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To design a Series Type Ohmmeter
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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.