• tutorial

Introduction

Hello! After the completion of the cycle on sensors, there were questions of a different plan for measuring the parameters of consumption of household and not very electrical appliances. Who consumes how much, how to connect what to measure, what are the subtleties, and so on. It's time to reveal all the cards in this area.
In this series of articles, we will consider the topic of measuring electricity parameters. These parameters are actually a very large number, which I will try to gradually talk about in small series.
There are three series in the pipeline so far:

• Electricity measurement.
• Electricity quality.
• Devices for measuring parameters of electricity.

In the process of analysis, we will solve certain practical problems on microcontrollers until the result is achieved. Of course, most of this cycle will be devoted to measuring AC voltage and can be useful to all those who like to control electrical appliances in their smart home.
Based on the results of the entire cycle, we will produce a kind of smart electric meter with Internet access. Absolutely notorious fans of controlling the electrical appliances of their smart home can provide all possible assistance in the implementation of the communication part based on, for example, MajorDomo. Let's make the OpenSource smart home better, so to speak.
In this series, in two parts, we will address the following questions:

• Connection of current and voltage sensors in DC devices, as well as single-phase and three-phase AC circuits;
• Measurement of effective values ​​of current and voltage;
• Power factor measurement;
• Apparent, active and reactive power;
• Electricity consumption;

By sliding you will find the answers to the first two questions of this list. I deliberately do not touch on the accuracy of measuring indicators and from this series I am only happy with the results obtained with an accuracy of plus or minus bast shoes. I will definitely devote a separate article to this issue in the third series.

1. Sensor connection

In the last cycle about voltage and current sensors, I talked about the types of sensors, but did not talk about how to use them and where to put them. It's time to fix it

Connecting DC sensors

It is clear that the whole cycle will be devoted to AC systems, but we will quickly go over DC circuits, as this can be useful to us when developing DC power supplies. Take for example a classic PWM buck converter:


Fig 1. PWM buck converter
Our task is to provide a stabilized output voltage. In addition, on the basis of information from the current sensor, it is possible to control the operating mode of the inductor L1, preventing its saturation, and also to implement the current protection of the converter. And frankly, there are no particular options for installing sensors.
A voltage sensor in the form of a resistive divider R1-R2, which is the only one capable of operating on direct current, is installed at the output of the converter. As a rule, a specialized converter microcircuit has a feedback input, and makes every effort to ensure that this input (3) has a certain voltage level prescribed in the documentation for the microcircuit. For example 1.25V. If our output voltage matches this level - all is well - we directly apply the output voltage to this input. If not, then set the divider. If we need to provide an output voltage of 5V, then the divider must provide a division factor of 4, i.e. For example, R1 = 30k, R2 = 10k.
The current sensor is usually installed between the power supply and the converter and on the chip. By the potential difference between points 1 and 2, and with a known resistance of the resistors Rs, it is possible to determine the current value of the current of our inductor. Installing a current sensor between sources and load is not a good idea, since the filter capacitor will be cut off by a resistor from consumers of pulsed currents. Installing a resistor in a break in the common wire also does not bode well - there will be two ground levels with which to mess around is still a pleasure.
Voltage drop problems can be avoided by using non-contact current sensors - e.g. hall sensors:


Fig 2. Non-contact current sensor
However, there is a trickier way to measure current. After all, the voltage drops on the transistor in the same way and the same current flows through it as the inductance. Therefore, the current value of the current can also be determined from the voltage drop across it. To be honest, if you look at the internal structure of converter microcircuits, for example, from Texas Instruments, then this method occurs as often as the previous ones. The accuracy of this method is certainly not the highest, but this is quite enough for the current cutoff to work.


Fig 3. Transistor as a current sensor
We do the same in other circuits of similar converters, whether it be boost or invert.
However, it is necessary to separately mention the transformer forward and flyback converters.


Figure 4. Connecting current sensors in flyback converters
They can also use either an external resistance or a transistor in its role.
On this, we are done with connecting sensors to DC converters. If you have suggestions for other options, I will gladly supplement the article with them.

1.2 Connecting sensors to single-phase AC circuits

In AC circuits, we have a much larger selection of possible sensors. Let's consider several options.
The simplest is to use a resistive voltage divider and a current shunt.


Figure 5. Connecting resistor sensors
However, it has a couple of significant drawbacks:
First, either we will provide a significant signal amplitude from the current shunt, allocating a large amount of power on it, or we will be content with a small signal amplitude and subsequently amplify it. And secondly, the resistor creates a potential difference between the neutral of the network and the neutral of the device. If the device is isolated, then it does not matter, but if the device has a ground terminal, then we risk being left without a signal from the current sensor, as we will short it out. Perhaps it is worth trying sensors that work on other principles.
For example, we will use current and voltage transformers, or a hall effect current sensor and a voltage transformer. There are much more opportunities for working with equipment, since the neutral wire has no losses, and most importantly, in both cases there is galvanic isolation of the measuring equipment, which can often come in handy. However, it must be taken into account that current and voltage transformer sensors have a limited frequency response, and if we want to measure the harmonic composition of distortions, then this is not a fact that will work out for us.


Fig 6. Connecting transformer and non-contact current and voltage sensors

1.3 Connecting sensors to multi-phase circuits of AC networks

In polyphase networks, our ability to connect current sensors is slightly less. This is due to the fact that the current shunt cannot be used at all, since the potential difference between the phase shunts will fluctuate within hundreds of volts, and I do not know of any general-purpose controller whose analog inputs can withstand such mockery.
One way to use current shunts is of course - for each channel you need to make a galvanically isolated analog input. But it is much easier and more reliable to use other sensors.
In my quality analyzer, I use resistive voltage dividers and remote current sensors based on the hall effect.

Fig 7. Current sensors in a three-phase network
As you can see from the figure, we are using a four-wire connection. Of course, instead of current sensors on the hall effect, you can take current transformers or Rogowski loops.
Instead of resistive dividers, voltage transformers can be used, both for a four-wire and for a three-wire system.
In the latter case, the primary windings of the voltage transformers are connected in a triangle, and the secondary windings in a star, the common point of which is the common point of the measuring circuit.


Figure 8. Use of voltage transformers in a three-phase network

2 Effective value of current and voltage

It's time to solve the problem of measuring our signals. Practical significance for us is primarily the effective value of current and voltage.
Let me remind you of the materiel from the sensor cycle. With the help of the ADC of our microcontroller, at regular intervals, we will record the instantaneous voltage value. Thus, for the measurement period, we will have an array of data on the level of the instantaneous voltage value (everything is similar for current).


Figure 9. A series of instantaneous voltage values
Our task is to calculate the effective value. First, let's use the integral formula:
(1)
In a digital system, you have to limit yourself to a certain quantum of time, so we go to the sum:
(2)
Where is the sampling period of our signal, and is the number of samples per measurement period. Somewhere here in the video I start to rub the game about the equality of the areas. I should have slept that day. =)
In the MSP430FE4252 microcontrollers, which are used in single-phase Mercury meters, 4096 readings are made for a measurement period of 1, 2 or 4 seconds. We will rely on T=1s and N=4096 in what follows. Moreover, 4096 points per second will allow us to use fast Fourier transform algorithms to determine the harmonic spectrum up to the 40th harmonic, as required by GOST. But more on that in the next episode.
Let's sketch an algorithm for our program. We need to ensure a stable start of the ADC every 1/8192 second, since we have two channels and we will measure these data alternately. To do this, set up a timer and the interrupt signal will automatically restart the ADC. All ADCs do this.
We will write the future program on arduino, since many have it at hand. Our interest is purely academic.
Having a system quartz frequency of 16 MHz and an 8-bit timer (so that life does not seem like honey), we need to ensure the frequency of operation of any timer interrupt with a frequency of 8192 Hz.
We are sad about the fact that 16MHz is not divided as much as we need and the final frequency of the timer is 8198Hz. We close our eyes to the error of 0.04% and still read 4096 samples per channel.
We are sad that the overflow interrupt in arduino is busy calculating the time (responsible for millis and delay, so this will stop working normally), so we use the comparison interrupt.
And we suddenly realize that the signal comes to us bipolar, and that msp430fe4252 copes with it perfectly. We are content with a unipolar ADC, so we assemble a simple bipolar-to-unipolar signal converter on an operational amplifier:


Fig 10. Bipolar to unipolar signal converter
Moreover, our task is to ensure the oscillation of our sinusoid relative to half of the reference voltage - then we will either subtract half of the range or activate the option in the ADC settings and get the sign values.
The Arduino has a 10-bit ADC, so from an unsigned result between 0-1023 we will subtract half and get -512-511.
We check the model assembled in LTSpiceIV and make sure that everything works as it should. In the video material, we are additionally convinced experimentally.


Figure 11. Simulation result. Green is the original signal, blue is the output

Sketch for Arduino for one channel

void setup() ( autoadcsetup(); DDRD |=(1<

The program was written in the Arduino IDE for the ATmega1280 microcontroller. On my debug board, the first 8 channels are routed for the internal needs of the board, so the ADC8 channel is used. It is possible to use this sketch for the board with ATmega168, but you need to select the correct channel.
Inside the interrupts, we distort a couple of service pins in order to visually see the working frequency of digitization.
A few words about where the coefficient 102 came from. At the first start, a signal of various amplitudes was supplied from the generator, the indication of the effective voltage value was read from the oscilloscope, and the calculated value was taken from the console in absolute ADC units.

Umax, V	Urms, V	Counted
3	2,08	212
2,5	1,73	176
2	1,38	141
1,5	1,03	106
1	0,684	71
0,5	0,358	36
0,25	0,179	19

Dividing the values ​​of the third column by the values ​​of the second, we get an average of 102. This will be our “calibration” coefficient. However, it can be seen that as the voltage decreases, the accuracy drops sharply. This is due to the low sensitivity of our ADC. In fact, 10 digits for accurate calculations is catastrophically small, and if it is quite possible to measure the voltage in the outlet in this way, then putting a 10-bit ADC to measure the current consumed by the load will be a crime against metrology.

At this point, we will break. In the next part, we will consider the other three questions of this series and we will smoothly move on to creating the device itself.

The presented firmware, as well as other firmware for this series (since I shoot videos faster than I prepare articles) can be found in the repository on GitHub.

Current measurement(abbreviated as current measurement) is a useful skill that will come in handy more than once in life. It is necessary to know the magnitude of the current when it is necessary to determine the power consumption. A device called an ammeter is used to measure current.

There are alternating current and direct current, therefore, various measuring instruments are used to measure them. Current is always denoted by the letter I, and its strength is measured in Amps and is denoted by the letter A. For example, I \u003d 2 A indicates that the current strength in the circuit under test is 2 Amps.

Let us consider in detail how various measuring instruments are marked for measuring different types of currents.

• On a measuring device for measuring direct current, the symbol "-" is applied in front of the letter A.
• On a measuring device for measuring alternating current, the symbol "~" is applied in the same place.

• ~ A device for measuring alternating current.
• -A device for measuring direct current.

Here is a photo of an ammeter designed for DC current measurements.

According to the law, the strength of the current flowing in a closed circuit, at any point in it, is equal to the same value. As a result, in order to measure the current, it is necessary to disconnect the circuit at any site convenient for connecting the measuring device.

It should be remembered that the magnitude of the voltage present in the electrical circuit does not have any effect on current measurement. The current source can be either a 220 V household power supply or a 1.5 V battery, etc.

If you are going to measure the current in a circuit, pay careful attention to whether the current flows in the circuit, direct or alternating. Take an appropriate measuring device and if you do not know the expected current strength in the circuit, set the current measurement switch to the maximum position.

Let us consider in detail how to measure the current strength with an electrical appliance.

For safety current consumption measurement electrical appliances we will make a homemade extension cord with two sockets. After assembly, we get an extension cord very similar to a standard store extension cord.

But if we disassemble and compare with each other, a home-made and store-bought extension cord, then we will clearly see the differences in the internal structure. The conclusions inside the sockets of a homemade extension cord are connected in series, and in the store they are connected in parallel.

The photo clearly shows that the upper terminals are interconnected by a yellow wire, and the mains voltage is supplied to the lower terminals of the sockets.

Now we start measuring the current, for this we insert the plug of the electrical appliance into one of the sockets, and the ammeter probes into the other socket. Before measuring current, do not forget the information you read about how to correctly and safely measure the current.

Now consider how to correctly interpret the readings of the pointer ammeter. At current consumption measurement instrument, the ammeter needle stopped at the division of 50, the switch was set to the maximum measurement limit of 3 amperes. The scale of my ammeter has 100 divisions. This means that it is easy to determine the measured current strength by the formula (3/100) X 50 \u003d 1.5 Ampere.

The formula for calculating the power of the device according to the consumed current.

Having data on the amount of current consumed by any electrical appliance (TV, refrigerator, iron, welding, etc.), you can easily determine what power consumption this appliance has. There is a physical pattern in the world that electricity always obeys. The discoverers of this pattern, Emil Lenz and James Joule, and in honor of them, it is now called the Joule-Lenz Law.

• I - current strength, measured in Amperes (A);
• U - voltage, measured in Volts (V);
• P is power, measured in Watts (W).

Let's carry out one of the current calculations.

I measured the current consumption of the refrigerator and it is equal to 7 amperes. The voltage in the network is 220 V. Therefore, the power consumption of the refrigerator is 220 V X 7 A \u003d 1540 W.

What can be done based on a small Attiny13 microcontroller? Many things. For example, a voltage, current, temperature meter, with results displayed on a HD44780 type display. So let's assemble this universal device that can be successfully used as a module in power supplies, chargers, UMZCH and in those places where very high accuracy is not required. The board size is only 35 x 16 mm.

U, I, T meter circuit on Attiny13

• Voltage measurement range 0-99V with resolution 0.1V.
• Current measurement range 0-9.99A with a resolution of 10 mA.
• Temperature measurement range 0-99C with resolution 0.1C.
• The current consumption of the meter itself is 35 mA.

First of all, you need to know in what voltage range the device will work. To establish this, it is necessary to calculate the voltage divider. For example, to get a measurement of 10V, the divider would be 1/10 (we multiply x 10 because the voltage would be 10 times the base 1V), for 30V it would be 1/30, and so on. Then you need to configure the program for this range. We multiply these 30 V by 640, and divide the result by 1023. The resulting number is approximately written at the beginning of the program, constant voltage and the program must be compiled (for the range of 100 V, 8.2k).

We can also set up the current measurement in a similar way, give a different divider, a different range, and list it, but I will not describe it. There is no analog temperature calibration here, because it seemed completely redundant.

We correct experimentally in the program, the const temp constant is responsible for this. The 1K resistor between the ground and the sensor output sets the voltage, it can even be reduced to 100 ohms.

How the scheme works

The voltage that we want to measure is applied to points V and V + on the board, we connect the GND point with the input of the mass of the power supply, and to point B - the output of the mass (the measurement takes place on the ground). Between the points GND and V - a shunt is connected. The meter is powered from the V and V+ point through the 7805 regulator. There is room on the board for a regulator in the TO252 package, but the larger 78L05 regulator in the TO92 package can also be used with success. The maximum voltage that can be specified for the V and V + points will be up to 35V for a regular 7805, for 78L05 it will, of course, be less, but not more than 30. In order to measure high voltages, the chip must be replenished separately - on the print side, interrupt the path under the voltage adjustment potentiometer, and apply power to point A. The system works with a 16x1 display with an HD44780 or 16x2 controller.

Video of the meter

When flashing the microcontroller, you must set pin reset as a normal pin (enable fusebit RSTDISBL). Before performing this operation, make sure that everything is well set, that after shutdown it is reset, and there is no access to the processor by a normal programmer! Sources, as well as all other documentation and files, are located

The device measures direct voltage from 0 to 51.1 V with a resolution of 0.1 V and direct current from 0 to 5.11 A with a resolution of 0.01 A. Its prototype was the meter described in, which is quite simple in design and has good parameters. The main idea implemented in it to use an inexpensive microcontroller deserves attention. However, the need to use an op amp capable of operating with a unipolar supply at an output voltage close to zero, as well as the presence of an additional power source, impose some restrictions on its use.

Digital voltage and current meter

In addition, the indicators on the prototype board are located inconveniently, it is better to install them in a row horizontally and reduce the dimensions of the front panel of the meter, bringing them closer to the dimensions of the indicators used. The circuit diagram of the meter is presented on the website www.site. Since it was not possible to find the 74HC595N chips used (shift registers with a storage register), 74HC164N chips were used, in which there is no storage register. Also, indicators were used that have a much higher brightness at low current, which made it possible to reduce the current consumed by the meter to 20 mA and to abandon the additional +5 V voltage regulator.

The signal from the current sensor (resistor R1) is fed to the input GP1 of the microcontroller through an inverting amplifier on the op-amp DA1. In contrast to (1J), a bipolar supply of the op-amp with a voltage of ± 8 V is used here, since not all op-amps have the rail to rail property and work correctly with a unipolar supply and almost zero output voltage. Bipolar supply makes it easy to solve this problem, allows the use There are many types of op-amps, since the voltage at the output of the op-amp can be in the range from 8 to 8 V. To protect the input of the microcontroller from overload, a restrictive circuit R10VD9 is used.

The trimmer resistor R8 adjusts the gain, and the trimmer resistor R11 sets zero voltage at the output of the op-amp. Diodes VD1 and VD2 protect the input of the op-amp from overload in the event of a break in the current sensor. Due to the relatively low resistance of the current sensor, the deviation of the voltage measurement result when the load current changes from zero to maximum (5.11 A) does not exceed 0.06 V. If the meter is built into a negative polarity voltage source. the current sensor can be switched on before the output voltage divider and its stabilizer.

In this case, the voltage drop across the current sensor will be compensated by the stabilizer feedback circuit. Since the current of the divider is usually small, it will have almost no effect on the readings of the ammeter, moreover, this effect can be compensated for by a subscript resistor R11. The meter is fed with the output voltage of the power supply rectifier through a converter on transistors VT1 and VT2. This is somewhat more complicated than in, since it requires the manufacture of a pulse transformer, but there are no problems with obtaining all the required voltage ratings. The voltage converter is the simplest push-pull oscillator. the scheme of which is borrowed from . The conversion frequency is about 80 kHz.

Due to the galvanic isolation between the input and output of the converter, the meter can be built into a voltage stabilizer of any polarity. With the transistors indicated in the diagram, it is operational at an input voltage of 30 to 44 V. At the same time, the output voltages vary from approximately 8 to 12 V. Due to the fact that the resistances of the resistors R5 and R6 are chosen quite large, the converter is not afraid of output short circuits. In such cases, the generation simply breaks down.

The voltage of 5 V for powering the digital part of the meter was obtained using an integral stabilizer DA2. It is not required to stabilize the supply voltage of the op amp, since it itself is sufficiently resistant to its changes. The ripple voltage with the conversion frequency is suppressed by RC filters at the inputs of the microcontroller DD1. If the ripples with a frequency of 100 Hz are too large, it is recommended to use the method of reducing them, described in. Here it is worth saying a few words about the instability of the low-order digit of the measurement result inherent in all digital meters.

It always randomly changes by one around the true value. These fluctuations are not due to a malfunction of the instrument, but they cannot be completely eliminated, they can only be reduced by averaging the results of a large number of measurements. The meter parts are mounted on three printed circuit boards made of insulating material foiled on one side. They are designed for installation of microcircuits in DIP packages. Indicators are mounted on one board (Fig. 2), digital microcircuits and a microcontroller are mounted on the second (Fig. 3). The converter, microcontroller supply voltage stabilizer and current sensor signal amplifier are installed on the third board (Fig. 4).

The placement of parts on the boards and board-to-board connections are shown in fig. 5. Red numbers on it indicate the numbers of the outputs of the pulse transformer T1 at the places of their connection to the board. The transformer itself is fixed on it with clamps made of insulated mounting wire. Blocking capacitors C13 and C14 are soldered directly to the power pins of the DD2 and DD3 microcircuits. As practice has shown, the meter works normally without these capacitors.

The boards of the microcontroller and indicators are connected by brackets made of galvanized steel 0.5 mm thick. The converter and amplifier board is fixed with two M2 screws. The distance between the boards is about 11 mm. This version of the device design (Fig. 6) takes up less space on the front panel of the power supply, in which this device must be built. Instead of OU KR140UD708, you can use, for example. KR140UD1408 and many other types of op amps It should be noted that they may require other correction circuits than KR140UD708. This should be taken into account when designing a printed circuit board.

Instead of shift registers 74HC164, you can use 74HC4015, but you will have to change the topology of the printed circuit board conductors. Diodes KD522B can be replaced by KD510A. Trimmer resistors R8 and R11 - SPZ19. R9 - imported. Permanent capacitors are also imported. Resistor R1 (current sensor) can be made from nichrome wire or used ready-made, as done in (1). I made it from a piece of nichrome tape with a cross section of 2.5 × 0.8 mm and a length (taking into account the tinned ends) of about 25 mm, removed from the thermal relay of the TRN.

Transformer T1 is wound on a ferrite ring with a size of 10x6x3 mm, removed from a faulty CFL. All windings are wound with PEV-2 wire with a diameter of 0.18 mm. Winding 2-3 contains 83 turns, windings 1-2 and 4-5 - 13 turns each, and winding 6-7-8 80 turns with a tap from the middle. If the output voltage of the rectifier is less than 30 V, the number of turns of winding 2-3 will have to be reduced to approximately 4 turns per volt. Windings 1-2-3 and 4-5 are insulated between each other with one layer of capacitor paper 0.1 mm thick, and from winding 6-7-8 - with two layers of such paper.

The microcontroller program was written in the MPLAB IDE v8.92 environment in the MPASM assembly language. Two options are offered. The files of the first option are in the folder "General. cathode" and are intended for a device with LED indicators with common discharge cathodes, including those indicated in the diagram in Fig. 1. Files of the second option from the folder "Common. anode” should be used when LED indicators with common discharge anodes are installed in the device. However, this version of the program has not been tested in practice. The programming of the microcontroller was done using the IC-prog program and the simple device described in (4).

The adjustment of the meter consists in setting the trimmer resistor R11 to zero at the output of the op-amp DA 1 in the absence of current in the measured circuit. Then current is applied to this circuit. close to the measurement limit, but less than it. By controlling the current with an exemplary ammeter, a trimming resistor R8 achieves equality in the readings of the exemplary and adjusted devices. Having applied and controlled the measured voltage with an exemplary voltmeter, set the corresponding readings on the indicator of the device with a trimming resistor R9. More details about adjustment are written in (1).

Voltage measurements in practice have to be performed quite often. Voltage is measured in radio engineering, electrical devices and circuits, etc. The type of alternating current can be pulsed or sinusoidal. Voltage sources are either current generators.

The pulse current voltage has the parameters of the amplitude and average voltage. Pulse generators can be sources of such voltage. Voltage is measured in volts and is designated "V" or "V". If the voltage is variable, then the symbol “ ~ ”, for constant voltage, the symbol “-” is indicated. The alternating voltage in the home household network is marked ~ 220 V.

These are devices designed to measure and control the characteristics of electrical signals. Oscilloscopes work on the principle of deflecting an electron beam, which produces an image of the values ​​of variables on the display.

AC voltage measurement

According to regulatory documents, the voltage value in the household network should be equal to 220 volts with a deviation accuracy of 10%, that is, the voltage can vary in the range of 198-242 volts. If the lighting in your house has become dimmer, the lamps began to fail frequently, or household devices began to work unstable, then to find out and fix these problems, you first need to measure the voltage in the network.

Before measuring, you should prepare your existing measuring device for work:

• Check the integrity of the insulation of the control wires with probes and tips.
• Set the switch to AC voltage, with an upper limit of 250 volts or higher.
• Insert the tips of the control wires into the sockets of the measuring device, for example, . In order not to be mistaken, it is better to look at the designations of the sockets on the case.
• Turn on the device.

It can be seen from the figure that the measurement limit of 300 volts is selected on the tester, and 700 volts on the multimeter. Some devices require several different switches to be set to the desired position to measure voltage: the type of current, the type of measurement, and also insert the wire lugs into certain sockets. The end of the black tip in the multimeter is plugged into the COM jack (common jack), the red tip is inserted into the socket marked “V”. This socket is common for measuring any kind of voltage. The socket marked "ma" is used for measuring small currents. The socket marked "10 A" is used to measure a significant amount of current, which can reach 10 amperes.

If you measure the voltage with the wire inserted into the “10 A” socket, the device will fail or the fuse will blow. Therefore, when performing measurement work, you should be careful. Most often, errors occur in cases where the resistance was first measured, and then, forgetting to switch to another mode, the voltage measurement begins. At the same time, a resistor responsible for measuring resistance burns inside the device.

After preparing the device, you can start measuring. If nothing appears on the indicator when you turn on the multimeter, this means that the battery located inside the device has expired and needs to be replaced. Most often in multimeters there is a "Krona", which produces a voltage of 9 volts. Its service life is about a year, depending on the manufacturer. If the multimeter has not been used for a long time, then the crown may still be faulty. If the battery is good, then the multimeter should show one.

The wire probes must be inserted into the socket or touched with bare wires.

On the display of the multimeter, the value of the mains voltage will immediately appear in digital form. On the pointer device, the arrow will deviate by a certain angle. The pointer tester has several graduated scales. If you carefully consider them, then everything becomes clear. Each scale is designed for specific measurements: current, voltage or resistance.

The measurement limit on the device was set to 300 volts, so you need to count on the second scale, which has a limit of 3, while the readings of the device must be multiplied by 100. The scale has a division value of 0.1 volts, so we get the result shown in the figure, about 235 volts. This result is within acceptable limits. If the readings of the instrument constantly change during the measurement, there may be poor contact in the electrical wiring connections, which can lead to sparking and malfunctions in the network.

DC voltage measurement

Sources of constant voltage are batteries, low-voltage or batteries, the voltage of which is not more than 24 volts. Therefore, touching the poles of the battery is not dangerous, and there is no need for special safety measures.

To assess the performance of a battery or other source, it is necessary to measure the voltage at its poles. For finger batteries, the power poles are located at the ends of the case. The positive pole is marked "+".

Direct current is measured in the same way as alternating current. The difference lies only in setting the device to the appropriate mode and observing the polarity of the outputs.

The battery voltage is usually marked on the case. But the result of the measurement does not yet indicate the health of the battery, since the electromotive force of the battery is measured in this case. The duration of operation of the device in which the battery will be installed depends on its capacity.

To accurately assess the performance of the battery, it is necessary to measure the voltage with the load connected. For a finger battery, a regular 1.5 volt flashlight bulb is suitable as a load. If the voltage drops slightly when the light is on, that is, no more than 15%, then the battery is suitable for use. If the voltage drops much more, then such a battery can still serve only in a wall clock, which consumes very little energy.