Exercise 2: Series-Opposing DC Sources EXERCISE OBJECTIVE When you have completed this exercise, you will be able to determine voltage by using series-opposing power connections. You will verify your results with a voltmeter. DISCUSSION When 2 cells are connected in series (series-aiding), the total load voltage equals the sum of the individual cell voltages. When two cells are connected with the voltages opposing each other, the cells are series-opposing rather than series-aiding. Cells connected either positive to positive or negative to negative are series-opposing. The total voltage in a circuit where two cells are connected as series-opposing is the difference between the 2 individual cell voltages. FACET by Lab-Volt 69
DC Fundamentals In this circuit, V1 and V2 are series-aiding. V3 is connected as series-opposing to V1 and V2. The total circuit voltage equals (V1 + V2) V3. In the circuit shown below, two cells are series-aiding and are then connected as series-opposing to a third cell. What is the total voltage of the circuit? a. 3V b. 1.5V c. 0.5V PROCEDURE Locate the BATTERIES circuit block. V5 and V6 are series-opposing. 70 FACET by Lab-Volt
NOTE: When connected into the circuit with a two-post connector, the YELLOW LED will light to indicate a positive total circuit voltage. The GREEN LED will light to indicate a negative total circuit voltage. Do not insert a two-post connector until instructed to do so. Measure the voltage of battery V5. Note the placement of the voltmeter. V5 = Vdc (Recall Value 1) Adjust the positive variable supply (V6) for a voltage reading equal to that of V5 Vdc (Step 2, Recall Value 1). NOTE: Use both FINE and COARSE supply controls and be precise. FACET by Lab-Volt 71
DC Fundamentals Measure the difference of potential between the positive terminals of V5 and V6. The meter indicates near 0V because the a. variable supply is connected to the RED meter lead. b. circuit batteries are series-aiding. c. circuit batteries are series-opposing. Adjust V6 for a difference of potential of -4 Vdc between the positive terminals of V5 and V6. NOTE: Use both FINE and COARSE supply controls and be precise. Measure the value of V6. Note the meter connections. V6 = Vdc (Recall Value 2) 72 FACET by Lab-Volt
Based on a total circuit voltage of -4 Vdc, which statement below is correct? a. The total voltage given is correct because the batteries are series-aiding. b. The total voltage given is correct because the batteries are series-opposing. c. Meter polarity is not given; therefore, you cannot determine total voltage. To determine total circuit voltage, the voltage values of V5 and V6 must be subtracted from each other. Why? a. V5 and V6 are series-opposing. b. V6 is variable and its voltage value is added to that of V5. c. V6 opposes the combined V5 and meter-generated voltages. Activate the circuit LEDs with a two-post connector. Adjust V6 until both LEDs are off. Why are the circuit LEDs off while V5 and V6 apply power to the circuit? a. V5 and V6 are about the same value and are series-opposing. b. V5 and V6 are series-opposing and place 11.2 Vdc across the LEDs. c. V5 and V6 are series-aiding and place 11.2 Vdc across the LEDs. NOTE: V5 and V6 are connected in series-opposing. The circuit YELLOW LED will light for a positive total circuit voltage. The GREEN LED will light for a negative total circuit voltage. Adjust V6 (the positive variable supply) controls to the maximum clockwise position. Why is the YELLOW LED on? a. V6 is higher in value than V5, generating a positive differential voltage. b. V6 is lower in value than V5, generating a negative differential voltage. c. V6 and V5 are now series-aiding. FACET by Lab-Volt 73
DC Fundamentals Adjust V6 (the positive variable supply) controls to the maximum counterclockwise position. Why is the GREEN LED on? a. V6 is higher in value than V5, generating a positive differential voltage. b. V6 is lower in value than V5, generating a negative differential voltage. c. V6 and V5 are now series-aiding. CONCLUSION When two batteries are connected in series-opposing circuits, the total circuit voltage equals the difference between each battery voltage. LEDs were used to determine the polarity of total circuit voltage produced by series-opposing batteries. REVIEW QUESTIONS 1. A series-opposing circuit is made up of two batteries connected a. positive to negative. b. negative to positive. c. negative to negative and positive to positive. d. either negative to negative or positive to positive. 2. In a series-opposing circuit, the total voltage equals the a. difference between individual battery voltages. b. sum of the individual battery voltages. c. way the batteries are connected. d. types of batteries used. 3. Two voltage sources of equal value are series-opposing. What is the output voltage? a. two-thirds b. double c. one-half d. zero 4. Three 1.5V dry cells are connected as shown. How much voltage is available at the output terminals? a. 0 Vdc b. 1.5 Vdc c. 3.0 Vdc d. 4.5 Vdc 74 FACET by Lab-Volt
5. If the overall circuit potential difference in the circuit shown is +5.20 Vdc, the value of battery V6 is a. +0.4 Vdc. b. +5.6 Vdc. c. +10.8 Vdc. d. +12 Vdc. FACET by Lab-Volt 75