Voltmeter- Principle and operation

June 19, 2016
Current is a flow of charge carriers. Voltage, or electromotive force (EMF), or potential difference, is the “pressure” that makes a current possible. Given a circuit whose resistance is constant, the current that will flow in the circuit is directly proportional to the voltage placed across it. Early electrical experimenters recognized that an ammeter could be used to measure voltage, since an ammeter is a form of constant-resistance circuit.
If you connect an ammeter directly across a source of voltage—a battery, say—the
meter needle will deflect. In fact, a milliammeter needle will probably be “pinned” if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect milliammeters or microammeters directly across voltage sources. An ammeter, perhaps with a range of 0-10 A, might not deflect to full scale if it is placed across a battery, but it’s still a bad idea to do this, because it will rapidly drain the battery.
Some batteries, such as automotive lead-acid cells, can explode under these conditions. This is because all ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across the power supply.
But if you place a large resistor in series with an ammeter, and then connect the
ammeter across a battery or other type of power supply, you no longer have a short circuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance to get a meaningful indication on the meter. Using a microammeter and a very large value of resistor in series, a voltmeter can be devised that will draw only a little current from the source.
A voltmeter can be made to have different ranges for the full-scale reading, by switching different values of resistance in series with the microammeter (Fig. 3-6). The internal resistance of the meter is large because the values of the resistors are large.
The greater the supply voltage, the larger the internal resistance of the meter, because the necessary series resistance increases as the voltage increases.


It’s always good when a voltmeter has a high internal resistance. The reason for this is that you don’t want the meter to draw much current from the power source. This current should go, as much as possible, towards working whatever circuit is hooked up to the supply, and not into just getting a reading of the voltage. Also, you might not want, or need, to have the voltmeter constantly connected in the circuit; you might need the voltmeter for testing many different circuits. You don’t want the behavior of the circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it will affect the behavior of anything that is working from the power supply.




Voltmeter- Principle and operation Voltmeter- Principle and operation Reviewed by Sikha on June 19, 2016 Rating: 5

Ammeters

June 19, 2016
Getting back to electromagnetic deflection, and the workings of the galvanometer, you might have thought by now that a magnetic compass doesn’t make a very convenient type of meter. It has to be lying flat, and the coil has to be aligned with the compass needle when there is no current. But of course, electrical and electronic devices aren’t all turned in just the right way, so as to be aligned with the north geomagnetic pole. That would not only be a great bother, but it would be ridiculous. Imagine a bunch of scientists running around, turning radios and other apparatus so the meters are all lying flat and are all lined up with the earth’s magnetic field! In the early days of electricity and electronics, when the phenomena were confined to scientific labs, this was indeed pretty much how things were.
  Then someone thought that the magnetic field could be provided by a permanent
magnet right inside the meter, instead of by the earth. This would supply a stronger

magnetic force, and would therefore make it possible to detect much weaker currents. It would let the meter be turned in any direction and the operation would not be affected. The coil could be attached right to the meter pointer, and suspended by means of a spring in the field of the magnet. This kind of meter, called a D’Arsonval movement, is still extensively used today. The assembly is shown in Fig. . This is the basic principle of the ammeter.

A variation of this is the attachment of the meter needle to a permanent magnet,
and the winding of the coil in a fixed form around the magnet. Current in the coil produces a magnetic field, and this in turn generates a force if the coil and magnet are aligned correctly with respect to each other. This meter movement is also sometimes called a D’Arsonval movement. This method will work, but the inertial mass of the permanent magnet causes a slower needle response. This kind of meter is also more prone to overshoot than the true DArsonval movement; the inertia of the magnet’s mass, once overcome by the magnetic force, causes the needle to fly past the actual current level before finally coming to rest at the correct reading.
 It is possible to use an electromagnet in place of the permanent magnet in the meter assembly. This electromagnet can be operated by the same current that flows in the coil attached to the meter needle. This gets rid of the need for a massive, permanent magnet inside the meter. It also eliminates the possibility that the meter sensitivity will change in case the strength of the permanent magnet deteriorates (such as might be caused by heat, or by severe mechanical vibration). The electromagnet can be either in series with, or in parallel with, the meter movement coil.
  The sensitivity of the D'Arsonval meter, and of its cousins, depends on several factors.
·        First is the strength of the permanent magnet, if the meter uses a permanent magnet.
·        Second is the number of turns in the coil. The stronger the magnet, and the larger the number of turns in the coil, the less current is needed in order to produce a given magnetic force.  If the meter is of the electromagnet type, the combined number of coil turns affects the sensitivity. Remember that the strength of a magnetomotive force is given in terms of ampere turns. For a given current (number of amperes), the force increases in direct proportion to the number of coil turns. The more force in a meter, the greater the needle deflection, and the smaller the amount of current that is needed to cause a certain amount of needle movement.
The most sensitive ammeters can detect currents of just a microampere or two.
The amount of current for full scale deflection (the needle goes all the way up without banging against the stop pin) can be as little as about 50 uA in commonly available meters.
  Thus you might see a microammeter, or a milliammeter, quite often in electronic
work. Meters that measure large currents are not a problem to make; it’s easy to make an insensitive device.
  Sometimes, it is desirable to have an ammeter that will allow for a wide range of
current measurements. The full-scale deflection of a meter assembly cannot easily be changed, since this would mean changing the number of coil turns and/or the strength of the magnet. But all ammeters have a certain amount of internal resistance. If a resistor, having the same internal resistance as the meter, is connected in parallel with the meter, the resistor will take half the current. Then it will take twice the current through the assembly to deflect the meter to full scale, as compared with the meter alone. By choosing a resistor of just the right value, the full-scale deflection of an ammeter can be increased by a factor of 10, or 100, or even 1000. This resistor must be capable of carrying the current without burning up. It might have to take practically all of the current flowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000 of the current. This is called a shunt resistance or meter shunt (Fig.).
Meter shunts are frequently used when it is necessary to measure very large currents, such as hundreds of amperes. They allow microammeters or milliammeters to be used in a versatile multimeter, with many current ranges.

Ammeters Ammeters Reviewed by Sikha on June 19, 2016 Rating: 5

Electrostatic deflection

June 19, 2016
Electric fields produce forces, just as do magnetic fields. You have probably noticed this when your hair feels like it’s standing on end in very dry or cold weather. You’ve probably heard that people’s hair really does stand straight out just before a lightning bolt hits nearby; this is no myth. Maybe you performed experiments in science classes to observe this effect.
  The most common device for demonstrating electrostatic forces is the electroscope. It consists of two foil leaves, attached to a conducting rod, and placed in a sealed container so that air currents will not move the foil leaves (Fig.). When a charged object is brought near, or touched to, the contact at the top of the rod, the leaves stand apart from each other. This is because the two leaves become charged with like electric poles—either an excess or a deficiency of electrons—and like poles always repel.

The extent to which the leaves stand apart depends on the amount of electric charge. It is somewhat difficult to actually measure this deflection and correlate it with charge quantity; electroscopes do not make very good meters. But variations on this theme can be employed, so that electrostatic forces can operate against tension springs or magnets, and in this way, electrostatic meters can be made.
An electrostatic device has the ability to measure alternating electric charges as
well as steady charges. This gives electrostatic meters an advantage over electromagnetic meters (galvanometers). If you connect ac to the coil of the galvanometer device ,the compass needle might vibrate, but will not give a clear deflection. This is because current in one direction pulls the meter needle one way, and current in the other direction will deflect the needle the opposite way. But if an alternating electric field is connected to an electrostatic meter, the plates will repel whether the charge is positive or negative. The deflection will be steady, therefore, with ac as well as with dc.
Most electroscopes aren’t sensitive enough to show much deflection with ordinary
117-V utility voltage. Don’t try connecting 117 V to an electroscope anyway; it might not deflect the foil leaves, but it can certainly present a danger to your body if you bring it out to points where you can readily come into physical contact with it.
   An electrostatic meter has another property that is sometimes an advantage in
electrical or electronic work. This is the fact that the device does not draw any current, except a tiny amount at first, needed to put a charge on the plates. Sometimes, an engineer or experimenter doesn’t want the measuring device to draw current, because this affects the behavior of the circuit under test. Galvanometers, by contrast, always need at least a little bit of current in order to operate. You can observe this effect by charging up a laboratory electroscope, say with a glass rod that has been rubbed against a cloth. When the rod is pulled away from the electroscope, the foil leaves will remain standing apart. The charge just sits there. If the electroscope drew any current, the leaves would immediately fall back together again, just as the galvanometer compass needle returns to magnetic north the instant you take the wire from the battery.





Electrostatic deflection Electrostatic deflection Reviewed by Sikha on June 19, 2016 Rating: 5

Measuring devices for Electrical quantities..

June 19, 2016

Many measuring devices work because electric and magnetic fields produce forces proportional to the intensity of the field. By using a tension spring against which the
electric or magnetic force can pull or push, a movable needle can be constructed. The needle can then be placed in front of a calibrated scale, allowing a direct reading of thequantity to be measured. These meters work by means of electromagnetic deflection or electrostatic deflection.
    Sometimes, electric current is measured by the extent of heat it produces in a resistance.
Such meters work by thermal heating principles.Some meters work by means of small motors whose speed depends on the measured quantity. The rotation rate, or the number of rotations in a given time, can be measured or counted. These are forms of rate meters. Still other kinds of meters actually count electronic pulses, sometimes in thousands,millions or billions. These are electronic counters. There are also various other metering methods.

Electromagnetic deflection

Early experimenters with electricity and magnetism noticed that an electric current produces a magnetic field. This discovery was probably an accident, but it was an accident that, given the curiosity of the scientist, was bound to happen. When a magnetic
compass is placed near a wire carrying a direct electric current, the compass doesn’t point toward magnetic north. The needle is displaced. The extent of the error depends on how close the compass is brought to the wire, and also on how much current the wire is carrying.
Scientific experimenters are like children. They like to play around with things. Most likely, when this effect was first observed, the scientist tried different arrangements to see how much the compass needle could be displaced, and how small a current could be detected. An attempt was made to obtain the greatest possible current-detecting sensitivity. Wrapping the wire in a coil around the compass resulted in a device that would indicate a tiny electric current (Fig.). This effect is known as galvanism, and the meter so devised was called a galvanometer.

Once this device was made, the scientist saw that the extent of the needle displacement increased with increasing current. Aha—a device for measuring current! Then, the only challenge was to calibrate the galvanometer somehow, and to set up some kind of standard so that a universal meter could be engineered.
You can easily make your own galvanometer. Just buy a cheap compass, about two
feet of insulated bell wire, and a six-volt lantern battery. Set it up as shown in Fig. 1. Wrap the wire around the compass four or five times, and align the compass so that the needle points right along the wire turns while the wire is disconnected from the battery. Connect one end of the wire to the minus (–) terminal of the battery. Touch the other end to the plus (+) terminal, intermittently, and watch the compass needle. Don’t leave the wire connected to the battery for any length of time unless you want to drain the battery in a hurry.
You can buy a resistor and a potentiometer at a place like Radio Shack, and set up
an experiment that shows how galvanometers measure current. For a 6-V lantern battery, the fixed resistor should have a value of at least 330 Ω at 1/4 watt, and the potentiometer should have a value of 10 KΩ (10,000 Ω) maximum. Connect the resistor and potentiometer in series between one end of the bell wire and one terminal of the battery, as shown in Fig. 2. The center contact of the potentiometer should be short-circuited to one of the end contacts, and the resulting two terminals used in the circuit. When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire. Early experimenters calibrated their meters by referring to the degree scale around the perimeter of the compass.


Measuring devices for Electrical quantities.. Measuring devices for Electrical quantities.. Reviewed by Sikha on June 19, 2016 Rating: 5

Convolution in digital signal processing

March 09, 2016
Image result for what is convolution in dspConvolution is a formal mathematical operation, just as multiplication, addition, and integration. Addition takes two numbers and produces a third number, while convolution takes two signals and produces a third signal. Convolution is used in the mathematics of many fields, such as probability and statistics. In linear systems, convolution is used to describe the relationship between three signals of interest: the input signal, the impulse response, and the output signal.
An input signal, x[n], enters a linear system with an impulse response, h[n], resulting in an output signal, y[n]. In equation form: x[n] * h[n] = y[n]. Expressed in words, the input signal convolved with the impulse response is equal to the output signal. Just as addition is represented by the plus, +, and multiplication by the cross, ×, convolution is represented by the star, *. It is unfortunate that most programming languages also use the star to indicate multiplication. A star in a computer program means multiplication, while a star in an equation means convolution.

Image result for what is convolution in dsp


Convolution in digital signal processing Convolution in digital signal processing Reviewed by Sikha on March 09, 2016 Rating: 5

How to open a circuit breaker

February 20, 2016


Once inside your box you should turn off the breaker being changed,There will be a black wire coming off the breaker - you need to loosen the screw and remove this wire. The wire goes to the circuit branch in your house that is controlled by the breaker. Set this wire end aside - it is not live but don't let it contact anything less you accidentally make it live!

The breaker is plugged in to  a rail in the bottom of the breaker box, the rail is live. You have to release breaker which has a detent that locks it to the rail by tilting and removing the breaker.

You then need to put the replacement breaker - must be the exact same rating or you risk burning down your house! - Make sure its off, then tilt it in and snap-lock it in place. Now replace the black wire to the circuit branch and turn the breaker on.

If you want to be extra safe, turn off the master breaker at the top before you change anything, then turn it back on after you've changed the breaker.

remember there will be some exposed electrical contacts when you do this. You'd better be able to recognize and avoid them.


How to open a circuit breaker How to open a circuit breaker Reviewed by Sikha on February 20, 2016 Rating: 5

Types of circuit breaker

February 20, 2016
According different criteria there are different types of circuit breaker. According to their arc quenching media the circuit breaker can be divided as-

  1. Oil circuit breaker.




    Oil circuit breaker
  1. Air circuit breaker.

3. SF6 circuit breaker.
4. Vacuum circuit breaker.
According to their services the circuit breaker can be divided as-
  1. Outdoor circuit breaker
  2. Indoor breaker.
According to the operating mechanism of circuit breaker they can be divided as-
  1. Spring operated circuit breaker.
  2. Pneumatic circuit breaker.
  3. Hydrolic circuit breaker.
According to the voltage level of installation types of circuit breaker are referred as-
  1. High voltage circuit breaker.
  2. Medium voltage circuit breaker.
  3. Low voltage circuit breaker.
Types of circuit breaker Types of circuit breaker Reviewed by Sikha on February 20, 2016 Rating: 5

Difference between Fuse and Circuit breaker

February 20, 2016
Electrical Fuse
Circuit breaker










• The fuse is a device working on the electrical and thermal properties of the conducting material, while a circuit breaker is a device working on the electromechanical principles.
• Once used a fuse has to be replaced but the circuit breaker can be reused after correction of the fault in the system.
• Fuses offer protection against only power overloads, while circuit breaker offers protection against power overloads and short circuits (voltage imbalances)
Difference between Fuse and Circuit breaker Difference between Fuse and Circuit breaker Reviewed by Sikha on February 20, 2016 Rating: 5

Difference between Isolator and Circuit breaker

February 20, 2016
 Here I am  telling about the difference between isolation switch as compared to a circuit breaker.
An isolating  switch is a switching device that may not be able to break a normal load current or a short circuit current.  Usually an isolation switch is used on the supply side of a circuit breaker to provide visual proof that the circuit is broken and there is no possibility of power flowing through.
A circuit breaker is a switching device that not only can interrupt normal current flowing through an electrical load, but can also protect the circuit by interrupting a fault current created by a short circuit.
Circuit breaker with parts
Circuit breaker

Isolator switch





Rotory isolator switch


















In household power and other lower power circuit breakers, there are sensing mechanisms within the circuit breakers to sense electrical currents above expected levels.  This internal sensing mechanism will "trip" the circuit breaker, interrupting the current and therefore protecting the circuit much like a fuse will do. 

The circuit breaker can be reset and energise the circuit again after the short circuit or fault is removed.

However, it should be noted that most circuit breaker manufacturers only test their molded case circuit breaker (the ones you use in your house) once for their short circuit rating.  This means theoretically that the circuit breaker may not be able to interrupt another short circuit if it had to interrupt one that is close to its maximum rating.

In higher voltages, safety concerns dictate that an isolator switch be used on the supply side of a circuit breaker so that workers can see and lock the isolator switch in the open position after using the circuit breaker to de-energise the circuit.

Difference between Isolator and Circuit breaker Difference between Isolator and Circuit breaker Reviewed by Sikha on February 20, 2016 Rating: 5

Physical significance of power factor

February 20, 2016
What is power factor?
Power factor = cosine of the angle between voltage and current= cosθ . This is the instant answer from all. The other definitions are , cosθ= ratio of active power to apparent power or it is the ratio of Resistance to the impedance in the circuit.

   These all are definitions and expressions that we all know. But, actually what is the physical significance of power factor? What does it actually mean?

  " Power factor is the measure of effectiveness of utilization of available power."

The power in an ac circuit is mainly of three types.Apparent power, Active power and Reactive power.
Apparent power is the total power supplied to the circuit . It is given by,

S= VI  Volt Ampere
  But  actually the circuit does not utilizes the applied power completely (if it is not a purely resistive circuit). The part of the applied power which is actually consumed by the circuit is called active power and it is given by,

 P= VI cosθ watts

The rest of the applied power is circulating in the circuit with out doing any actual work. It s called reactive power and is given by,
 Q=  VI sinθ Volt Ampere Reactive (VAR)

Thus, the power factor is the measure of  percentage of the  power consumed .

 i.e,   " Power factor is the measure of effectiveness of utilization of available power."

The power factor can be also measured in terms of percentage.

  In a purely resistive circuit, the power factor is 1, or 100%. This means that, in a pure resistive circuit, the applied power is completely utilized by the circuit. (Reactive power is zero)

  If  in a circuit, the power factor is 0.5, or 50 % it means that only 50% of the applied power is utilized by the circuit.

Physical significance of power factor Physical significance of power factor Reviewed by Sikha on February 20, 2016 Rating: 5

Electrical power cable

February 19, 2016
Power cables are mainly used for power transmission & distribution purpose. It is an assembly of one or more individually insulated electrical conductors, usually held together with an overall sheath. Electrical power cables may be installed as permanent wiring within buildings, buried in the ground and run overhead or exposed. Flexible power cables are used for portable devices, mobile tools and machinery.

Construction of Power Cable
There are various parts of a cable to be taken care of during construction. The power cable mainly consists of
1. CONDUCTOR
2. INSULATION
3. LAY for Multicore cables only
4. BEDDING
5. BRAIDING/ARMOURING (IF REQUIRED)
6. OUTER SHEATH 


CONDUCTOR
 Conductors are the only power carrying path in a power cable. Conductors are of different materials. Mainly in cable industry we use copper (ATC, ABC) and aluminium conductors for power cables. There are different types of conductor as Class 1: solid, Class 2 stranded, Class 5 flexible, Class 6 Extra flexible (Mostly used for cords & welding) etc. Conductor sizes are identified with conductor resistance.
INSULATION The insulation provided on each conductor of a cable by mainly PVC (POLY VINYL CLORIDE ), XLPE (CROSSLINKED POLYETHYELENE), RUBBER (VARIUS TYPES OF RUBBER ). Insulating material is based on operating temperature.
Cores are identified by colour coding by using different colours on insulation or by number printing on cores
BEDDING (INNER SHEATH) This portion of the cable is also known as inner sheath. Mostly it is used in Multi core cables. It works as binder for insulated conductors together in multi-core power cables and provides bedding to armour/braid. This portion of the cable is mainly made of PVC( PVC ST-1, PVC ST-2 ), RUBBER (CSP SE-3, CSP SE-4 & PCP SE-3, PCP SE-4, HOFR SE-3 HOFR SE-4, HD HOFR SE-3 ETC)
ARMOURING There are mainly G.I. WIRE ARMOURING, G.I. STEEL STRIP armouring. It is done by placing G.I. WIREs, GI or STEEL STRIPs one by one on inner sheath. Armouring is a process which is done mainly for providing earthing shield to the current carrying conductors as well as it is also used for earthing purpose of the cable for safety. When there is any insulation failure in the conductor, the fault current gets enough paths to flow through the armour if it is properly earthed. Providing extra mechanical protection and strength to cable an important added advantage of armouring. In MINING CABLES it is done for conductance
BRAIDING ANNEALED TINNED COPPER WIRE , NYLON BRAID , COTTON BRAID are mainly used for this purpose. Braiding is the process which gives high mechanical protection to cable and also used for earthing purpose. Significance of braiding is it is more flexible in comparison to armouring.
OUTER SHEATH This is outermost cover of the cable normally made of PVC (POLYVINYL CLORIDE ), RUBBER (VARIUS TYPES OF RUBBER) and often the same material as the bedding. It is provided over the armour for overall mechanical, weather, chemical and electrical protection. Outer sheath is protection offered to cable not much electrically but more mechanically.


Electrical power cable Electrical power cable Reviewed by Sikha on February 19, 2016 Rating: 5
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