TechStudy

Ideal diode vs Practical diode

2022-09-18T17:22:00.000+05:30

Ideal diode

The ideal diode is a perfect two state device which exhibit zero impedance when forward bias and infinite impedance when reverse Biased. Since either current or voltage is zero at any instant, no power is dissipated by an ideal diode.

Practical diode

Practical diode have not zero impedance when forward bias and do not exhibit infinite impedance when reverse based.

What is Semiconductor Diode.

2021-09-19T17:55:00.002+05:30

1. What is Semiconductor Diode?

A diode is an electronic device made up of, when some doping is done in semiconductor like Silicon and Germanium. Diode has a property that is pass the current in one direction only (work as short circuits) and do not pass the current on other side (work like the open circuits).

Content:-

1.1 Semiconductor Diode.

1.2 Biasing and working of Semiconductor Diode.

1.3 Terminology used in Semiconductor Diode.

1.4 Voltage- Current Characteristics of Semiconductor Diode.

1.1 Semiconductor Diode:

When a single crystal of semiconductor is doped with acceptors on one side and donors on the other side a p-n junction is formed. A schematic diagram of a p-n junction is shown in fig. 1.1. In this figure donor ions are denoted by plus signs and the electrons they donate are represented by the small filled circles. The holes are shown by unfilled small circles and acceptor ions by the minus signs.

Fig. (1.1)

Fig (1.2)

A semiconductor diode contains two terminals. When it is connected in circuit, one thing to decide is whether the diode is forward or reverse biased. The p-side of the diode is always the positive terminal for forward bias and designated the anode. The n-side is called cathode and is the negative terminal when the device is forward biased. The holes and electrons are the mobile charges, while the ions are immobile charges.

1.2 Biasing and Working of Semiconductor Diode

1.2.1 Forward Bias:- When an external voltage is applied with the polarity i.e. + V to the positive terminal and – V to the negative terminal of the diode, this condition is known as Forward Bias. An ideal p-n diode has a zero ohm voltage drop across the body of the crystal. For such a diode, the height of the potential barrier at the junction is lowered by the applied forward voltage V. The equilibrium initially establishes between the forces tending to produce diffusion of majority carriers and the restraining influence of the potential energy barrier at the junction will be disturbed. Therefore, the holes cross the junction from the p-type to the n-type and the electrons cross the junction in the opposite direction. These majority carriers can then travel around the closed circuit and a relatively larger current will flow.

The semiconductor diode works or conducts current in the forward bias only. The mechanism of current flow in a forward biased p-n junction can be described in the following way:-

(i) The free electrons from the negative terminal continue to pour the n-region while the free electrons in the n-region move towards the junction.

(ii) The electron travels through the n-region as free electrons i.e., current in n-region by free electrons.

(iii) When these electrons reach the junction, they combine with holes and become valence electrons.

(iv) The electrons travel through p-region as valence electrons i.e., current in the p-region is by holes.

(v) When these valence electrons reach the left end of crystal, they flow into the positive terminal of the battery.

From this we conclude that in n-type region, current is carried by free electrons whereas in p-type region, it is carried by holes. However, in the external connecting wires the current is carried by electrons.

1.2.2 Reverse Bias:- When an external voltage is applied with the reverse polarity i.e + V to the negative terminal and – V to the positive terminal of the diode, this condition is known as Reverse Bias or blocking bias. Due to this type of connection, holes in the p-type and electrons in the n-type move away from the junction. As a consequence, the region of negative charge density spread to the left of the junction and positive charge density region is spread to the right. However, this process cannot go on indefinitely because in order to have a steady flow of holes to the left, these holes must be supplied across the junction from the n-type germanium and there are very few holes in the n-type side. Therefore, nominally, zero current results.

In fact, a small current does flow because a small number of hole-electron pairs are produced throughout the crystal due to thermal energy. The holes formed in the n-type germanium will wander over to the junction. A similar remark applies to the electrons thermally produced in the p-type germanium. This small current is known as the diode reverse saturation current. This reverse current will increase with the increasing temperature. Therefore, back resistance of the crystal diode decreases on increasing the temperature.

This mechanism of conduction in the reverse direction can be understood in another way. Let us say, when no voltage is given to the p-n diode, the potential barrier across this junction exists. If a voltage V is given to the diode in the reverse direction, then height of the potential-energy barrier is increased by the amount eV. Due to this increase in barrier height , the flow of majority carriers is reduced. But the minority carriers are unaffected by the increased height of the barrier.

1.3 Terminology used in Semiconductor Diode.

1.3.1 Depletion Layer:-

The depletion layer is the layer where to the two semiconductor meet together and form the parallel plates of opposite charges, this is formed due to the uncovered charges with the depletion region. Therefore it is called depletion layer. The depletion layer behaves like an insulator. The depletion layer possesses capacitance because of the presence of fixed charge plates. The width of the depletion layer depends upon the doping level of the immunity in n-type and p-type semiconductors. The higher the doping level, the thinner will be the depletion layer.

1.3.2 Barrier Potential:-

The depletion layer of a p-n junction contains fixed plates of oppositely charged ions on its two sides. Because of this charge separation, an electric potential is established across the junction, even when the junction is not connected to any external voltage source. This electric potential is known as barrier potential.

The potential barrier exerts a repelling force on the mobile charge carriers, trying to crossover the junction. This force stops the mobile charge carriers to crossover the junction, unless the energy is supplied from an external source.

1.3.3 Transition Capacitance:-

The transition capacitance represents the change in charge stored in the depletion region with respect to a change in junction voltage. The increase in the level of reverse bias caused the width of the depletion region, W to increase. An increase in the width of the depletion region, W is accompanied by additional uncovered ions in the space charge or transition region. Because positive ions exist on one side of the junction and negative ions on the other, the transition capacitance, C_T is analogous to a

C_T = A_ (F)

Where,

A= Junction Area.

€= Permittivity of the semiconductor.

C_T=Transition capacitance.

W= Width of the depletion layer which is inversely proportional to the square root of the reverse bias voltage.

We must note that W is a function of the reverse biased voltage so that transition capacitance, C_T is voltage dependent. For a step-graded junction the width of the depletion region, W is inversely proportional to the square root of the reverse bias voltage. Under forward biased conditions, the value of transition capacitance is so small compared to diffusion capacitance that it is generally neglected. Similarly in a reverse biased diode, a small amount of carrier diffusion exists, but this capacitance is negligible when compared to transition capacitance.

1.3.4 Peak Inverse Voltage (PIV):-

During the negative half cycle of the input, the diode is reverse biased. The whole of the input voltage appears across the diode. When the input reaches its peak value, Vm in the negative half cycle, the voltage across the diode is also maximum. This maximum voltage is called the peak inverse voltage. It represents that the maximum voltage of the diode must withstand during the negative half cycle of the input.

When diode is used as a rectifier, the peak inverse voltage is extremely important. In rectifier service, it has to be ensured that reverse voltage across the diode does not exceed its peak inverse voltage during the negative half cycle of input A.C. voltage.

1.3.5 Breakdown Voltage:-

It is the minimum reverse voltage at which p-n junction breaks down with sudden rise in reverse current. Even at room temperature, some hole-electron pairs (minority carriers are produced in the depletion layer. With reverse bias, the electron move towards the positive terminal of supply. At large reverse voltage, these electrons acquire high enough velocities to dislodge valence electrons from semiconductor atoms. The newly liberated electrons in turn free other valence electrons. In this way, we get an avalanche of free electrons. Therefore, the p-n junction conducts a very large reverse current.

1.3.6 Knee Voltage:-

It is the forward voltage at which the current through the junction starts to increase rapidly. When a diode is forward biased, it conducts current very slowly until we overcome the potential barrier. For silicon p-n junction, the potential barrier is 0.7 V whereas it is 0.3 V for germanium junction.

Diagram1.3.6

1.4 Voltage- Current Characteristics of Semiconductor Diode:-

For a p-n junction, the current I is related to the applied voltage V by the equation as follows:-

I = I_o( ^V/^V_T– 1) -------------- (i)

Where,

I_o = Reverse saturation current

V_T = Volt equivalent of temperature.

A positive value of current I mean that current flows from the p-side to n-side. The diode is forward biased if voltage V is positive and indicating that the p-side of the junction is positive with respect to the n-side. The symbole is 1 for germanium and is approximatiely 2 for silicon at the rated current. The volt equivalent of temperature is obtained by the equation as

V_T = T / 11600 ------------------------- (ii)

Where T is temperature in degree Kelvin, i.e. (^oK). At room temperature, T = 27 ^oC = (27+273)^oK= 300 ^oK, V_T = 300/ 11600 = 0.026 V = 26 mV.

The volt-ampere characteristic described by equation (i) is shown in fig. When voltage V is positive and several times V_T, the unity within the bracket in equation (i) may be neglected. Hence the current I increase exponentially with voltage V except for very small values of voltage V. When the diode is reverse biased and IVI is several times V_T, then the current approximately equals to the negative value of reverse saturation current i.e. I - I_o. The reverse current is therefore constant, independent of the applying reverse bias. As a consequence I_o is referred to as the reverse saturation current.

Diagram 1.4

The voltage at which the current starts to increase rapidly is called the cut-in voltage or knee voltage (V_o) of the diode, whose value is 0.7V for silicon and 0.3V for germanium. In the reverse bias, the diode current is very small, only few µA for germanium diodes and only a few nA for silicon diodes. It remains small and almost constant for all voltage less than the break down voltage V_Z. At breakdown the current increase rapidly for small increase in voltage.

Diode Biasing

2021-01-16T13:58:00.000+05:30

When a single crystal of semiconductor is doped with acceptors on one side and donors on the other side a p-n junction is formed. A schematic diagram of a p-n is shown in fig (a) . In this figure donor ions are denoted by plus signs and the electrons they donate are represented by the small filled circles. The holes are shown by unfilled small circles and acceptor ions by the minus signs.

A p-n junction cannot be formed by simply joining the two pieces together because it would produce a discontinuous crystal structure. Special fabrication techniques are needed to form a p-n junction. An important property of semiconductor diode is that it allows the conduction current in only one direction. For this it is used as a rectifier. A schematic symbol of crystal diode is depleted in fig (b). The arrow in the symbol represent the direction of easier conventional current flow. A semiconductor diode contains two terminals. When it is connected in the circuit, one this is to decided is whether the diode is forward or reverse biased. The p-side of the diode is always the positive terminal for forward bias and designated the anode. The n- side is called the cathode and is the negative terminal when the device is forward biased. The holes and electrons are the mobile charges, while the ions are immoblies charges.

WORKING OF A SEMICONDUCTOR DIODE

Forward Bias :- When an external voltage is applied with the polarity i.e positive terminal of baterry is connected with positive terminal of diode and negative terminal of the battery is connected with negative terminal of the diode, it is known as forward bias. An ideal p-n diode has a zero ohmic voltage drop across the body of the crystal. For such a diode, the height of the potention barrier at the juntion is lowered by the applied forward voltage V.

The equilibrium initially establishes between the forces tending to produce diffusion of majority cariers and the restraining influence of the potential energy barrier at the junction will be disturbed. Therefore, the holes cross the junction from the p-type to the n-type and the electrons cross the junction in the opposite direction. These majority carries can then travell around the closed circuit and a relatively large current will flow.

Reverse Bias :- The applied voltage in the direction i.e negative terminal of baterry is connected with positive terminal of diode and positive terminal of the battery is connected with negative terminal of the diode, it is known as reverse or blocking bias. Due to this type of connection, holes in the p-type and electrons in the n-type move away from the junction. As a consequene, the region of negative-charge density is spread to the left of the junction and positive charge density region is spread to the right. However, this process cannot go on indefinitely because in order to have a steady flow of holes to the left, these holes must be supplied across the junction from the n-type germanium and there are very few holes in the n-type side. Therefore, nominally, zero current results.

In fact, a small current does flow because a small number of holes - electron pairs are produced throughout the crystal due to thermal energy. The holes formed in the n-type germanium will wander over to the junction. A similar remark applies to the electrons thermally produced in the p-type germanium. This small current is known as the diode revers saturation current. This reverse current will increase with increasing temperature. Therefore, back resistance of the crystal diode decreases on increasing the temperature.

The another way to be understand is :- When no voltage is given to the p-n diode , the potential barrier across this junction exists. If a voltage v is given to the diode in the reverse direction, then height of the potential- energy barrier is increased by the amount eV. Due to this increase in barrier height , the flow of majority carriers is reduced. But the minority carriers are unaffected by the increased height of the barrier.

Transistor, PNP , NPN Transistor and its working principal

2020-09-15T17:22:00.001+05:30

Introduction:- Transistor is a device which works on the phenomena of transfer of resistance. Transistor is a device which are made by sandwich the to p-n diode.

Types of Transistor:-

There are two types of transistors:

1) N-P-N Transistor.

2) P-N-P Transistor.

1) N-P-N Symbolic Diagram

Fig. (1)

2) P-N-P Symbolic Diagram:-

Fig. (2)

Working of Transistor:-

Fig. (3)

The systematic diagram of transistor is shown is in fig. (3) .

1) For working of transistor the emitter is forward bias w.r.t base and collector is reverse bias w.r.t base.

2) The forward bias are at the emitter base Junction reduce the barrier potential and narrow the depletion region. However the relative Lite drop bass light doped base and collector regions produce a width depletion region under the reverse bias. The worth nothing point is that the effective base width w b between the depletion region is very narrow.

Electron are injected into the emitter region by the emitter bias supply voltage emitter base. conduction band electrons have enough energy to overcome the emitter base barrier potential. The injected electrons enter the very thin, lightly doped this region. because the base is very lightly doped related to the emitter region, only a few of the electrons are We combine with the holes dropped into the base.

injection of electrons make the electrons consideration on the amateur junction very large and on the collector junction the constitution of the electrons is extremely small and as such the gradients of the electrons concentration is very large in the base. Injected electrons diffuse into the collective region to to extremely small thickness of base which is much less than the diffusion length. Most of the electrons process into the collector region for stop collector is Reverse Biased and creates a strong electrostatic field between the base and collector. The field and immediately immediately Ee collects the diffuse electrons which enter the collector Junction. Flow of electrons into the base region when confronted with the holds a few electrons se 12 5% combined and utilise and rest of the electron se 95 to 95% of the injected electron diffuse into the collector region and collected by the collector electrodes. To maintain the base neutrality the base electrodes provide equal number of electrons which have combined with holes and result in a base current. Does the emitter current I is equal to the sum of collector current IC and base current IB. The ratio of collector current ic2 emitter current I is called alpha and it is measured of possible current amplification. In a transistor Alpha cannot be higher than Unity, but practical value of 0.95 to 0.9 and are attend in commercial transistor.

The operation of both PNP and NPN transistor are same on the basis of above as explained.

How to find Emitter, Base & Collector Terminal:-

To find the emitter, base and collector terminal the following step to be taken:

1) First we have to find out the type of transistors. As we know that transistor is made up of by sandwiching to P-N junction diode. So first find out the terminal "Base".

2) Base is a common terminal between emitter and collector. It's resistance is intermediate with emitter and collector. Put the negative terminal of the multi meter on one terminal at positive terminal on the other two terminal one by one. If the multi meter shows continuity with both the rest of terminal than this terminal is base. You have to check like this with all three terminal and find out the base.

3) Emitter is a terminal with the resistance grater than base and lower than collector. Similarly Collector is a terminal with the resistance more grater than base and emitter . Put the terminal of the multi meter with the other's two terminal and find out the resistance value. The lower resistance value with respect to base shows the " Emitter Terminal" and the grater resistance value with respect to base shows the " Collector Terminal".

Application:-

Transistor is used in many application such as switches, amplifiers etc.

Operating condition for a transistor

1. Forward reverse biasing- For normal operation of a transistor emitter base Junction is always forward biased and collector base junction is always reverse biased full stop this type of icing is known as forward reverse biasing for the purpose of this battery e v e p is connected between ammeter and base while battery vcb is connected between collector and base. In forward reverse biasing the transistor is in region and the collector current Ic depend upon the emitter current Ie. Generally transistor is operated in this region for amplification.

2. Forward-Forward Biasing- In this by saying both the collector and emitter a foreword by. The transistor operated in saturation region. In this region the collector current comes independent of the base current. Thus the transition act like a closed switch.

3. Reverse reverse biasing:- In this biasing both junctions are reverse biased first off this by saying the transistor are practically zero current because the amateur does not emit charge carrier into the base and no charge carrier are collected by the collector, accept a few thermally generated minority carriers. Thus the transistor acts like and open switches.

4. Reverse forward Biasing:- In this emitter - base junction is reverse biased while base-collector junction is forward biased. So the emitter dose not emit any charge carrier hence no any carrier injected to base as the depletion layer between the emitter - base is broader wide. On the other side the depletion region between the base-collector is very thin but no majority charge carriers injected to base therefore little bit carriers enter in the collector and negligible current flow through the collector.

Binary to Octal Conversation

2020-08-12T15:34:00.003+05:30

Let's learn how to convert binary numbers into octal numbers system.

Example: - (100111001)2 into ( )8

Step 1: - Make a pair of 3 bits of binary numbers.

100 111 001

Step 2: - Write down the octal equivalent of the paired binary numbers.

100 111 001

| | |

4 7 1

Step 3: - The octal equivalent of the paired binary numbers is (471)8.

Binary to Hexadecimal Conversation

2020-08-12T15:17:00.007+05:30

Let us learn how to convert binary numbers into hexadecimal numbers.

Example: -(101011001001)10 into ( )16

Step 1: - Make a pair 4 bits binary number from LSB side to MSB.

1010 1100 1001

Step 2: - Write down the decimal equivalent of the paired binary numbers.

1010 1100 1001

| | |

10 12 9

Step 3: - Write down the hexadecimal equivalent of the decimal numbers.

1010 1100 1001

| | |

10 12 9

| | |

A C 9

Step 4: - So the hexadecimal equivalent of the binary numbers comes (AC9)16.

Binary to Decimal Conversation

2020-08-09T22:28:00.003+05:30

To convert binary numbers into decimal numbers were have to follow the following steps

Example 1 : - (010101011)2 into (Decimal)10

Step 1: - Mark the MSB (Most Significant Bit) and LSB (Least Significant Bit.). The LSB is the number from the right hand side. In the given example the LSB is 1. Similarly we have to mark MSB. The MSB is a number from left hand side. In the given example the MSB is 0.

010101011

| |

MSB LSB

Step 2:- Multiply the each binary numbers with its base i. e 2 with the cumulative power from 0 to n.

0x2^8+1x2^7+0x2^6+1x2^5+0x2^4+1x2^3+0x2^2+1x2^1+1x2^0

Step 3: - Now solve the equation.

=0x2^8+1x2^7+0x2^6+1x2^5+0x2^4+1x2^3+0x2^2+1x2^1+1x2^0

=0+128+0+32+0+8+0+2+1

=(171)10

Step 4: - The (171)10 is the answer.

Now we take another example in this we learn how to convert binary numbers after decimal into decimal number system.

Example 2: - (1100.100)2 into (Decimal)10

Step 1: - Mark the MSB (Most Significant Bit) and LSB (Least Significant Bit.). In this example the seperate MSB and LSB marked for the number before the decimal and after the decimal. For the number before decimal the LSB is 0 and the number after the decimal the LSB is 1 from the right & left hand side respectively. Similarly In the given example for the number before decimal the MSB is 1 and the number after the decimal the MSB is 0 from the left & right hand side respectively.

1 1 0 0 . 1 0 0

| | | |

MSB LSB LSB MSB

Step 2:- Multiply the each binary numbers with its base i. e 2 with the cumulative power from 0 to n.

1x2^3+1x2^2+0x2^1+0x2^0.1x2^-1+0x2^-2+0x2^-3

Step 3: - Now solve the equation.

1x2^3+1x2^2+0x2^1+0x2^0.1x2^-1+0x2^-2+0x2^-3

= 8+4+0+0 (.) 1/2 + 1/4 + 1/8

= 8+4+0+0(.) (0.5+0.25+0.125)

= (12.875)10

Step 4: - The (12.875)10 is the answer.

Energy Gap, Valence Shell and Valence electrons, Forbidden Energy Gap

2020-08-05T22:35:00.001+05:30

Energy Gap

The Energy gap between the valence band and conduction band is known as forbidden energy gap. Forbidden energy gap is a region in which no electron can stay as there is no allowed energhy state. The greater the forbidden energy gap more tightly the valence electrons are bound to the nucleus.

Valence Shell and Valence electrons

The outermost shell of an atom is called the valence shell and electrons in this shell are called the valence electrons. Formation of energy bands occur owing to overlapping of energy levels of these valence electrons in valence shells. With the decrease in interatomic distance between the atoms in a crystal, the energy levels of electrons in outermost shells of atoms overlap to form energy bands.

Forbidden Energy Gap

The energy gap between the valence band and conductions band is known as forbidden energy gap. It is a region in which no electron can stay as there is no allowed energy state. Magnitude of forbidden energy gap in germanium and silicon is 0.72 eV and 1.12 eV respectively at 300 K and 0.785 eV and 1.21 eV respectively at absolute zerro temperature.

Magnetic Loss in Transformer

2020-08-05T22:34:00.006+05:30

All transformers dissipate some energy in the process of transformation. Operating principles dedicate that the magnetic core is always magnetized to the same level, regardless the load. This means that even when transformer works with no load, the magnetic losses are almost the same as when it is operating at full load.

At full load, the efficiency of large power transformers can be very high- above 99% in some cases. However, because the magnetic loss is piratically fixed, efficiency reduces with the load. Inevitable, under no load conditions any transformer operates with 0% efficiency- it consumes some energy to remain energized, but it delivers no energy to any load.

The electricity transmission grid and all the transformers in it are exposed to varying load. At night the load is relatively light, so the transformers operate at much less the nominal conditions, but they keep dissipating exactly the same energy in magnetic losses.

When transformers were first invented, magnetic cores were made from so-called "soft iron". This material was quite loss and ever since, engineers and scientists have been striving to improve efficiency by introducing a number of technological modification- to chemical structure, mechanical processing, annealing, post-processing, etc.

Now a days, the most commonly used material for magnetic cores of power transformers is electrical steel, which is a descendant of soft steel. However, the power loss of the best grade of electrical steel now in use is around 50 times lower than that of soft steel. This has had a very significant impact on the overall efficiency of power transformers.

In some cases, medium voltage power transformers are still in use after 50 years of operation. Because these old transformers were made from inferior magnetic material, their losses are much higher and replacing them with modern units would most likely bring noticeable saving in the energy distribution network- a transformer made 50 years ago could lose 5-10 times more energy ( for magnetic loss ) than a modern equivalent with the same rating.

The production of the grain - oriented electrical steel is a very complex process ( perhaps a topic for a future article). Further improvements are on the horizon, but currently are too expensive to implement. However, another approach is possible - different material can be used.

To ensure correct operation, several properties of transformers must be tested regularly, electric insulation, degradation of oil, turn and voltage ratio, etc. Higher losses in the magnetic core can influence to some degree the voltage ration or the phase shifts between the test signals. However, this is usually not a problem because correctly designed transformer testers can cope with these effects, even for older transformers.

Study and research continue in the problem of losses in magnetic materials. Manufacturers of electrical steel and amorphous ribbon sponsor many of these studies are done and in progress in "magnetic" where the newest developments are presented.

In future we can expect even better and more efficient magnetic cores. The improvements might not be ground breaking, but every single percentage point counts for efficiency , for running cost and for the environment.

Block diagram of Optical Fiber communication system

2020-08-01T23:47:00.001+05:30

1. The sound signals from different subscribers are multiplexed and converted into digital form by ADC/ Encoder.

2. The signal is then modulated using a pulse modulation technique and given by electrical to optical (E-O) converter. This device produce modulated optical signals.These signals are transmitted through an optical fibre to the desired destination.

3. At the receiver end, an optical detector is is employed to convert optical signals to electrical form. These signals are amplified and the modulated, decoded / demultiplex. The resultant digital signals are converted into analogue form by the digital to analog converter and given to the subscribers.

What are negative and positive feedback systems

2020-07-24T20:08:00.004+05:30

Feedback: -The feedback is the process of taking a part of output signal and feeding it back to the input circuit.

Depending upon whether the feedback signal increases or decreases the input signal, there are two basic types of feedback in amplifiers, namely positive feedback and negative feedback.

Positive feedback - Is the feedback voltage or current is applied in such a manner that it is in phase with input signal and thus increases its, then it is known as positive or regenerative feed back. The positive feedback increases the gain of amplifier. However, it reduce excessive dispersion due to which it is seldom used in amplifier. Because of its capability of increasing the power of original signal it is used in oscillator circuits. The positive feedback has the disadvantage of increased dispersion and instability.

Negative feedback - If the feedback voltage or current is applied in such a way that it is out of phase with the input signal and thus increases it, then it is known as negative feedback for this generator feedback. The negative feedback reduce gain of amplifier but it has numerous advantages such as gain stability reduction in non-linear distortion, reduction in noise, increase in bandwith, increase in input impedance and decrease in output impedance. A negative feedback is frequently used in a small signal as well as large signal amplifier circuits. The only one disadvantage or demerits of negative feedback is that it reduce the amplifier gain.

The Other advantage of negative feedback is that the task again Af of the Amplifier with feedback can be stabilized against variation of the h-parameters of transistor. Also the significant improvement in the frequency response and in linear t of operation of the feedback amplifier compared with that of the amplifier without feedback.

Advantages of fibre optic system over coaxial cable a microwave system.

2020-07-20T18:48:00.001+05:30

1. Lightweight: - Fiber cables are very thin, smaller and weight lesser then coaxial/ metallic cables.

2. Large information carrying capacity: - Fiber optic system use light frequency (10^5 to 10^6 GHz) as carries, and does have much larger bandwidth.

3. Less space and easy installation: - Fiber cable require less space fiber cable are safer and easy install without worrying about volatile liquids and glass.

4. Immunity to Interface: - Fiber system also immune to Static interference caused by electrical noise sources like electrical motor etc full stop in fiber cable there is no magnetic induction between fiber cables and there is no possibility of cross talks.

5. No capacitance and induction formation: - metallic cable form capacitance between them and induction along their conductor which act like low pass filters and limit the bandwidth of the system. fiber cables are free from this limitation.

6. Information typing: - It is Virtually impossible to tap information from a fiber cable.

7. Resistivity to temperature and environmental changes: - The fiber cable can operate over a larger temperature and are less affected by corrosive liquid and gases.

8. Fibre cable are cheaper: - The fiber optic system are cheaper than their coaxial cable a microwave links.

Peak Inverse Voltage (PIV)

2020-07-19T22:31:00.001+05:30

During the negative half cycle of the input the diode is Reverse biased. The whole of the input voltage appears across the diode when the input reaches its peak value, Vm in the negative half cycle, the voltage across the diode is also maximum. This maximum voltage is called the Peak Inverse Voltage. It represents that the maximum voltage of the diode must withstand the negative half cycle of the input.

When the diode is used as a rectifier the peak inverse voltage is extremely important. In rectifier service, it has to be ensured that reverse voltage across the diode does not exceed its peak inverse voltage during the negative half cycle of input A.C.voltage.

General Science Quiz

2020-07-14T19:37:00.001+05:30

Choose the correct option from the choices given under the question.

1. Which of the following reactions can be employed for getting unsymmetrical alkalies in good
yield?

1. Wurtz reaction 2. Corey-House reaction
3. Both 4. None.

RIGHT ANSWER: 2

2. Which of the following salts is hexahydrate?

1. Blue vitriol 2. Green vitriol
3. White vitriol 4. Mohrs salt.

RIGHT ANSWER: 4

3. Which of the following substance does not give dinitrogen gas with
nitrous acid ?

1. Acetamide 2. Methyl amine
3. Diethyl amine 4. Isoprpyl amine.

RIGHT ANSWER: 3

4. Which of the following sulphides is yellow in colour?

1. ZnS 2. NiS
3. CdS 4. MgS.

RIGHT ANSWER: 3

5. Which of the following unit cells has the least symmetry?

1. Tetragonal 2. Triclinic
3. Monoclinic 4. Cubic.

RIGHT ANSWER: 2

6. Which of the following will show disproportionate when treated with 50% aqueous NaOH?

1. Benzyl alcohol 2. Ethanal
3. Phenyl ethanal 4. m-nitro benzaldehyde.

RIGHT ANSWER: 4

7. Which one of the following does not show different oxidation states?

1. Iron 2. Copper
3. Zinc 4. Manganese.

RIGHT ANSWER: 3

8. Which one of the following pairs are chemically dissimilar?

1. Na and K 2. Ba and Sr
3. Zr and Hf 4. Ca and Zn.

RIGHT ANSWER: 4

9. What is the force that pulls the freely falling body towards the earth is called____

1. Frictional Force 2. Gravitational Force
3. Magnetic Force 4. Traditional Force

RIGHT ANSWER: 2

10. The units in terms of which the units of all other physical quantities can be derived are called _____ units

1. physical 2. derived
3. basic 4. underived

RIGHT ANSWER: 3

11. In MKS system the units of length are

1. centimetres 2. milli metres
3. metres 4. nm

RIGHT ANSWER: 3

12. In FPS system the units of mass

1. kilograms 2. tons
3. quintals 4. pounds

RIGHT ANSWER: 4

13. The distance of seperation between two points in space is defined as _____

1. length 2. area
3. volume 4. none

RIGHT ANSWER: 1

14. In a gas a burning splinter is put out with a pop sound. The gas is :

1. Hydrogen 2. Oxygen
3. Carbon dioxide 4. Nitrogen

RIGHT ANSWER: 1

15. The width of a river can be found by :

1. Measuring tape 2. Survey chain
3. Method of triangulation 4. Scale

RIGHT ANSWER: 3

16. Existence of harmful substances in atmosphere causes_______

1. Pleasant smell 2. Pleasant atmosphere
3. Pollution 4. None

RIGHT ANSWER: 3

17. Which of the following is not a fuel?

1. Charcoal 2. Kerosene
3. Mercury 4. petrol

RIGHT ANSWER: 3

18. The scale marked on the nut is

1. Pitch Scale 2. Head Scale
3. Vernier Scale 4. Main Scale

RIGHT ANSWER: 2

19. If the zeroth division of the head scale is above the index line

of the pitch scale then the correction is

1. Positive 2. Negative
3. No correction 4. None

RIGHT ANSWER: 1

20. Generally L.C. of a screw gauge is _____

1. 0.1mm 2. 0.01mm
3. 0.001mm 4. 0.01cm

RIGHT ANSWER: 2

21. Conversion of glucose to pyruvic acid yields a net gain of :

1. 2 molecules of ATP 2. 36 molecules of ATP
3. 4 molecules of ATP 4. 38 molecules of ATP

RIGHT ANSWER: 1

22. Blood of one person is transfused into another through his :

1. muscle 2. trachea
3. vein 4. artery

RIGHT ANSWER: 3

23. Endocrine glands are present in

1. amoeba 2. earth worm
3. euglena 4. man

RIGHT ANSWER: 4

24. Which pair of species exist as monoatomic molecules?

1. Helium and argon 2. Argon and chlorine
3. Hydrogen and helium 4. Ozone and nitrogen.

RIGHT ANSWER: 1

25. Which transition metal has the lowest density?

1. Os 2. Zn
3. Sc 4. La.

IR Remote Control

2012-07-15T16:07:00.001+05:30

This circuit will allow you to turn on any piece of equipment that operates on 115 volts ac. The reciever circuit is based on the Radio Shack infrared receiver module(MOD), part number 276-137. It is also available from some of the other sources listed on my Links page. The MOD accepts a 40khz IR signal that is modulated at 4 khz. When a signal is recieved the MOD will go low. The sensitivity of the MOD is set by different values for R1 and C1.
The values for R1 may need to be as high as 10,000 ohms and for C1 40uf. This will prevent the unit from turning on under normal lighting conditions. You will need to experiment with the vaules that work best for you. The output of the 4013 chip a flip flop toggles on and off with the reception of a IR pulse. The output of the 4013 turns on the MOC optical coupler which in turn switches on the triac and supplies power to the AC load.

IR Remote Transmitter

IR Remote Controller

Electrolytic Capacitor

2010-02-15T13:27:00.000+05:30

An electrolytic capacitor is a type of capacitor typically with a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output, and especially in the absence of rechargeable batteries that can provide similar low-frequency current capacity. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not; the large value of the capacitance allows them to pass very low frequencies.

The electrolytic capacitor was invented in 1886 by Charles Pollak. It was largely responsible for the development of mains-powered radio receivers, since it permitted the filtering of the 50-60 hertz power supplied to residences, after it was rectified to power the radio tubes. This was not practical without the small volume and low cost of electrolytic capacitors.

Contents

1 Construction

2 Polarity

3 Electrolyte

4 Electrical behavior of electrolytics

5 Capacitance

6 Variants

Construction

Aluminum electrolytic capacitors are constructed from two conducting aluminum foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture.

Polarity

In aluminum electrolytic capacitors, the layer of insulating aluminum oxide on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that allows for a relatively high capacitance in a small volume. The aluminum oxide layer can withstand an electric field strength of the order of 109 volts per meter. The combination of high capacitance and high voltage result in high energy density.

Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, denoting the adjacent terminal that should be more negative than the other. This is necessary because a reverse-bias voltage will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Without the dielectric material the capacitor will short circuit, and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.

Modern capacitors have a safety valve, typically either a scored section of the can, or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic. Electrolytic can withstand a reverse bias for a short period of time, but they will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.

These are the different schematic symbols for electrolytic capacitors. The minus or N marked side of the physical capacitor is equivalent to the node opposite to the plus sign on its symbolic equivalent. Tip: Take notice of the shape of the symbols and the placement of the positive and negative nodes, because most schematics do not print the "+", but rely on the symbol itself instead. note: caps in metal can have the color mark at the minus side !

Polarity of caps with wires:

axial: the minus wire is connected to the case, the plus wire is isolated.

radial = single ended: a vertical color stripe indicates the minus side.

Electrolyte

The electrolyte is usually boric acid or sodium borate in aqueous solution together with various sugars or ethylene glycol which are added to retard evaporation. Care should be taken to avoid ingestion of or eye contact with the electrolyte, and any areas of the body where skin contact has occurred should be washed in good time. It is important to follow safe working practice and to use appropriate protective equipment, notably gloves and safety glasses, when working with the electrolyte. Some very old tantalum electrolytic, often called "Wet-slug", contain the more hazardous sulfuric acid, however most of these are no longer in service due to corrosion.

Electrical behavior of electrolytic

A common modeling circuit for an electrolytic capacitor has the following schematic:where Rleakage is the leakage resistance, RESR is the equivalent series resistance, LESL the equivalent series inductance (L being the conventional symbol for inductance).

RESR must be as small as possible since it determines the loss power when the capacitor is used to smooth voltage. Loss power scales quadratically with the ripple current flowing through and linearly with RESR. Low ESR capacitors are imperative for high efficiencies in power supplies.

It should be pointed out that this is only a simple model and does not include dielectric absorption (soakage) and other non-ideal effects associated with real electrolytic capacitors.

Since the electrolytes evaporate, design life is most often rated in hours at a set temperature. For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower, reaching 15 years at 45 degrees. Most Electrolytic capacitors are rated for 85 degrees Celsius maximum.

Capacitance

The capacitance value of any capacitor is a measure of the amount of electric charge stored per unit of potential difference between the plates. The basic unit of capacitance is a farad, however this unit has been too large for general use until the invention of the Double-layer capacitor, so microfarad, nanofarad and picofarad are more commonly used. These are usually abbreviated to μF or uF, nF and pF.

Many conditions determine a capacitor's value, such as the thickness of the dielectric and the plate area. In the manufacturing process, electrolytic capacitors are made to conform to a set of preferred numbers. By multiplying these base numbers by a power of ten, any practical capacitor value can be achieved, which is suitable for most applications.

A standardized set of capacitor base numbers was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers 1.0, 1.5, 2.2, 3.3, 4.7 or 6.8 by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).

Most electrolytic capacitors have a tolerance range of 20 %, meaning that the manufacturer is stating that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance.

Variants

Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct current else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.

Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.

As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:

Aluminum electrolytic capacitor: compact but lossy, these are available in the range of ESL

values are a few nH.[1]

Tantalum: compact, low-voltage devices up to several hundred µF, these have a lower? energy density and are more accurate than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode electrode is formed of sintered tantalum grains, with the dielectric electro chemically formed as a thin layer of oxide. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The cathode electrode is formed either of a liquid electrolyte connecting the outer can or of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypropylene) that reduces internal resistance and eliminates a self-ignition failure[2].

Compared to aluminum electrolytic, tantalum capacitors have very stable capacitance, little DC leakage, and very low impedance at high frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected in the circuit backwards or exposed to spikes above their voltage rating.

Tantalum capacitors are more expensive than aluminum-based capacitors and generally only usable at low voltage, but because of their higher capacitance per unit volume and lower impedance at high frequencies, they are popular in miniature applications such as cellular telephones.

PAPER CAPACITORS

2010-02-15T12:39:00.000+05:30

First of all capacitors is a term that was not always used. The word condensers was used for a very long time and some place along the way it was changed to capacitors. Most people call them "caps", so when you read that a radio has been "recaped", that means that the capacitors have been replaced.

The performance of the radio depends on the conditon of these "caps". Most often they will be badly deteriorated and they should ALL be replaced and NOT JUST the easy one's to get at. They are cylinders filled with wax on the ends with a wire lead coming out of both ends.
Capacitors are found in all areas of the radio circuit for general coupling, decoupling and filtering. They are NON-POLARITY sensitive and are usually clearly labeled with the value and voltage rating. They often have a black band around one end. This end was connected to the less sensitive side of the circuit and for non-electrolytic ones it does NOT make any difference which end you connect.

Damage to the caps is usually indicated by melting, bubbling or darkening of the outer wax covering and loss of wax on one end. Sometimes they look ok and test ok, but have electrical leakage and can act more like a resistor.
It is hotly debated whether or not you should replace them all as purists and collectors feel they should only be replaced if absolutely necessary. Paper-wax capacitors are VERY prone to failure and it is just a matter of time before they fail, so I believe they should ALL be replaced. Since the work is under the chassis and not visible, it should have little impact on the value. People who look at my radios have never asked me to take the chassis out.
Before you expend a lot of effort trying to find the exact values, you come as close as you can to the values. For example, a 0.05 cap you use a 0.047 cap, for a 0.02 cap use a 0.022 cap and so on. The voltage I use is 630 Volts on everything, that way you don't have to keep as many of them in stock.

Difference between a compiled and an interpreted program?

2010-01-23T19:18:00.001+05:30

Most programs are written in a high-level language such as C, Perl, or Java. Just as a human language makes it easy for people to communicate with one another, so computer languages simplify the job of telling a computer what to do. However, because a computer only understands numbers, talking to one is like speaking to someone with whom you don't share a language. You need a translator to properly communicate, and that's what interpreters and compilers do.

The difference between an interpreted and a compiled language lies in the result of the process of interpreting or compiling. An interpreter produces a result from a program, while a compiler produces a program written in assembly language. The assembler of architecture then turns the resulting program into binary code. Assembly language varies for each individual computer, depending upon its architecture. Consequently, compiled programs can only run on computers that have the same architecture as the computer on which they were compiled.

A compiled program is not human readable, but instead is in an architecture-specific machine language. Creating a compiled program requires several steps. First, the programmer, using a development tool or even a simple text editor, writes the source code in a chosen computer language. If the program is complex, pieces of it may be spread across several files. The programmer then compiles the program, sorting and linking the modules and translating it all into machine code that the computer understands.

Because different kinds of computers do not speak each others' machine languages, a compiled program will only work on the platform it was designed for. For example, a program written for HP-UX normally will not work on a Mac OS computer or a computer running Solaris. Despite this drawback, compiled programs are faster than those that must be run through an interpreter. Also, it is often possible to recompile the program so that it will run on different platforms. Examples of languages that are normally used to produce compiled programs include C, Fortran, and COBOL.

In an interpreted program, on the other hand, the source code typically is the program. Programs of this type (often known as scripts) require an interpreter, which parses the commands in the program and then executes them. Some interpreters, such as the Unix shells (sh, csh, ksh, etc.), read and then immediately execute each command, while others, such as Perl, analyze the entire script before sending the corresponding machine language instructions. The advantage of a script is that it is very portable. Any computer that has the appropriate interpreter installed may run the program more or less unchanged. This is a disadvantage as well, because the program will not run at all if the interpreter is not available. In general, interpreted programs are slower than compiled programs, but are easier to debug and revise. Other examples of interpreted languages include JavaScript and Python.

Intermediate to computer-specific compiled programs and interpreted scripts are programs designed for runtime environments. Java and Smalltalk programs are executed in this fashion. Constructing programs for runtime environments is similar to writing traditional compiled programs. The difference is that instead of compiling the source code into a machine language, it is output into byte code for the runtime environment's "virtual machine". This virtual machine intercepts the byte code instructions and translates them into computer-specific commands. The advantage of this approach is that the runtime environment quickly compiles only the needed pieces of the code (some parts of the program may never need to be executed). This is called just-in-time compiling. The major disadvantage with runtime environments is that a program that is not designed well will force the runtime environment to compile almost all of the code up front and then make redundant calls to the interpreter. This makes the program slower to load and run.

Geostationary satellite

2010-01-23T18:59:00.000+05:30

Definition
According to Kepler's Third Law, the orbital period of a satellite in a circular orbit increases with increasing altitude. Space stations and shuttles in Low Earth orbit (LEO), typically two or four hundred miles above the Earth's surface make between fifteen and sixteen revolutions per day. The Moon, at an altitude of about 240,000 miles (385,000 km), takes thirty days to make a complete rotation. Between those extremes lies the "magic" altitude of 22,300 miles (35,786 km) at which a satellite's orbital period matches, or is an integral part of, the period at which the Earth rotates: once every sidereal day (23 hours 56 minutes). In that case, the satellite is said to be geosynchronous.

If a geosynchronous satellite's orbit is not exactly aligned with the equator, the orbit is known as an inclined orbit. It will appear (when viewed by someone on the ground) to oscillate daily around a fixed point in the sky. As the angle between the orbit and the equator decreases, the magnitude of this oscillation becomes smaller; when the orbit lies entirely over the equator, the satellite remains stationary relative to the Earth's surface – it is said to be geostationary.

Phase Plug Technology

2010-01-23T18:42:00.000+05:30

In any conventional loudspeaker, the highest frequencies of audio emanate from the area around the center of the cone and the lower frequencies are produced by the area of the cone that is farther from the center. In fact, the distance from the center at which a sound wave will come off a loudspeaker cone is directly related to its frequency. This helps explain why larger cone loudspeakers are usually capable of producing more bass.

Due to the shape of the conventional loudspeaker cone, the higher frequency sound waves tend to collide at a focal point. This causes phase distortion and a loss in clarity. As a result, accuracy and transparency suffer. This also has detrimental effects on the realism of the sound environments created by today's high fidelity digital audio and surround sound systems.

The phase plug pole piece extension improves driver performance and clarity by deflecting delicate midrange audio out into the sound space and minimizing distortion producing collisions

Types of potentiometer

2010-01-19T00:35:00.000+05:30

Types of potentiometer

--> -->Their are different types of potentiometers as given below:-

Constant current potentiometer
Constant resistance potentiometer
Microvolt potentiometer
Thermocouple potentiometer

--> --> --> Constant current potentiometer:- --> This is used for measuring voltages below 1.5 volts. In this circuit, the unknown voltage is connected across a section of resistance wire the ends of which are connected to a standard electrochemical cell that provides a constant current through the wire, The unknown emf, in series with a galvanometer, is then connected across a variable-length section of the resistance wire using a sliding contact(s). The sliding contact is moved until no current flows into or out of the standard cell, as indicated by a galvanometer in series with the unknown emf. The voltage across the selected section of wire is then equal to the unknown voltage. All that remains is to calculate the unknown voltage from the current and the fraction of the length of the resistance wire that was connected to the unknown emf. The galvanometer does not need to be calibrated, as its only function is to read zero. When the galvanometer reads zero, no current is drawn from the unknown electromotive force and so the reading is independent of the source's internal resistance.
Because the resistance wire can be made very uniform in cross-section and resistivity, and the position of the wiper can be measured easily, this method can be analyzed to accurately determine the uncertainties in the measurement. When measuring potentials larger than that produced by a standard cell, an external voltage divider is used to scale the measured voltage down to approximately 1 volt for measurement by the potentiometer; the uncertainties due to the voltage divider construction and the load placed on the source by the voltage divider then become part of the uncertainty of the overall measurement.

Constant resistance potentiometer:-

--> The constant resistance potentiometer is a variation of the basic idea in which a variable current is fed through a fixed resistor. These are used primarily for measurements in the millivolt and microvolt range.
Microvolt potentiometer:-
--> This is a form of the constant resistance potentiometer described above but designed to minimize the effects of contact resistance and thermal emf. This equipment is satisfactorily used down to readings of 1000 nV or so.

Thermocouple potentiometer:-

--> Another development of the standard types was the 'thermocouple potentiometer' especially adapted for temperature measurement with thermocouples. Potentiometers for use with thermocouples also measure the temperature at which the thermocouple wires are connected, so that cold-junction compensation may be applied to correct the apparent measured EMF to the standard cold-junction temperature of 0 degrees C.

Potentiometer

2010-01-19T00:22:00.001+05:30

A potentiometer is a variable tapped resistor that can be used as a voltage divider. The same term is applied both to an electrial component and to a measuring instrument.

As an electrical component, a potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If all three terminals are used, it can act as a variable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used as controls for electrical devices such as a volume control of a radio. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick.

A potentiometer instrument for measuring the potential (or voltage) in a circuit taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The sliding tap of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding tap and the unknown potential; the deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology.

Features and Application of PLCC

2009-10-15T12:09:00.001+05:30

FEATURES

· 20 to 1 frequency range with an external resistor.

· Logic compatible output with the 100-mA current sinking capability.

· Bandwidth adjustable from 0 to 14 %.

· High rejection of out of band signals and noise.

· Immunity to false signals.

· Highly stable center frequency.

· Center frequency adjustable from 0.01hz to 500khz.

APPLICATIONS

Touch tone decoding.
Precision oscillator.
Frequency control and monitoring.
Wide band FSK demodulation.
Ultrasonic controls.

DESCRIPTION ABOUT PLCC

2009-10-15T11:48:00.001+05:30

DESCRIPTION ABOUT PLCC (VIEW BLOCK DIAGRAM)

Power Supply Section

For the functioning of various sections of the PLT unit, pure and stable dc voltage is required. Pure and stable dc bias is received from the power supply section of PLT unit for the functioning of the various sections and circuit. The power supply section of the PLT unit receives ac supply from the ac main wires and covert into the dc and after regulating it gives dc supplies of 9 volts.

Approximately 220 volts (50) Hz ac supply enters into the PLT unit from ac power line through the ac main wires. The supply is directly fed to the primary winding of the main transformer of the PLT unit. Main transformer is a step down transformer.

Microphone Preamplifier

Impedance microphones have an extremely small output voltage, which in normal use is unlikely to ever exceed 1m-vrms. This means that the preamplifier must have high voltage gain and a very low noise input if a high signal to noise ratio is to be achieved. This circuit requires only 200 mv rms at the input for an output level 0.1 vrms and the unweighted signal to noise ratio (input open circuit) is a little under –66 dB.

Many dynamic microphones have an integral step up transformer which gives increased output voltage, but the available output current is of course decreased, and so a preamplifier having a higher input impedance (usually 50k) is required.

Audio Amplifier Section

This section consists of audio amplifier and speaker. Input of the audio amplifier is very low. It is required to amplify this signal and increase the signal to noise level. The output of the audio amplifier goes to speaker.

Line Trap

Line trap unit is fixed between coupling capacitor to the lines .it is a parallel tuned circuit containing inductance and capacitance. It has less than 0.1 V resistor.

The line trap unit provides a protection or prevention of high frequency signal from entering the neighboring line and carrier current.

Protection Circuit

There are many types of protection circuits used in the system. They prevent high voltage, noise, and interference. They are as follows: -

A) To Protect IFT

This protection circuit is used to protect IFT which is connected parallel with the 230 volt ac. when noise spikes exceed from some predetermined values, one of the diode gets short -circuited and protects the IFT.

B) To Provide Constant Voltage

To protect constant voltage for the protection of whole circuit, zener diode is used. Zener diode is connected in parallel with the bridge circuit. When the voltage across the bridge increases from the 9 volts, zener diode gets short-circuited and hence protects the whole circuit.

C) Other Protection

There are various protection circuits connected with the IC's according to IC's specifications.

Signaling Network

In this block, IC CD 4093 is used as a signaling network. This IC acts as schmitt trigger, which provides the train of pulses to generate beep sound.

Matching Network

This network is used to match the frequency of transmitter and receiver .it is also used to avoid the interference between the modulating signal and power line signal of 50 Hz.

TONE DECODER

The LM 567 and LM567C are general-purpose tone decoders designed to provide a saturation transistor switch to ground when an input signal is present within the pass band. The circuit consists of an I & Q detector driven by a voltage controlled oscillator, which determines the center frequency of the decoder. External components are used to independently set the central frequency, bandwidth and output delay.

Power Line Technique equipment

2009-10-15T10:18:00.000+05:30

PLT equipment

The PLT equipment comprises the following:

Line trap unit
Coupling capacitor
Tuning unit
Transmitter, receiver and other electronic equipment

For Circuit Diagram Follow this link
For List of Component Follow this link

Coupling Capacitor

The carrier equipment is connected to the transmission line through, coupling capacitor' which is of such a capacitance that it offers low reactance (1/vc) to the carrier frequency but high reactance to power frequency. For example, 2000pf capacitor offers 1.5 MV to 50 Hz and 150V to 500 kHz.

Thus coupling capacitor allows carrier frequency signals to enter the carrier equipment but does not allow 50 Hz power frequency currents to enter the carrier equipment. To reduce impedance further, low impedance is connected in series with coupling capacitor to form a resonance at carrier frequency.

Line Trap Unit

It is a parallel tuned circuit comprising L and C. It has low impedance (less than 0.1 ohm) to 50 Hz and high impedance to carrier frequencies. This unit prevents the high frequency signals from entering the neighbouring line, and the carrier current flow only in the protected line.

(A) The function of the line trap is to prevent these high frequency

signals from going towards undesired directions. The line traps are connected in series with the high voltage lines on the station side and must be designed for the following ratings:

Normal power frequency current
Short-time short-circuit current
Basic insulation level characterized by the normal power frequency voltage, lightning impulse withstand voltage and switching impulse withstand voltage.

(B) Constructional Features

The line trap unit comprises of the following three main parts:

Main Coil
Tuning Unit
Over-Voltage Arrestor

The main coil carries the full line current and is designed to carry normal current continuously with temperature rise within specified limits. It should also be capable of carrying the specified short-circuit current for specified short duration (say 1second). The coil consists of aluminum conductors wound perpendicular to the axis of the coil. The coil should have high mechanical strength. Several Conductors are wired in parallel to obtain desired current rating. The individual windings or the conductors are separated with fiber glass reinforced spacers. These spacers absorb the stresses occurring during short circuits.

A rigid aluminum crosses arm provided at each bend of the coil. The two aluminum crosses are held together by one or more glass fiber tension rods.

The coil is air insulated and not cast in resin. Hence, self-capacitance is low.

Low self-capacitance is advantageous in high frequency applications. The open-air ventilated coils have better coiling than the molded coils. The main coil is of low weight and is strong.

Parts of magnetic material are not used in the construction.

(C) Tuning Unit

The tuning unit is mounted inside the main coil on the tension rod. It is easily attachable and detachable. The tuning unit is designed for one of the following:

1. Single Wave

2. Double Wave

3. Wide Band

4. Adjustable Tuning

Over-Voltage Arrester

It is located inside the main coil and it protects the tuning unit from impulse voltage.

Electronic Equipment:

There are generally identical units at each end:

(i) Transmitter unit

(ii) Receiver unit

(iii) Relay unit

Transmitter Unit:

Frequencies between 50 to 600 kHz are employed in different frequency bands. Each band has certain bandwidth (say 150-300khz, 90-115khz). Carrier frequencies are generated in oscillators .The oscillator can be tuned to a particular frequency selected for the application or it can be crystal oscillator with which the operation for a particular bandwidth can be achieved by selecting an appropriate crystal.

The output voltage of the oscillator is held constant by voltage stabilizer. The output of the amplifiers fed into the amplifier to overcome the losses in the transmission path between the transmitter and the receiver at remote end of the line.

The control of transmitter can be achieved by different methods depending upon the type of protection desired.

Receiving Unit

Receiver receives the high frequency signals arriving from remote end. The receiver receives the signal and feeds to carrier relay unit. Relay unit comprises -

An atteneuator which reduces the signals to a safer value
Band pass filter, which restricts the acceptance of unwanted signal
Matching transformer on matching element to match the impedance of line and receiving unit

Frequency Spacing

Differential frequencies are used in adjacent line sections. The wave traps ensured at the carrier signals do not enter the next line section. The receiver filters filter out other frequencies.

Modulation Of Higher Frequency Signal

Modulator modulates the voice signals and the modulated signal is fed to the amplifier and is then transmitted via coupling unit.

The process involves taking half cycle of current and producing the requisite blocks of carrier by tuning the oscillator ON. The level of the line current at which the oscillator is made onto produce the carrier blocks should be theoretically constant.

3.5 Modulation Techniques For PLC

Different types of modulation have been utilised for PLT -
Frequency modulation
Amplitude modulation
Narrow Band such as ASK
Non-Coherent FSK, and DPSK
Spread-Spectrum Schemes such as FSK.

Power line carrier systems amplitude modulate the carrier frequency. Full amplitude modulation (AM) has a frequency spectrum of sideboards symmetrical about the carrier frequency as shown in figure. These side bands contain all the information being transmitted and the carrier frequency is only the bearer of the messages. It is therefore possible to achieve savings in transmitted power without degrading signal performance by reducing the power of carrier frequency and by deleting one of the side bands. This is known as single side band (SSB) transmission. The carrier is not fully suppressed because it is used to synchronize the remote end receiver with corresponding transmitter. The transmitter and receiver circuits therefore tend to be slightly more complex than those used for normal broadcast AM transmission because of the filtering and accurate synchronizing involved. The lowest range (~30kHz to 200kHz) of carrier frequencies are used on long transmission lines and the higher ranges (~200kHz to 500kHz) on shorter lines. This helps to offset the attenuation effects of long lines with high frequency transmission. Computer simulation may be used to optimize the best frequency band to employ on given overhead line taking into account interference from any adjacent circuits.