Potentiometers and Rheostats DC Fundamentals Exercise 1: The Rheostat EXERCISE OBJECTIVE When you have completed this exercise, you will be able to vary current by using a rheostat. You will verify your results with a multimeter. DISCUSSION through that load. When R2 is in the fully CCW position (maximum resistance in the circuit), total circuit resistance equals R1 + R2, or 300. 226 FACET by Lab-Volt
DC Fundamentals Potentiometers and Rheostats When R2 is in the fully CW position (minimum resistance in the circuit), total circuit resistance equals R1, or 100. The power dissipated in each element shown is determined by I T 2 x R1 and I T 2 x R2. All power equation forms can be applied to the circuit elements: 2 V R / R V R x IT With R2 set to the full CW position, power dissipation in load R1 is a. maximum. b. 0. c. minimum. With R2 set to the full CCW position, power dissipation in load R1 is a. maximum. b. 0. c. minimum. FACET by Lab-Volt 227
Potentiometers and Rheostats DC Fundamentals PROCEDURE The LINEAR/NONLINEAR VARIABLE RESISTOR circuit block consists of two variable resistors, R2A and R2B. S1 selects either R2A (a linear pot) or R2B (a log pot). resistors. Locate the LINEAR/NONLINEAR VARIABLE RESISTOR circuit block, and connect the circuit shown. Set S1 to R2A to select the linear potentiometer. On your circuit, measure the resistance value of R1. R1 = (Recall Value 1) 228 FACET by Lab-Volt
DC Fundamentals Potentiometers and Rheostats On your circuit, measure the resistance of R2A at 50%. R2A at 50% = (Recall Value 2) On your circuit, measure the resistance of R2A at 100%. R2A at 100% = (Recall Value 3) Energize your circuit block. Measure the voltage of R1 with R2A set for 0% (V R2A is 0 Vdc with R2A set for 0%). V R1 = Vdc (Recall Value 4) Measure the voltage of R1, with R2A set for 50% (midpoint). V R1 = Vdc (Recall Value 5) Measure the voltage of R2A, with R2A set for 50% (midpoint). V R2 at 50% = Vdc (Recall Value 6) Measure the voltage of R1, with R2A set for 100% (full value). V R1 = Vdc (Recall Value 7) FACET by Lab-Volt 229
Potentiometers and Rheostats DC Fundamentals Measure the voltage of R2A, with R2A set for 100% (full value). V R2 at 100% = Vdc (Recall Value 8) Based on your data recalled below, circuit a. voltages change because the rheostat affects total resistance and current. b. voltages change because the potentiometer affects only circuit current. c. current does not change; only resistance and voltage change. R2A V R1 V R2A 0% Vdc (Step 5, Recall Value 4) 0.00 Vdc 50% Vdc (Step 6, Recall Value 5) Vdc (Step 7 Recall Value 6) 100% Vdc (Step 8, Recall Value 7) Vdc (Step 9, Recall Value 8) In your circuit, a CW shaft rotation of R2A a. decreases total resistance and decreases circuit current. b. increases total resistance but does not affect circuit current. c. increases total resistance and decreases circuit current. In your circuit, when the value of R2A is over 910 ohms, rheostat power dissipation is: a. not affected by the value of R2A. b. inversely related to the value of R2A. c. directly related to the value of R2A. Which rheostat setting produces the maximum power dissipation in the circuit load? a. maximum CW (maximum rheostat resistance) b. maximum CCW (minimum rheostat resistance) c. a midpoint setting of the rheostat CONCLUSION A rheostat varies total circuit resistance and controls total circuit or load current. A rheostat controls the total current through a circuit load and, therefore, determines the power dissipation of the load. REVIEW QUESTIONS 1. A rheostat in an electrical circuit primarily controls the load a. voltage. b. current. c. resistance. d. All of the above. 2. When using a carbon-composition pot as a rheostat, you should a. connect it in series. b. connect it in parallel. c. connect only the two end terminals. d. use CCW rotation to decrease circuit current. 230 FACET by Lab-Volt
DC Fundamentals Potentiometers and Rheostats 3. In this circuit, with R2A set at 50%, total circuit resistance is a. 3410. b. 2500. c. 2160. d. 1250. 4. In this circuit, with R2 set at 1200, the load current is a. 4.54 ma. b. 6.25 ma. c. 7.89 ma. d. 8.33 ma. 5. In this circuit, if an ammeter were connected between R1 and R2, how much current would be indicated if R2 were set to the maximum CCW position? a. 16.66 ma b. 8.33 ma c. 6.25 ma d. 5.68 ma FACET by Lab-Volt 231