Rheostats and Potentiometers

20/07/2013 12:43


Author Name:                              Vasilis Leandrou

 

OBJECTIVES

This report aims to:

  • Recognize the final features of a potentiometer.
  • Know how to use a potentiometer and have control over potential levels.
  • Get the knowledge of using a potentiometer levels.
  • Realizing when a potentiometer is being used in the form of a straight line or nonlinear control element. 

BACKGROUND THEORY

In the experiments will be used the DMM, a linear carbon potentiometer, rheostat and power supply to measure the potentiometer level.    

 

·         The DMM read resistance, voltage and current with a digital display.

·         Power supplies gives a source of power, rated as maximum voltage and current output

·         The potentiometer is a three terminal device that is used foremost to control potential voltage levels. The control can be a straight line or nonlinear and can stretch from minimum to maximum available voltage.

·         The rheostat is a two terminal variable resistance apparatus whose final characteristics can also be a straight line or non linear. It is used in series with the load to control the voltage and current levels. 

 

EQUIPMENT

 

·         Digital Multimeter                   (Brand: Good Will Instruments Co. Ltd, Model: GDM-8135, Serial Number: CF-922334)

·         Power supply

·         Linear carbon potentiometer

·         Resistors 470Ω, 1ΚΩ, 10ΚΩ, 100ΚΩ

EXPERIMENTAL METHOD AND PROCEDURE

Part 1

The DMM was connected to the two outside terminals of the potentiometer. The measure values of the potentiometer were taken (table 1.1). Then the DMM was connected to the one terminal and the center wiper arm. The measure values of the potentiometer were taken (table 1.2).

Part 2

The power supply of 10V was connected with the potentiometer of 1ΚΩ resistance and to the DMM. The measure of 5V and 3.8V was taken (table 2.1). Then a parallel resistor of different value was adding between the potentiometer and to the DMM. The voltage measure of each resistor was taken (table 2.2).

Part 3

The power supply of 10V was connected to the potentiometer witch is used as a rheostat and resistor with different values. The measure of the minimum and maximum volts of different resistors was taken (table 3.1)  

Part 4

The power supply of 12V was connected to the DMM, to the potentiometer of 1ΚΩ resistance and to a 470Ω resistor. The measure of the resistance using the potentiometer as a rheostat and the volts witch pass throw the 470Ω resistance changing the values of the current was taken (table 4.1).

OBSERVATIONS

Table 1.1 outside terminal

R(full CW)

R(full CCW)

R(random)

970Ω

970Ω

970Ω

 

Table 1.2 one outside terminal and the center wiper arm

R(full CW)

R(full CCW)

R(full range

R(CW-other terminal)

R(CCW-other terminal)

R(full range-other terminal)

9Ω

980Ω

971Ω

9Ω

980Ω

971Ω

 

Table 2.1 potential voltage levels

V(range)

R(5V)

R(3.8V)

0-10V

518Ω

3.98Ω

 

Table 2.2 potentiometer and resistors

V(100ΚΩ)

V(10ΚΩ)

V(1ΚΩ)

V(470Ω)

4.99V

4.87V

3.91V

3.16V

 

Table 3.1 minimum and maximum values

R

V(minimum)

V(maximum)

R(meas. Vminimum)

V(calculate)

470Ω

3,86V

11,62V

987Ω

8,13v

1ΚΩ

6,01V

11,93V

-

-

10ΚΩ

11V

12,07V

-

-

100ΚΩ

11,92V

12,09V

-

-

 

Table 4.1 current control with a rheostat

I(470Ω)

Rrheostat

V(470Ω)

I=V/R(470Ω)

I measure

21mA

0Ω

10V

0,02Α

21mA

18mA

78,6Ω

8,56V

0,018Α

18,3mA

14mA

244Ω

6,57V

0,014Α

14,04mA

10mA

527Ω

4,53V

0,01Α

10,02mA

8mA

781,25Ω

3,75V

0,007Α

8,01mA

 

 

Data discussion

On the table1.1 the resistance between the two outside terminals of a potentiometer as the wiper arm is moved to any position between the two is not change. On the table1.2 the range resistance between one outside terminal and the center wiper arm is increase at the same rate as the resistance between the other outside terminal and the center wiper arm is decrease.

  On the table2.1 in regard to the control of voltage levels by a potentiometer as the voltage increase the resistance increase. On the table 2.2 when the resistance decreases the voltage decrease too.

On the table 3.1 the minimum and the maximum value of voltage each different resistor.

On the table 4.1 as the amperes decrease the resistance of the rheostat increase as a result the voltage of the 470Ω is decrease.

 

RECOMMENDATIONS

 

The resistance between the two outside terminals of a potentiometer as the wiper arm is moved to any position between the two is not change. The resistance is 970Ω in any position of the potentiometer (table1.1). The range resistance between one outside terminal and the center wiper arm is increase at the same rate as the resistance between the other outside terminal and the center wiper arm is decrease. The resistance of the potentiometer when is full CW is 9Ω and when is CCW is 980Ω so the range is 971Ω. The opposite values when change the outside terminal CW 980Ω CCW 9Ω (table 1.2).

 

In regard to the control of voltage levels by a potentiometer as the voltage increase the resistance increase, when is 5V the resistance is 518Ω and when is 3.8V the resistance is 398Ω (table 2.1)

 

We can use the potentiometer as a rheostat by using two of its terminals. Using the outside terminals is known that the resistance remains fixed. Using the wiper arm and only the one outside terminal the resistance can be varied through the full range of the potentiometer. When the resistance increases the potentiometer tours more values of voltage.

 

When the rheostat of 1ΚΩ resistance and a resistor seeing that as the resistor has smaller resistance so more different values of voltage it has. When is connect to the resistor of 470Ω the minimum voltage value is 3,86V and the maximum is 11,62V and when is connect the resistor of 10ΚΩ the minimum value is 11V and the maximum is 12,07V (table 3.1)

 

The amperes decrease of a potentiometer of 1ΚΩ resistance to a resistor of 470Ω the resistance of the rheostat is increase but the voltage through the resistor is decrease. When 21mA, the rheostat has not resistance and the voltage through the resistor is 10V but when it is 8mA the rheostat has 781,25Ω resistance and the voltage through the resistor is 3,75V (table 4.1). 

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