A voltmeter is a device that is used to measure voltage across a section of a circuit. How to work with this device correctly, what needs to be taken into account when choosing a voltmeter, what other devices are there for measuring voltage in the network, let's figure it out.

Voltage

Voltage is a physical quantity that expresses the work that was expended on a test electric charge from one electrical circuit points to another. Or in other words, it is the energy expended in moving a positive charge from a point of low potential to a point of high potential.

It comes in two types: constant and variable. Constant voltage is typical for electrostatic circuits or DC, and alternating – for circuits with alternating and suisoidal current. This physical quantity is measured in volts, and its designation is U.

This value can be found using the following formulas:

• U=I*R
• U=P/I
• U=√P*R

Where U is voltage, I is current, R is resistance, P is power.

But the value of U can be found without using these formulas if you carry out special measurements. To do this, you just need to know how to use a voltmeter.

It is the simplest device for measuring voltage. In physics lessons in schools, children are often told about the features of this device, teach how to check the voltage in an electrical circuit. Using it, you can find out not only the voltage, but also the resistance, if you know the special formulas. The voltmeter is easy to use and simple to use, so the voltmeter remains the best way to measure U at home.

Classification of voltmeters

They can be electromechanical (such devices are the most sensitive and accurate), electronic, the operating principle of which is AC voltage conversion permanent and digital.

Based on their purpose, voltmeters can be pulse, direct or alternating current. And according to the principle of application - panel and portable. Before using the device, you need to check which type they are in order to make the correct measurements.

A little history

The first voltmeter in history was invented by the Russian physicist G.V. Richmann in 1754 and was called “electric force indicator”. Modern electrostatic voltmeters are based on the principles of this device.

Structure of a voltmeter

Before you start measuring voltage, you should learn how a voltmeter works.

Its main elements are the body, terminal, pointer and scale. The terminals usually have a plus or minus sign or are marked with a color (plus is red, minus is blue or black). Often on this device you can see the letter "V". When the device is used for circuits with alternating current, a wavy line is depicted on the dial, and when for circuits with direct current, a straight line is displayed. Sometimes the designations AC (to measure alternating current) and DC (to measure direct current) are used. In devices for alternating current there is no polarity.

The classic voltmeter, which is now a little outdated, consists of a coil of thin horseshoe-shaped wire with an iron pointer, which is located between the ends of a magnet. The arrow moves on the axis. Current flows through the coil, and the magnetized needle moves due to the current. The greater the current, the more the needle deflects. You can see that the design of this device is not very complicated. Its entire principle is based on simple laws of physics.

How to use a voltmeter

The voltmeter is always connected in parallel to a section of the circuit, since such a connection reduces the current. The device can measure voltage only in a certain section of the electrical circuit. When working with it, you must always observe polarity. Wires are screwed to screws with nuts. For devices designed for constant voltage, the contacts are marked with plus and minus signs. This concerns the pointer voltmeter. In electronic models everything is much simpler: there are no wires. You can learn more about the operating principle of a voltmeter by watching the video.

How to use a voltmeter

Before taking measurements, you need to check whether the device is suitable for them. First of all, it is necessary to determine the maximum permissible measurement value for a given voltmeter. To do this, simply find the largest numerical value on the voltmeter scale. Next should be clarified, in what units the voltmeter measures. These can be volts, microvolts or millivolts. Neglecting this point may result in the device starting to smoke after connecting to a network whose voltage value is many times higher than the permissible value.

If the voltage in the electrical circuit is already known and exceeds sixty volts, then you need to use special dielectric gloves and probes with good insulation. Safe voltage for humans is about 42 volts under normal conditions and about 11 in unfavorable conditions (high humidity, high temperature, iron objects nearby, etc.).

Voltmeter and car

In a car, this device is used for two main reasons: to monitor the charging of the battery and to monitor voltage drops in the on-board network. For complete control power sags, you can install two voltmeters: one to connect to the battery, and the second to connect to the amplifier terminals.

With it you can measure the current in the car network. The car body has a negative charge (the “-” sign), which means that a terminal with a negative pole is connected to it. The positive terminal is connected to the "positive" generator. This is how the voltage in the car is measured. It usually has a value of about fourteen volts. It is better to use thick wires for connections: they reduce the error in measurements. Basic voltage standards:

• For a stopped engine 12.2 - 12.6 volts
• For a running engine 13.6 - 14.4 volts

Multimeter

A multimeter can also measure voltage. Before using this device, be sure to read the instructions.

Multimeters can generally measure three basic quantities: current, resistance, and voltage. They can be analog and digital.

Some multimeters can also measure:

Thus, the capabilities of a multimeter are determined by its model and type. Absolutely any multimeter can measure voltage, current (constant) and resistance.

The classic voltmeter is easy to use and simple in design. It is always connected in parallel to a section of the circuit. It is always better to assemble the circuit first and then connect the voltmeter to it. When working with this device It is very important to observe polarity. It can be used to measure the voltage in a car. We should never forget that voltage (both high and low) is dangerous not only to health, but also to human life.

Therefore, when working with electrical appliances, you must follow safety precautions: use special gloves, work only under normal conditions, etc. Before use, you need to check the device.

Current strength is measured in amperes, therefore, a device that measures current is called ammeter. Voltage (or the potential difference between two points in an electrical circuit) is measured in volts, therefore the device that measures voltage is called voltmeter. To have a quantitative idea of ​​the magnitudes of voltages and currents flowing in different devices, we will give several examples.

1. An incandescent lamp with a power of P = 60 W, operating from a network with a voltage of U = 220 V. The current through such a lamp can be calculated using formula A.

2. Electric kettle with power P = 2200 W (U = 220 V). The current strength is A.

3. The current flowing through a resistor with a resistance to which a voltage of 1.5 V is applied (from a regular battery) is equal to Ohm’s law A.

Three examples are enough to convince us of the very wide range of current (and voltage) values ​​and the need to have ammeters (and voltmeters) with different division values. To measure small currents, milliammeters, designated by the symbol (see Fig. 16a), and microammeters (see Fig. 16b), designated.

The devices shown in Fig. 16 have different measurement ranges. As can be seen from the figure, the maximum current for a milliammeter is only 5 mA, and for a microammeter 50 μA. If the current is greater than these values, then the needle will go off scale, that is, it will go as far to the right as possible and stop, held by the stop. It should be especially noted that when turned off, the arrow of the device should point to the zero division. If the device is “knocked down”, that is, in the absence of current it shows a non-zero value, then you need to use a screwdriver or just your fingernail to turn the zero adjustment wheel (see Fig. 16c).

Now let's look at devices whose measurement range can be changed.

Fig. 17. Dual-range ammeter (a) and 4-range voltmeter (b)

Fig. 17a shows a 2-range ammeter, for which, using a switch, you can set the maximum current to 1 A or 2 A (the white dot on the base of the key indicates 1 A), which will correspond to a maximum value of 100 divisions on the ammeter scale. By moving the ammeter key from the “1 A” position to the “2 A” position, we change the division value from w = 0.01 A to w = 0.02 A.

Fig. 17b shows a 4-range voltmeter with four possible voltage measurement limits of 7.5 V; 15 V; 30V and 60V (the white dot on the base of the key indicates 15V). Considering that the voltmeter scale is designed for 150 divisions, then by moving the voltmeter key from one position to another, we change the division value w as follows:

"7.5V" (w=0.05 V); "15V" (w=0.1 V); "30V" (w=0.2 V); "60V" (w=0.4 V).

I remember such a case from my experience as a novice electrician - somehow I wanted to assemble a simple electronic transmitter circuit. I assembled the parts, wound the coils, and soldered everything together according to the existing electrical circuit. I turn it on, but it doesn't work. Naturally, I became interested in what was what and what processes were taking place inside the circuit. I see on the diagram the operating current values ​​​​are indicated with a crossed out cross on the circuits. I couldn't figure out what kind of schematic symbol this was. A friend came to me and told me what was what. After measurements, I found an error and the transmitter started working. Now I want to share with the same beginners the “great secret” of measuring electric current, and explain how to correctly measure the current strength.

So, electric current is the ordered movement of charged particles inside a particular electrical conductor. This is akin to the flow of water inside a water pipe. If in the case of water moving blades are installed that rotate this flow, then in the case of electric current the flow of charged particles is passed through an additional electrical circuit located in the meters for this purpose. Certain electrical parameters appear on this circuit, such as the voltage drop in a given area (a certain value of the potential difference) and resistance. Since these voltage values ​​already belong to the meter’s circuit, it can easily convert them into a numerical, visual form.

In practice, electric current measurements are made as follows. In the case of measuring constant current, the measuring device (ammeter) is connected to the break of a specific section of the electrical circuit (this is what is indicated by the cross on the circuit diagram indicating the normal operating value of the current, which I mentioned above) in which the measurements are made. Current begins to flow through the electrical element of the ammeter and reacts to changes in electrical parameters within itself. In the case of measuring alternating current, another measurement method arises - using current clamps.

They work like this - the main part is presented in the form of a sliding transformer that wraps around the current-carrying wire. There is an alternating electromagnetic field around a conductor carrying alternating current, which, when flowing around the magnetic circuit, induces a magnetic flux in it. At the other end of this transformer there is a measuring coil on which the voltage value appears. It is converted and displayed on the screen.

How do electricians actually measure current? They have two types of ammeters. For electrical measurements of current strength of relatively small values ​​(in conventional electrical control circuits of electrical equipment), the electrician has with him a conventional multimeter, inside of which there is a function for measuring current strength (both alternating and direct) through an open circuit. On such devices, the maximum current value is within 20 amperes. If there is a need to measure currents of large values, provided that the electrical system is working, and without the possibility of breaking one or another part of the electrical circuit, then current clamps come to the rescue. They just need to wrap around the desired current-carrying wire or bus, and they will immediately show the operating value of the alternating current strength in a given section of the power circuit.

It should not be forgotten that the ammeter itself, when connected to an open section of the circuit, introduces additional electrical resistance into the circuit. If it is not critical for the operation of the system, then you can ignore it. But if even small voltage drops are critical to the operation of electrical equipment, then ammeters with minimal internal resistance should be used. And do not forget, after measuring the current, to switch the multimeter leads to the voltage measurement terminals, since otherwise it may turn out that you are measuring the voltage with a device whose internal resistance is zero. This will naturally cause a short circuit and trouble.

To measure the EMF, or the voltage on a specific current source, use a device called a Voltmeter. To connect the Voltmeter to the terminals of the measurement source, remote probes are used. By type, indicators are divided into digital and pointer.

In order to measure alternating or direct current, various instruments are used. The devices can be universal and measure both one and another type of current. These include a voltmeter brand “E533”

Both direct and alternating voltage are measured in Volts. In Latin it is designated “V”, in Russian “V”. If the voltage is constant, the symbol “-” is placed before the letter; if the voltage is variable “ ~ " Let's say the AC network is designated in two ways: ~220V or 220V. On accumulators and batteries the marking is applied without a sign.

The battery voltage is indicated 1.5V or 1.5V. The vehicle network is specified as follows 12V, 12V. The positive terminal must be marked with a “+” sign. Different instruments are needed to measure each type of current. This is due to the fact that the polarity of direct current does not change over time, but that of alternating current does. For example, we have a household network that changes 50 times every 1 second. The frequency of changes is measured in Hertz, 1 Hz is equal to 1 change in voltage polarity per 1 second.

How to measure voltage in household electrical wiring

The requirements of GOST 13109-97 state that the voltage in the electrical network should not exceed 220V±10%. The minimum voltage in this case will be 198 V, the maximum - 242 V. If household appliances do not work stably, the light bulbs burn dimly or burn out, the first thing you should do is measure the voltage of the electrical wiring.

Before measurements, prepare the device:

The picture shows that the measurement limit in the tester is set to 300 V, in the multimeter - 700 V. Many tester models require switches to be set to several positions: type of measurement (Ohms, A, B); type of current (-, ~), and also install the ends of the probes into the required sockets. The multimeter requires installing a black probe in the COM port (whatever the measurement may be), a red one in V (measurement of voltage, frequency, resistance). The ma socket is designed to measure small currents, the 10 A socket is for currents not exceeding 10 A.

Carefully! If you insert a plug into the 10 A socket and measure the voltage, the device will fail. If there is a fuse, this will save the device. If not, you will have to purchase a new one. This happens quite often. I've come across quite a few devices with burnt out resistors. After everything has been done, you can start measuring.

If there are no numbers on the display when you turn on the device, the batteries are not inserted or are damaged. Most often, multimeters use Krona, which provides 9 V power. This battery will last for a year. Therefore, if the device was not used for a long time, the battery could be discharged. In stationary conditions, it is better to use a ~220V/–9V adapter instead of a crown. The ends of the probes are inserted into the socket.

The multimeter will start working, but you need to be able to read the readings of the dial gauge. At first glance, this is a rather complicated operation. The “TL-4” device, which I have had for over 40 years, has 5 scales. The upper scale is for readings that are multiples of 1 (0.1, 1, and so on).

The scale is lower for numbers that are multiples of 3 (0.3, 3, and so on). If alternating current is measured, the value of which is 1 V, 3 V, there are 2 auxiliary scales. A special scale is applied under the resistance. All testers are made according to this principle, only the multiplicity of numbers may differ.

We will take readings from the second scale, multiplying them by 100. Because the probe is inserted at “~300V”. The small division price is 0.1. Therefore, 2.3 + take into account that the arrow is between the strokes, it turns out 2.35 * 100 = 235 V. This voltage is within the permissible range. If a constant deviation of the arrow is observed during measurements, it is necessary to check the contact of the connections. If it's bad, have it reviewed.

How to Measure the DC Voltage of a Battery
battery or power supply

Since the current source is no more than 24 V, which does not threaten human life, there is no need to adhere to safety measures. To determine the suitability of a battery, power supply or accumulator for further operation, you should measure the voltage at the terminals. On the battery, the terminals are located at the ends. Positive is marked as a “+” sign

Measuring direct current is no different from measuring alternating current. You just need to install the device in the required measuring range and measure, maintaining the polarity.

To make a more accurate assessment of the capacitance, it is necessary to measure the voltage under the load that is applied to the poles. For a battery with a voltage of 1.5 V, a load in the form of a 1.5 V incandescent lamp is suitable. To make it convenient to carry out test work, it can be soldered to the battery through conductors. If the voltage deviation from the norm is no more than 15%, the battery is suitable.

If the device is not available, you can determine the degree of discharge by the glow of the light bulb. However, this method will not provide a guarantee. It will only confirm that the battery can be used at this time. If the light bulb is dim, do not throw away the battery. It can be installed in a wall clock, in which it will serve for a long time. This is because the current consumption of the clock is too small.

Voltages, currents and resistances are unlikely to immediately answer this question: what does a voltmeter measure? Since the answer is obvious if you just read the second half of the title of the article, we will expand on this topic in a little more detail. In particular, we will consider measuring voltage in everyday life, the characteristics of devices, and the principle of operation.

Definition

A voltmeter is a measuring device that allows you to obtain a voltage value in either a direct or alternating current circuit. The range of the device can extend up to 1000 V or more. It all depends on its purpose. To better understand what this is, consider the definition of electromotive force. Since it is very often confused with the mains voltage, they should be separated from each other.

EMF and voltage: the difference

So, EMF characterizes the work performed by any non-electrical forces to move a single positive charge along the contour under consideration. In the most common case, it shows the ability of an energy source to create one or another potential difference at two separated points in the circuit. Like voltage, it is measured in volts. It differs from it in that it characterizes the power source at idle, that is, without connecting to the network.

When there is current in the circuit, that is, it is closed, another more familiar concept appears - voltage. Moreover, it can be taken both for the power source itself at its terminals, and in any part of the circuit. Voltage measurement is the identification of the potential difference between two separated points. For a power source, it is usually somewhat less than the electromotive force when it is included in the consumption circuit. In fact, both EMF and voltage are the same thing, with the only difference being what physical process gives rise to the potential difference between the two points at which the measurement is carried out.

Types of voltmeters

There are two main types to be distinguished here: portable and stationary. The portable voltage measuring device can not only be carried by hand. It usually includes functionality for checking currents and resistance in the circuit, as well as conductor temperatures, etc. Stationary devices are often structurally integrated with the network itself, in which the measurement is carried out, for example, in electrical distribution boards, panels, etc.

As for the classification according to the principle of operation, we can distinguish several types of electromechanical voltmeters and two types of electronic ones. The latter are analog and digital. Electromechanical voltmeters use a magnetic transducer system to obtain voltage values. Electronic devices digitize the received signal using an ADC. Voltmeter readings, depending on the principle of data presentation, are shown either by a dial indicator or on a special digital display.

Another classification is by purpose. It allows you to divide devices into DC and AC meters, as well as phase-sensitive, pulse and universal ones. For the latter, almost the entire range of signals is available, the voltage of which needs to be known.

Main technical characteristics

Depends on the purpose of the device. For example, a DC voltmeter usually has several measurement ranges, so their number will be one of the most important technical characteristics. In addition, almost all devices have a certain input resistance, which, by the way, depends on the limits within which the voltage of the section of the circuit under study lies.

Other important characteristics are, of course, errors, as well as scale resolution or the minimum reading step if we are talking about a digital voltmeter. If the user has a universal device for measuring voltage, the above can be supplemented by limits on the quantities with which it is capable of working, for example, currents, resistances, temperatures, as well as ranges, errors and frequencies for variable signals.

Operating principle

Digital voltmeters, which have recently become common in everyday life, have a large number of electronics under the cover. This is due to the conversion of the analog signal received at the input of the device into digital form using an ADC. Those who are really interested can read a large amount of literature on the topic. Such a voltmeter, the price of which ranges from several hundred rubles, of course, does not pretend to have a wide range of capabilities and great accuracy, but it is quite capable of measuring the voltage at the terminals of a car battery or in a 220 V network.

Circuit connection

A voltmeter in a circuit is always connected in parallel to the load or power source if you need to measure its EMF or voltage at the terminals. That is why there are such strict requirements for the input resistance of the device, since such a connection implies the appearance of additional conductivity in the circuit.

Under no circumstances should you connect a voltmeter, the price of which is even remotely high, into the circuit in series. Otherwise, the user risks simply burning the device, without having time to find out what it all started for. Even if he withstands such barbaric treatment, his readings should not be trusted, since the current in the circuit undergoes serious changes when the voltmeter is connected in this way, again pushing into the unknown the possibility of knowing the actual voltage of the section in which the measurements are made.

Safety precautions during operation

Since the resistance of the voltmeter itself is in most cases quite high, and the connection circuit, accordingly, is parallel, the risk of getting something serious in a low-voltage circuit is minimal. However, if we are talking about industrial devices, especially stationary ones, this implies huge values ​​of the measured quantities, both voltages and, most likely, currents. Therefore, safety precautions here must be at the highest level, and without sufficient knowledge, rubber gloves, mats and other appropriate measures, naturally, any activity is contraindicated. What the voltmeter measures is most likely very life-threatening, so we recommend not to tempt fate. In any case, you should not touch it, even if the circuit is by definition low-voltage.

Conclusion

Answering the question of what a voltmeter measures, we examined in general terms its structure, main technical characteristics, and classification. Of course, this short review does not pretend to be comprehensive, especially given the reader’s insufficient basic knowledge of electrical engineering.

To summarize, it should be noted that the voltmeter, of course, measures the voltage on the section of the circuit to which it is connected in parallel. Otherwise, his readings will be unreliable, not to mention the danger of possible loss of expensive equipment. The reader should be careful when operating the device at home, since contact with high-voltage circuits is dangerous to life.

1. What are the peak, average, average rectified and root mean square voltage values?

2. What coefficients establish the relationship between amplitude and root mean square, root mean square and average voltage values? What are these coefficients for a harmonic waveform?

3. What can cause a methodological error when measuring a non-sinusoidal signal? Give examples of devices in which such an error is observed?

4. What types are voltmeters classified into?

5. What are the features of measuring electrical voltage?

6. Why is a converter of electrical voltage into a proportional value of electric current used when constructing a voltmeter?

7. What functions does an additional resistor perform?

9. What requirements must be observed when choosing a device in order to reduce methodological error?

10. What systems of measuring mechanisms are used to measure DC voltage, power frequency voltage, audio and high frequency current voltage?

11. Which analog voltmeters are most often used to measure DC voltage? Why?

12. What are the features of measuring voltage with an electrostatic voltmeter?

13. What systems of instruments are used to measure power frequency voltage?

14. Features of measuring electrical voltage with electrodynamic system devices.

15. Describe the design features and operating principle of a voltage measuring transformer.

16. Name the features of measuring AC voltage at LF.

17. Name the features of measuring AC voltage at HF ​​and microwave.

18. Give the basic diagrams for constructing electronic analog voltmeters and their differences.

19. Explain the method of number - pulse conversion of voltage into code.

20. Explain the bitwise coding method.

21. What is a digital measuring instrument (DMI)?

22. What main elements of the circuit does any CIP contain?

23. What is the purpose of an analog-to-digital converter (ADC)?

24. What transformations are carried out in the ADC?

25. What is time sampling?

26. What is level sampling?

27. What does the quantization process come down to?

28. Is measurement error introduced into the result when sampling by time and sampling by level? Explain.

29. Why were digital devices called digital devices?

30. What is coding?

31. What is a digital readout device (DRO) used for?

32. In what device is a digital code converted to a decimal number system?

33. List the advantages and disadvantages of the CIP.

V – SECTION

Topic: General information and methods of frequency measurement

1. What is AC frequency?

2. What is 1Hz equal to?

3. How are frequency and wavelength related?

4. What methods of frequency measurement are there?

5. What determines the choice of frequency measurement method?

6. What methods and instruments are used to measure the frequency of the audio range?

7. What methods and instruments are used to measure frequency in the LF range?

8. Describe the principle of frequency measurement with electrodynamic and ferrodynamic ratiometers.

9. Describe the design and operating principle of a vibration frequency meter.

10. At what frequencies is the comparison method used?

11. What are oscillographic frequency measurement methods?

12. Describe the features of frequency measurement using the Lissajous Figures method.

13. Describe the features of frequency measurement using the circular sweep method.

14. What is the operating principle of a heterodyne frequency meter? (Zero beat method).

15. What is the operating principle of a vibration frequency meter?

16. What is the main application of the resonant frequency measurement method?

17. What is the operating principle of a digital electronic frequency counter based on?

18. Describe the principle of constructing a block diagram and the principle of operation of a digital electronic frequency counter.

TOPIC: PHASE SHIFT MEASUREMENT. BASIC INFORMATION.

1. What is the current phase?

2. What is the initial phase?

3. What is the phase shift between two harmonic signals?

4. How is the phase shift determined?

5. Between what electrical signals is the phase shift measured?

6. For what reasons can a phase shift occur in an electrical circuit?

7. What is the phase shift between common-mode signals?

8. What is the phase shift between signals that are in antiphase and quadrature?

9. What are the names of devices for measuring phase differences? Give their classification.

10. What are the names of devices designed to artificially introduce a phase shift?

11. In what units of measurement is the phase shift expressed?

12. What measurement methods are used to measure phase shift?

13. Describe the features of measuring phase shift with electromechanical phase meters.

14. What are oscillographic methods for measuring phase shift?

15. Describe the linear sweep method.

16. What is the sine sweep method?

17. Describe the principle of determining the phase shift by the ellipse method.

18. Describe the principle of determining the phase shift using the circular scanning method.

Topic: General information and methods for measuring power and energy

1. What is electrical power?

2. Through what basic electrical quantities is power determined?

3. What is the difference in power in alternating current circuits of sinusoidal oscillations?

4. What is pulse power?

5. How is average power determined?

6. What is the power level?

7. Define instantaneous power.

8. In what units of measurement is power expressed?

9. What are the advantages of using relative power units?

10. Describe the features of the ammeter and voltmeter method in determining power.

11. How is the power of direct and alternating single-phase current measured with wattmeters?

12. What determines the error in measuring power in DC circuits using a wattmeter?

13. What factors determine the operating frequency range of electromechanical wattmeters?

14. Explain the principle of operation and circuits of rectifier and thermoelectric wattmeters.

15. Explain the principle of operation of a wattmeter in an electrodynamic system.

16. Describe the features of power measurement at audio frequencies.

17. Describe the features of power measurement at high frequencies.

Topic: Measuring the parameters of electronic semiconductor devices and microelectronic circuits

1. Explain the features of measuring the parameters of electron tubes.

2. Explain the features of measuring the parameters of semiconductor diodes.

3. How do you measure the main parameters of diodes and take their volt-ampere characteristics?

4. How are the capacitance and quality factor of varicaps measured?

5. What are the properties of tunnel diodes and the circuits for measuring their characteristics and parameters?

6. What are the properties of switching diodes and the circuits for measuring their characteristics and parameters?

7. How do you perform a simple test of the performance of diodes and transistors?

8. How are transistor parameters measured?

9. Give the definition of h - parameters of transistors, explain the method for measuring these parameters.

10. What are the properties and characteristics of field-effect transistors of different structures?

11. What is oscillography of the volt-ampere characteristics of diodes?

12. How to reproduce the output characteristics of transistors on the oscilloscope screen?

13. Explain the features of measuring the parameters of integrated circuits.

14. What are the main parameters that characterize digital integrated circuits?

Topic: Methods for measuring resistance, capacitance and inductance

1. What is the purpose of resistors, capacitors and inductors?

2. What must be taken into account when measuring resistance, capacitance, and inductance?

4. What is the operating principle of direct-reading ohmmeters based on?

5. What switching circuits does a single-frame ohmmeter of a magnetoelectric system have?

6. Describe the sequential block diagram and operating principle of a device built according to this scheme.

7. Describe the features of measuring small resistances with a magnetoelectric ohmmeter.

8. Name the main disadvantage of single-frame ohmmeters of a magnetoelectric system.

9. Describe the principle of measuring resistance with a ratiometric device?

10. How are the parameters of resistors, capacitors and inductors measured using the voltmeter-ammeter method?

11. What is the bridge measurement method?

12. What are the features of the bridge method for measuring resistance, capacitance and inductance on direct and alternating current?

13. Explain the construction principle and operating principle of a digital device for measuring capacitance, inductance, and resistance.

14. Explain the principle of operation of an electrodynamic microfaradometer.

Voltmeters, millivoltmeters, and microvoltmeters of various systems are used to measure voltage. These devices are connected in parallel to the load, so their resistance should be as high as possible (about two orders of magnitude greater than the resistance of any circuit element).

Figure 6 Figure 7

To expand the measurement limits of the voltmeter (in k times) in DC circuits with voltages up to 500V, additional resistances are usually used R d , connected in series with a voltmeter.

From the relation
let's define
,

Where U max is the highest voltage value that can be measured by a voltmeter with additional resistance;

U int - the limiting (nominal) value of the voltmeter scale in the absence of Rd.

The value of the actually measured voltage U is determined from the relationship:

;
,

where U in - voltmeter reading.

In alternating current circuits, voltage transformers are used to change the measurement limits of the voltmeter.

Power measurement. Power measurement in direct and single-phase current circuits

The power in DC circuits consumed by this section of the electrical circuit is equal to:

and can be measured with an ammeter and voltmeter.

In addition to the inconvenience of simultaneous reading of two instruments, power measurement by this method is carried out with inevitable error. It is more convenient to measure power in DC circuits with a wattmeter.

It is impossible to measure active power in an alternating current circuit with an ammeter and voltmeter, because the power of such a circuit also depends on cosφ:

Therefore, in AC circuits, active power is measured only with a wattmeter.

Figure 8

Fixed winding 1-1 (current) is connected in series, and movable winding 2-2 (voltage winding) is connected in parallel with the load.

To correctly turn on the wattmeter, one of the current winding terminals and one of the voltage winding terminals are marked with an asterisk (*). These terminals, called generator terminals, must be turned on from the power supply side by combining them together. In this case, the wattmeter will show the power coming from the network (generator) to the electrical energy receiver.

Measurement of active power in three-phase current circuits

When measuring the power of three-phase current, various circuits for connecting wattmeters are used depending on:

wiring systems (three- or four-wire);

load (uniform or uneven);

load connection diagrams (star or delta).

a) power measurement under symmetrical load; three- or four-wire wiring system:

Figure 9 Figure 10

In this case, the power of the entire circuit can be measured with one wattmeter (Figures 9,10), which will show the power of one phase P=3P f =3U f I f cosφ

b) with an asymmetric load, the power of a three-phase consumer can be measured with three wattmeters:

Figure 11

The total power of the consumer is equal to:

c) power measurement using the two-wattmeter method:

Figure 12

It is used in 3-wire three-phase current systems with symmetrical and asymmetrical loads and any method of connecting consumers. In this case, the current windings of the wattmeters are connected to phases A and B (for example), and the parallel windings to linear voltages U AC and U BC (or A and C  U AB and U CA), (Fig. 12).

Total power P=P 1 +P 2.