Reproduce the Series Type Ohmmeter Using Pmmc of Ifsd = 30 Μa for Range of 0-10 Kω.

UNIVERSITI TEKNIKAL MALAYSIA MELAKA (UTeM)
FALKULTI KEJURUTERAAN ELEKTRONIK & KEJURUTERAAN KOMPUTER (FKEKK)
BACHELOR DEGREE OF ELECTRONIC ENGINEERING (TELECOMUNICATION ELECTRONIC) (BENT)
BENE 1183 ELECTRONIC INSTRUMENTATION

TITLE : SERIES TYPE OHMMETER
SUBJECT : BENE 1183 ELECTRONIC INSTRUMENTATION

1.0 Title:
Reproduce the series type Ohmmeter using PMMC of Ifsd = 30 μA for range of 0-10 kΩ.

2.0 Objectives : 1. To design a series type Ohmmeter using PMMC. 2. To understand the properties of series type Ohmmeter. 3. To design a series type ohmmeter when Ifsd=30μA is given. 4. To calculate the half scale deflection and full scale deflection of the series type ohmmeter from zero to infinity.

3.0 Equipment and component: No. | Equipment | Quantity | No. | Component | Quantity | 1. | DMM CD771(Series no:IN4/04/1260 ) | 1 | 5. | Crocodile clip | 4 | 2. | DC Ammeter(Series no:81BA3893 ) | 1 | 6. | Decade Resistance Box(Series no:VR1/1030/INST ) | 1 | 3. | Power supply | 1 | 7. | Resistor 1.0kΩ | 2 | 4. | Project board | 1 | 8. | Resistor 10kΩ | 1 |

4.0 Theory

Moving Coil Instruments
Moving Coil Instruments (MC): The moving coil instruments can be of permanent magnet type (PMMC) or dynamometer type. The permanent magnet, polarized instruments are used for the measurement of DC voltage/current while the dynamometer type instruments are used for the measurement of power in both DC and AC circuits.

The operation of PMMC instrument is based on the principle that: when a current carrying conductor is placed in a magnetic field, it is acted upon by a force, which tends to move it to one side, out of the field. These instruments consist of a permanent magnet, in the air gap of which a rectangular coil of many turns wound on light former is placed. The coil is supported by bearings and a light pointer is attached to the coil. Deflecting torque is due to the current flowing in the coil. The damping is by eddy currents generated in the core and the controlling torque is by springs. These springs also function as the current leads.

Figure

Ohmmeter 1. An ohmmeter is an instrument used to measure resistance and check the continuity of electrical circuits and component. This resistance reading is indicated through a meter movement.

2. The ohmmeter must then have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow desired current to flow through the movement at any given resistance.

3. Two types of schemes are used to design an ohmmeter – series type and shunt type.

4. 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.

Series type ohmmeter
In this Figure1, R1 is the current limiting resistor, R2 is the zero adjust resistor, RX is the unknown resistor, E is the internal battery voltage and Rm is the internal resistance of the D’Arsonval movement. A and B are the output terminals of the ohmmeter across which an unknown resistor is connected.

Figure1: Basic series type ohmmeter

When RX = 0 (short circuit), R2 is adjusted to get full-scale current through the movement. Then, I = Ifsd. The pointer will be deflected to its maximum position on the scale. Therefore, this full-scale current reading is marked 0 ohms; When RX = ∞ (open circuit), I = 0. The pointer will read zero. Therefore, the zero current reading is marked ∞ ohms.

By connecting different values of RX, intermediate values are marked. The overall accuracy of the scale markings depends on the repeating accuracy of the movement and tolerance of the resistors used for calibration. Figure 2 shows a typical scale of the series type ohmmeter. Note that the scale is logarithmic – “expended” at the low end of the scale and “compressed” at the high end to be able to span a wide range from zero to infinite resistance.

Figure2: Typical scale of series type ohmmeter

To Calculate R1 and R2

R1 and R2 used in Figure 1 can be determined by using a value of RX corresponding to half the deflection of the meter. For the given movement, Ifsd and Rm are known. Let Rh be the half deflection resistance. For this value of RX, I = Ifsd/2.

Further, at half deflection, Rh = internal resistance of the circuit looking from terminals A and B =R1+R2RmR2+Rm

Battery current needed to supply half-scale deflection is given by: Ih=E2Rh

Total current, It, is supplied by the battery for full-scale deflection is double of this current, i.e. It=ERh and I2=It+Ifsd

Using KVL:
I2R2=IfsdRm
Solving these equations, we get: R2=IfsdRmRhE-IfsdRh

Shunt type ohmmeter

Figure 3 shows the basic circuit of the shunt-type ohmmeter where movement mechanism is connected parallel to the unknown resistance. In this circuit it is necessary to use a switch, otherwise current will always flow in the movement mechanism.

Figure3: Basic shunt-type ohmmeter
Resistor Rsh is used to bypass excess current. Let the switch be closed. When RX = 0 (short circuit), the pointer reads zero because full current flows through Rx and no current flows through the meter and Rsh. Therefore, zero current reading is marked 0 ohms. When RX = ∞ (open circuit), no current flows through RX. Resistor R1 is adjusted so that full-scale current flows through the meter. Therefore, maximum current reading is marked ∞ ohms. Comparison of series and shunt ohmmeter scales is shown in Figure 4.

Figure4: Ohmmeter scales: (a) series scale and (b) shunt scale
To Calculate R1 and Rsh Again, we can use the concept of half-scale deflection. Let Rh be the half-deflection resistance. For this value of RX, Im = Ifsd /2. Further, at half deflection, current through Rh is equal to sum of the currents through Rsh and Rm, i.e.
Ix=Ish-Im
Also, Ix=ERh-ImRmRh
Solving for Ish, we get Ish=Im(Rm-RhRh)
Therefore,
Rsh=ImRmIsh
Now,
It=Ish+Im+Ix=2(Ish+Im) =2ImRmRh
Therefore:
R1=E-ImRmIt

5.0 Procedure 1) The Ammeter was chosen to have Ifsd = 30 μA. The internal resistance of of Ifsd = 30 μA ammeter was measured as shown in the figure below:

2) A 1kΩ resistor was measured before the circuit was set up as shown in the figure below:

3) The value of current flow and R2, which is the potentiometer was calculated using multisim.

4) The circuit in multisim was opened to ensure current flow throughout the circuit is zero.

5) The circuit in multisim was then closed to ensure the total current flow throughout the circuit is 30μA.

6) A 1kΩ resistor was then inserted parallel to the circuit, a half value of the total current was displayed in the ammeter.

7) A 10kΩ resistor was then replaced the 1kΩ resistor, the reading of the ammeter was displayed.

8) The series type ohmmeter circuit was set up. The 1kΩ resistor was set as R1, which is a fixed value resistor. A decade resistance box was set as R2, which is a vary resistance in order to make full scale deflection for the Ifsd = 30 μA ammeter.

9) The voltage was fixed to 9.5V throughout the experiment. The circuit was opened to ensure no current flow through the ammeter.

10) The circuit was then closed, R2 was varies until the ammeter was deflected to the maximum range of 30μA.

11) A 1kΩ resistor was then connected to the circuit in series, and a half scale deflection of the ammeter was obtained.

12) A 10kΩ resistor is used to replace 1kΩ resistor, the deflection of the ammeter was then captured as shown below:

13) The measurement taken was recorded.

6.0 Results Resistor (Ω) | Measurement value (Ω) | Theoretical value (Ω) | Internal resistor of ammeter, Rm | 4.70k | 4.70k | R1 | 0.999k | 1k | R2 | 15 | 15 |

Calculation:
* Percentage Error (%) = Expected value-measured valueexpected value ×100%
%e=15.83-15.5015.83*100%
= 2.08%
%e=2.744-2.8502.744*100%
= 3.86%
Scales:
From theory:
Full scale:30 μA ≈ 0Ω, 0A ≈ ∞Ω
Half scale15 μA≈1k Ω
From calculation:
Rx = Rh for the half of full scale deflection, since
R1=Rh-If.s.d.RmRhV
R1=Rh(1-If.s.d.RmV)
Rh=R11-If.s.d.RmV
Rh=0.999kΩ1-30µA*4.7kΩ9.5V = 1.014kΩ
Current for Rh is Ih = V/2Rh , since V=9.5V and 2Ih = If.s.d.= 30µA
Then Rh = 9.5/2*30µ = 15.83µA
From the calculation we can know that half scale deflection is 15.83µA equivalent 1.014kΩ Resistor Value (kΩ) | Half scale deflection,Rh | Percentage Error (%) | | Measured Value (µA) | Stimulated Value(µA) | | 1 | 15.50 | 15.83 | 2.08 | 10 | 2.850 | 2.744 | 3.86 |

From the table, the percentage of error is in the range of the tolerance of resistors which is ± 10%

7.0 Discussions

1) During the experiment, all resistors were measured using digital multimeter(DMM). However, the resistance displayed by DMM doesn’t show the exact value of the resistor. This is due to the high sensitivity of the DMM as well as the tolerance presents in every resistor.

2) The experiment was asked to reproduce a series type ohmmeter using PMMC of Ifsd = 30 μA for range of 0-10 kΩ. R1 and R2 are determined by the value of Rx, which gives half the full scale deflection.

3) However, the half deflection was a little skewed to the right. It is not a perfect half scale deflection. This is because some amount of voltage was lost in the form of heat energy during the flowing of current in the circuit.

4) The Rx = 1 kΩ is then changed to 10 kΩ. The deflection of the PMMC of Ifsd = 30 μA shows a moderate deflection. Theoretically, there should be a very small deflection of PMMC and the pointer will not approaches to zero.

5) This phenomenon happens due to the tolerance present in the 10 kΩ resistor. The resistor can’t perform with respect to its actual value. Therefore, a few amount of current will still able to pass through the circuit and hence, giving reading to the PMMC of Ifsd = 30 μA.

6) However, when a 10kΩ resistor is slotted into the circuit, there should be a little deflection in the PMMC. It is impossible for the deflection pointed to infinity.

7) In order to minimize the error in reading, the power supply was switched off for every measurement taken. This is to prevent voltage loss in the form of heat energy to the surrounding as the circuit cannot stand with constant current flow and will eventually heated up.

8) If the varying resistor such as rheostat is used in the experiment, insertion effects will happen, therefore, we can’t obtain the actual zero ohm for the short circuit measurement. With that, a resistant decade box was highly recommended to be used as a variable resistor for the circuit.

8.0 Conclusion

1) From this project, we achieved the aim of this assignment where the objective is to reproduce a series type Ohmmeter using PMMC of Ifsd = 30 μA for range of 0-10 kΩ. 2) To set up a range. A half scale deflection must be done to the PMMC via varying the value of voltages and resistors before proceed to the maximum range measurement. 3) The stimulated value of the half deflection is approximately equal to the experimental result. Therefore, the production of the series type ohmmeter for range 0-10kΩ is proven to be true.

9.0 References http://www.tpub.com/neets/book16/68d.htm http://www.hirstbrook.com/netw1/06%20dArsonval.pdf http://books.google.com.my/books?id=rcHGGErSS3UC&pg=SA1-PA55&lpg=SA1-PA55&dq=multi+range+voltmeter&source=bl&ots=o0P1ojimdq&sig=bbPJ-iCmDiSszHhOjxgtkOtY2wo&hl=en&ei=mZvgToKWFpDyrQe9u7WnCw&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEgQ6AEwBDgo#v=onepage&q=multi%20range%20voltmeter&f=false A module of Electronic Instrumentation – Module 8(page 28-29)

References: http://www.tpub.com/neets/book16/68d.htm http://www.hirstbrook.com/netw1/06%20dArsonval.pdf http://books.google.com.my/books?id=rcHGGErSS3UC&pg=SA1-PA55&lpg=SA1-PA55&dq=multi+range+voltmeter&source=bl&ots=o0P1ojimdq&sig=bbPJ-iCmDiSszHhOjxgtkOtY2wo&hl=en&ei=mZvgToKWFpDyrQe9u7WnCw&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEgQ6AEwBDgo#v=onepage&q=multi%20range%20voltmeter&f=false A module of Electronic Instrumentation – Module 8(page 28-29)