Indirect resistance measurement is carried out using a voltmeter. How to Measure DC Electrical Resistance

Electrical resistance - basic electrical characteristic conductor, a value characterizing the resistance of an electrical circuit or its section to electric current. Resistance can also be called a part (it is more often called a resistor) that provides electrical resistance current Electrical resistance is caused by the conversion of electrical energy into other forms of energy and is measured in Ohms.

Measurement using ammeter and voltmeter method. The resistance of any electrical installation or section of an electrical circuit can be determined using an ammeter and voltmeter using Ohm's law. When switching on the devices according to the diagram in Fig. 1.2, (a) not only the measured current I x passes through the ammeter, but also the current I v flows through the voltmeter. Therefore the resistance

R x = U / (I - U/R v ) (110)

Where R v - voltmeter resistance.

When switching on the devices according to the diagram in Fig. 1.2, b, the voltmeter will measure not only the voltage drop Ux at a certain resistance, but also the voltage drop in the ammeter winding U A = IR A. Therefore

R x = U/I - R A (111)

Where R A - ammeter resistance.

In cases where the resistance of devices is unknown and, therefore, cannot be taken into account, it is necessary to use the circuit in Fig. 1 when measuring small resistances. 1.2a, and when measuring high resistances - with the circuit in Fig. 1.2, b. In this case, the measurement error, determined in the first circuit by the current I v, and in the second by the voltage drop UA, will be small compared to the current I x and voltage U x.

Resistance measurement with electric bridges. The bridge circuit (Fig. 1.3a) consists of a power source, a sensitive device (galvanometer G) and four resistors included in the arms of the bridge: with an unknown resistance R x (R4) and known resistances R1, R2, R3, which can be used during measurements change. The device is connected to one of the bridge diagonals (measuring), and the power source is connected to the other (supply).

Resistances R1 R2 and R3 can be selected such that when contact B is closed, the readings of the device will be equal to zero (in this case, it is customary to say that the bridge is balanced). At the same time, unknown resistance

R x = (R 1 /R 2 )R 3 (112)

Rice. 1.2

Rice. 1.3.

In some bridges, the ratio of the arms R1/R2 is set constant, and the balance of the bridge is achieved only by selecting the resistance R3. In others, on the contrary, the resistance R3 is constant, and equilibrium is achieved by selecting the resistances R1 and R2.

Resistance measurement with a DC bridge is carried out as follows. An unknown resistance R x (for example, the winding of an electrical machine or apparatus) is connected to terminals 1 and 2, a galvanometer is connected to terminals 3 and 4, and a power source (dry galvanic cell or battery) is connected to terminals 5 and 6. Then, by changing the resistances R1, R2 and R3 (which are used as resistance stores switched by the corresponding contacts), they achieve bridge equilibrium, which is determined by the zero reading of the galvanometer (at closed contact IN).

There are various designs of DC bridges, the use of which does not require calculations, since the unknown resistance R x is measured on the instrument scale. The resistance stores mounted in them allow you to measure resistances from 10 to 100,000 Ohms.

When measuring small resistances with conventional bridges, the resistances of connecting wires and contact connections introduce large errors into the measurement results. To eliminate them, double DC bridges are used (Fig. 1.3, b). In these bridges, the wires connecting a resistor with a measured resistance R x and some standard resistor with a resistance R0 with other resistors of the bridge, and their contact connections are connected in series with the resistors of the corresponding arms, the resistance of which is set to at least 10 Ohms. Therefore, they have virtually no effect on the measurement results. The wires connecting resistors with resistances R x and R0 are included in the power circuit and do not affect the equilibrium conditions of the bridge. Therefore, the accuracy of measuring small resistances is quite high. The bridge is designed so that when adjusting it, the following conditions are met: R1 = R2 and R3 = R4. In this case

R x = R 0 R 1 /R 4 (113)

Double bridges allow you to measure resistances from 10 to 0.000001 ohms.

If the bridge is not balanced, then the needle in the galvanometer will deviate from the zero position, since the current of the measuring diagonal at constant values ​​of resistances R1, R2, R3, etc. d.s. the current source will depend only on the change in resistance R x. This allows you to calibrate the galvanometer scale in units of resistance R x or any other units (temperature, pressure, etc.) on which this resistance depends. Therefore, unbalanced DC bridge is widely used in various devices for measuring non-electrical quantities using electrical methods.

Various AC bridges are also used, which make it possible to measure inductance and capacitance with great accuracy.

Measuring with an ohmmeter. The ohmmeter is a milliammeter 1 with a magnetoelectric measuring mechanism and is connected in series with the measured resistance R x (Fig. 1.4.) and an additional resistor R D in the DC circuit. At constant e. d.s. source and resistance of the resistor R D the current in the circuit depends only on the resistance R x. This allows you to calibrate the instrument scale directly in ohms. If the output terminals of the device 2 and 3 are short-circuited (see the dashed line), then the current I in the circuit is maximum and the arrow of the device deviates to the right at the greatest angle; on the scale this corresponds to a resistance of zero. If the device circuit is open, then I = 0 and the arrow is at the beginning of the scale; this position corresponds to a resistance equal to infinity.

The device is powered from a dry galvanic cell 4, which is installed in the device body. The device will give correct readings only if the current source has a constant e. d.s. (the same as when calibrating the instrument scale). Some ohmmeters have two or more measurement ranges, such as 0 to 100 ohms and 0 to 10,000 ohms. Depending on this, a resistor with measured resistance R x is connected to different terminals.

Measuring high resistances with megaohmmeters. To measure insulation resistance, megohmmeters of the magnetoelectric system are most often used. They use logometer 2 as a measuring mechanism (Fig. 1.5), the readings of which do not depend on the voltage of the current source supplying the measuring circuits. Coils 1 and 3 of the device are located in the magnetic field of a permanent magnet and are connected to a common power source 4.

Rice. 1.4.

Rice. 1.5.

An additional resistor R d is connected in series with one coil, and a resistor with resistance R x is connected in the circuit of the other coil.

A small DC generator 4 called an inductor is usually used as a current source; The generator armature is rotated by a handle connected to it through a gearbox. Inductors have significant voltages from 250 to 2500 V, thanks to which large resistances can be measured with a megohmmeter.

When the currents I1 and I2 flow through the coils interact with the magnetic field of a permanent magnet, two oppositely directed moments M1 and M2 are created, under the influence of which the moving part of the device and the pointer will occupy a certain position. As was shown in § 100, the position of the moving part of the ratiometer depends on the ratio I1/I2. Therefore, when R x changes, will the angle change? arrow deviations. The megohmmeter scale is calibrated directly in kilo-ohms or mega-ohms (Fig. 1.6, a).

Rice. 1.6.

To measure the insulation resistance between the wires, you need to disconnect them from the current source (from the network) and connect one wire to terminal L (line) (Fig. 1.6b), and the other to terminal 3 (ground). Then, by rotating the handle of the inductor 1 megohmmeter, the insulation resistance is determined on the scale of the ratiometer 2. Switch 3 in the device allows you to change the measurement limits. The voltage of the inductor, and therefore the speed of rotation of its handle, theoretically does not affect the measurement results, but in practice it is recommended to rotate it more or less evenly.

When measuring the insulation resistance between the windings of an electric machine, disconnect them from each other and connect one of them to terminal L and the other to terminal 3, after which, by rotating the inductor handle, the insulation resistance is determined. When measuring the insulation resistance of the winding relative to the housing, it is connected to terminal 3, and the winding to terminal L.

During the manufacture, installation and operation of electrical and radio engineering devices and installations, it is necessary to measure electrical resistance.

In practice, various methods are used to measure resistance, depending on the nature of the objects and measurement conditions (for example, solid and liquid conductors, grounding conductors, electrical insulation); on requirements for accuracy and speed of measurement; on the value of the measured resistances.

Methods for measuring small resistances differ significantly from methods for measuring large resistances, since in the first case it is necessary to take measures to eliminate the influence of the resistance of connecting wires and transition contacts on the measurement results.

Measuring mechanisms of ohmmeters. For direct resistance measurement, single- and double-frame magnetoelectric measuring mechanisms are used.

A single frame mechanism can be used to measure resistances. For this purpose, an additional resistor with a constant resistance is introduced into the device

and supply it with a power source (for example, a dry cell battery). The resistance being measured is connected with the meter in series (Fig. 1) or in parallel.

When connected in series, the current in the meter , Where

- meter resistance; - power supply voltage.

Considering that

, Where - current sensitivity of the device (constant value), we find that the angle of deflection of the device needle at depends only on the value of the measured resistance:

If the scale is calibrated using this expression in units of resistance, then the device will be an ohmmeter. The voltage of dry elements decreases over time, so an error is introduced into the measurements, the greater the greater the actual voltage differs from the voltage at which the scale was calibrated.

An error from the variability of the supply source voltage does not occur if the measuring mechanism has two windings located on a common axis at a certain angle to each other (Fig. 2.).

Rice. 1. Fig. 2.

In a two-frame measuring mechanism, which is called a ratiometer, there are no counteracting springs; the rotating and counteracting moments are created by electromagnetic forces. Therefore, in the absence of current in the windings, the well-balanced moving part of the device is in indifferent equilibrium (the needle stops at any scale mark). When there is current in the coils, two electromagnetic moments directed in opposite directions act on the moving part.

The magnetic circuit of the measuring mechanism is designed so that the magnetic induction along the air gap is distributed unevenly, but in such a way that when the moving part is turned in any direction, the torque decreases and the counteracting moment increases (depending on the direction of rotation, the role of the moments changes).

The moving part stops when

or . It follows that the position of the arrow on the scale depends on the ratio of the currents in the windings, i.e. , but does not depend on the voltage of the supply source.

In the diagram fig. 2. It can be seen that the measured resistance

is included in the circuit of one of the logometer coils, therefore the current in it, as well as the deflection of the instrument needle, clearly depends on the value .

Using this dependence, the scale is calibrated in units of resistance and then the device is an ohmmeter. Ohmmeters for measuring insulation resistance are supplied with a power source with a voltage of up to 1000 V in order to carry out the measurement at a voltage approximately equal to the operating voltage of the installation. Such a source can be a built-in magnetoelectric generator with a manual drive or a transformer with a rectifier connected to the alternating current network.

Ohmmeters designed to measure high resistances (more than 1 MOhm) are called megaohmmeters.

Indirect methods for measuring resistance. The resistance of a resistor or other element of an electrical circuit can be determined from the readings of a voltmeter and ammeter (at constant current) using Ohm's law:

(diagrams Fig. 3, a, b). According to the diagram in Fig. 4 determine the resistance using the readings of one voltmeter. In switch position 1 P the voltmeter measures the network voltage, and in position 2 - voltage at the voltmeter terminals. In the latter case . From here

Indirect methods are used to measure medium resistances, and high resistances are also measured with one voltmeter. The accuracy of these methods depends significantly on the ratio of the values ​​of the measured resistance

and internal resistances of the ammeter and voltmeter. The measurement results can be considered satisfactory in accuracy if the following conditions are met: (see diagram Fig. 3, a); (see diagram Fig. 3, b); (see diagram Fig. 4).

Rice. 3 Fig. 4

Methods and comparison devices. To measure small and medium resistances, the method of comparing the measured resistance is used

with exemplary . These two resistances in the diagram in Fig. 5 are connected in series, so the current in them is the same. Its value is adjusted using a resistor so that it does not exceed the permissible current for resistances and . From here . Unknown voltage drops are measured with a voltmeter or potentiometer. The measurement results are more accurate if the resistances are of the same order, and the resistance of the voltmeter is large enough so that connecting it does not affect the mode of the main circuit.

When measuring small resistances using this method, the voltmeter is connected using potential clamps, which make it possible to exclude the resistance of the main circuit contacts from the measurement results.

In this article we will try to learn how to measure small resistances. Radio amateurs sometimes have a need to accurately determine the resistance of a shunt when making or repairing an ammeter, so that it, in turn, also accurately displays its units of measurement or for other purposes. But how to do this when the multimeter does not have a milli-ohm scale, the markings are either absent or completely unknown and incomprehensible? Most measuring instruments have a minimum scale of 200 Ohms for measuring resistance and 3.5 - 4 digits, when you short-circuit the probes there is already about 0.7 Ohms, when measuring resistance 0.1 Ohms nothing changes, trouble. Let's fix it now.

I suggest using a bridge measurement circuit for this purpose. Everyone should understand what a bridge is; we won’t dwell on that. Let's make a bridge of resistors, apply some voltage to it and measure it, although we can also measure the current, it won't make a difference; we choose what is more accurate at hand. So what does low resistance measurement have to do with it? Patience, everything is in order from afar. There is such a wonderful thing as bridge balance. The product of the resistances of the opposite arms of the bridge, provided it is balanced, will be the same. And voltages and currents, when the bridge is balanced, will cancel each other out and give a total of 0.

(Let R0 be R3 and Rx be R4)

So, based on the above, if instead of one of the resistors we put our small resistance of an arbitrary value in the bridge, and make the other resistor variable or tuning (according to the diagram, we use two variable resistors to accurately balance the bridge, especially in the case when there are no multi-turn variables at hand resistors) to achieve bridge balance. This circuit can be used to measure shunts and small resistances:

It was lazy to assemble the circuit, especially since it takes a lot of time to make the board, so an experimental sample of the circuit was made by hanging installation. Here, resistors R1 and R2 are not 1%, but they were selected as close as possible to the resistance of a given value, the resistance error did not exceed 0.5% at room conditions.

But you need to know how to get the exact value of the measured resistance. Firstly, the main feature of such a circuit is that it “multiplies” the measured resistance. This means that there is no need for a milli-ohm scale in a multimeter. A resistance of 0.1 Ohm can already be measured on a kilo Ohm scale. Only the measurement will now be not direct, but indirect; you will have to use a little mathematics and calculate the final result of the measurement.

Let's decide what range of ratings we will measure (meaning low resistance or shunt resistance). To do this, you need to select the values ​​of the variable resistors:

According to the circuit, we use two variable resistors for greater interaction accuracy, 1 kOhm and 100 Ohm. This resistance of variable resistors will allow you to measure the maximum resistance of 1.1 ohms, the minimum while maintaining measurement accuracy of 0.01 ohms (with Rx = 0.01 ohms R0 should be 10 ohms, which also need to be measured quite accurately with your multimeter)

And the values ​​of constant resistors, so that the bridge can be easily balanced and it is convenient to calculate the value of the shunt or small resistance:

The multiplicity of resistors relative to each other is best taken exactly like this - 10, 100, 1000, in order to quickly calculate the final result, although no one forbids taking non-round numbers, so that you can also count with a calculator. According to the scheme, this is the ratio of 100,000 to 100, that is, a multiplier of 1000.

Let's put together a diagram. You can use any tuning or variable resistors, but for greater accuracy I advise you to take multi-turn tuning or variable resistors, and use constant ones with a tolerance of no more than 1%, or better yet, even less. The circuit uses a 9-volt Krona as a battery; it can be replaced with any other source. Capacitors in case of using power supplies for filtering. The circuit in our resistance configuration consumes 90 mA from a 9 V battery, so for frequent measurements, of course, it is more advisable to use a power supply. The circuit has been assembled, now we are studying the measurement technique. After connecting the resistance being measured, it is necessary to apply voltage to the circuit, no matter what, but the higher it is, the greater the accuracy, set the meter to the limit of 200 mV and begin the process of balancing the bridge by rotating the trimming resistor until complete zero on the voltmeter. This means that the bridge is balanced and all expressions are now valid for our circuit. Next, we measure the resistance of the tuning resistor and calculate the value of the small resistance:

or more beautifully like this

(219 Ohm * 100 Ohm)/100 kOhm we get 0.219 Ohm shunt resistance (see video).

Or, more simply, the result obtained must be divided by 1000 (since 100 kOhm/100 Ohm will be 1000 - our multiplier) in our case. So what do we see? Yes! This is the resistance that we measured 0.219 Ohm (~0.22 Ohm). Within the limits of good accuracy, and if you take into account errors in measurement and interaction with the circuit, it’s ideal.

Now you won’t have to rack your brains when the need for such measurements arises. The scheme is simple, but not many people know about it.

Attached to the article is a printed circuit board for making a mini multimeter attachment and a project for those who are curious to check this miracle, but are too lazy to assemble the circuit.

List of radioelements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
R1	Resistor	100 kOhm	1	1%		To notepad
R2	Resistor	100 Ohm	1	1%		To notepad
R0(1)	Trimmer resistor	1 kOhm	1	3296W		To notepad
R0(2)	Trimmer resistor	100 Ohm	1	3296W		To notepad
C1	Electrolytic capacitor	220 µF	1	Other denomination possible		To notepad
C2	Capacitor	100 nF	1

- This measuring device, used to determine the resistance value in electrical circuits. Resistance is measured in Omaha and is denoted by the Latin letter R. What Ohm is in a popular form is described in the website article “The Law of Current Strength”.

Block diagram and designation on Ohmmeter diagrams

The measuring device ohmmeter is structurally a pointer or digital indicator with a battery or power supply connected in series as shown in the photograph.

All combined instruments - pointer testers and digital multimeters - have the function of measuring resistance.

In practice, a device that measures only resistance is used for special cases, for example, to measure insulation resistance at elevated voltages, ground loop resistance, or as a reference device for testing other low-precision ohmmeters.

On electrical measuring circuits, an ohmmeter is designated by the Greek letter omega enclosed in a circle, as shown in the photograph.

Preparing an Ohmmeter for Measurements

Repair of electrical wiring, electrical and radio engineering products consists of checking the integrity of the wires and searching for contact failure in their connections.

In some cases, the resistance must be equal to infinity, for example, insulation resistance. And in others it is zero, for example, the resistance of wires and their connections. And in some cases it is equal to a certain value, for example, the resistance of the filament of a light bulb or heating element.

Attention! In order to avoid failure of the Ohmmeter, it is allowed to measure the resistance of circuits only when they are completely de-energized. You must unplug the plug from the socket or remove the batteries from the compartment. If the circuit contains electrolytic capacitors of larger capacity, then they must be discharged by short-circuiting the capacitor leads through a resistance rated at about 100 kOhm for a few seconds.

As with voltage measurements, before measuring resistance, it is necessary to prepare the device. To do this, you need to set the device switch to the position corresponding to the minimum measurement of the resistance value.

Before measurements, you should check the functionality of the device, since the batteries may be bad and the Ohmmeter may not work. To do this, you need to connect the ends of the probes together.

In this case, the tester’s needle should be set exactly to the zero mark; if it is not, then you can turn the “Set” knob. 0". If this does not work, you need to replace the batteries.

For dialing electrical circuits, for example, when checking light bulb incandescent, you can use a device whose batteries are dead and the needle does not set to 0, but reacts at least a little when the probes are connected. It will be possible to judge the integrity of the circuit by the fact that the arrow is deflected. Digital devices should also show zero readings, a deviation in tenths of ohms is possible due to the resistance of the probes and the transition resistance in the contacts connecting them to the terminals of the device.

When the ends of the probes are open, the tester arrow should be set to the point indicated on the scale ∞, and in digital instruments, the overload will flash or the number will be displayed 1 on the indicator on the left side.

The ohmmeter is ready for use. If you touch the ends of the probes to the conductor, then if it is intact, the device will show zero resistance, in otherwise, the readings will not change.

Expensive models of multimeters have a circuit continuity function with audio indication, indicated in the resistance measurement sector with a diode symbol. It is very convenient for testing low-impedance circuits, such as cable wires twisted pairs for Internet or household electrical wiring. If the wire is intact, then the continuity is accompanied by sound signal, which eliminates the need to read readings from the multimeter indicator.

Examples from practice of measuring resistance of products

In theory, everything is usually clear, but in practice questions often arise that can best be answered by examples of checking the most common products with an ohmmeter.

Checking incandescent lamps

The incandescent light bulb in a lamp or car on-board devices has stopped shining, how can I find out the reason? The switch, electric socket or wiring may be faulty. Using the tester, any incandescent lamp from a home lamp or car headlight, filament of fluorescent lamps and energy-saving lamps can be easily checked. To check, just set the device switch to the minimum resistance measurement position and touch the ends of the probes to the terminals of the light bulb base.

The resistance of the light bulb filament was 51 Ohms, which indicates its serviceability. If the thread were broken, the device would show infinite resistance. The resistance of a 220 V halogen light bulb with a power of 50 watts when illuminated is about 968 Ohms, and a 12 volt car light bulb with a power of 100 watts is about 1.44 Ohms.

It is worth noting that the resistance of an incandescent lamp filament in a cold state (when the light bulb is not lit) is several times less than in a warm state. This is due to the physical property of tungsten. Its resistance increases nonlinearly with heating. Therefore, incandescent lamps usually burn out the moment they are turned on.

Checking sound-reproducing headphones

It happens with headphones in one of the emitters, or in both at once, the sound is distorted, periodically disappears or is absent. There are two possible options: either the headphones or the device from which the signal is received are faulty. Using an ohmmeter it is easy to check what the cause is and localize the location of the fault.

To check the headphones, you need to connect the ends of the probes to their connector. Typically, headphones are connected to the equipment using a 3.5 mm jack connector, shown in the photo.

One end of the probe touches the common terminal, and the other, in turn, touches the terminals of the right and left channels. The resistance should be the same and be about 40 ohms. Usually, the resistance is indicated in the passport for headphones.

If the resistance of the channels is very different, then there may be a short circuit or a broken wire in the wires. It is easy to verify this; just connect the ends of the probes to the terminals of the right and left channels. The resistance should be twice that of one earphone, that is, already 80 Ohms. In practice, the total resistance of series-connected emitters is measured.

If the resistance changes when the conductors move during measurements, it means that the wire is frayed in some place. Usually the wires fray where they exit the Jack or emitters.

To localize the location of the wire break, during measurements it is necessary to bend the wire locally, fixing the rest of it. Based on the instability of the ohmmeter readings, you will determine the location of the defect. If it’s a Jack, then you need to purchase a detachable connector, bite off the old one with a section of bad wire and solder the wire to the contacts of the new Jack.

If the break is located at the entrance to the headphones, then you need to disassemble them, remove the defective part of the wire, strip the ends and solder them to the same contacts to which the wires were soldered before. In the website article “How to solder with a soldering iron” you can learn about the art of soldering.

Measuring the resistor value (resistance)

Resistors (resistance) are widely used in electrical diagrams. Therefore, when repairing electronic devices there is a need to check the serviceability of the resistor or determine its value.

On electrical diagrams, a resistor is designated as a rectangle, inside which its power is sometimes written in Roman numerals. I – one watt, II – two watts, IV – four watts, V – five watts.

You can check the resistor (resistance) and determine its value using a multimeter turned on in resistance measurement mode. In the resistance measurement mode sector, there are several switch positions. This is done in order to increase the accuracy of the measurement results.

For example, position 200 allows you to measure resistances up to 200 Ohms. 2k – up to 2000 Ohm (up to 2 kOhm). 2M – up to 2,000,000 Ohm. (up to 2 MOhm). The letter k after the numbers denotes the prefix kilo - the need to multiply the number by 1000, M stands for Mega, and the number needs to be multiplied by 1,000,000.

If the switch is set to position 2k, then when measuring a resistor with a nominal value of 300 kOhm, the device will show an overload. It is necessary to switch it to position 2M. In contrast to measuring voltage, it does not matter what position the switch is in; you can always switch it during the measurement process.

Online calculators for determining resistor values
by color marking

Sometimes when checking a resistor, the ohmmeter shows some resistance, but if the resistor, as a result of overloads, has changed its resistance and it no longer corresponds to the marking, then such a resistor should not be used. Modern resistors are marked using colored rings. The most convenient way to determine the value of a resistor marked with colored rings is to use an online calculator.

marked with 4 colored rings

Online calculator for determining the resistance of resistors
marked with 5 colored rings

Checking diodes with a multimeter or tester

Semiconductor diodes are widely used in electrical circuits to convert alternating current to direct current, and usually when repairing products, after external inspection printed circuit board First of all, diodes are checked. Diodes are made from germanium, silicon and other semiconductor materials.

By appearance diodes come in different shapes, transparent and colored, in metal, glass or plastic housings. But they always have two conclusions and immediately catch the eye. The circuits mainly use rectifier diodes, zener diodes and LEDs.

The symbol for diodes in the diagram is an arrow pointing to a straight line segment. A diode is designated by the Latin letters VD, with the exception of LEDs, which are designated by the letters HL. Depending on the purpose of the diodes, additional elements are added to the designation scheme, which is reflected in the drawing above. Since there is more than one diode in a circuit, for convenience, a serial number is added after the letters VD or HL.

It is much easier to check a diode if you understand how it works. And the diode works like a nipple. When you inflate a ball, rubber boat or car tire, air enters it, but the nipple does not allow it back.

A diode works exactly the same. It only allows air to pass in one direction, not electricity. Therefore, to check the diode, you need a direct current source, which can be a multimeter or a pointer tester, since they have a battery installed.

Above is a block diagram of the operation of a multimeter or tester in resistance measurement mode. As you can see, a DC voltage of a certain polarity is supplied to the terminals. It is customary to apply the plus to the red terminal, and the minus to the black. When you touch the diode terminals in such a way that the positive output of the device is on the anode terminal of the diode, and the negative output is on the cathode of the diode, then current will flow through the diode. If the probes are swapped, the diode will not pass current.

A diode can usually have three states - good, broken or broken. During a breakdown, the diode turns into a piece of wire; it will pass current no matter the order in which the probes touch. If there is a break, on the contrary, the current will never flow. Rarely, but there is another condition when the transition resistance changes. Such a malfunction can be determined by the readings on the display.

Using the above instructions, you can check rectifier diodes, zener diodes, Schottky diodes and LEDs, both with leads and in SMD version. Let's look at how to test diodes in practice.

First of all, it is necessary, observing the color coding, to insert the probes into the multimeter. Usually a black wire is inserted into COM, and a red wire into V/R/f (this is the positive terminal of the battery). Next, you need to set the operating mode switch to the dialing position (if there is such a measurement function), as in the photo, or to the 2kOm position. Turn on the device, close the ends of the probes and make sure it is working.

We’ll start the practice by checking the ancient germanium diode D7, this specimen is already 53 years old. Germanium-based diodes are now practically not produced due to high cost germanium itself and a low maximum operating temperature, only 80-100°C. But these diodes have the smallest voltage drop and noise level. They are highly valued by tube amplifier builders. In direct connection, the voltage drop across a germanium diode is only 0.129 V. The dial tester will show approximately 130 Ohms. When the polarity is changed, the multimeter shows 1, the dial tester will show infinity, which means a very high resistance. This diode is OK.

The procedure for checking silicon diodes is no different from checking those made of germanium. The cathode terminal is usually marked on the diode body; it can be a circle, line or dot. In direct connection, the drop across the diode junction is about 0.5 V. For powerful diodes, the drop voltage is less, and is about 0.4 V. Zener diodes and Schottky diodes are checked in the same way. The voltage drop of Schottky diodes is about 0.2 V.

U powerful LEDs on direct transition more than 2 V drops and the device may show 1. But here the LED itself is an indicator of serviceability. If, when turned on directly, you can see even the faintest glow of the LED, then it is working.

It should be noted that some types of high-power LEDs consist of a chain of several LEDs connected in series and this is not noticeable from the outside. Such LEDs sometimes have a voltage drop of up to 30 V, and they can only be tested from a power supply with an output voltage of more than 30 V and a current-limiting resistor connected in series with the LED.

Checking electrolytic capacitors

There are two main types of capacitors, simple and electrolytic. Simple capacitors can be included in the circuit in any way you like, but electrolytic capacitors can only be connected with polarity, otherwise the capacitor will fail.

On electrical diagrams, a capacitor is indicated by two parallel lines. When designating an electrolytic capacitor, its connection polarity must be indicated with a “+” sign.

Electrolytic capacitors have low reliability and are the most common cause of failure of electronic components of products. A swollen capacitor in the power supply of a computer or other device is not a rare sight.

Using a tester or multimeter in resistance measurement mode, you can successfully check the serviceability of electrolytic capacitors, or, as they say, ring. The capacitor must be removed from the printed circuit board and be sure to be discharged so as not to damage the device. To do this, you need to short-circuit its terminals with a metal object, such as tweezers. To test the capacitor, the switch on the device must be set to resistance measurement mode in the range of hundreds of kilo-ohms or mega-ohms.

Next, you need to touch the terminals of the capacitor with the probes. At the moment of contact, the instrument needle should sharply deviate along the scale and slowly return to the position of infinite resistance. The speed at which the needle deflects depends on the capacitance value of the capacitor. The larger the capacitor capacity, the slower the shooter will return to its place. A digital device (multimeter), when touching the probes to the terminals of the capacitor, will first show a small resistance, and then increasingly increasing up to hundreds of megohms.

If the behavior of the devices differs from that described above, for example, the resistance of the capacitor is zero Ohm or infinity, then in the first case there is a breakdown between the windings of the capacitor, and in the second, a break. Such a capacitor is faulty and cannot be used.

Selecting a measurement methoddepends on what is expected measured resistance values And required accuracy. Main methods for measuring DC resistance are indirect, direct assessment method and pavement.

Figure 1. Probe circuits for measuring large (a) and small (b) resistances

Figure 2. Circuits for measuring large (a) and small (b) resistances ammeter-voltmeter method In the basic circuits of the indirect method, voltage and current meters are used.

Figure 1, a shows a circuit suitable for measuring resistances of the same order as the input resistance Rv of the voltmeter Rн. Having measured the voltage U0 with short-circuited Rx, the resistance Rx is determined by the formula Rx = Rу(U0/Ux-1).

When measured according to the diagram in Fig. 5.1, b resistors of high resistance are connected in series with the meter, and small resistors are connected in parallel.

For the first case, Rx = (Ri + Rd)(Ii/Ix-1), where Ii is the current through the meter with Rx short-circuited; for the second case

where Ii is the current through the meter in the absence of Rx, Rd is an additional resistor.

The ammeter-voltmeter method is more universal, allowing one to measure resistance under certain operating modes, which is important when measuring nonlinear resistance (see Fig. 2).

For the diagram in Fig. 2, a

For the diagram in Fig. 2, b

Relative methodological measurement error:

Ra and Rv are the resistances of the ammeter and voltmeter.

Rice. 3. Circuits of ohmmeters with serial (a) and parallel (b) measurement circuits

Rice. 4. Bridge circuits for measuring resistance: a - single bridge, b - double.

From the expressions for the relative error it is clear that the diagram in Fig. 2, a provides less error when measuring large resistances, and the circuit in Fig. 2, b - when measuring small ones.

The measurement error using the ammeter-voltmeter method is calculated using the formula

where gв, ga are the accuracy classes of the voltmeter and ammeter; Up, Ip - measurement limits of the voltmeter and ammeter.

Direct measurement of DC resistance is carried out with ohmmeters. If resistance values ​​are more than 1 Ohm, ohmmeters with a series measurement circuit are used, and for measuring low resistances - with a parallel circuit. When using an ohmmeter in order to compensate for changes in supply voltage, it is necessary to set the arrow of the device. For a series circuit, the arrow is set to zero when the measured resistance is shunted. (Bypassing is usually performed using a button specially provided in the device). For a parallel circuit, before starting the measurement, the arrow is set to the “infinity” mark.

To cover the range of small and large resistances, they build ohmmeters in parallel-series circuit. In this case, there are two Rx reading scales.

The highest accuracy can be achieved using the bridge measurement method. Medium resistances (10 Ohm - 1 MOhm) are measured using a single bridge, and small ones - using a double bridge.

The measured resistance Rx is included in one of the arms of the bridge, the diagonals of which are connected respectively to the power source and the null indicator; as the latter, a galvanometer, a microammeter with a zero in the middle of the scale, etc. can be used.

Figure 5. Circuits for measuring large (a) and small (b) alternating current resistances

The equilibrium condition of both bridges is determined by the expression

Shoulders R1 and R3 are usually made in the form resistance stores (shop bridge). Using R3, a range of R3/R2 ratios are set, usually multiples of 10, and using R1, the bridge is balanced. The measured resistance is counted according to the value set by the handles of the resistance magazines. The bridge can also be balanced smooth change ratio of resistors R3/R2, made in the form of a flux chord, with a certain value R1 (linear bridge).

For repeated measurements of the degree of resistance compliance with a certain set value Rн is used unbalanced bridges. They are balanced at Rx=Rн. Using the indicator scale, you can determine the deviation of Rx from Rн as a percentage.

They work on the principle of self-balancing automatic bridges. The voltage that occurs when there is an imbalance at the ends of the diagonal of the bridge, after being amplified, acts on the electric motor that mixes the slider slider. When balancing the bridge, the engine stops, and the position of the slider determines measured resistance value.