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Lab #3 Potentiometers and Rheostats1 Name:________________________________________ Objective The objective of this exercise is to examine the practical workings of adjustable resistances, namely the potentiometer and rheostat. Their usage in adjustable voltage and current control will be investigated. Theory Overview A potentiometer is a three terminal resistive device. The outer terminals present a constant resistance which is the nominal value of the device. A third terminal, called the wiper arm, is in essence a contact point that can be moved along the resistance. Thus, the resistance seen from one outer terminal to the wiper plus the resistance from the wiper to the other outer terminal will always equal the nominal resistance of the device. This three terminal configuration is used typically to adjust voltage via the voltage divider rule, hence the name potentiometer, or pot for short. While the resistance change is often linear with rotation (i.e., rotating the shaft 50% yields 50% resistance), other schemes, called tapers, are also found. One common non-linear taper is the logarithmic taper. It is important to note that linearity can be compromised (sometimes on purpose) if the resistance loading the potentiometer is not significantly larger in value than the potentiometer itself. If only a single outer terminal and the wiper are used, the device is merely an adjustable resistor and is referred to as a rheostat. These may be placed in-line with a load to control the load current, the greater the resistance, the smaller the current. Equipment (1) IDL-600 Analog Trainer (1) Digital Multimeter (1) 1 kΩ potentiometer (1) 100 Ω resistor 1 DC Electrical Circuits Laboratory Manual, James M Fiore (copyrighted under the terms of a Create Commons License) – Modified for ET 207 Equipment 1 Schematics Figure 9.2 Fig 9.1 Procedure 1. A typical potentiometer is shown in Figure 9.1. First Measure the resistance from terminal 1 to terminal 3 and record. Using a 1k pot, first rotate the knob fully counter-clockwise and using the DMM, measure the resistance from terminal 1 to the wiper arm (2). Then measure the value from the wiper arm (2) to terminal 3. Record these values in Table 9.1. Add the two readings, placing the result in the final column. 2. Rotate the knob 1/4 turn clockwise and repeat the measurements of step 1. Repeat this process for the remaining knob positions in Table 9.1. Note that the results of the final column should all equal the nominal value of the potentiometer. 3. Construct the circuit of Figure 9.2 using V = 2 volts, a 1 k potentiometer with a 100Ω resistor in series. Rotate the knob fully counter-clockwise and measure the voltage from the wiper to ground, Wiper to top side of Pot and across the 100Ω resistor. Record these values in Table 9.2. Continue taking and recording voltages as the knob is rotated to the other four positions in Table 9.2. Data Tables Resistance of Pot from term 1 to terminal 3: ________________________________________ Position Full CCW ¼ ½ ¾ Full CW R1,2 R2,3 R1,2+R2,3 Table 9.1 Power supply voltage (measured -> aprox 2V):___________________________________ Position Full CCW ¼ ½ ¾ Full CW VR1 V1,2 Table 9.2 2 V2,3 I Questions 1. In Table 9.1, does the total resistance always equal the nominal resistance of the potentiometer? Speculate why this is so? 2. If we treat R1,2 and R2,3 as two separate resistors, what would the schematic look like for the potentiometer? 3. In Table 9.2, is the load voltage always directly proportional to the knob position? Is the progression linear (show the graph, Position vs V1,2 and Position vs V2,3)? 4. Does the VR1+V1,2+V2,3 equal the power supply voltage? 5. Using I, V form Table 9.2, calculate the Resistance for each row using Ohm’s law: Position Full CCW ¼ ½ ¾ Full CW R1 R1,2 R2,3 Table 9.3 6. Now graph R1,2 from table 9.3, vs V1,2 from table 9.2 and R2,3, from table 9.3 vs V2,3 from table 9.2. 7. Since the Voltage is a constant (2V) and the measured current is approximately the same for each row in table 9.2, what can we say about the total resistance of the circuit? What is that resistance? 3 Fig 9.3 - Position vs V1,2 Fig 9.4 - Position vs V2,3 4 Fig 9.5 - R1,2 vs V1,2 Fig 9.6 - R2,3 vs V2,3 5