KING ABDULAZIZ UNIVERSITY

THE COLLEGE OF ENGINEERING, THE KINGDOM OF

SAUDI ARABIA

EE303

ELECTRICAL MEASRMENT & INSTRMENT SPRING 2013

EXPERIMENT # 6

Series Type Ohmmeter

GROUP # 4

Team Member Group ID Section

Fahad Mohammad Al-Jdanni 1008538 DA

Faisal Alawi Baroom 1007396 DA

Mazen Almuqati 1007539 DA

Abdulaziz Hammouda 1007055 DA

Lab. Instructor: Mohammad Mottahir

Experiment Date: 2. April. 2013

Lab. Time: Sunday 11:00 1:00

1

Experiment No.6

Introduction

This lab will be about design ohmmeter. The ohmmeter is an instrument used to measure

resistance and check the continuity of electrical circuits and component. Two types of

schemes are used to design an ohmmeter series type and shunt type. The series type of

ohmmeter is used for measuring relatively high values of resistance, while the shunt type

is used for measuring low values of the resistance. The type of ohmmeter we will design

in this lab is series type ohmmeter.

Objective The objective of this experiment is to construct an elementary series-type ohmmeter.

Theory

Ohmmeter

Two instruments are commonly used to check the continuity or to measure the

resistance of a circuit or circuit element. These instruments are the ohmmeter and the

megger, or megohmmeter. The ohmmeter is widely used to measure resistance and to

check the continuity of electrical circuits and devices. Its range usually extends to a

few megohms. The megger is widely used for measuring insulation resistance, such

as the resistance between the windings and the frame of electric machinery, and for

measuring the insulation resistance of cables, insulators, and bushings. Its range may

extend to more than 1,000 megohms. When measuring very high resistances of this

nature, it is not necessary to find the exact value of resistance, but rather to know that

the insulation is either above or below a certain standard. When precision

measurements are required, some type of bridge circuit is used. Ohmmeters may be of

the series or shunt type.

Series-type Ohmmeters

A simplified schematic of an ohmmeter is shown in figure 1. E is a source of EMF;

R1 is a variable resistor used to zero the meter; R2 is a fixed resistor used to limit the

current in the meter movement; and A and B are test terminals across which the

resistance to be measured is placed. If A and B are connected together (short

circuited), the meter, the battery, and resistors R1 and R2 form a simple series circuit.

With R1 adjusted so that the total resistance in the circuit is 4,500 ohms, the current

through the meter is 1 ma. and the needle deflects full scale. Since there is no

resistance between A and B, this position of the needle is labeled zero (figure 8-138).

If a resistance equal to 4,500 ohms is placed between terminals A and B, the total

resistance is 9,000 ohms and the current is 0.5 ma. This causes the needle to deflect

half scale. This half scale reading, labeled 4.5 K ohms, is equal to the internal

resistance of the meter, in this instance 4,500 ohms. If a resistance of 9,000 ohms is

2

placed between terminals A and B, the needle deflects one-third scale. Resistances of

13.5 K and 1.5 K placed between terminals A and B will cause a deflection of one-

fourth and three-fourths scale, respectively. If terminals A and B are not connected

(open circuited), no current flows and the needle does not move. The left side of the

scale is, therefore, labeled infinity to indicate an infinite resistance. A typical

ohmmeter scale is shown in figure 8-138. Note that the scale is not linear and is

crowded at the high resistance end. For this reason, it is good practice to use an

ohmmeter range in which the readings are not too far from mid scale. A good rule is

to use a range in which the reading obtained does not exceed ten times, or is not less

than one-tenth, the mid scale reading. The useful range of the scale shown is, by this

rule, from 450 ohms to 45,000 ohms. Most ohmmeters have more than one scale.

Additional scales are made possible by using various values of limiting resistors and

battery voltages. Some ohmmeters have a special scale called a low ohm scale for

reading low resistances. A shunt-type ohmmeter circuit is used for this scale.

Figure 1. ohmmeter circuit

Circuit diagram

Circuit diagram for experiment 6 is shown in figure 2.

Figure2

3

Lab Safety:

Before moving ahead, we have to take a look at the following instructions that we should

follow it during the lab time.

General safety instructions

1. Move carefully in the lab. 2. No eating, no drinking and no smoking in the lab. 3. Use appropriate and available safety precautions and tools. 4. Never use chairs or boxes to reach to high places. 5. Concentrate to your experiment and equipment. 6. Don't keep any liquid close or on-top of any electrical device. 7. While doing experiment , be sure of the followings : Connect circuit wiring carefully, let lab engineer check it. Keep away any wire or equipment not used. If you need to do any change in your circuit, switch power off, do necessary

modifications, double check it, then switch power on. Component and hand should be dry while doing the experiment. If you got unexpected results ask assistance of responsible. Never touches or play with denuded wires or cables. After finishing the experiment, switch-OFF the equipment then botches put back all

components and wires in their places.

Safety rules of electrical laboratories: 1. Proper the grounding for lab .Power-Supply and equipment. 2. Fire extinguisher fixed in appropriate places. 3. Emergency exit signs for any emergency case. 4. Coincidence of power cables /plugs and current loads regular check of cables and

wires insulation. 5. Cables should be in insulated trunks. 6. Warning tags for high voltage or radiating equipment. 7. First aid kit in appropriate place and well equipped.

Equipments:

1mA meter movement DC Power supply Two variable resistor boxes Wires for connection

4

Steps:

1. Connect the positive terminal of the power supply to the variable resistor box (RB1).

2. Connect the negative terminal from the variable resistor box (RB1) to the positive of

the 1mA meter movement. Set the resistor box to be 10K.

3. Connect the circuit by connecting the negative terminal of the 1mA meter movement

to the negative terminal of the power supply

4. Adjust the power supply to 10 volts. Note that the meter movement will not give 1mA

exactly which means there is internal resistor of meter movement.

5. Decrease the variable resistor box (RB1) until the meter movement reach of full scale

deflection (1mA).

6. Connect another variable resistor box (RB2) parallel to the 1mA meter movement.

7. Increase the variable resistor box (RB2) until the pointer of 1mA meter movement

become in the half of the full scale deflection (0.5mA). Note the value of the second

variable resistor box (RB2) represents the internal resistor value (Rm).

8. Adjust the power supply to 5 volt.

9. Calculate RT (total resistance) by using the law: RT = = 5000.

10. Calculate RR (range resistance) by using the law: RT = RR + Rm.

11. Adjust the variable resistor box (RB1) to value of RR.

12. Connect variable resistor box (RB2) series with variable resistor box (RB1).

13. Calculate Rx (the resistance that to be measured) by using the law: D= , D is

relative deflection and it is given in each row of the result table.

14. Adjust the variable resistor box (RB2) to value of Rx.

15. Measure the relative deflection of series type ohmmeter (that was 1mA meter

movement).

16. Calculate the percentage of error by using: % error =

.

5

Result & Calculation The ammeter resistance was measured by:

Rm = (166) RR = 5000- Rm

= 5000 - (166) = 4834 Relative deflection (D) was given

Deflection % was calculated as : Relative deflection (D) 100

Rx = RT (( ) , where RT : is total resistance

error = | | 100

Example in how the calculation was done when D= 0.2 as shown :

Deflection % = (0.2) 100 = 20%

Rx= 5000 (( ) = 20000

By observing the meter, the percentage of the deflection measured as 18.5%

error = | | 100 = 7.5%

Then, the following table (Table.1) was completed in the same manner Mentioned above

Table.1 : Results of the experiment

Error % Deflection%

Measured RX Deflection %

Relative

deflection (D)

0% 0% 0% 0.0

0% 10% 45000 10% 0.1

7.5% 18.5% 20000 20% 0.2

6.67% 26% 11667 30% 0.3

5% 38% 7500 40% 0.4

3% 48.5% 5000 50% 0.5

3.33% 58% 3333 60% 0.6

4.29% 67% 2143 70% 0.7

1.25% 79% 1250 90% 0.8

1.11% 91% 556 90% 0.9

2% 102% 0 100% 1

6

Comment

RR is calculated then used to build the Ohmmeter circuit

Rx represent the unknown resistance to be measured.

It is clear that the obtained values of the deflection percentage aren't 100% correct, it has

some error.

Conclusion

After finishing this experiment, and after we have learned that the internal resistance of

any measurement device effects in the readings from the previous experiments, we used

in this experiment the internal resistance and variable resistances in order to help us to

build a series ohmmeter measurement device. Although in this experiment there are some

errors in the readings, it still extremely helpful in order to let students know about

manipulating in the devices functions. Finally, we hope that this report gained the

acceptance.