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

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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.


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...
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