Digital readout meters-principle and operation

June 19, 2016

Increasingly, metering devices are being designed so that they provide a direct readout, and there’s no need (or possibility) for interpolation. The number on the meter is the indication.Its that simple. Such a meter is called a digital meter.
The advantage of a digital meter is that it’s easy for anybody to read, and there is no chance for interpolation errors. This is ideal for utility meters, clocks, and so me kinds of ammeters, voltmeters and wattmeters. It works very well when the value of the quantity does not change very often or very fast.
But there are some situations in which a digital meter is a disadvantage. One good example is the signal-strength indicator in a radio receiver. This meter bounces up and down as signals fade, or as you tune the radio, or sometimes even as the signal modulates.
A digital meter would show nothing but a constantly changing, meaningless set of numerals. Digital meters require a certain length of time to “lock in” to the current, voltage, power or other quantity being measured. If this quantity never settles at any one value for a long enough time, the meter can never lock in.
Meters with a scale and pointer are known as analog meters. Their main advantages are that they allow interpolation, they give the operator a sense of the quantity relative to other possible values, and they follow along when a quantity changes. Some engineers and technicians prefer the “feel” of an analog meter, even in situations where a digital meter would work just as well.
One problem you might have with digital meters is being certain of where the decimal point goes. If you’re off by one decimal place, the error will be by a factor of 10.

Also, you need to be sure you know what the units are; for example, a frequency indicator might be reading out in megahertz, and you might forget and think it is giving you a reading in kilohertz. That’s a mistake by a factor of 1000. Of course this latter type of error can happen with an analog meter, too.
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Watt-hour meters- principle and operation

June 19, 2016



The utility company is not too interested in how much power you’re using with one appliance,or even how much power a single household is drawing, at any given time. Byfar the greater concern is the total energy that is used over a day, a week, a month or ayear. Electrical energy is measured in watt hours, or, more commonly for utility purposes,in kilowatt hours (kWh). The device that indicates this is the watt-hour meteror kilowatt-hour meter.


The most often-used means of measuring electrical energy is by using a small electricmotor device, whose speed depends on the current, and thereby on the power at aconstant voltage. The number of turns of the motor shaft, in a given length of time, is directly proportional to the number of kilowatt hours consumed. The motor is placed at the point where the utility wires enter the house, apartment or building. This is usually at a point where the voltage is 234 V. This is split into some circuits with 234 V, for heavy-duty appliances such as the oven, washer and dryer, and the general household fines for lamps, clock radios and, television sets.
You’ve surely seen the little disk in the utility meter going around and around,

sometimes fast, other times slowly. Its speed depends on the power you’re using. The total number of turns of this little disk, every month, determines the size of the bill you will get—as a function also, of course, of the cost per kilowatt hour for electricity. Kilowatt-hour meters count the number of disk turns by means of geared, rotary drums or pointers. The drum type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise. Reading a pointer type utility meter is a little tricky, because you must think in whatever direction (clockwise or counterclockwise) the scale goes. An example of a pointer type utility meter is shown in Fig. shown below. 




Read from left to right. For each little meter, take down the number that the pointer has most recently passed. Write down the rest as you go. The meter in the figure reads 3875 kWh. If you want to be really precise, you can say it reads 3875-1/2 kWh.
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Wattmeter -principle and working

June 19, 2016

The measurement of electrical power requires that voltage and current both be measured simultaneously. Remember that power is the product of the voltage and current. That is, watts (P) equals volts (E) times amperes (I), written as P = EI.
 In fact, watts are sometimes called volt-amperes in a dc circuit. You might think that you can just connect a voltmeter in parallel with a circuit, thereby getting a reading of the voltage across it, and also hook up an ammeter in series to get a reading of the current through the circuit, and then multiply volts times amperes to get watts consumed by the circuit. And in fact, for practically all dc circuits,this is an excellent way to measure power (Fig.).


Quite often, however, it’s simpler than that. In many cases, the voltage from the
power supply is constant and predictable. Utility power is a good example. The effectivevoltage is always very close to 117 V. Although it’s ac, and not dc, power can be measured in the same way as with dc: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done.
Then, rather than 1 A, the meter would show a reading of 117 W, because P= EI=117×1=117 W.
If the meter reading were 300 W, the current would be 300/117=2.56 A.An electric iron might consume 1000 W, or a current of 1000/117 _ 8.55 A. And a
large heating unit might gobble up 2000 W, requiring a current of
 2000/117=17. 1A.
This might blow a fuse or breaker, since these devices are often rated for only 15 A. You’ve probably had an experience where you hooked up too many appliances to a single circuit, blowing the fuse or breaker. The reason was that the appliances, combined, drew too much current for the house wiring to safely handle, and the fuse or breaker, detecting the excess current, opened the circuit.
Specialized wattmeters are necessary for the measurement of radio-frequency
(RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized applications. But almost all of these meters, whatever the associated circuitry, use simple ammeters as their indicating devices.


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FET and vacuum-tube voltmeters

June 19, 2016
It was mentioned that a good voltmeter will disturb the circuit under test as little as possible,and this  requires that the meter have a high internal resistance. Besides the electrostatictype voltmeter, there is another way to get an extremely high internal resistance. This is to sample a tiny, tiny current, far too small for any meter to directly indicate, and then amplify this current so that a meter will show it. When a miniscule amount of current is drawn from a circuit, the equivalent resistance is always extremely high.




    The most effective way to accomplish the amplification, while making sure that the current drawn really is tiny, is to use either a vacuum tube or a field-effect transistor (FET). You needn’t worry about how such amplifiers work right now; that subject will come much later in this book. A voltmeter that uses a vacuum tube amplifier to minimize current drain is known as a vacuum-tube voltmeter (VTVM). If an FET is used, the meter is called a FET voltmeter (FETVM). Either of these devices provide anextremely high input resistance along with good sensitivity and amplification. And they allow measurement of lower voltages, in general, than electrostatic voltmeters.

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Multimeter- Principle and operation

June 19, 2016
In the electronics lab, a common piece of test equipment is the multimeter, in which different kinds of meters are combined into a single unit. The volt-ohm-milliammeter (VOM) is the most often used. As its name implies, it combines voltage, resistance and current measuring capabilities.
 You should not have too much trouble envisioning how a single milliammeter can be used for measuring voltage, current and resistance. The preceding discussions for measurements of these quantities have all included methods in which a current meter can be used to measure the intended quantity.

   Commercially available multimeters have certain limits in the values they can measure. The maximum voltage is around 1000 V; larger voltages require special leads and heavily insulated wires, as well as other safety precautions. The maximum current that a common VOM can measure is about 1 A. The maximum resistance is on the order of several mega ohms or tens of mega ohms. The lower limit of resistance indication is about an ohm.

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Ohmmeter- basic principle and working

June 19, 2016
You  know that the current through a circuit depends on the resistance. This principle can be used to manufacture a voltmeter using an ammeter and a resistor. The larger the value of the resistance in series with the meter, the more voltage is needed to produce a reading of full scale. This has a converse, or a “flip side”: Given a constant voltage, the current through the meter will vary if the resistance varies. This provides a means for measuring resistances.

  An ohmmeter is almost always constructed by means of a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage (Fig.). By selecting the resistances appropriately, the meter will give indications in ohms over any desired range. Usually, zero on the meter is assigned the value of infinity ohms, meaning a perfect insulator. The full-scale value is set at a certain minimum, such as 1 Ω, 100 Ω, or 10 KΩ (10,000 Ω).

Ohmmeters must be precalibrated at the factory where they are made. A slight error in the values of the series resistors can cause gigantic errors in measured resistance. Therefore, precise tolerances are needed for these resistors. It is also necessary that the battery be exactly the right kind, and that it be reasonably fresh so that it will provide the appropriate voltage. The smallest deviation from the required voltage can cause a big error in the meter indication.
The scale of an ohmmeter is nonlinear. That is, the graduations are not the same
everywhere. Values tend to be squashed together towards the “infinity” end of the scale. It can be difficult to interpolate for high values of resistance, unless the right scale is selected.
 Engineers and technicians usually connect an ohmmeter in a circuit with the
meter set for the highest resistance range first; then they switch the range until the
is in a part of the scale that is easy to read. Finally, the reading is taken, and is multiplied(or divided) by the appropriate amount as indicated on the range switch. Figure Below  shows an ohmmeter reading.


The meter itself says 4.7, but the range switch says 1 KΩ. This indicates a resistance of 4.7 KΩ, or 4700 Ω.
Ohmmeters will give inaccurate readings if there is a voltage between the points
where the meter is connected. This is because such a voltage either adds to, or subtracts from, the ohmmeter battery voltage. This in effect changes the battery voltage, and the meter reading is thrown way off. Sometimes the meter might even read “more than infinity” ohms; the needle will hit the pin at the left end of the scale. Therefore, when using an ohmmeter to measure resistance, you need to be sure that there is no voltage between the points under test. The best way to do this is to switch off the equipment.


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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.




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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.

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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.





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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.


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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


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