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DC Ammeter

Figure 1: A dc ammeter consists of a PMMC instrument and a low resistance shuntWhere Rm = internal resistance of the coil Rsh = resistance of the shunt Im = full-scale deflection current of the coil Ish = shunt current I = full-scale current of the ammeter including the shunt

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Example An ammeter exists with Rm = 99 and FSD current of 0.1 mA, also Rs = 1 . Determine the total current passing through the ammeter at (a) FSD, (b) 0.5 FSD,(c) 0.25 FSD.

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Multirange Ammeter

To increase the measuring capacity Protecting the instrument from excessive current flow

Figure 2: Multirange ammeter using switched shunts

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Figure 3: An Ayrton shunt used with an ammeter consists of several series-connected resistors all connected in parallel with the PMMC instrument

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Precaution of using ammeter

Never connect an ammeter across a source of emf Observe the correct polarity When using a multirange meter, first use the highest current range. Always use therange that will give you a reading as near to fullscale as possible

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DC Voltmeter

Figure 4: A dc voltmeter is made up of a PMMC instrument and a series multiplierresistor. Where Im = deflection current of the meter Rm = internal resistance of the meter Rs = multiplier resistance V = full-range voltage of the meter

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Example A PMMC instrument with FSD = 100 A and Rm = 1 k , to be converted into voltmeter. Determine Rs if the voltmeter is to measure 50 V at full scale. Also, calculate the applied voltage when the instrument indicates 0.8 FSD.

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Multirange Voltmeter

Figure 5(a): Multirange voltmeter using switched multiplier resistors

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Figure 5(b): Multirange voltmeter using series-connected multiplier resistors

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Example A PMMC instrument with FSD = 50 A and Rm = 1700 is to be employed as a voltmeter with ranges of 10 V, 50 V, 100 V. Calculate required values of multiplier resistors for the above circuits.

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Series Ohmmeter

Figure 6: Basic series ohmmeter circuit consisting of a PMMC instrument and a standard resistor (R1) Where Eb = battery Rx = resistance to be measured Rm = meter resistance R1 = standard resistor

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Eb Im ! R x R1 Rm

For Rx = 0 , the pointer indicates 0 max. current will flow For Rx = , the pointerindicates no current will flow Pointer position depends on the relationship between Rx and (R1 + Rm)

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Example A series ohmmeter has 1.5 V battery, a 100 A meter and R1 which makes (R1+ Rm) = 15 k. (a) Determine the instrument indication when Rx = 0 (b) Determinehow the resistance scale should be marked at 0.5 FSD, 0.25 FSD and 0.75 FSD.

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Ohmmeter with Zero AdjustSeries ohmmeter will operate satisfactorily as long as the battery remains exactly at its value. When the battery voltage falls, the instrument scale is no longer correct. Falling battery voltage can be taken care of by an adjustable resistor connected in parallel with the meter.

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Figure 7: An adjustable resistor (R2) connected in parallel with the meter provides an ohmmeter zero control. The ohmmeter terminals are initially short-circuited and the zero control (R2) is adjusted to give zero-ohms reading.

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Example For the above figure, Eb = 1.5 V, R1 = 15 k , Rm = 50 , R2 = 50 and FSD= 50 A. Determine the meter reading at 0.5 FSD, and new value of R2 when Eb fallsto 1.3 V. Also, determine the meter reading at 0.5 FSD when Eb = 1.3 V. Ans: 15k , 68.18 , 15 k

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Shunt Ohmmeter - MultirangeMajor inconvenience of series ohmmeter: large adjustment of zero control would have to be made. Using shunt ohmmeter, only zero adjustment is needed.

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Figure 8: Circuit, scale and range switch for a typical multirange shunt ohmmeter

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Example (a) In the above figure, calculate Im when Rx = 0 at the range of Rx1. Also, calculate Im when Rx = 24 . (b) With the range of Rx10, calculate Im for Rx= 0 and Rx = 70 . Ans: (a) 37.5 A, 18.72 A; (b) 37.53 A, 29.04 A.

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Megger

Figure 9: Megger

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Measuring very high resistance. It has hand-driven dc generator supplies high voltage. Coil a tends to move the pointer clockwise, and coil b tends to move the pointer counterclockwise. Coil a is connected in series with R3 and Rx, and connected across the generator. Coil b is connected in series with R2, and also connected across the generator. There are no restraining springs therefore the pointer floats freely.

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If the test leads are open-circuited (Rx = ), No current flows in coil a Currentflows in coil b Pointer is deflected to infinite resistance If the test leads are shorted (Rx = 0), Pointer rests at zero because the current in coil a is relatively high comparing to coil b If 0

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