ECE lab9 Rheostats and Potentiometers PDF

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Temple University College of Engineering Department of Electrical and Computer Engineering

Lab Report Cover Page ECE 2113 Section 005 Experiment # 9: Rheostats and Potentiometers Nelson Chan Alan Henry

TuID# 915414898 TuID# 914301616 November 5, 2018

I.

Introduction

Resistors are used as adjusting or controlling elements for signals in mechanical, electronic, and electrical devices such as the volume of sound, intensity of light, and the temperature of heat. There are two kinds of resistors: Potentiometer which is also known as “POT,” a resistor with three terminals and a sliding contact with an adjustable voltage divider. Rheostat which is a variable resistor with two terminals. Potentiometers are conceptually equivalent to rheostats, an older circuit element constructed by placing a sliding contact (S) on a long uniformly wrapped coil of resistive wire. The resistance of the whole coil (between the end contacts 1 and 2) is a constant value, but that resistance is divided into two pieces by the sliding contact. This has the effect of creating two variable resistors, one between contacts 1 and S, the other between contacts 2 and S. A virtue of the rheostat is that it produced a linear dependence of the variable resistance on the distance that the sliding contact had traveled. This was usually controlled by a rack and pin type arrangement, which convert the turns of a knob into linear motion and hence resistance. Variable resistors can be made even smaller than this, but they become difficult to manipulate. Analogous to the rheostat, a potentiometer has three contacts and a knob. The resistance across two of the contacts will be a constant value and the third contact will produce the variable resistance with reference to the others. Alternatively, a potentiometer can be used alone as a variable voltage supply. The working of POT is the same as that of a variable resistor. The construction is also the same. It has a resistive element as the track and a sliding contact called the wiper and it is connected with the help of another sliding contact to another terminal. The position of the wiper depends on the type of potentiometer used. The resistive element has a terminal on both the ends and can be linear or logarithmic. There are single turn POTs which changes its entire resistance in one rotation while more accurate potentiometers called multi-turn POTs are also present. They only need about 20 to 30 rotations to change the entire resistance which makes them much more accurate. A rheostat is also a variable resistor and is a 2-terminal device. It is commonly used for handling higher currents and voltages while one terminal will be connected to the end of the track and the other to a moveable wiper. When the wiper moves from one end to the other, the resistance changes from zero to maximum. A rheostat can be made from a potentiometer as the same mechanism is used except that the terminal that is not used will be connected to the wiper. This helps in reducing the variation in resistance. II.

Procedure

Chapter 8 Part 1 Potentiometer Characteristics (a) The basic construction and electrical symbol for a potentiometer are shown on Figure 1.1.

Figure 1.1

(b) Connect an ohmmeter across the outside two terminals of the 1 kΩ POT as shown on Figure 1.2 with the wiper arm, terminal B, left open. Then turn the control knob as far as it will go clockwise direction and record the resistance.

Figure 1.2

(c) Turn the control arm in the counterclockwise direction as far as it will go and record the resistance. (d) Then turn the control arm to any position between the two maximum and minimum resistance and record the resulting resistance. (e) Connect the ohmmeter to an outside terminal and the wiper arm shown on Figure 1.3 and turn the knob fully clockwise and record the resistance. Then turn the knob fully counterclockwise and record its resistance. Determine and record the full range of the resistance between the center contacts and one outside contact.

Figure 1.3

(f) Maintain the connection to the wiper arm but switch the outside terminal from one end to another. Turn the control knob to the full clockwise position and record the resistance. Then turn the knob fully counterclockwise and record the resistance. Determine the full range of resistance for these connection. (g) Measure and record the resistance with the wiper arm set to any random position between the full Cw and the full CCW position. Part 2 Potentiometer Control of Potential (Voltage) Levels (a) Construct a circuit show on Figure 1.4. Set the voltage to 10 V. Turn the control arm and record the range of voltage.

Figure 1.4

(b) Adjust the wiper arm until the voltmeter reads 5 V. Then turn off the power supply and measure and record the resistance between the two terminals of the potentiometer.

(c) Repeat step (b) with voltage of 3.8 V. (d) Construct a circuit shown on Figure 1.5 and set the voltage to 5 V, then connect a 100 kΩ. Measure and record the voltage across the 100 kΩ.

Figure 1.5

(e) Repeat step (d) with 10 kΩ, 1 kΩ and 470 Ω resistors. Part 3 Using the Potentiometer as a Rheostat. (a) Construct a circuit shown on Figure 1.6 and set the supply voltage to 12 V using the DMM voltmeter. Then connect the voltmeter acorss the 470 Ω resistor and find the minimum and maximum voltage using the control arm of the rheostat and record the voltage.

Figure 1.6

(b) Set the output voltage to it minimum value and turn off the power supply. Remove the POT and measure and record the terminal resistance. Using rheostat resistance value and the measure value of 470 Ω, calculate the output voltage using a 12 V power supply. (c) Repeat step (b) using a 1 kΩ, 10 kΩ resistor and 100 kΩ. III.

Results and Discussion Chapter 8: 1B.

Conclusion: Between the the two outside terminals, the resistance remains the same regardless of wiper position 1C-1D.

C. Turning the wiper to half rotation generates approximately half the maximum resistance. There is a linear relationship between the resistance and wiper arm rotation.

D. The full range is the same as the range using the original connections. Conclusion: The resistance range between both the outside terminal and center terminal is the full resistance. 1E.

Conclusion: The resistance between the center and outside terminals will always equal the maximum resistance. 2A.

The resistance is about half the total voltage. 366/960*100=38.1% → Yes, 38% of total resistance matches 38% of total voltage. Conclusion: Any voltage across the outside terminal can be set from zero and the applied voltage between the center and outside terminals by adjusting the wiper. 2B.

Voltage drop with 100kΩ: 0.02V Can be ignored for most applications Voltage drop with 10kΩ: 0.07V Can be ignored for most applications Voltage drop with 1kΩ: 0.98V Change cannot be ignored (% difference>20%) Voltage drop with 470Ω: 1.99V (39.8%) Conclusion: The resistance level should be at least 10x the max. potentiometer level when applying a load. This allows you to have a close values between available

voltage

and resistance level. 3A. Terminal resistance (theoretical): 0Ω so there is no voltage drop Terminal resistance at minimum: full 1kΩ so there is maximum voltage drop

VL=(461Ω*11.9V)/(461Ω+1013Ω)=3.72V This is very close to the VL (min.) value of 3.73V Conclusion: A potentiometer used as a rheostat can control voltage but cannot control very low voltages because the minimum is sensitive to the applied load and max rheostat resistance. Results for 1kΩ are as expected. Since the load resistance is close to the max., VL is expected to have a min. value of about half the applied voltage. Results for the 10kΩ are as expected. Since the range is significantly less, the load resistor should increase. Results for the 100kΩ are as expected. Since the min. is close to the max. available voltage, voltage control is mostly gone. Conclusion: Larger ratio of applied load to max. rheostat resistance=less ability to control. Max. resistance should be at least 10x greater than applied load for best control. IV. Conclusion For this lab, we looked at potentiometers. In Part 1, we looked at the resistances of the terminals. We found that, for the outside terminals, the resistance remains the same regardless of the position of the wiper. Between the outside and center terminals, the resistance ranged between about zero and the full range of the outside terminals, the value of which corresponded to how much the wiper was rotated. We also found that the sum of the center and outside terminals were equal to the maximum resistance (full range). In Part 2, we looked at controlling voltage and found that the percent of total resistance was the same as the percent of total voltage. We also looked at how much resistance was needed for a voltage drop to be insignificant when applying a load. In Part 3, we looked at using a potentiometer as a rheostat. Our results were as expected, and we found that as the resistance increases, the voltage range decreased. This indicated decreasing ability to control the voltage....