BME/ISE 3511 Bioelectronics I - Laboratory Exercise #4 Variable Resistors (Potentiometers and Rheostats) Introduction: Variable resistors are known by several names (potentiometer, rheostat, variable resistor, trim pots). Technically speaking, a three-terminal resistor with a sliding contact (wiper) configured for use as an adjustable voltage divider is referred to as a potentiometer. A two-terminal resistor configured for use as a variable resistor (current control) is referred to as a rheostat. Potentiometers are often used as volume controls in audio equipment or as position transducers as in joysticks or servo motors. The wiper may move in a circular path (typically less that 360 ) or in a straight line. The resistive media may be made from various materials such as carbon composite or even a wire-wound coil. The relationship between the wiper position and the resulting resistance is referred to as the taper. The taper is generally either linear or logarithmic. Logarithmic tapers mimic the loudness sensation experienced by human hearing and are often referred to as audio taper. Linear tapers with straight line wipers are often called faders. Potentiometers are commonly available in ranges from 25 ohms to 10M ohms. The power rating is usually less than one watt. Rheostats are often high current devices and range in values from 10 ohms to 100K ohms and from power ratings from 1 to over 100 amps. Objectives: Become familiar with the basic characteristics of potentiometers and rheostats. Laboratory Equipment and Supplies: Background: DMM's DC Variable Power Supply Fixed and Variable Resistors Breadboard Refer to the Digital Multimeter DMM Model 72-7940 Functions, Ranges, Resolutions, and Accuracy (page 4 of the BME/ISE 3511 Bioelectronics I - Laboratory Exercise #1 procedures guidelines) for specifications and identification of the various functions and range of measurement values. Refer to the DMM itself for examples of the function labels; starting at the top center and moving clockwise: Off, AC Voltage, DC Current, Square Wave Output, Battery Tester, Diode Test, Resistance, and DC Voltage. AC current measurement is not supported. Note: The maximum allowable DC current is 200 ma. Page 1 of 4.
Resistance, Voltage, and Current Measurements Lab Procedures: Caution: VERY IMPORTANT for your safety and the condition of the equipment. Before making any voltage measurements, be sure the DMM function switch is set to the VDC Function. Before making any resistance measurements, be sure the power supply is turned off and disconnected from the circuit and the DMM function is set to the Resistance Function. Potentiometer See Figure A, page 4, for an illustration of a variable resistor configured as a potentiometer. Note the nominal resistance value can be expressed as the sum of two resistors R 1 + R 2, which forms a voltage divider, such that V out = V s [ R 2 / (R 1 + R 2 ) ]. Adapt the following convention when viewed from the shaft side with the terminals A, C, B, as indicated. Full clockwise rotation should yield minimum resistance (and hence minimum voltage) between terminals A & C; and maximum resistance (and hence maximum voltage) between terminals C & B. Likewise full counterclockwise rotation should yield maximum resistance between terminals A & C; and minimum resistance between terminals C & B. Use the DMM to measure the resistance between points A & B for full CW and for full CCW, record this value, Table 1. Use the DMM to measure the resistance between points A & C at maximum and minimum values and between C & B at maximum and minimum, record all values, Table 1. Calculate maximum circuit current for V s = 5 volts DC at minimum resistance A to B, record your calculation, Table 1, Calculated Values. Construct a simple voltage divider potentiometer circuit as in Figure A. Pre-set potentiometer to full counterclockwise rotation. Set the DC power supply to 5 VDC; record the power supply ammeter reading (circuit current). Adjust the potentiometer rotation from full counterclockwise to ¼, ½, ¾, and full clockwise rotation: record resistance, current, and voltage readings, Table 2, Potentiometer Values. Use the measured resistance readings to calculate V out = V s [ R cb / (R ac + R cb ) ], Table 2. Page 2 of 4.
Rheostat See Figure B, page 4, for an illustration of a variable resistor configured as a rheostat. Note the resistance value is equal to the proportion of the nominal value that is contained between the two active terminals. Therefore the range of resistance value can vary from a maximum equal to the nominal value to zero; hence the need for the current limiting resistor R 1. Adapt the following convention when viewed from the shaft side with the terminals A, C, B, as indicated. Full clockwise rotation should yield minimum resistance (i.e., maximum current) between terminals A & C; and full counterclockwise rotation should yield maximum resistance (i.e., minimum current between terminals A & C. Note, terminal B is not a factor. Measure and record the value for R 1, Table 1. Calculate maximum circuit current for V s = 5 volts DC at minimum resistance (including R 1 ), record your calculation, Table 1. Calculate minimum circuit current for V s = 5 volts DC at maximum resistance (including R 1 ), record your calculation, Table 1. Construct a simple rheostat circuit (with current limiting resistor R 1 ) as shown in Figure B. Pre-set rheostat to full counterclockwise rotation. Set the DC power supply to 5 VDC; record the power supply ammeter reading (circuit current). Adjust the rotation from full counterclockwise to ¼, ½, ¾, and full clockwise rotation: record resistance, current, and voltage readings, Table 3, Rheostat Values. Conclusions: You must annotate any references you consulted in answering the following questions. You may wish to use a word processor to complete your answers and attach the printout to your Potentiometer: Discuss any major discrepancies between your predicted/expected values and your measured values. Explain (you may use a schematic if you wish) how to configure the circuit so that a full clockwise rotation would result in maximum output voltage and full counterclockwise rotation would result in minimum output voltage. Explain why only the voltage changes but not the current when rotating the potentiometer shaft. Rheostat: Discuss any major discrepancies between your predicted/expected values and your measured values. Explain (you may use a schematic if you wish) how to configure the circuit so that a full counterclockwise rotation would result in minimum circuit current and full clockwise rotation would result in maximum circuit current. Explain why both the voltage and the current change when rotating the rheostat shaft. Page 3 of 4.
BME/ISE 3511 Bioelectronics I Laboratory Exercise #4 Report Form Variable Resistors (Potentiometers and Rheostats) I affirm that I personally participated in the collection and analysis of the data for this laboratory exercise and that I personally contributed to the completion of this laboratory report. Student Name: Signature: Date: I affirm that I personally participated in the collection and analysis of the data for this laboratory exercise and that I personally contributed to the completion of this laboratory report. Student Name: Signature: Date: Grade: Grader Comments:
Potentiometer Rheostat V S V S A to B Full CCW R 1 A to B Full CW R var A to C (Full CCW) A to C Min R var A to C (Full CW) A to C Max I s Min C to B Min I s Max C to B Max I s Max (A to B) Table 1 Measured and Calculated Values BME/ISE 3511 Bioelectronics I Laboratory Exercise #4 Report Form Position V s I s Resistance Voltage Vout Full CCW ¼ Rotation ½ Rotation ¾ Rotation Full CW Table 2 Potentiometer Resistance, Current, and Voltage Values Position V s I s Resistance Voltage A to C A to C Full CCW ¼ Rotation ½ Rotation ¾ Rotation Full CW Table 3 Rheostat Resistance, Current, and Voltage Values A to B A to C C to B A to B A to C C to B