· Web viewAC bridges are often used to measure the value of unknown impedance (self/mutual inductance of inductors or capacitance of capacitors accurately). A large number of AC bridges

Electrical and Electronic measurements & instrumentation 10EE35

Question Bank with solution

PART-A

Unit-1

1. Derive the dimensional equation for jan14, july13, jan13, jan15i) EMF in SI units ii) Magnetizing force in SI units iii) Capacitance in SI units iv) MMF in LMTI v) Flux density in LMTI vi) Resistivity and conductivity in SI units

Capacitance C = q/v = [M1/2 L3/2 T-1 є1/2]/ [M1/2L1/2T-1є-1/2] = [єL]

Resistance R = V/I = [M1/2L1/2 T-1 є-1/2]/[M1/2L3/2T-2є1/2] = [L-1Tє-1]

Where f = frequency, Bm = Max. flux density, d= diameter of wire, ρ – resistivity of

material. Find the values a, b,c,and g using L,M,T,I system

P = k fa Bmb dc ρg

[P] = [I1L-1 ]

[f] = [T-1 ]

[Bm] = [M1T-2I-1 ]

[d] = [L]

[ρ] = [M1 L3 T-3I-2]

[I1L-1 ] = k [T-1 ] a [M1T-2I-1 ] b [L] c [M1 L3 T-3I-2] g

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[I1L-1 ] = k [T-a ] [MbT-2bI-b ] [L c ] [Mg L3g T-3gI-2g]

By comparing and solving a = 2, b = 2, g=1 , c=4

2. Expression for mean torque of an electrodynamometer type of wattmeter isgiven by TdαMaEbZc where M=mutual incductance, E= applied vtg,Z=impedance. Determine the values of a,b,c . jan14

Torque, moment or moment of force (see the terminology below), is the tendency of a force to rotate an object about an axis,[1] fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object. Mathematically, torque is defined as the cross product of the lever-arm distance vector and the force vector, which tends to produce rotation.Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.The symbol for torque is typically τ, the Greek letter tau. When it is called moment, it is commonly denoted M.The magnitude of torque depends on three quantities: the force applied, the length of the lever arm[2] connecting the axis to the point of force application, and the angle between the force vector and the lever arm. In symbols:

whereτ is the torque vector and τ is the magnitude of the torque,r is the displacement vector (a vector from the point from which torque is measured to the point where force is applied),F is the force vector,× denotes the cross product,θ is the angle between the force vector and the lever arm vector.

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The length of the lever arm is particularly important; choosing this length appropriately lies behind the operation of levers,pulleys, gears, and most other simple machines involving a mechanical advantage.The SI unit for torque is the newton metre (N·m).

3. Derive the balancing equ for Kelvins double bridge jan14, july13

An interesting variation of the Wheatstone bridge is the Kelvin Double bridge, used for measuring very low resistances (typically less than 1/10 of an ohm). Its schematic diagram is as such:

The low-value resistors are represented by thick-line symbols, and the wires connecting them to the voltage source (carrying high current) are likewise drawn thickly in the schematic. This oddly-configured bridge is perhaps best understood by beginning with a standard Wheatstone bridge set up for measuring low resistance, and evolving it step-by-step into its final form in an effort to overcome certain problems encountered in the standard Wheatstone configuration.

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If we were to use a standard Wheatstone bridge to measure low resistance, it would look something like this:

When the null detector indicates zero voltage, we know that the bridge is balanced and that the ratios Ra/Rx and RM/RN are mathematically equal to each other. Knowing the values of Ra, RM, and RN therefore provides us with the necessary data to solve for Rx . . . almost.We have a problem, in that the connections and connecting wires between Ra and Rx possess resistance as well, and this stray resistance may be substantial compared to the low resistances of Ra and Rx. These stray resistances will drop substantial voltage, given the high current through them, and thus will affect the null detector's indication and thus the balance of the bridge:

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Since we don't want to measure these stray wire and connection resistances, but only measure Rx, we must find some way to connect the null detector so that it won't be influenced by voltage dropped across them. If we connect the null detector and RM/RN ratio arms directly across the ends of Ra and Rx, this gets us closer to a practical solution:

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Now the top two Ewire voltage drops are of no effect to the null detector, and do not influence the accuracy of Rx's resistance measurement. However, the two remaining Ewire voltage drops will cause problems, as the wire connecting the lower end of Ra with the top end of Rx is now shunting across those two voltage drops, and will conduct substantial current, introducing stray voltage drops along its own length as well.Knowing that the left side of the null detector must connect to the two near ends of Ra and Rx in order to avoid introducing those Ewire voltage drops into the null detector's loop, and that any direct wire connecting those ends of Ra and Rx will itself carry substantial current and create more stray voltage drops, the only way out of this predicament is to make the connecting path between the lower end of Ra and the upper end of Rx substantially resistive:

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We can manage the stray voltage drops between Ra and Rx by sizing the two new resistors so that their ratio from upper to lower is the same ratio as the two ratio arms on the other side of the null detector. This is why these resistors were labeled Rm and Rn in the original Kelvin Double bridge schematic: to signify their proportionality with RM and RN:

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With ratio Rm/Rn set equal to ratio RM/RN, rheostat arm resistor Ra is adjusted until the null detector indicates balance, and then we can say that Ra/Rx is equal to RM/RN, or simply find Rx by the following equation:

The actual balance equation of the Kelvin Double bridge is as follows (Rwire is the resistance of the thick, connecting wire between the low-resistance standard Ra and the test resistance Rx):

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5. Obtain wheatstone bridge sensitivity interms of parameters of the bridge

jan13The bridge consists of four resistive arms together with a source of e.m.f. and a nulldetector. The galvanometer is used as a null detector.

The arms consisting the resistances R] and R2 are called ratio arms. The arm consisting the standard known resistance R3 is called standard arm. The resistance R4 is the unknown resistance to be measured. The battery is connected between A and C while galvanometer is connected between Band D.

6. Explain the neat sketch how megger is used for the measurement of very high resistance. Jan13, jan15

The important construction features of Megger consist of following parts:

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1) Control and Deflecting coil: They are normally mounted at right angle to each other and connected parallel to the generator. The polarities are such that the torque produced by them is in opposite direction.

2) Permanent Magnet: Permanent magnet with north and south poles to produce magnetic effect for deflection of pointer.

3) Pointer and scale: A pointer is attached to the coils and end of the pointer floats on a scale which is in the range from ―zero‖ to ―infinity‖. The unit for this is ―ohms‖.

4) D.C generator or battery connection: Testing voltage is supplied by hand operated D.C generator for manual operated Megger and a battery and electronic voltage charger for automatic type Megger.

5) Pressure coil and current coil: Provided for preventing damage to the instrument in case of low external source resistance.

Working: -

The voltage for testing is supplied by a hand generator incorporated in the instrument or by battery or electronic voltage charger. It is usually 250V or 500V and is smaller in size.

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- A test volt of 500V D.C is suitable for testing ship‘s equipment operating at 440V A.C. Test voltage of 1000V to 5000V is used onboard for high voltage system onboard.

- The current carrying coil (deflecting coil) is connected in series and carries the current taken by the circuit under test. The pressure coil (control coil) is connected across the circuit.

- Current limiting resistor – CCR and PCR are connected in series with pressure and current coil to prevent damage in case of low resistance in external source.

- In hand generator, the armature is moving in the field of permanent magnet or vice versa, to generate a test voltage by electromagnetic induction effect.

- With an increase of potential voltage across the external circuit, the deflection of the pointer increases; and with an increase of current, the deflection of pointer decrease so the resultant torque on the movement is directly proportional to the potential difference and inversely proportional to the resistance.

- When the external circuit is open, torque due to voltage coil will be maximum and the pointer will read ―infinity‖. When there is short circuit the pointer will read ―0‖.

Unit-2

1. Explain the sources and detectors used in AC bridges. jan14, jan15

One way to maximize the effectiveness of audio headphones as a null detector is to connect them to the signal source through an impedance-matching transformer. Headphone speakers are typically low-impedance units (8 Ω), requiring substantial current to drive, and so a step-down transformer helps ―match‖ low-current signals to the impedance of the

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headphone speakers. An audio output transformer works well for this purpose: (Figure below)

―Modern‖ low-Ohm headphones require an impedance matching transformer for use as a sensitive null detector.

Using a pair of headphones that completely surround the ears (the ―closed-cup‖ type), I've been able to detect currents of less than 0.1 µA with this simple detector circuit. Roughly equal performance was obtained using two different step-down transformers: a small power transformer (120/6 volt ratio), and an audio output transformer (1000:8 ohm impedance ratio). With the pushbutton switch in place to interrupt current, this circuit is usable for detecting signals from DC to over 2 MHz: even if the frequency is far above or below the audio range, a ―click‖ will be heard from the headphones each time the switch is pressed and released.

Connected to a resistive bridge, the whole circuit looks like Figure below.

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Bridge with sensitive AC null detector.

Listening to the headphones as one or more of the resistor ―arms‖ of the bridge is adjusted, a condition of balance will be realized when the headphones fail to produce ―clicks‖ (or tones, if the bridge's power source frequency is within audio range) as the switch is actuated.

When describing general AC bridges, where impedances and not just resistances must be in proper ratio for balance, it is sometimes helpful to draw the respective bridge legs in the form of box-shaped components, each one with a certain impedance: (Figure below)

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Generalized AC impedance bridge: Z = nonspecific complex impedance.

For this general form of AC bridge to balance, the impedance ratios of each branch must be equal:

Again, it must be stressed that the impedance quantities in the above equation must be complex, accounting for both magnitude and phase angle. It is insufficient that the impedance magnitudes alone be balanced; without phase angles in balance as well, there will still be voltage across the terminals of the null detector and the bridge will not be balanced.

Bridge circuits can be constructed to measure just about any device value desired, be it capacitance, inductance, resistance, or even ―Q.‖ As always in bridge measurement circuits, the unknown quantity is always ―balanced‖ against a known standard, obtained from a high-quality, calibrated component that can be adjusted in value until the null detector device indicates a condition of balance. Depending on how the bridge is set up, the unknown component's value may be determined directly from the setting of the calibrated standard, or derived from that standard through a mathematical formula.

A couple of simple bridge circuits are shown below, one for inductance (Figure below) and one for capacitance: (Figure below)

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Symmetrical bridge measures unknown inductor by comparison to a standard inductor.

Symmetrical bridge measures unknown capacitor by comparison to a standard capacitor.

Simple ―symmetrical‖ bridges such as these are so named because they exhibit symmetry (mirror-image similarity) from left to right. The two bridge circuits shown above are balanced by adjusting the calibrated reactive component (Ls or Cs). They are a bit simplified from their real-life

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counterparts, as practical symmetrical bridge circuits often have a calibrated, variable resistor in series or parallel with the reactive component to balance out stray resistance in the unknown component. But, in the hypothetical world of perfect components, these simple bridge circuits do just fine to illustrate the basic concept.

An example of a little extra complexity added to compensate for real-world effects can be found in the so-called Wien bridge, which uses a parallel capacitor-resistor standard impedance to balance out an unknown series capacitor-resistor combination. (Figure below) All capacitors have some amount of internal resistance, be it literal or equivalent (in the form of dielectric heating losses) which tend to spoil their otherwise perfectly reactive natures. This internal resistance may be of interest to measure, and so the Wien bridge attempts to do so by providing a balancing impedance that isn't ―pure‖ either:

Wein Bridge measures both capacitive Cx and resistive Rx components of―real‖ capacitor.

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Being that there are two standard components to be adjusted (a resistor and a capacitor) this bridge will take a little more time to balance than the others we've seen so far. The combined effect of Rs and Cs is to alter the magnitude and phase angle until the bridge achieves a condition of balance. Once that balance is achieved, the settings of Rs and Cs can be read from their calibrated knobs, the parallel impedance of the two determined mathematically, and the unknown capacitance and resistance determined mathematically from the balance equation (Z1/Z2 = Z3/Z4).

It is assumed in the operation of the Wien bridge that the standard capacitor has negligible internal resistance, or at least that resistance is already known so that it can be factored into the balance equation. Wien bridges are useful for determining the values of ―lossy‖ capacitor designs like electrolytics, where the internal resistance is relatively high. They are also used as frequency meters, because the balance of the bridge is frequency-dependent. When used in this fashion, the capacitors are made fixed (and usually of equal value) and the top two resistors are made variable and are adjusted by means of the same knob.

An interesting variation on this theme is found in the next bridge circuit, used to precisely measure inductances.

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Maxwell-Wein bridge measures an inductor in terms of a capacitor standard.

This ingenious bridge circuit is known as the Maxwell-Wien bridge (sometimes known plainly as the Maxwell bridge), and is used to measure unknown inductances in terms of calibrated resistance and capacitance. (Figure above) Calibration-grade inductors are more difficult to manufacture than capacitors of similar precision, and so the use of a simple

―symmetrical‖ inductance bridge is not always practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge, as they are here.

Another advantage of using a Maxwell bridge to measure inductance rather than a symmetrical inductance bridge is the elimination of measurement error due to mutual inductance between two inductors. Magnetic fields can be difficult to shield, and even a small amount of coupling between coils in a bridge can introduce substantial errors in certain conditions. With no second inductor to react with in the Maxwell bridge, this problem is eliminated.

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For easiest operation, the standard capacitor (Cs) and the resistor in parallel with it (Rs) are made variable, and both must be adjusted to achieve balance. However, the bridge can be made to work if the capacitor is fixed (non-variable) and more than one resistor made variable (at least the resistor in parallel with the capacitor, and one of the other two). However, in the latter configuration it takes more trial-and-error adjustment to achieve balance, as the different variable resistors interact in balancing magnitude and phase.

Unlike the plain Wien bridge, the balance of the Maxwell-Wien bridge is independent of source frequency, and in some cases this bridge can be made to balance in the presence of mixed frequencies from the AC voltage source, the limiting factor being the inductor's stability over a wide frequency range.

There are more variations beyond these designs, but a full discussion is not warranted here. General-purpose impedance bridge circuits are manufactured which can be switched into more than one configuration for maximum flexibility of use.

A potential problem in sensitive AC bridge circuits is that of stray capacitance between either end of the null detector unit and ground (earth) potential. Because capacitances can ―conduct‖ alternating current by charging and discharging, they form stray current paths to the AC voltage source which may affect bridge balance: (Figure below)

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2. Derive the balance eqn for Anderson bridge. jan14, july13, jan13

AC bridges are often used to measure the value of unknown impedance (self/mutual inductance of inductors or capacitance of capacitors accurately). A large number of AC bridges are available and Anderson's Bridge is an AC bridge used to measure self inductance of the coil. It is a modification of Wheatstones Bridge. It enables us to measure the inductance of a coil using capacitor and resistors and does not require repeated balancing of the bridge. The connections are shown in Fig: 1.

The bridge is balanced by a steady current by replacing the headphone H by moving coil galvanometer and A.C source by a battery. This is done by

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adjusting the variable resistance, r. After a steady balance has been obtained, inductive balance is obtained by using the A.C source and headphone.

The condition for balance is that the potentials at the terminals D and E are same. Then the current flowing through branch AB is I1, through branch AE and EB is I2. The current flowing through branches AD and DC is I3, while that through branch BC is I1+I2. No current flows through branch DE.

Circuit DetailsConsider the mesh ABCDA

(1)This shows that potential drop along ABC is equal to that along ADC.

Consider the mesh ABEA, there is no e.m.f.

(2) Consider the mesh AEDA,

(3)i.e. potential difference from A to E is equal to that from A to D. From (3) we get,

3. (4) Now substitute the value of I3 from (1) in (4)

(5)Dividing (5) by (2)

4.

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(6)Multiply and divide by R in the L.H.S of (6) and rearrange,

5.

(7)Equating real parts on both sides of (7)

(8)Equation (8) represents the condition for balancing of the bridge.

Equating imaginary parts on both sides of (7)

(9)Substituting :

From (8) and (9) gives us

(10)At this condition of balancing there is minimum sound in the headphone. Further we can make P=Q

(11)

The inductive reactance can be calculated by

(11)

3.Explain how capacitance and dissipation factor is measured using Scheringbridge. jan13

The Schering Bridge is an electrical circuit used for measuring theinsulating properties of electrical cables and equipment.[1] It is

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an AC bridge circuit, developed by Harald Schering. It has the advantage that the balance equation is independent of frequency.The connections of the Schering bridge under balance conditions are shown in the figure below.

In this diagram:C1 = capacitor whose capacitance is to be determined,R1 = a series resistance representing the loss in the capacitor C1, C2 = a standard capacitor,R3 = a non-inductive resistance,C4 = a variable capacitor,R4 = a variable non-inductive resistance in parallel with the variable capacitor C4.

4. Obtain the balance eqn for maxwell‘s inductance , capacitance bridge used for measurement of unknown inductance. jan15

A Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type of Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and capacitance. It is areal product bridge.

It uses the principle that the positive phase angle of an inductive impedance can be compensated by the negative phase angle of a capacitive impedance when put in the opposite arm and the circuit is at resonance; i.e., no potential difference across the detector and hence no current flowing through it. The unknown inductance then becomes known in terms of this capacitance.

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With reference to the picture, in a typical application and are known fixed entities, and and are known variable entities. and are adjusted until the bridge is balanced.

and can then be calculated based on the values of the other components:

5. With neat diagram explain the Kelvin Double bridge, obtain the balancing equation. jan15

In the Wheatstone bridge, the bridge contact and lead resistance causes significant error, while measuring low resistances. Thus for measuring the values of resistance below 1 -n, the modified form of Wh~tstone bridge is used, known as Kelvin bridge. The consideration of the effect of contact and lead resistances is the basic aim of the Kelvin bridge.

The resistance Rv represents the resistance of the connecting leads from R., to R,. The resistance Rx is the unknown resistance to be measured. The galvanometer can be connected to either terminal a, b or terminal c. When it is connected to a, the lead resistance Ry gets added to Rx hence the value measured by the bridge, indicates much higher value of Rx.

If the galvanometer is connected to terminal c, then Ry gets added to R3. This results in the measurement of Rx much lower than the actual value.

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The point b is in between the points a and c, in such a way that the ratio of the resistance from c to b and that from a to b is equal to the ratio of R] and R2.

Unit-3

1. Explain clearly how shunts and multipliers are used to extend the range of instruments jan14

The theory follows from Ohm's, and Kirchoff''s laws. In the case of the multiplier, the same current flows through the meter and the multiplier resistance. The meter resistance can sometimes be ignored, because it is very small compared to the multiplier.

In the case of the shunt, the same voltage is applied across the shunt and the meter resistance. The meter resistance can not be ignored.

This theory does not work with digital panel meters because the input resistance is extremely high and is unknown. In addition, they are intrinsically voltmeters - not ammeters or microammeters like a mechanical meter with a d'Arsonval movement. To calculate the shunt for a digital meter - just use Ohm's law to see what resistor (R) will give you the required V (2Volts or 200mV - depending) for the current (I) to be measured.

Digital meters and valve voltmeters use a potentiometer voltage divider - see potentiometer software next..

Formulae For Shunt:

I Rshunt=Rm FSD

whence:

Rshunt= IRm FSD

For Multiplier:

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FSD (Rmult+Rm)=V

and so:

Rmult=(VFSD)−Rm

Where:FSD = Meter Full Scale DeflectionI = Current RangeV = Voltage RangeRshunt = Shunt ResistanceRmult = MultiplierResistanceRm = Meter Resistance

2. What are the advantages of instrument transformer? jan14, jan15

Advantages:

Single range ammeters and voltmeters can measure a wide range of currents and voltages, if used in conjunction with suitable Current Transformers (CTs) and Potential Transformers (PTs)

The measuring instruments like ammeter, voltmeter and wattmeters etc are incorporated in the secondary circuit and hence they are totally segregated from the high voltage, thereby ensuring safety for the operator and observer

The meter need not be insulated for high voltages which would be the case if they are directly included in a high voltage circuit

Using current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbarwithout having to break the conductor carrying current. The core of the Current Transformer (CT) is opened at the hinge, the current carrying conductor is introduced in the center of the core through a opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer

3. Explain the theory and operation of the comparative deflection method of testing CT silsbels method. july13

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Silsbee‘s Method: The arrangement for Silsbee‘s deflectional method is shown in Fig.1. Here the ratio and phase angle of the test transformer ‗X‘ are determined in terms of that of a standard transformer ‗S‘ having the same nominal ratio.

Procedure:

The two transformers are connected with their primaries in series. An adjustable burden is put in the secondary circuit of the transformer under test. An ammeter is included in the secondary circuit of the standard transformer so that the current may be set to desired value. W1 is a wattmeter whose current coil is connected to carry the secondary current of the standard transformer. The current coil of wattmeter W2 carries a current _I which is the difference between the secondary currents of the standard and test transformer. The voltage circuits of wattmeters are supplied in parallel from a phase shifting transformer at a constant voltage V.

4. Explain the CT and PT july13, jan13, jan15

Current Transformers (CT) and Potential Transformers (PT) are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively.

A CT has large number of turns on its secondary winding, but very few turns on its primary winding. The primary winding is connected in series with the load so that it carries full load current. A low voltage range ammeter (0-5A) is connected across the secondary winding terminals. Secondary of the CT is practically short circuited since the ammeter resistance is very low. It should be remembered that

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secondary of the CT should not be made open as it draws heavy current and damages the primary winding of the CTA PT has large number of turns in the primary and fewer turns in the secondary and hence it steps down the voltage. The primary winding is connected across the supply voltage and low range voltmeter (0-110V) is connected across the secondary winding terminalsSome of the main difference between current transformers (CT) and potential transformers (PT) are given below:

The secondary of the CT is almost short circuit, whereas the secondary of the PT is practically a open circuit

The primary winding of the CT is connected in series with the load so that it carries the full line current, but there is only a small voltage across it. However the primary winding of the PT has the full supply voltage applied across it

In CT the excitation current I0 and flux density vary over a wide range whereas in PT, they vary over a limited range only

Unit-4

1. Explain with a neat figure construction and working of dynamometer type wattmeter. jan14, july13, jan15.

A watt meter is used to measure the electric power of a circuit, or sometime it also measures the rate of energy transferred from one circuit to another circuit. When a moving coil (that is free to rotate) is kept under the influence of a current carrying conductor, then automatically a mechanical force will be applied to the moving

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coil, and this force will make a little deflection of the moving coil. If a pointer is connected with the moving coil, which will move of a scale, then the deflection can be easily measured by connecting the moving coil with that pointer. This is the principle of operation of all dynamo meter type instruments, and this principle is equally applicable for dynamo meter type watt meter also.

This type of watt meter consists of two types of coil, more specifically current coil and voltage coil. There are two current coils which are kept at constant position and the measurable current will flow through those current coils. A voltage coil is placed inside those two current coils, and this voltage coil is totally free to rotate. The current coils are arranged such a way, that they are connected with the circuit in series. And the voltage coil is connected in parallel with the circuit. As simple as other voltmeter and ammeter connection. In fact, a watt meter is a package of an ammeter and a voltmeter, because the product of voltage and current is the power, which is the measurable quantity of a watt meter.

When current flows through the current coils, then automatically a magnetic field is developed around those coils. Under the influence of the electromagnetic field, voltage coil also carries some amount of current as it is connected with the circuit in parallel. In this way, the deflection of the pointer will proportional to both current and voltage of the circuit. In this way, Watt = Current × Voltage equation is satisfied and the deflection shows the value of power inside the circuit. A dynamo meter type watt meter is used in various applications where the power or energy transfer has to be measured.

2. Explain with the help of neat sketch the construction of induction type energy meter. jan14, jan13

The principle of working and construction of induction type meter is very simple and easy to

understand that's why these are widely used in measuring energy in domestic as well as industrial world. In all induction meters we have two fluxes which are produced by two different alternating currents on a metallic disc. Due to alternating fluxes there is an induced emf, the emf produced at one point (as shown in the figure given below) interacts with the alternating current of the other side resulting in the production of torque.

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Induction Type Meter

Similarly, the emf produced at the point two interacts with the alternating current at point one, resulting in the production of torque again but in opposite direction. Hence due to these two torques which are in different directions, the metallic disc moves. This is basic principle of working of aninduction type meters. Now let us derive the mathematical expression for deflecting torque. Let us take flux produced at point one be equal to F1and the flux and at point two be equal to F2. Now the instantaneous values of these two flux can written as:

where Fm1 and Fm2 are respectively the maximum values of fluxes F1 and F2, B is phase difference between two fluxes.

We can also write the expression for induced emf's at point one be

at point two. Thus we have the expression foreddy currents at point one is

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where K is some constant and f is frequency.

3. Write a short note on electronic energy meter. Jan14, july13

Electronic meters display the energy used on an LCD or LED display, and some can also transmit readings to remote places. In addition to measuring energy used, electronic meters can also record other parameters of the load and supply such as instantaneous and maximum rate of usage demands, voltages, power factor andreactive power used etc. They can also support time-of-day billing, for example, recording the amount of energy used during on-peak and off-peak hours.

4. Explain how 3phase reactive power is measured. Jan13

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In a star (wye) connected topology, with rotation sequence L1 - L2 - L3, the time-varying instantaneous voltages can be calculated for each phase A,C,B respectively by:

where: is the peak voltage,

is the phase angle in radians is the time in seconds is the frequency in cycles per second and

voltages L1-N, L2-N and L3-N are referenced to the star connection point.

5. Explain the working and operation of LPF wattmeter. July13

with the wattmeter now placed in series and the transmitter off, measure the forward RF power. Here's how:

1. Set the meter's FUNCTION switch to the POWER position. 2. Set the RANGE switch to the appropriate setting. (For the SX100, use the

30W range; and for the SX200, use the 20W range). 3. Verify that the TX and ANT output connections are secure. 4. Turn on the transmitter and turn up the output power to 10 watts.

5. Set the POWER switch on the wattmeter to FWD (forward) and record the reading. If you're using the TR6000 transmitter, the power-adjust dial is labeled on the front of the unit. If you have the earlier TR20 (Phase II) transmitter, open the transmitter lid and adjust the blue power potentiometer, located vertically just above the power-switch. (A properly tuned antenna should allow a reading of 10 watts.)

6. Record reflected power by turning the meter's POWER switch to REF. This reading should be less than 1, and less than 1/10 the forward power reading.

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6.With the necessary figures, explain the calibration of single phase energy meter. jan15

Energy meter is an instrument which measures electrical energy. It is also known as watt hour

(Wh) meter. It is an integrating meter. There are several types of energy meters. Single phase

induction type energy meters is very commonly used to measure electrical energy consumed in

domestic and commercial installations. Electrical energy is measured in kilo watt hours (kWh)

by these energy meters.

In this experiment the purpose is to calibrate the energy meter. This means we have to find out the error/ correction in the energy meter readings. This calibration is possible only if some other standard instrument is available to know the correct reading.

Wattmeter:- Wattmeter is an instrument which measures instantaneous power consumed by a circuit . It consists of two coils:-

1. Fixed coil, divided in two parts is connected in series with load and produces a flux proportional to the current.

2. Movable coil is suspended on the pivot and jeweled bearings, produces the flux proportional to the voltage across the load. Deflection of the pointer is the result of the change in the mutual inductance between the fixed and the moving coils.

PROCEDURE:

1. Connect the circuit as shown in figure and apply a rated constant AC voltage.

2. Switch on one of the loads.

3. Record the time taken for 10 revolutions of the disc of the energy meter with the help of stop watch.

4. Take voltmeter, ammeter and wattmeter readings.

5. Repeat for more number of readings (4 readings) for different loads.

6. Record the readings as per table.

7. Note the multiplication factor of the wattmeter.

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Unit-5

1. Write a note on true RMS reading voltmeter. Jan14, jan13A modern digital electronic wattmeter/energy meter samples the voltage

and current thousands of times a second. For each sample, the voltage is multiplied by the current at the same instant; the average over at least one cycle is the real power. The real power divided by the apparent volt-amperes (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The readings may be displayed on the device, retained to provide a log and calculate averages, or transmitted to other equipment for further use. Wattmeters vary considerably in correctly calculating energy consumption, especially when real power is much lower than VA (highlyreactiveloads, e.g. electric motors). Simplemeters may be calibrated to meet specified accuracy onlyfor sinusoidalwaveforms. Waveforms for switched-mode powersupplies as used for much electronic equipment may be very far from sinusoidal, leading to unknown and possibly large errors at any power. This may not be specified in the meter's manual.

2. Explain the operation of Successive approximation type of digital voltmeter. jan14

Successive-approximation DVM

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A D/A converter is used to provide the estimates. The "equal to or greater than" or "less than" decision is made by a comparator. The D/A converter provides the estimate and is compared to the input signal. A special shift register called a successive-approximation register (SAR) is used to control the D/A converter and consequentially the estimates. At the beginning of the conversion all the outputs from the SAR are at logic zero. If the estimate is greater than the input, the comparator output is high and the first SAR output reverses state and the second output changes to a logic "one." If the comparator output is low, indicating that the estimate is lower than the input signal, the first output remains in the logic one state and the second output assumes the logic state one. This continues to all the states until the conversion is complete.

3. Explain with the neat sketch the working of Electronic multimeter.July13

A multimeter or a multitester, also known as a VOM (Volt-Ohm meter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter would include basic features such as the ability to measure voltage, current, and resistance. Analog multimeters use a microammeter whose pointer moves over a scale calibrated for all the different measurements that can be made. Digital multimeters (DMM, DVOM) display the measured value in numerals, and may also display a bar of a length proportional to the quantity being measured. Digital multimeters are now far more common than analog ones, but analog multimeters are still preferable in some cases, for example when monitoring a rapidly varying value.

A multimeter can be a hand-held device useful for basic fault finding and field service work, or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as electronic equipment, motor controls, domestic appliances, power supplies, and wiring systems.

A multimeter is a combination of a multirange DC voltmeter, multirange AC voltmeter, multirange ammeter, and multirange ohmmeter. An un-amplified analog multimeter combines a meter movement, range resistors and switches.

For an analog meter movement, DC voltage is measured with a series resistor connected between the meter movement and the circuit under test. A set of switches allows greater resistance to be inserted for higher voltage ranges. The

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product of the basic full-scale deflection current of the movement, and the sum of the series resistance and the movement's own resistance, gives the full-scale voltage of the range. As an example, a meter movement that required 1 milliampere for full scale deflection, with an internal resistance of 500 ohms, would, on a 10-volt range of the multimeter, have 9,500 ohms of series resistance.[3]

For analog current ranges, low-resistance shunts are connected in parallel with the meter movement to divert most of the current around the coil. Again for the case of a hypothetical 1-mA, 500-ohm movement on a 1-Ampere range, the shunt resistance would be just over 0.5 ohms.

Moving coil instruments respond only to the average value of the current through them. To measure alternating current, a rectifier diode is inserted in the circuit so that the average value of current is non-zero. Since the rectified average value and the root-mean-square value of a waveform need not be the same, simple rectifier-type circuits may only be accurate for sinusoidal waveforms. Other wave shapes require a different calibration factor to relate RMS and average value. Since practical rectifiers have non-zero voltage drop, accuracy and sensitivity is poor at low values.

To measure resistance, a small battery within the instrument passes a current through the device under test and the meter coil. Since the current available depends on the state of charge of the battery, a multimeter usually has an adjustment for the ohms scale to zero it. In the usual circuit found in analog multimeters, the meter deflection is inversely proportional to the resistance; so full-scale is 0 ohms, and high resistance corresponds to smaller deflections. The ohms scale is compressed, so resolution is better at lower resistance values.

Amplified instruments simplify the design of the series and shunt resistor networks. The internal resistance of the coil is decoupled from the selection of the series and shunt range resistors; the series network becomes a voltage divider. Where AC measurements are required, the rectifier can be placed after the amplifier stage, improving precision at low range.

Digital instruments, which necessarily incorporate amplifiers, use the same principles as analog instruments for range resistors. For resistance measurements, usually a small constant current is passed through the device under test and the digital multimeter reads the resultant voltage drop; this eliminates the scale compression found in analog meters, but requires a source of significant current.

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An autoranging digital multimeter can automatically adjust the scaling network so that the measurement uses the full precision of the A/D converter.

In all types of multimeters, the quality of the switching elements is critical to stable and accurate measurements. Stability of the resistors is a limiting factor in the long-term accuracy and precision of the instrument.

4. With a neat sketch explain the construction and operating principal of single phase power factor meter. Jan13

AC power flow has the three components: real power (also known as active power) (P), measured in watts (W); apparent power (S), measured in volt-amperes (VA); and reactive power (Q), measured in reactive volt-amperes(var).[6]

The power factor is defined as:

In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as vectors that form a vectortriangle such that:

If is the phase angle between the current and voltage, then the power factor is equal to the cosine of the angle, , and:

Since the units are consistent, the power factor is by definitiona dimensionless number between −1 and 1. When power factor is equal

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to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source on each cycle. When the power factor is 1, all the energy supplied by the source is consumed by the load. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle. Capacitive loads are leading (current leads voltage), and inductive loads are lagging (current lags voltage).

5. Write the short notes on Q meter. jan15

A Q meter is a piece of equipment used in the testing of radio frequency circuits. It has been largely replaced in professional laboratories by other types of impedance measuring device, though it is still in use among radio amateurs. It was developed at Boonton Radio Corporation in Boonton, New Jersey in 1934 by William D. Loughlin.

A Q meter measures Q, the quality factor of a circuit, which expresses how much energy is dissipated per cycle in a non-ideal reactive circuit:

This expression applies to an RF and microwave filter, bandpass LC filter, or any resonator. It also can be applied to an inductor or capacitor at a chosen frequency. For inductors

Where is the reactance of the inductor, is the inductance, is the angular frequency and is the resistance of the inductor. The resistance represents the loss in the inductor, mainly due to the resistance of the wire.Q meter works on the principle of series resonance.

For LC band pass circuits and filters:

Where is the resonant frequency (center frequency) and is the filter bandwidth. In a band pass filter using an LC resonant circuit, when the loss (resistance) of the inductor increases, its Q is reduced, and so the bandwidth of the filter is increased. In a coaxial cavity filter, there are no inductors and capacitors, but

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the cavity has an equivalent LC model with losses (resistance) and the Q factor can be applied as well.

Unit-6

1. Explain with the help of block diagram working of digital storage oscilloscope. jan14, jan15

the basic oscilloscope, as shown in the illustration, is typically divided into four sections: the display, vertical controls, horizontal controls and trigger controls. The display is usually a CRT or LCD panel which is laid out with both horizontal and vertical reference lines referred to as the graticule. In addition to the screen, most display sections are equipped with three basic controls: a focus knob, an intensity knob and a beam finder button.

The vertical section controls the amplitude of the displayed signal. This section carries a Volts-per-Division (Volts/Div) selector knob, an AC/DC/Ground selector switch and the vertical (primary) input for the instrument. Additionally, this section is typically equipped with the vertical beam position knob.

The horizontal section controls the time base or "sweep" of the instrument. The primary control is the Seconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-Y axis signals. The horizontal beam position knob is generally located in this section.

The trigger section controls the start event of the sweep. The trigger can be set to automatically restart after each sweep or it can be configured to respond to an internal or external event. The principal controls of this section will be the source and coupling selector switches. An external trigger input (EXT Input) and level adjustment will also be included.

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In addition to the basic instrument, most oscilloscopes are supplied with a probe as shown. The probe will connect to any input on the instrument and typically has a resistor of ten times the oscilloscope's input impedance. This results in a .1 (-10X) attenuation factor, but helps to isolate the capacitive load presented by the probe cable from the signal being measured. Some probes have a switch allowing the operator to bypass the resistor when appropriate

2. Explain with the help of block diagram working of dual trace oscilloscope. jan14, july13, jan15

A digital storage oscilloscope is an oscilloscope which stores and analyses the signal digitally rather than using analoguetechniques. It is now the most common type of oscilloscope in use because of the advanced trigger, storage, display and measurement features which it typically provides

The input analogue signal is sampled and then converted into a digital record of the amplitude of the signal at each sample time. The sampling frequency should be not less than the Nyquist rate to avoid aliasing. These digital values are then turned back into an analogue signal for display on a cathode ray tube (CRT), or transformed as needed for the various possible types of output—liquid crystal display, chart recorder, plotter or network interface.

Digital storage oscilloscope costs vary widely; bench-top self-contained instruments (complete with displays) start at US$300or even less, with high-performance models selling for tens of thousands of dollars. Small, pocket-size models.

3. Explain the measurement of phase and frequency using lissajous patterns. jan14, jan15

In mathematics, a Lissajous curve /ˈlɪsəʒuː/, also known as Lissajousfigure or Bowditch curve /ˈbaʊdɪtʃ/, is the graph of a system of parametricequations

which describe complex harmonic motion. This family of curves was investigated by Nathaniel Bowditch in 1815, and later in more detail by Jules Antoine Lissajous in 1857.

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The appearance of the figure is highly sensitive to the ratio a/b. For a ratio of 1,the figure is an ellipse, with special casesincluding circles (A = B, δ = π/2 radians) and lines (δ = 0). Another simpleLissajous figure is the parabola (a/b = 2, δ = π/4). Other ratios produce more complicated curves, which are closed only if a/b isrational. The visual form of these curves is often suggestive of a three-dimensional knot, and indeed many kinds of knots, including those known as Lissajous knots, project to the plane as Lissajous figures.

Visually, the ratio a/b determines the number of "lobes" of the figure. For example, a ratio of 3/1 or 1/3 produces a figure with three major lobes (see image). Similarly, a ratio of 5/4 produces a figure with 5 horizontal lobes and 4 vertical lobes. Rational ratios produce closed (connected) or "still" figures, while irrational ratios produce figures that appear to rotate. The ratio A/B determines the relative width-to-height ratio of the curve. For example, a ratio of 2/1 produces a figure that is twice as wide as it is high. Finally, the value of δ determines the apparent "rotation" angle of the figure, viewed as if it were actually a three-dimensional curve. For example, δ=0 produces x and y components that are exactly in phase, so the resulting figure appears as an apparent three-dimensional figure viewed from straight on (0°). In contrast, any non-zero δ produces a figure that appears to be rotated, either as a left/right or an up/down rotation (depending on the ratio a/b).

Lissajous figure on an oscilloscope, displaying a 1:3 relationship between the frequencies of the vertical and horizontal sinusoidal inputs, respectively.

Lissajous figures where a = 1, b = N (N is a natural number) and

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are Chebyshev polynomials of the first kind of degree N. This property is exploited to produce a set of points, calledPadua points, at which a function may be sampled in order to compute either a bivariate interpolation or quadrature of the function over the domain [-1,1]×[-1,1].

4. Classify transducers with an example each. Jan13

Transducer ClassificationSome of the common methods of classifying transducers are given below.

Based on their application.

Based on the method of converting the non-electric signal into electric signal.

Based on the output electrical quantity to be produced.

Based on the electrical phenomenon or parameter that may be changed due to the whole process. Some of the most commonly electrical quantities in a transducer are resistance, capacitance, voltage, current or inductance. Thus, during transduction, there may be changes in resistance, capacitance and induction, which in turn change the output voltage or current.

Based on whether the transducer is active or passive.

5. Explain the construction and operating principal of LVDT with necessary sketches, jan13, jan14, july13

The linear variable differential transformer (LVDT) (also called justa differential transformer linear variable displacementtransformer, or linear variable displacement transducer is a type of

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electrical transformer used for measuring linear displacement (position). A counterpart to this device that is used for measuring rotary displacement is called a rotary variable differential transformer (RVDT).

6. List out the temperature detectors. Explain resistance temperature detector. Jan13.

Thermistor- Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature. Negative Temperature Coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and Positive Temperature Coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature.

Thermocouple Resistance thermometer Silicon bandgap temperature sensor

Unit-7& 8

1. Explain the photoconductive and photovoltaic cell. Jan14, july13

PHOTO- VOLTAIC IN SEMICONDUCTORS:

The height of the potential barrier is an open circuited dark (non-illuminated) P-N junction adjusts itself such that resultant current is zero. Under this condition, the electric field at the junction is in such a diretion so as to repel the majority carriers. When light is incident on diode surface, minority carriers get injected & hence the minority current increases. But since the diode is open circuited, the resultant current must remain zero. Therefore majority current should increase by the same amount as the minority carrier current. This increase in majority current is possible if the retarding electric field at the junction is reduced resulting in the lowering of the barrier height. Therefore across the diode terminals there appears voltage which is equal the decrease in the barrier potential. This constitutes the photovoltaic e.m.f. & is of the order of 0.1 volts for the Ge cell & 0.5 volt for Si cell.

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DERIVATION OF EXPRESSION:We have seen that the photovoltaic e.m.f. Vp appears across the diode when the net current I in the diode is zero.Substituting I= 0. In the volt–ampere characteristics of a photo diode given by

I = Is + I0 ( 1- eVe/ ηkT )We get Is + I0 ( 1- eVe/ ηkT ) = 0

1+ ( Is/I0) = e Vpe/ ηkTlog ( 1+ ( Is/I0) ) = Vpe/ηkT .

Therefore photo-voltaic e.m.f.Vp =(ηkT/e) log ( 1+ ( Is/I0) )

But Is/I0 >> 1, except for extremely small light intensities.

Vp = (ηkT/e) log ( 1+ ( Is/I0) )

This equation shows that the photovoltaic e.m.f Vp increases algorithmically with Is and hence with illumination it has been shown diagrammatically.PHOTOVOLTAIC CELLS:When a pair of electrodes is immersed in an electrolyte & light is allowed to incident on one of them, a potential difference is created between the electrodes this phenomenon is called photovoltaic effect. Devices based on this effect are known as photovoltaic cells. In a photovoltaic cells light energy is used to create a potential difference the potential difference so developed is directly proportional to the frequency & intensity of incident light.CONSTRUCTION & WORKING:A basic photovoltaic cells consist of peace of semi conducting materials bonded to a metal plate. Materials like selenium & silicon are mostly used for preparing photovoltaic cells.

When light is made to fall on semi conducting material, valence electron holes are liberated from its crystal structures the electrons so liberated move towards the metal plate where as holes flow in opposite directions thus a potential difference is created between the semi conducting materials and the metal plate. Consequently a conventional current flows in the external circuit through a load resistor R .In actual form of photovoltaic cells a thin metallic film of silver,gold or platinum is deposited on a semi conducting layer like cuprous oxide (Cu2O) or iron selenide. The whole arrangement is than attached to a metal based plate (copper) as shown in the figure.

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When external light is allowed to fall on metallic film F, it penetrates easily and at the barrier layer between the metallic film and the semiconductor, photo-electric emission occurs. The photoelectrons so emitted from the layer, move towards the metallic film. Consequently the metallic film F becomes negatively charged and the copper based plate positively charged. Hence a potential difference is developed between two and a current flows in the external circuit.The strength is proportional to the intensity of light and flows without any bias i.e. without any external source of e.m.f.USES:These cells are used in devices like

1. Photographic exposure metre.

2. Direct reading illuminations metre.

3. Operation of relays.

SOLAR CELLS:A solar cells or solar battery is basically a P–N junction diode which converts solar energy into electrical energy. It is also called a solar energy converter and is simply a photo diode operated zero bias voltage.

CONSTRUCTION:A solar cell consists of a P–N junction diode generally made of Ge or Si. It may also be constructed with many other semi conducting materials like GaAs, indium arsenide and cadmium arsenide. The P–N diode so formed is packed in a can with glass windows on top so that light may fall upon P & N type materials. The thickness of P region of is kept very small so that electrons generated in this region can deffuse to the junction before the recombination takes place. Thickness of N region is also kept small to allow holes generated near the surface to diffuse to the junctions before they recombine. A heavy doping of P and N regions is recommended to obtain a large photo voltage. A nickel plated ring is provided around the P layer which acts as the positive output terminal. A metal contact at the bottom serves as the negative output terminal.

WORKING:The working of solar cells may be understood with reference of figure When light is allowed to fall on a P-N junction diode, photons collide with valence electrons and impart them sufficient energy enabling them to leave there parent atoms. Thus

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electrons hole pairs are generated in both the P and the N sides of the junctions . These electrons and holes reach the depletion region W by diffusion and are then separated by a strong barrier field existing between there. However the minority carriers, electrons in the p-side , slide down the barrier potential to reach the Inside and the holes in the N-side move to P-side. Their flow constitutes the minority current which is directly proportional to the illumination and also depends on the surface area being exposed light.

The accumulation of electrons and holes on the two sides of the jnction gives rise to an open circuit voltage Voc which is a function of illumination. The open circuit voltage produced for a silicon solar cells is typically 0.6 volt & the short circuit current is about 40 m A / cm2 in bright noon day sun light power conversion efficiency of about 15% are obtained with a thin N diffused layer into a P wafer. Many such cells are interconnected to provide large quantities of electrical power. Solar panels providing 5watt at 12 volt have been built to operate 24 hrs a day by recharging the batteries during day light hrs.

Characteristics:Typical V- I characteristics of a solar cell corresponding to different levels of illuminations are shown in the figure. It may be seen that for 100 m W/cm2 illuminations the open circuit voltage is about 0.57 volt while the short circuit current is 50 m A. maximum power output is however obtained when the cell is operated at the knee of the curve.

2. What are the selection criteria for the transducer. jan14

The selection of a transducer is the result of a technical and economic trade-off, considering the transducer as well as the associated subsystems. All aspects of an application must therefore be taken into account during transducer selection and system design, with particular attention to the following:electrical requirements - including power supply requirements, peak measurement, response time, di/dt, dv/dtmechanical requirements - including aperture size, overall dimensions, mass, materials, mountingthermal considerations - including current profile versus time, maximum RMS measurement, thermal resistances, cooling methodsenvironmental considerations - including vibration, temperature, proximity of other conductors or magnetic fieldsThe transducer development process includes a full regimen of tests that comprise the characterization report. These tests follow the scientific method, varying individual parameters to characterize the transducer's response to each. During

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production, a quality plan indicates the tests to be carried out on each product or batch of products to verify compliance with specifications. Unless otherwise specified, performance is tested under nominal conditions of current, voltage, temperature, and other parameters in the production and laboratory environments. In the actual application, several factors can act simultaneously and potentially produce unexpected results. It is therefore necessary to assess the transducer in these specific conditions to verify acceptable performance. This assessment is typically not difficult if the working conditions are well known.The operating temperature range is based on the materials and construction of the selected transducer. Minimum temperatures are typically in the range of -40 °C to - 10 °C and maximum temperatures in the range of 50 °C to 105 °C.

3. Classify electrical transducers. july13

Electrical TransducersHere are some of commonly used electrical transducers:1) Potentiometers: They convert the change in displacement into change in the resistance, which can be measured easily. 2) Bridge circuits: These convert the physical quantity to be measured into the voltage. 3) Wheatstone bridge: It converts the displacement produced by the physical quantity to the current in the circuit. 4) Capacitive sensors or Variable Capacitance Transducers: These comprise of the two parallel plates between which there is dielectric material like air. The change in distance between the two plates produced by the displacement results in change in capacitance, which can be easily measured. 5) Resistive sensors or Variable Resistance Transducers: There is change in the resistance of these sensors when certain physical quantity is applied to it. It is most commonly used in resistance thermometers or thermistors for measurement of temperature. 6) Magnetic sensors: The input given to these sensors is in the form of displacement and the output obtained is in the form of change in inductance or reluctance and production of the eddy currents. 7) Piezoelectric transducers:When force is applied to these transducers, they produce voltage that can be measured easily. They are used for measurement of pressure, acceleration and force. 8) Strain gauges: When strain gauges are strained or stretched there is change in their resistance. They consist of the long wire and are able to detect very small displacements produced by the applied force or pressure.

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9) Photo electric transducers: When the light is applied to these transducers they produce voltage. 10) Linear variable differential transformer (LVDT): LVDT is the transformer consisting of the primary and the secondary coil. It converts the displacement into the change in resistance. 11) Ultrasonic Transducers: These transducers use the ultrasonic or ultrasound waves to measure parameters like fluid level, flow rate etc.

4. What do you mean by DIC explain with a help of block diagram. jan13, jan14, july13.

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5. With the help of neat sketch the working of XY recorder . jan14, jan13

x-y recorder is an instrument which gives a graphic record of the relationship between two variables. In x-y recorder's emf is plotted as a function of another emf. This is done by one self balancing potentiometer which controls the position of paper or chart roll while another self balancing potentiometer controls the position of the recording pen. The emf used for the operation of x-y recorders not necessarily measure only voltage. the measure emf may be the output of the transducer which may be measure of displacement ,force,pressure,strain or any other physical quantities.

6. With the help of neat diagram explain the working of function generator. jan14, july13, jan15

A function generator is usually a piece of electronic testequipment or software used to generate different types ofelectrical waveforms over a wide range of frequencies. Some of the most common waveforms produced by the function generator are the sine, square, triangular and sawtooth shapes. These waveforms can be either repetitive or single-shot (which requires an internal or external trigger source).[1] Integrated circuits used to generate waveforms may also be described as function generator ICs.

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Although function generators cover both audio and RF frequencies, they are usually not suitable for applications that need low distortion or stable frequency signals. When those traits are required, other signal generators would be more appropriate.Working

Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps.[3] This triangular wave is used as the basis for all of its other outputs. The triangular wave is generated by repeatedly charging and discharging a capacitor from a constant current source. This produces a linearly ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the charging and discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequencies may be obtained. Sawtooth waves can be produced by charging the capacitor slowly, using a current, but using a diode over the current source to discharge quickly - the polarity of the diode changes the polarity of the resulting sawtooth, i.e. slow rise and fast fall, or fast rise and slow fall.

A 50% duty cycle square wave is easily obtained by noting whether the capacitor is being charged or discharged, which is reflected in the current switching comparator output. Other duty cycles (theoretically from 0% to 100%) can be obtained by using a comparator and the sawtooth or triangle signal. Most function generators also contain a non-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave by rounding off the corners of the triangle wave in a process similar to clipping in audio systems.

A typical function generator can provide frequencies up to 20 MHz. RF generators for higher frequencies are not function generators in the strict sense since they typically produce pure or modulated sine signals only.

Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.

Some function generators can also generate white or pink noise.[citation needed]

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More advanced function generators are called arbitrary waveform generators (AWG). They use direct digital synthesis (DDS) techniques to generate any waveform that can be described by a table of amplitudes.

7. Write a note on display devices. July13.A display device is an output device for presentation

of information in visual or tactile form (the latter used for examplein tactile electronic displays for blind people). When the input information is supplied as an electrical signal, the display is called an electronic display

Applications

Full-area 2-dimensional displays are used in, for example:

Television sets Computer monitors Head-mounted displayBroadcast reference monitor Medical monitors

8. Write short notes on LED and LCD display. jan15 , jan13.

LCD stands for ―liquid crystal display‖ and technically, both LED and LCD TVs are liquid crystal displays. The basic technology is the same in that both television types have two layers of polarized glass through which the liquid crystals both block and pass light. So really, LED TVs are a subset of LCD TVs.

LED, which stands for ―light emitting diodes,‖ differs from general LCD TVs in that LCDs use fluorescent lights while LEDs use those light emitting diodes. Also, the placement of the lights on an LED TV can differ. The fluorescent lights in an LCD TV are always behind the screen. On an LED TV, the light emitting diodes can be placed either behind the screen or around its edges. The difference in lights and in lighting placement has generally meant that LED TVs can be thinner than LCDs, although this is starting to change. It has also meant that LED TVs run with greater energy efficiency and can provide a clearer, better picture than the general LCD TVs.

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LED TVs provide a better picture for two basic reasons. First, LED TVs work with a color wheel or distinct RGB-colored lights (red, green, blue) to produce more realistic and sharper colors. Second, light emitting diodes can be dimmed. The dimming capability on the back lighting in an LED TV allows the picture to display with a truer black by darkening the lights and blocking more light from passing through the panel. This capability is not present on edge-lit LED TVs; however, edge-lit LED TVs can display a truer white than the fluorescent LED TVs.

Because all these LCD TVs are thin-screen, each has particular angle-viewing and anti-glare issues. The backlit TVs provide better, cleaner angle viewing than the edge-lit LED TV. However, the backlit LED TV will usually have better angle viewing than the standard LCD TV. Both LED and LCD TVs have good reputations for their playback and gaming quality.

9. Write a note on Weston frequency meter. jan13, jan15

The main principle of working of weston type frequency meter is that "when an electric current flows through the two coils which are perpendicular to each other, due to these currents some magnetic fields will produce and thus the magnetic needle will deflects towards the stronger magnetic field showing the measurement of frequency on the meter". Construction of weston frequency is as compared to ferrodynamic type of frequency meter. In order to construct a circuit diagram we need two coils, three inductors and two resistors. Given below is the circuit diagram for the weston type frequency meter.

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Axis of both coils are marked as shown. Scale of the meter is calibrated such that at standard frequency the pointer will take position at 45°. Coil 1 contains a series resistor marked R1 and reactance coil marked as L1, while the coil 2 has a series reactance coil marked as L2 and parallel resistor marked as R2. The indcuctor which is marked as L0 is connected in series with the supplyvoltage in order to reduce the higher harmonic means here this inductor is working as a filter circuit. Let us look at the working of this meter.

Now when we apply voltage at standard frequency then the pointer will take normal position, if there increase the frequency of the applied voltage then we will see that the pointer will moves towards left marked as higher side as shown in the circuit diagram. Again we reduce the frequency the pointer will start moving towards the right side, if lower the frequency below the normal frequency then it cross the normal position to move towards left side marked lower side as shown in the figure.

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Now let us look at the internal working of this meter. Voltage drop across an inductor is directly proportion to frequency of the source voltage, as we increase the frequency of the applied voltage the voltage drop across the inductor L1 increase that means the voltage impressed between the coil 1 is increased hence the current through the coil 1 increase while the current through the coil 2 decreases. Since the electric current through the coil 1 increases the magnetic field also increases and the magnetic needle attracts more towards the left side showing the increment in the frequency. Similar action will takes if decrease the frequency but in this the pointer will moves towards the left side.

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Documents

· Web viewAC bridges are often used to measure the value of unknown impedance (self/mutual inductance of inductors or capacitance of capacitors accurately). A large number of AC bridges