ac dc metrers-1

UNIT 08:DC & AC METERSPART-I

**RUSAYL INSTITUTE LLC

RUSAYL INSTITUTE LLC

CHAPTER OUTLINE3.1 Introduction to DC meters3.2 dArsonval meter movement3.3 DC Ammeter3.4 DC Voltmeter3.5 DC Ohmeter3.6 Introduction to AC meter3.7 dArsonval meter movement (half-wave rectification)3.8 dArsonval meter movement (full-wave rectification)3.9 Loading effects of AC meter**RUSAYL INSTITUTE LLC


OBJECTIVESAt the end of this chapter, students should be able to:Explain in detail the principles of operation of the pmmc or dArsonval meter movementExplain the purpose of shunts across a meter and multipliers in series with a meterExplain and calculate the voltmeter loading effectsAnalyze a circuit in terms of Voltmeter Loading Effect Explain the purpose of Ohmmeter Describe the construction and operation of a basic Ohmmeter



Describe the operation of half-wave rectifier circuitTrace the current path in a full-wave bridge rectifier circuitCalculate ac sensitivity and the value of multiplier resistors for half-wave and full-wave rectification

OBJECTIVES**RUSAYL INSTITUTE LLC


3.1 INTRODUCTION TO DC METERS

A meter is any device built to detect accurately and display an electrical quantity in a form readable by a human being Usually this "readable form" is visual: motion of a pointer on a scale, a series of lights arranged to form a "bar graph," or some sort of display composed of numerical figuresMost modern meters are "digital" in designOlder designs of meters are mechanical in nature, using some kind of pointer device to show quantity of measurement



3.1 INTRODUCTION TO DC METERS

The display mechanism of a meter is often referred as a movement, borrowing from its mechanical nature to move a pointer along a scale so that a measured value may be read.Most mechanical movements are based on the principle of electromagnetism:- electric current through a conductor produces a magnetic field perpendicular to the axis of electron flow. The greater the electric current, the stronger the magnetic field produced.



If the magnetic field formed by the conductor is allowed to interact with another magnetic field, a physical force will be generated between the two sources of fieldsIf one of these sources is free to move with respect to the other, it will do so as current is conducted through the wire, the motion (usually against the resistance of a spring) being proportional to the strength of currentPractical electromagnetic meter movements can be made now where a pivoting wire coil is suspended in a strong magnetic field, shielded from the majority of outside influences Such an instrument design is generally known as a permanent-magnet moving coil (PMMC) movement or dArsonval meter movement

3.1 INTRODUCTION TO DC METERS **RUSAYL INSTITUTE LLC


The dArsonval meter movement is a current responding device which used very widely nowadaysCurrent from a measured circuit passes through the windings of the moving coils causes it to behave as an electromagnetic.The poles of EMT interact with the poles of PM, causing the coils to rotate.The pointer deflects up scale whenever current flows in proper direction in the coil. For this reason, all DC meter movements show polarity markings.The moving coil responds to the amount of current through its windings.3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


In the picture above, the meter movement "needle" shown is pointing somewhere around 35 percent of full-scale, zero being full to the left of the arc and full-scale being completely to the right An increase in measured current will drive the needle to point further to the right and a decrease will cause the needle to drop back down toward its resting point on the left

3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


3.2 dARSONVAL METER MOVEMENTThe arc on the meter display is labeled with numbers to indicate the value of the quantity being measuredIn other words, if it takes 50 A current to drive the needle fully to the right (making a "50 A full-scale movement"), the scale would have 0 A written at the very left end and 50 A at the very right, 25 A being marked in the middle of the scale In all likelihood, the scale would be divided into much smaller graduating marks, probably every 5 or 1 A, to allow whoever is viewing the movement to infer a more precise reading from the needle's position The basic principle of this device is the interaction of magnetic fields from a permanent magnet and the field around a conductor (a simple electromagnet)



A permanent-magnet moving-coil (PMMC) movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as shown in figure below.3.2 dARSONVAL METER MOVEMENT When the switch is closed, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil in Figure 2(a) will be the north pole of this electromagnet. Since opposite poles attract, the coil will move to the position shown in Figure 2(b). **RUSAYL INSTITUTE LLC


To use pmmc as a meter, 2 problems must be solved: find a way to return the coil to its original position when there is no current through the coil find a method to indicate the amount of coil movement 3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


The first problem is solved by the: use of hairsprings attached to each end of the coil these hairsprings can also be used to make the electrical connections to the coil with the hairsprings, the coil will return to its initial position when there is no current the springs will also tend to resist the movement of the coil when there is current through the coil3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


As the current through the coil increases, the magnetic field generated around the coil increases The stronger the magnetic field around the coil, the farther the coil will move (good basis for a meter) But, how will you know how far the coil moves? If a pointer is attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the scale can be marked to indicate the amount of current through the coil. 3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


2 other features are used to increase the accuracy& efficiency of this meter. First, an iron core is placed inside the coil to concentrate with the magnetic fields. Second, curved pole pieces are attached to the magnet to ensure that the turning force on the coil increases steadily as the current increases. The meter movement as it appears when fully assembled is shown in this figure.3.2 dARSONVAL METER MOVEMENT**RUSAYL INSTITUTE LLC


POP QUIZLabel the figure appropriately **RUSAYL INSTITUTE LLC


3.3 DC AMMETERWhat are meters?Meters are used to measure current and voltage. Normally the meter will be a single low range meter such as 0 - 1 mA full deflection meter of the D'Arsonval type. The d'Arsonval type meter works on the principle that a coil of wire to which a pointer is attached is pivoted between the poles of a permanent magnet. When current flows through the coil, it sets up a magnetic field that interacts with the field of the magnet to cause the coil to turn.**RUSAYL INSTITUTE LLC


The meter pointer deflects in direct proportion to the current. This meter is called an ammeter. 3.3 DC AMMETERFigure 1: A typical 0 to 1mA ammeter.**RUSAYL INSTITUTE LLC


a device used to measure currentput in series/ parallel with the circuitvery common in labuse unit Ampere (A)/ mAused the principle of the dArsonval meter movement with slight modificationplacing a LOW resistance in PARALLEL with the meter movement resistance to increase the range of current that can be measured by the meter

3.3 DC AMMETER**RUSAYL INSTITUTE LLC


DC ammeterPMMC galvanometer constitutes the basic movement of a dc ammeterSince the coil winding of a basic movement is small and light, it can carry only very small currentsWhen large currents are to be measured, it is necessary to bypass a major part of the current through a resistance called a shuntThe resistance of shunt can be calculated using conventional circuit analysisRsh = shunt resistorRm = internal resistance of the movementIsh shunt currentIm full-scale deflection current of meter movementI full-scale current of ammeter + shunt (total current)



3.3 DC AMMETERBasic dc Ammeter+-+-DArsonval movementSince Rsh is in parallel with the meter movement, the voltage drop across the shunt and movement must be the same. Therefore,**RUSAYL INSTITUTE LLC


EXAMPLE 3.1A 1mA meter movement with an internal resistance of 100 is to be converted into a 0-100mA. Calculate the value of shunt resistance required.



Multirange ammeter To obtain a multirange ammeter, a number of shunts are connected across the movement with a multi-position switchReferring to the figure, the circuit has 4 shunts Ra, Rb, Rc and Rd which can be placed in parallel with the movement to give four different current ranges



Also known as universal shunt.Used on a multiple range ammeter.It eliminates the possibility of the meter movements being in the circuit without any shunt resistance protect the deflection instrument of the ammeter from an excessive current flow when switching between shunts. Advantage - maybe used as a wide range of meter movements.When the switch is in position 1, Ra is in parallel with the series combination of Rb, Rc and the meter movement.3.3 DC AMMETERAyrton ShuntFig. 3: An Ammeter using Ayrton shunt.123m-++-SAB**RUSAYL INSTITUTE LLC


3.3 DC AMMETERHence the current through the shunt is more than the current through the meter movement, protecting the meter movement and reducing its sensitivity

If the switch is connected to position 2, Ra and Rb are together in parallel with series combination of Rc and the meter movementNow the current through the meter is more than the current through the shunt resistance



3.3 DC AMMETERIf the switch is connected to position 3, Ra, Rb and Rc are together in parallel with the meterHence max current flows through the meter movement and very little through the shunt destroy the meter or blow a fuse increases the sensitivity



EXAMPLE 3.3Design an Aryton shunt (**figure below) to provide an ammeter with a current range of 0-1mA, 0-10mA, 0-50mA and 0-100mA. DArsonval movement with an internal resistance of 100 and full scale current of 50A is used.**RUSAYL INSTITUTE LLC


EXAMPLE 3.3 (solution)(3.1)**RUSAYL INSTITUTE LLC


EXAMPLE 3.3 (SOLUTION)(3.4)(3.2)(3.3)**RUSAYL INSTITUTE LLC


EXAMPLE 3.3 (SOLUTION)**RUSAYL INSTITUTE LLC


EXAMPLE 3.3 (SOLUTION)Hence, the value of shunts are

R1 = 0.05263R3 = 0.4147R2 = 0.05263R4 = 4.734**RUSAYL INSTITUTE LLC


Fig. 3: An Ammeter using Ayrton shunt.Compute the value of the shunt resistors for the circuit. Given that Rm = 1k, Im = 100 mA, I1=10mA, I2=100mA, I3=1A.Check :Rsh = Ra + Rb + Rc always!EXAMPLE 3.4**RUSAYL INSTITUTE LLC


WPmax

AIsin

VUd

Ra

Rb

Rm

Im

I

Rc

I1

I2

I3

I - Im

+

-

1A

5A

10A

3.3 DC AMMETERAmmeter insertion effects All ammeters contain some external resistance, which may range from a low to a greater value Inserting ammeter in a circuit always increase the resistance of the circuit and therefore reduces the current in the circuit. Without ammeter, the current flows in the circuit shown below can be calculated as

Current without ammeter insertion effect. **RUSAYL INSTITUTE LLC


R1

E

3.3 DC AMMETER However, inserting the ammeter as shown below will reduce the current in the circuit to:Circuit with ammeter insertion effect**RUSAYL INSTITUTE LLC


EXAMPLE 3.5Determine the insertion error in circuit shown below if E=100V, R1=100, and Rm=100.**RUSAYL INSTITUTE LLC


In this sub-topic, we have discussed about :Introduction to electrical metersShunt resistor in a single-range AmmeterUniversal shunt in multiple-range AmmeterCalculation of shunt resistorsAmmeter insertion effectsSummary**RUSAYL INSTITUTE LLC


3.4 DC VOLTMETER

To use the basic meter as a dc voltmeter, it is necessary to know the amount of current required to deflect the basic meter to full scale known as IfsdFor example, suppose a 50A current is required for full scale deflection. This full scale value will produce a voltmeter with a sensitivity of 20k per V.

sensitivity = Hence, a 0-1mA would have a sensitivity of????1k/V**RUSAYL INSTITUTE LLC


3.4 DC VOLTMETERA basic DArsonval movement can be converted into a dc voltmeter by adding a series resistor known as multiplier (Rs)The function of multiplier is to limit the current through the movement so that the current does not exceed the full scale deflection valueA dc voltmeter measures the potential between two points in a dc circuit or a circuit component

Basic dc voltmeter**RUSAYL INSTITUTE LLC


WPmax

AIsin

VUd

Rs

Rm

Im

+

-

3.4 DC VOLTMETERThe value of Rs required is calculated as follows:Im = full scale deflection current of the movement (Ifsd)**RUSAYL INSTITUTE LLC


A basic DArsonval movement with a full scale deflection of 50A and internal resistance of 500 is used as a voltmeter. Determine the value of the multiplier resistance needed to measure a voltage range of 0-10V.EXAMPLE 3.6**RUSAYL INSTITUTE LLC


To measure the potential difference between two points in a dc circuit/component, a dc voltmeter is always connected across them with proper polarity

3.4 DC VOLTMETER**RUSAYL INSTITUTE LLC


Multirange voltmeterA dc voltmeter can be converted into a multirange voltmeter by connecting a number of resistors (multipliers) along with a range switch to provide a greater number of workable rangesFigure below shows a multirange voltmeter using four position switch and 4 multipliers R1, R2, R3, and R4 for voltage values V1, V2, V3 and V4.



EXAMPLE 3.7Calculate the values of Rs for the multiple- range DC Voltmeter circuit as shown below:**RUSAYL INSTITUTE LLC


EXAMPLE 3.7 (solution)**RUSAYL INSTITUTE LLC


Voltmeter loading effects

When selecting a meter for a certain voltage measurement, it is important to consider the sensitivity of a dc voltmeterA low sensitivity meter may give a correct reading when measuring voltages in low resistance circuit but produce unreliable reading in a high resistance circuitA voltmeter when connected across two points in a highly resistive circuits, acts as a shunt, reducing the total equivalent resistance of that portion (Inserting voltmeter always increase the resistance and decrease the current flowing through the circuit) The meter then indicates a lower reading than what existed before the meter was connectedThis is called voltmeter loading effect and is caused mainly by low sensitivity instrument



EXAMPLE 3.8Two different voltmeters are used to measure the voltage across RB in the circuit below. The meters are: Meter A : S= 1k/V;Rm=0.2k; Range =10V Meter B : S=20k/V;Rm=1.5k; Range = 10V Calculate:Voltage across RB without any meter.Voltage across RB when meter A is used.Voltage across RB when meter B is used.Loading Errors in both voltmeter readings.**RUSAYL INSTITUTE LLC


EXAMPLE 3.8 (solution)The voltage across the resistance RB, without any meter connected is calculated using the voltage divider formula:

ii) Starting with meter A, having sensitivity S = 1k/V. Therefore, the total resistance it presents to the circuit is:

The total resistance across RB is RB in parallel with meter resistance, Rm1:**RUSAYL INSTITUTE LLC


EXAMPLE 3.8 (solution)Therefore, the voltage reading obtained with meter 1 using the voltage divider equation is:

iii)The total resistance that meter 2 presents to the circuit is:

The parallel combination of RB and meter 2 gives:



EXAMPLE 3.8 (solution)Therefore, the voltage reading obtained with meter 2 using the voltage divider equation is:

iv) The error in the reading of the voltmeter is given by:



EXAMPLE 3.9Find the voltage reading and the percentage of loading error of each reading obtained with a voltmeter on: Its 5-V range.Its 10-V rangeIts 50-V range.The meter has a 20-k/V sensitivity and connected across RA.**RUSAYL INSTITUTE LLC


SummaryIn this sub-topic, we have learned about: The purpose of multipliers put in series with a meter movements.Calculation of the multiplier resistance of a VoltmeterVoltmeter loading effectsThe basic dArsonval meter movement can be converted to a DC Voltmeter by connecting a Multiplier (Rs) with the meter movement. Sensitivity, S is the reciprocal of the full-scale deflection current. Therefore, it is desirable to make the voltmeter resistance much-much more higher than the circuit resistance.



3.5 DC OHMETERThe purpose of an Ohmmeter is to measure resistanceResistance reading is indicated trough a mechanical meter movement which operates on electric current.Thus, Ohmmeter must have an internal source of voltage to create current necessary to operate the movement.It also must have an appropriate ranging resistors to allow just the right amount of current.A simple Ohmmeter comprises battery and meter movement as in figure below:



3.5 DC OHMETER

When there is infinite resistance (no continuity between test leads), there is zero current through the meter movement, and the needle points toward the far left of the scale.In this regard, the ohmmeter indication is "backwards" because maximum indication (infinity) is on the left of the scale



3.5 DC OHMETERIf the test leads of the Ohmmeter are directly shorted together (measuring zero ), the meter movement will have a maximum amount of current through it, limited only by the battery voltage and the movement's internal resistance:With 9 volts of battery and only 500 of internal movement resistance, current will be 18mA, which is far beyond the full-scale rating of the movement will likely damage the meter.



3.5 DC OHMETERSo, to avoid this, add series resistance to the meters circuit so that the movement just registers full-scale when the test leads are shorted together

To determine the proper value for R, calculate the Rtotal needed to limit current to only 1mA (full-scale) with 9V of potential from the battery, then subtract the movement's internal resistance:



3.5 DC OHMETERNow, we're still having a problem of meter range.On the left side of the scale we have "infinity" and on the right side we have zero.One might wonder, What does middle-of-scale represent? What figure lies exactly between zero and infinity?.Infinity is more than just a very big amount: it is an incalculable quantity, larger than any definite number ever could be.



3.5 DC OHMETERIf half-scale indication on any other type of meter represents 1/2 of the full-scale range value, then what is half of infinity on an ohmmeter scale? The answer to this paradox is a logarithmic scale!.

With a logarithmic scale, the amount of resistance spanned for any given distance on the scale increases as the scale progresses toward infinity, making infinity an attainable goal.



3.5 DC OHMETERWe still have a question of range for our ohmmeter, though. What value of resistance between the test leads will cause exactly 1/2 scale deflection of the needle?If we know that the movement has a full-scale rating of 1 mA, then 0.5 mA (500 A) must be the value needed for half-scale deflection. Following our design with the 9 volt battery as a source we get:



3.5 DC OHMETER

With an internal movement resistance of 500 and a series range resistor of 8.5 k, this leaves 9 k for an external (lead-to-lead) test resistance at 1/2 scale. In other words, the test resistance giving 1/2 scale deflection in an ohmmeter is equal in value to the (internal) series total resistance of the meter circuit. Using Ohm's Law a few more times, we can determine the test resistance value for 1/4 and 3/4 scale deflection as well:



3.5 DC OHMETER1/4 scale deflection (0.25 mA of meter current):

3/4 scale deflection (0.75 mA of meter current):**RUSAYL INSTITUTE LLC


3.5 DC OHMETERSo, the scale for this ohmmeter looks something like this:

One major problem with this design is its reliance upon a stable battery voltage for accurate resistance reading. If the battery voltage decreases (as all chemical batteries do with age and use), the ohmmeter scale will lose accuracy.



3.5 DC OHMETEROne thing that needs to be mentioned with regard to ohmmeters: they only function correctly when measuring resistance that is not being powered by a voltage or current source. In other words, you cannot measure resistance with an ohmmeter on a "live" circuit! The reason for this is simple: the ohmmeter's accurate indication depends on the only source of voltage being its internal battery. The presence of any voltage across the component to be measured will interfere with the ohmmeter's operation. If the voltage is large enough, it may even damage the ohmmeter



SUMMARY In this sub-topic, we have learned about: Ohmmeters contain internal sources of voltage to supply power in taking resistance measurements. An analog ohmmeter scale is "backwards" from that of a voltmeter or ammeter, the movement needle reading zero resistance at full-scale and infinite resistance at rest. Analog ohmmeters also have logarithmic scales, "expanded" at the low end of the scale and "compressed" at the high end to be able to span from zero to infinite resistance. Ohmmeters should never be connected to an energized circuit (that is, a circuit with its own source of voltage). Any voltage applied to the test leads of an ohmmeter will invalidate its reading.



EVALUATION

Find the value of R, scale, scale and scale of this Ohmmeter?**RUSAYL INSTITUTE LLC


THANK YOU..**RUSAYL INSTITUTE LLC


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