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Файл:Professional English for Electrical Specialties. Учебное пособие для СПО
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4. Электронные вольтметры переменного тока строятся по двум схемам:
преобразование переменного напряжения в постоянное и дальнейшее усиление
постоянного напряжения и усиление переменного напряжения и дальнейшее
преобразование переменного напряжения в постоянное.
5. По сравнению с электромеханическими вольтметрами аналоговые
электронные вольтметры имеют следующие достоинства: широкий частотный
диапазон измеряемого напряжения (от единиц герц до сотен мегагерц); слабую
зависимость показаний от частоты измеряемого напряжения в рабочем диапазоне
частот; высокую чувствительность, практически постоянную в рабочем диапазоне
частот; широкий динамический диапазон (от десятых долей до сотен вольт)
благодаря применению усилителей и делителей напряжения; ничтожно малую
мощность потребления, т. к. имеют большое входное сопротивление Rвх (10-10
МОм), малую входную емкость Cвх (1-4 пФ), но и в тоже время развивают
мощность, достаточную для приведения в действие выходного
магнитоэлектрического измерителя.
6. Выбор электронного вольтметра определяется схемами входа и
преобразователя, полным входным сопротивлением, градуировкой шкалы, пределом
измерений, чувствительностью, погрешностью и зависимостью показаний
вольтметра от формы кривой и частоты измеряемого напряжения.
7. Если цифровые приборы отличаются точностью показаний то типы
вольтметров, относящиеся к аналоговым (стрелочным) приборам, способны
реагировать на малейшие отклонения параметров, не определяемых цифровым
прибором.
8. Вольтметр включают в цепь параллельно нагрузке и источнику напряжения,
это делается для того чтобы высокое сопротивление, используемое в приборе не
оказывало влияние на показания прибора.
9. Нормальная работа вольтметра возможна при температуре воздуха не
превышающая 25 – 30 °C с относительной влажностью воздуха до 80 % при
атмосферном давлении 630 – 800 мм рт. ст.
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10. Если шкала вашего прибора не предусматривает возможности отклонения
стрелки в двух направлениях, то для измерения отрицательного напряжения нужно,
например, красным щупом коснуться точки, которой перед этим касались белым
щупом и наоборот (цвета щупов и проводов могут быть произвольными).
Exercise 23.
a. Make an outline of the text “Voltmeters”.
b. Retell the text “Voltmeters” according to the written outline.
Exercise 24. Read the text and then make questions so that the words in bold
provide answers.
Safety Considerations
Don't use a meter with a cracked housing or probes with bare wires showing.
Never use the ohm setting on a multimeter on live voltage. You will damage the
meter.
Use a voltage probe or test light if you just want to check if a circuit is live.
Extreme care is required in using the ammeter function of any multimeter. If you
attempt to use the multimeter as a voltmeter when it has been left in the ammeter
function, the internal fuse will be destroyed!
A complete circuit is needed before electric current will flow, a convenient feature
for working safely with laboratory circuitry. If you do not plug in the power supply or
turn it on, you can work on most circuits without fear of being shocked. Therefore, when
setting up a circuit, turning on the power should be the last step, and turning off the power
is the first step before touching or changing any section of the circuit.
Another safety guideline is to always work with one hand behind your back or
safely out of the way; i.e., do not use both hands for wiring. Damaging current flow
through your upper chest may result if your body serves to complete a circuit between
your right and left hands. Most death by electrocution is caused by fibrillation, disruption
of the body's nerve signals controlling rhythmic beating of the heart, induced by modest
current flow through the chest area.
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Analogue meters (containing moving needles) must be used very carefully. The
meter has an overall low resistance so as not to affect the circuit in which it is placed. An
ammeter connected in parallel may draw a large current and be ruined.
The meter must be placed with its negative (black) terminal connected to the low
voltage side of the circuit and the positive (red) terminal to the high voltage side.
A meter should always be set to its highest possible reading when first connected in
the circuit. If the needle does not deflect enough to make an accurate measurement, select
a lower value in the current range.
Exercise 25. Compose a dialogue between a professor and a student using the
words from the text “Safety Considerations”.
3.4 Ohmmeters
1. What do ohmmeters measure?
2. How do ohmmeters work?
Before you start
Exercise 26. Read and translate the text.
If all other factors are held constant, the current through a circuit depends on the
resistance. This provides us with a means for measuring resistance. An ohmmeter can be
constructed by placing a milliammeter or microammeter in series with a set of fixed,
switchable resistances and a battery that provides a known, constant voltage (figure 37).
By selecting the resistances appropriately, the
meter gives indications in ohms over any desired
range. The zero point on the milliammeter or
microammeter 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, 1 kΩ, or 10 kΩ. Figure 37
An ohmmeter must be calibrated at the factory where it is made, or in an electronics
lab. 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. That means their
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values must actually be what the manufacturer claims they are, to within a fraction of 1
percent if possible. It is also necessary that the battery provide exactly the right voltage.
The scale of an ohmmeter is nonlinear. That means the graduations are not of the
same width everywhere on the meter scale. The graduations tend to be squashed together
toward the infinity end of the scale. Because of this, it is difficult to interpolate for high
values of resistance unless the appropriate meter range 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 down until the meter needle 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 38
shows an ohmmeter reading. The meter itself indicates
approximately 4.7, but the range switch says 1 kΩ. This
indicates a resistance of about 4.7 kΩ, or 4700 Ω.
Figure 38
Ohmmeters 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’s own battery voltage. Sometimes, in this type of situation, an ohmmeter might
tell you that a circuit has “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 must
always be sure that there is no voltage between the points under test. The best way to do
this is to switch off the equipment in question.
Exercise 27. Give the definitions of the following words and expressions.
Switchable
Indication
Desired range
Assign
Infinity ohms
A perfect insulator
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Calibrate
Manufacturer claim
To interpolate for high values of resistance
The appropriate meter range
Exercise 28. Find the sentences in the text where these words and phrases were
used. Comment on them.
Exercise 29. Insert the missing words into the sentences below.
1. An ….. can be constructed by placing a milliammeter or microammeter in series
with a set of fixed, switchable …… and a battery that provides a known, constant …...
2. The zero point on the milliammeter or microammeter is assigned the ….. of
infinity ….., meaning a perfect …….
3. The scale of an ohmmeter is …….
4. Engineers and technicians usually connect an ohmmeter in a …. with the meter
set for the highest ….. range first.
5. Finally, the reading is taken, and is ……. (or divided) by the appropriate amount
as indicated on the range …..
6. Ohmmeters give ….. readings if there is a voltage between the points where the
….. is connected.
7. Sometimes, in this type of situation, an …… might tell you that a circuit has
“more than infinity” ohms!
8. Therefore, when using an ohmmeter to measure resistance, you must always be
sure that there is no voltage between the …… under …...
9. It is difficult to …… for high values of resistance unless the ……. meter range is
selected.
10. The …… tend to be squashed together toward the infinity end of the ……..
Exercise 30. Ask 7 questions using the omitted words from the previous
exercise. Discuss the answers with your partner.
Exercise 31. Make up sentences. Compose a dialogue using the words from the
table.
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Table 6
1. A meter
2. An ammeter
3. An ohmmeter
4. A voltmeter
5. A galvanometer
measures
the resistance
very small currents
electrical values
the current
the potential difference in
volts
1. The voltage
2. The current
3. The resistance
is
measured
in ohms
in volts
in amperes
Exercise 32. Solve the problems. Discuss them with your partner.
1. Draw dependences of absolute, ratio and reduced errors of ohmmeter having the
measurement range from 0 to 500 Ohm, the scale length 200 mm and the accuracy rating
of device 1.5.
2. Determine the absolute error of ohmmeter which is characterized as follows: the
accuracy rating is equal to 3.0; the limit of range is equal to 2000 Ohm; the length of scale
is equal to 150 mm.
3. A moving-coil ohmmeter with a series circuit is supplied with voltage 5V, its
inner resistance of the measuring mechanism being equal to 2 kOhm, the resistance of the
restrictive resistor being equal to 78 kOhm. The full deflection angle of pointer is equal
100°. Determine the maximum current of ohmmeter and do plot at the scale the deflection
angle against resistance measured, if maximal value RX MAX= 1000 kOhm.
Exercise 33. Translate the following sentences into English.
1. Электронные омметры (подгруппа Е6) широко используются для измерения
активных сопротивлений в диапазоне 10Е-4 - 10Е12 Ом при измерении
сопротивлений резисторов, изоляции, контактов, поверхностных и объемных
сопротивлений и в других случаях.
2. Омметры с последовательной схемой соединения более пригодны для
измерения больших сопротивлений, а с параллельной схемой — малых.
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3. Для регулировки омметра с последовательной схемой включения перед
измерением замыкают накоротко его зажимы с надписью «Rx», и в том случае, если
стрелка не устанавливается на отметке «О», перемещают ее до этой отметки с
помощью — шунта.
4. Регулировка омметра с параллельной схемой включения производится при
отключенном резисторе Rx при этом вращением рукоятки шунта указатель
устанавливают на отмётку шкалы соответствующую значению Rx= ∞ .
5. Конструктивно омметры с логометром выполняют весьма разно образно в
зависимости от требуемого предела измерения, назначения (щитовой или
переносный прибор) и т. п.
6. Точность омметров при линейной шкале характеризуется приведенной
погрешностью по отношению к пределу измерения.
7. При нелинейной (гиперболической) шкале погрешности прибора
характеризуются, также приведенной погрешностью, %, но по отношению к длине
шкалы, выраженной в миллиметрах, т. е; γ=(∆l/lшк)100.
8. Параллельную схему включения прибора используют для измерения
сравнительно малых сопротивлений, так как большие сопротивления будут мало
влиять на показания прибора.
9. В омметрах такого типа противодействующий момент создается
электрическим путем за счет того, что подвижная часть измерительного механизма
состоит из двух жестко скрепленных между собой рамок.
10. Одни концы этих проводов вставляются в два разъёма которые
присутствуют в омметре, а другие концы подносятся к выводам любой детали в
которой присутствует электрический ток, к примеру та же самая батарейка, к
выводам которой можно так же подсоединить омметр и он покажет результат.
Exercise 34.
a. Make an outline of the text “Ohmmeters”.
b. Retell the text “Ohmmeters” according to the written outline.
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3.5 Wattmeters
Before you start
1. What do wattmeters measure?
2. How do wattmeters work?
Exercise 35. Read and translate the text.
The measurement of electrical power requires that voltage and current both be
measured simultaneously. Remember that in a dc circuit, the power (P) in watts is the
product of the voltage (E) in volts and the current (I) in amperes. That is, P = EI. In fact,
watts are sometimes called volt-amperes in dc circuits.
Do you think you can 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? Well, you can. For
most dc circuits, this is an excellent way to measure
power, as shown in Figure 39. Figure 39
Sometimes, it’s simpler yet. In many cases, the voltage from the power supply is
constant and predictable. Utility power is a good example. The effective voltage is always
very close to 117 V. Although it’s ac, and not dc, power in most utility circuits can be
measured in the same way as power is measured in dc circuits: 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 will show a reading of 117 W,
because P = EI = 117 × 1 = 117 W. If the meter reading is 300 W, the current is I = P/E =
300/117 = 2.56 A. An electric iron might consume 1000 W, or a current of 1000/117 =
8.55 A. A large heating unit might gobble up 2000 W, requiring a current of 2000/117 =
17.1 A. You should not be surprised if this blows a fuse or trips a circuit breaker, because
these devices are often rated for 15 A.
88
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, milliammeters, or microammeters as their indicating devices.
Exercise 36. Match the words to their definitions.
simultaneously to expend; use up
hook up
multiply to connect a mechanism and a source of
power
predictable a device that trips like a switch and
opens the circuit when overloaded
consume the electronic reproduction of sound,
especially from broadcast or recorded
sources, with minimal distortion
gobble up at the same instant
a circuit breaker a safety device that protects an electric
circuit from excessive current,
consisting of or containing a metal
element that melts when current
exceeds a specific amperage, thereby
opening the circuit
a high-fidelity to grow in amount, number, or degree
a fuse use a large amount of (something) very
quickly
application capable of being foretold
Exercise 37.Compose 7 sentences using the words in the previous exercise. Each
sentence should contain 2 words or word expressions above.
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Exercise 38. Complete the sentences, choosing the right variants.
1. The ampere is an electrical unit
a) for measuring the strength of electric
current in a circuit.
b) for measuring the external force
applied to a circuit.
2. The ohm is an electrical unit
a) for measuring the external force
applied to a circuit.
b) for measuring the resistance or
opposition to the flow of current.
3. The watt is an electrical unit
a) for measuring the strength of electric
current in a circuit.
b) for measuring electric power.
4. The volt is and electrical unit
a) for measuring the external force
applied to a circuit.
b) for measuring electric power.
5. The device used for measuring intensity
of an electric current is called
a) ammeter.
b) ohmmeter.
c) voltmeter.
d) wattmeter.
6. The device used for measuring
resistance is called
a) ammeter.
b) ohmmeter.
c) voltmeter.
d) wattmeter.
7. The device used for measuring
insulation resistance is called
a) ammeter.
b) ohmmeter.
c) wattmeter.
d) megohmmeter.
8. The device used for measuring delivery
of electric energy is called
a) ammeter.
b) ohmmeter.
c) watthourmeter.
d) megohmmeter.
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