Primary and secondary cells

June 19, 2016


Primary and secondary cells
Some electrical cells, once their potential (chemical) energy has all been changed to electricity and used up, must be thrown away. They are no good anymore. These arecalled primary cells.


Other kinds of cells, like the lead-and-acid unit depicted above, can get their chemical
energy back again. Such a cell is a secondary cell.

Primary cells include the ones you usually put in a flashlight, in a transistor radio,
and in various other consumer devices. They use dry electrolyte pastes along with
metal electrodes. They go by names such as dry cell, zinc-carbon cell, alkaline cell,
and others. Go into a department store and find the panel of batteries, and you’ll see
various sizes and types of primary cells, such as AAA batteries, D batteries, camera batteries, and watch batteries. You should know by now that these things are cells, not truebatteries. This is a good example of a misnomer that has gotten so widespread that
store clerks might look at you funny if you ask for a couple of cells. You’ll also see real
batteries, such as the little 9-V transistor batteries and the large 6-V lantern batteries.
Secondary cells can also be found increasingly in consumer stores. Nickel-cadmium
(Ni-Cd or NICAD) cells are probably the most common. They’re available in
some of the same sizes as nonrechargeable dry cells. The most common sizes are AA, C,and D. These cost several times as much as ordinary dry cells, and a charging unit also costs a few dollars. But if you take care of them, these rechargeable cells can be usedhundreds of times and will pay for themselves several times over if you use a lot of “batteries” in your everyday life.
The battery in your car is made from secondary cells connected in series. These cells
recharge from the alternator or from an outside charging unit. This battery has cells likethe one in Fig.



. It is extremely dangerous to short-circuit the terminals of such a battery,
because the acid (sulfuric acid) can “boil” out and burn your skin and eyes.
An important note is worth making here: Never short-circuit any cell or battery, because

it might burst or explode.
Lead acid battery
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Cells and batteries

June 19, 2016

ONE OF THE MOST COMMON AND MOST VERSATILE SOURCES OF DC IS THE CELL.
The term cell means self-contained compartment, and it can refer to any of various different things in (and out of) science. In electricity and electronics, a cell is a unit source of dc energy. There are dozens of different types of electrical cells.
When two or more cells are connected in series, the result is known as a battery.
Kinetic and potential energy .
Energy can exist in either of two main forms. Kinetic energy is the kind you probably think of right away when you imagine energy. A person running, a car moving down a freeway, a speeding aircraft, a chamber of superheated gas—all these things are visible manifestations of kinetic energy, or energy in action. The dissipation of electrical power, over time, is a form of kinetic energy too.
Potential energy is not as vividly apparent. When you raise a block of concrete into
the air, you are creating potential energy. You remember the units called foot pounds, the best way to measure such energy, from school physics classes. If you raise a one-pound weight a foot, it gains one foot pound of potential energy. If you raise it 100 feet, it gains 100 foot pounds. If you raise a 100-pound weight 100 feet, it will gain 100 × 100, or 10,000, foot pounds of potential energy. This energy becomes spectacularly evident if you happen to drop a 100-pound weight from a tenth-story window.
Electrochemical energy
In electricity, one important form of potential energy exists in the atoms and molecules of some chemicals under special conditions. Early in the history of electrical science, laboratory physicists found that when metals came into contact with certain chemical solutions, voltages appeared between the pieces of metal. These were the first electrochemical cells.

   A piece of lead and a piece of lead dioxide immersed in an acid solution (Fig.)will show a persistent voltage. This can be detected by connecting a galvanometer between the pieces of metal. A resistor of about 1,000 ohms should always be used in series with the galvanometer in experiments of this kind; connecting the galvanometer directly will cause too much current to flow, possibly damaging the galvanometer and causing the acid to “boil.”

The chemicals and the metal have an inherent ability to produce a constant exchange of charge carriers. If the galvanometer and resistor are left hooked up between the two pieces of metal for a long time, the current will gradually decrease, and the electrodes will become coated. The acid will change, also. The chemical energy, a form of potential energy in the acid, will run out. All of the potential energy in the acid will have been turned into kinetic electrical energy as current in the wire and galvanometer. In turn, this current will have heated the resistor (another form of kinetic energy), and escaped into the air and into space.

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Bar-graph meters

June 19, 2016

A cheap, simple kind of meter can be made using a string of light-emitting diodes (LEDs) or a liquid-crystal display (LCD) along with a digital scale, to indicate approximate levels of current, voltage or power. This type of meter has no moving parts to break, just like a digital meter. But it also offers the relative-reading feeling you get with an analog meter.
Figure below is an example of a bar-graph meter that is used to show the power output, in kilowatts, for a radio transmitter. It indicates 0.8 kW or 800 watts, approximately.




The chief disadvantage of the bar-graph meter is that it isn’t very accurate. For this reason it is not generally used in laboratory testing. The LED or LCD devices sometimes also flicker when the level is “between” two values given by the bars. This can be annoying to some people.

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Pen recorders

June 19, 2016




A meter movement can be equipped with a marking device, usually a pen, to keep a graphic record of the level of some quantity with respect to time. Such a device is called a pen recorder. The paper, with a calibrated scale, is taped to a rotating drum. The drum, driven by a clock motor, turns at a slow rate, such as one revolution per hour or one revolution in 24 hours. A simplified drawing of a pen recorder is shown in Fig.



A device of this kind, along with a wattmeter, might be employed to get a reading
of the power consumed by your household at various times during the day. In this way you might tell when you use the most power, and at what particular times you might be using too much.

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Light meters or illumination meter./Lux meter

June 19, 2016

Light intensity is measured by means of a light meter or illumination meter. You might think that it’s easy to make this kind of meter by connecting a milliammeter to a solar (photovoltaic) cell. And this is, in fact, a good way to construct an inexpensive light meter (Fig.).




 More sophisticated devices might use dc amplifiers to enhance sensitivity and to allow for several different ranges of readings. One problem with this design is that solar cells are not sensitive to light at exactly the same wavelengths as human eyes. This can be overcome by placing a colored filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as human eyes. Another problem is calibrating the meter. This must usually be done at the factory, in units such as lumens or candela. Its not important that you know the precise definitions of these units in electricity and electronics.

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VU and decibel meters

June 19, 2016

Image result for VU and decibel meters




In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loudness meters are sometimes used. These are calibrated in decibels, a unit that you will sometimes encounter in reference to electronic signal levels. A decibel is an increase or decrease in sound or signal level that you can just barely detect, if you are expecting the change.
Audio loudness is given in volume units (VU), and the meter that indicates it is called a VU meter. Usually, such meters have a zero marker with a red line to the right and a black line to the left, and they are calibrated in decibels (dB) above and below this zero marker (ref fig).

The meter might also be calibrated in watts rms, an expression for audio power.
As music is played through the system, or as a voice comes over it, the VU meter
needle will kick up. The amplifier volume should be kept down so that the meter doesn’t go past the zero mark and into the red range. If the meter does kick up into the red  scale, it means that distortion is probably taking place within the amplifier circuit.

Sound level in general can be measured by means of a sound-level meter, calibrated in decibels (dB) and connected to the output of a precision amplifier with a microphone of known, standardized sensitivity (Fig. 3-13). You have perhaps heard that a vacuum cleaner will produce 80 dB of sound, and a large truck going by might subject your ears to 90 dB. These figures are determined by a sound-level meter. A VU meter is a special form of sound-level meter.
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Frequency counters

June 19, 2016



The measurement of energy used by your home is an application to which digital metering is well suited. It’s easier to read the drum type, digital kilowatt-hour meter than to read the pointer type meter. When measuring frequencies of signals, digital metering is not only more convenient, but far more accurate.

The frequency counter measures by actually counting pulses, in a manner similar
to the way the utility meter counts the number of turns of a motor. But the frequency counter works electronically, without any moving parts. It can keep track  of thousands, millions or even billions of pulses per second, and it shows the rate on a digital display that is as easy to read as a digital watch. It measures frequency directly by tallying up the number of pulses in an oscillating wave, even when the number of pulses per second is huge.


The accuracy of the frequency counter is a function of the lock-in time. Lock-in is usually done in 0.1 second, 1 second or 10 seconds. Increasing the lock-in time by a factor of 10 will cause the accuracy to be good by one additional digit. Modern frequency counters are good to six, seven or eight digits; sophisticated lab devices will show frequency to nine or ten digits.
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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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