or ) at the lower end of R3? Connect the BLACK multimeter lead at the junction of R2 and R3. What is the indicated voltage polarity at the top of R1? What is the indicated polarity at the junction of R1 and R2? FACET by Lab-Volt 143
Series Resistive Circuits DC Fundamentals What is the indicated polarity at the lower end of R3? Based on your results, which statement best describes the effects of a change in circuit reference point? a. The expected and measured polarities agree. b. The expected and measured polarities do not agree. c. The polarities cannot agree because circuit common was changed. Based on the voltage drops of R1, R2, and R3, what is the relationship between current and resistance with respect to the magnitude of the voltage drop? a. Based on Ohm s law, there is no direct relationship. b. For a given circuit current, the greatest drop occurs across the lower value resistor. c. For a given circuit current, the greatest drop occurs across the highest value resistor. 144 FACET by Lab-Volt
DC Fundamentals Series Resistive Circuits CONCLUSION The polarity of your reading is determined by the location of the circuit common when the multimeter is connected to your circuit. The polarity of your reading is also determined by which multimeter lead is placed on circuit common. In a series circuit, the sum of the voltage drops equals the source voltage. In a series circuit, the voltage drop of a resistor is directly related to the current through the resistor and the value of resistance. REVIEW QUESTIONS 1. In a series circuit, the voltage source a. is equal to the sum of the voltages across the components. b. is applied directly across each component. c. applies different amounts of current through each component. d. is always divided equally among all components. 2. Two 50 resistors are connected in series across a 10 Vdc source. The voltage drop of each resistor is a. 10V. b. 7.5V. c. 5V. d. 2.5V. 3. Locate the SERIES CIRCUIT circuit block and connect the diagram shown. Toggle CM1 on and off. Which of the following is true when CM1 is toggled on? a. R3 increased to 500 and circuit current decreased. b. R2 decreased to 500 and circuit current increased. c. R1 decreased to 500 and circuit current increased. d. R1 increased to 500 and circuit current increased. 4. With the same circuit connected, toggle CM1 on and off. What is the total circuit current and voltage across R2 when CM1 is toggled on? a. 6 ma and 0V b. 6 ma and 7.8 Vdc c. 0V and 6 ma d. 0 ma and 0 V FACET by Lab-Volt 145
Series Resistive Circuits DC Fundamentals 5. When you connect a voltmeter in a FACET series circuit, the a. polarity is not important when measuring voltage. b. meter is connected in series with the circuit. c. negative lead is connected to the most positive end of the component. d. negative lead is connected to the most negative end of the component. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. 146 FACET by Lab-Volt