INTRODUCTION

Electrical resistance DC is the main parameter of resistors. It also serves as an important indicator of the serviceability and quality of operation of many other elements of electrical radio circuits - connecting wires, switching devices, various types of coils and windings, etc. Possible resistance values, the need to measure which arises in radio engineering practice, lie within a wide range - from thousandths ohm or less (resistance of sections of conductors, contact junctions, shielding, shunts, etc.) up to thousands of megohms or more (insulation resistance and leakage of capacitors, surface and volume resistance of electrical insulating materials, etc.). Most often it is necessary to measure resistance of average values ​​- from approximately 1 Ohm to 1 MOhm.

Depending on the limits of the measured resistance, resistance meters are divided into milliohm meters (with a lower limit of tenths of milliohms); ohmmeters (with a lower limit in Ohm units); kiloohmmeters (with an upper limit of about 1 MOhm); megohmmeters (with an upper limit of up to 1000 MOhm); teraohmmeters (with an upper limit greater than 106 MOhm).

The purpose of this course project is to design an Ohmmeter that measures resistance at 200 Ohm and 2 Mohm.

RESISTANCE MEASUREMENT METHODS

Direct assessment methods

Method for converting resistance to time interval

Fig.1.

Working principle:

IN starting position the switch is in position “0”, the capacitor is charged to voltage U0, the output signal of the comparing device (SU) has a zero level. The measurement start signal (time t1) moves the switch to position “1”, while the voltage at the non-inverting input of the control system at the first moment of time exceeds the voltage acting at the inverting input, and the output signal of the control system takes on a unity level. During the discharge of the capacitor, the voltage at the non-inverting input continuously drops and at the time when it is lower (t2), the output signal of the control system returns to the original zero level.

As a result, a signal with a duration directly proportional to the value of the measured resistance will appear at the output of the control system.

Conversion equation:

Advantages:

The output quantity is time, a quantity convenient for quantization;

Sufficiently high accuracy;

Wide measuring range;

No high resistance reference resistors required;

Flaws:

Can only be used for measuring practically non-reactive resistances;

Inability to measure voltage-dependent resistances (non-wire resistors, dielectrics);

Bulky.

Methods for converting resistance to current

Fig.3. Block diagram of converting resistance to current

Working principle:

The circuit contains a reference voltage source, the circuit of which includes the measured resistance. A voltage applied to the resistance being measured causes a current Ix in the circuit, inversely proportional to the resistance being measured.

Conversion equation:

Advantages:

Simplicity;

High accuracy of further current measurement;

No exemplary high-impedance resistor required

Flaws:

The inverse dependence of the current in the circuit on the measured resistance.

Fig.3.

Working principle:

A high voltage source creates a current in the circuit: in a circuit with additional resistance R0 current I0, and in a circuit with measured resistance Rx-Ix; The ratio of these currents is proportional to the measured resistance.

Conversion equation:

Advantages:

Simplicity;

Flaws:

Nonlinear scale;

Need for a high voltage generator;

Limited accuracy.

Methods for converting resistance to voltage

a) using an ideal current generator

Fig.4. Block diagram of resistance to voltage conversion

Working principle:

The circuit contains a reference current source with a very high input resistance, in the circuit of which the measured resistance is connected. The voltage across the resistor is directly proportional to the measured resistance.

Conversion equation:

Flaws:

The need for a current source with a very large output current;

The need for an amplifier with a very high resistance during subsequent voltage conversion.

Advantages:

Greater sensitivity;

Simplicity.

b) using a real current source

Fig.5.

Working principle:

The current I0 is created by the voltage source U0 and is equal to U0 / R0; when the input resistance of the amplifier is much greater than the measured one, almost all of it flows through Rx, and the voltage at the output of the amplifier will be proportional to the measured resistance.

Conversion equation:

Flaws:

The need for an amplifier with a very high input impedance;

Low sensitivity;

Advantages:

Direct dependence of the output voltage on the measured resistance;

Simplicity.

c) divider method (voltage is removed from Rx)

Fig.6.

Working principle:

A stabilized direct voltage U0 is supplied to the input of the divider formed by the measured resistance Rx and the reference resistance R0 >> Rx; A voltage proportional to the measured resistance is removed from the resistor Rx.

Conversion equation:

Advantages:

Simplicity

Flaws:

The need for high-resistance resistance greater than that measured;

The need for an amplifier with a very high input impedance for further voltage conversion.

d) divider method (voltage is removed from R0)

Fig.7.

Working principle:

Similar to (c), with the difference that a voltage proportional to the measured resistance is removed from the reference resistance R0<< Rх.

Conversion equation:

Advantages:

There is no need for high resistance reference resistance;

Simplicity

Flaws:

Low accuracy of further voltage measurements;

Inverse relationship between voltage and measured resistance

Comparison methods

Bridge method

Working principle:

By changing the ratio R1/R2 and resistance R3, equilibrium is achieved, determined by the absence of current in the null indicator circuit. In this case, the measurement result is determined by the values ​​of R1/R2 and resistance R3.

resistance voltage amplifier error

Fig.8. Block diagram

Equilibrium condition:

Advantages:

Greater accuracy;

High sensitivity;

Flaws:

Need for high-resistance reference measures;

During the manufacture, installation and operation of electrical and radio engineering devices and installations, it is necessary to change the 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); from requirements for accuracy and speed of change; 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.

Single frame mechanism, can be used to measure resistance. For this purpose, an additional resistor with a constant resistance R d is introduced into the device and supplied with a power source (for example, a dry-cell battery). The measured resistance R x is connected with the meter in series (Fig. 6.16) or in parallel.

With a series connection, the current in the meter is I=U/(R and +R d +R x) where R and is the resistance of the meter; U is the voltage of the power source.

Taking into account formula (6.2), we find that the angle of deflection of the instrument needle at U = const depends only on the value of the measured resistance R x:

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 voltage does not occur if the measuring mechanism has two windings located on a common axis at a certain angle to each other (Fig. 6.17).

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 division). When there is current in the coils, the moving part is acted upon by two electromagnetic moments directed in opposite directions.

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 at M 1 Bp = M 2 ap or N 1 SB 1 I 1k = N 2 SB 2k I 2k. It follows that the position of the arrow on the scale depends on the ratio of the currents in the windings, i.e. α=f (I 1 k /I 2 k), but does not depend on the voltage of the supply source.

In the diagram fig. 6.17 it can be seen that the measured resistance R x is included in the circuit of one of the ratiometer coils, therefore the current in it, as well as the deflection of the device needle, uniquely depends on the value of R x.

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 megohmmeters.

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: R X =U/I (circuits Fig. 6.18, a, b). According to the diagram in Fig. 6.19 determine the resistance R x according to the readings of one voltmeter. In position 1 of switch P, the voltmeter measures the network voltage U, and in position 2 - the voltage at the voltmeter terminals U V. In the latter case, U B /R B = U x /R x. 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 R x and the internal resistances of the ammeter (R a) and voltmeter (R B). The measurement results can be considered satisfactory in terms of accuracy if the following conditions are met: R x ≥100R a (see diagram Fig. 6.18, a); R x ≤R in /100 (see diagram Fig. 6.18, 6); R X ≤ R B (see diagram Fig. 6.19).

Methods and comparison devices. To measure small and medium resistances, the method of comparing the measured resistance R x with the reference R o is used. These two resistances in the diagram in Fig. 6.20 are connected in series, so the current in them is the same. Its value is adjusted using a resistor R p so that it does not exceed the permissible current for resistances Rx and Ro U x /R x =Uo/Ro- Hence R X = R O U X /U 0. The unknown voltage drops U x and Uo are measured with a voltmeter or potentiometer. The measurement results are more accurate if the resistances R x and Ro 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.

Medium and high resistances can be measured by the substitution method (Fig. 6.21). Ammeter A measures the current by setting switch P to position 1 and then 2. The voltage at the input terminals of the circuit is the same, so U - I x R x = IoRo. Hence R x = R o I o /I x

When measuring large resistances, the ammeter is replaced with a galvanometer with a shunt, which significantly increases the accuracy of the measurement.

The most accurate results when measuring resistance are provided by bridge circuits, which in practice are used in various versions depending on the values ​​of the measured resistances and the required measurement accuracy.

More often than others you can find a device built according to the scheme (Fig. 6.22), which in practice is called a “single bridge”. In this case, the bridge circuit includes resistances R 1 ;R 2 ;R;R x , which form a closed loop. A, B, C, D of four branches (they are called “bridge arms”).

One diagonal of the circuit includes a direct current source, the other a galvanometer with a double-sided scale (zero in the middle of the scale).

Let us assume that for a certain resistance R x other resistances are selected so that the current in the measuring diagonal I g = 0, i.e. the potentials V B and V r are the same when the switches K 1 and K 2 are closed. In this case, I 1 =I 2;I x =I;I 1 R 1 =I x R x;I 2 R 2 =IR.

Using these equalities, it is easy to obtain an expression for the measured resistance R X = RR 1 / R 2. If the resistances R 1 and R 2 are the same in value, then R X = R. In an industrial device, R is a set of resistors (resistance store), compiled according to the ten-day principle. On the top cover there are switches with which you can dial, within certain limits, any value of resistance with an accuracy that is determined by the smallest step of resistance change.

To expand the measurement limits, the values ​​of R 1 and R 2 are selected so that their ratio can also be changed using the decimal system (for example, R/R 2 = 100; 10; 1; 0.1; 0.01; 0.001; 0.0001 ).

Single bridges are used mainly for measuring average resistances. When measuring small resistances, the element being measured is connected according to a special circuit or special bridges designed for this purpose are used.

ELECTRIC MACHINES

General information

Electrical machines whose actions are based on electromagnetic phenomena and which serve to convert mechanical energy and electrical energy are called electric machine generators, and those that convert electrical energy into mechanical energy are called electric motors. Electrical machines are also used to convert electrical energy from one parameter to another, which are called converters. The following can be converted: type of current, frequency, voltage, number of phases and other parameters of electricity.

Electric generators are driven by steam and water turbines, internal combustion engines, etc. Electric motors are used to drive machine tools, various machines, transport equipment, etc.

Electrical machines include transformers - static devices that do not have moving parts, but in their structure and principle of operation they have much in common with electrical machines.

Electric machines have the property of reversibility, that is, they can operate as a generator. If they are rotated by some kind of motor or electrical power is supplied to them, they can be used as electric motors. However, when designing electric machines, the requirements imposed by the peculiarities of their operation by a generator or electric motor are taken into account.

Electrical machines are divided into AC and DC machines.

AC electrical machines are divided into synchronous, asynchronous, and collector.

The most widely used are three-phase synchronous alternating current generators and three-phase asynchronous electric motors. AC brushed motors have limited use due to the complexity of the device, maintenance and higher cost. Their main advantage is the ability to control the rotation speed over a wide range, which is difficult in asynchronous motors.

DC electrical machines are a combination of AC machines with a mechanical rectifier-collector being an integral part of these machines. With the help of a collector, alternating current is converted into direct current.

DC electric machines have a limited scope due to the higher cost of these machines and the complexity of their operation compared to AC machines.

Transformers

A transformer is a device designed to convert alternating current voltage of one value into alternating current voltage of another value.

The simplest transformer (Fig. 2.1) consists of a closed core made up of separate sheets of transformer steel, insulated from each other. The windings are placed on the core. The winding that is connected to the AC source is called the primary winding. The winding to which the load is connected is called secondary.

Alternating current flowing through the primary winding creates a magnetic flux F. It penetrates all windings simultaneously and in each of them induces a measured emf, the magnitude of which is proportional to the number of turns in the winding. The more turns, the greater the EMF:

where E ( - EMF of the primary winding (self-induction EMF); E 2 - EMF of the secondary winding (mutual induction EMF); 1, and 2 - the number of turns in the primary and secondary windings.

The controlled EMF in the windings is equal to the voltages acting on the primary and secondary windings:

Consequently, the greater the number of turns it has, the greater the voltage on the secondary winding. Voltage ratio

at the terminals of the primary winding to the voltage on the secondary winding is called the transformation ratio K:

A transformer is called a step-down transformer if the voltage on the secondary winding is less than the voltage on the primary winding (K>1).

A transformer is called a step-up transformer if the voltage on the secondary winding is greater than the voltage on the primary winding (K<1).

When a consumer is connected, a current I 2 will flow through the secondary winding, which will create a magnetic flux directed towards the magnetic flux of the primary winding. The flow of the primary winding will decrease, this will cause a decrease in the self-induction emf E 1 in it, as a result of which the current I 1 in the primary winding will increase. This will happen until the magnetic flux of the primary winding of the transformer becomes the same.

Thus, as the current in the secondary winding increases, the current in the primary winding increases, and as the current in the secondary winding decreases, the current in the primary winding decreases.

If we do not take into account losses in the transformer windings, then we can consider the powers of the primary and secondary windings to be the same:

hence,

This means that in a step-up transformer, an increase in voltage in the secondary winding occurs due to a decrease in the current in it, and in a step-down transformer, a decrease in voltage occurs due to an increase in the current in the secondary winding.

The efficiency of the transformer is high and ranges from 80-99%. Sometimes autotransformers are used instead of transformers. An autotransformer is a transformer in which the alternating current source and consumer are connected to different points of the same winding (Fig. 2.1b). An autotransformer works in the same way as a regular transformer.

In construction conditions, transformers are used: for transmitting electricity; welding works; power tools; electrical heating of concrete and soil; measuring

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.

With a series connection, 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 based on 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.

Introduction………………………………………………………………………………2

DC Resistance Measurement…………………..…….3

Ammeter-voltmeter method……………………………………………………….……3

Direct assessment method…………………………………………………………..4

Bridges for measuring DC resistance………………...6

Measuring very high resistances……………………………………9

AC Resistance Measurement………………….…...10

Imitance meter…………………………………………..………………...10

Measuring line……………………………………………………………..……….11

Measuring ultra-low resistances…………………………..…………13

Conclusions………………………………………………………………….………..…14

Introduction

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

Resistance (often denoted by the letter R) is considered, within certain limits, to be a constant value for a given conductor and can be defined as

R - resistance;

U is the electrical potential difference at the ends of the conductor, measured in volts;

I is the current flowing between the ends of the conductor under the influence of a potential difference, measured in amperes.

For practical measurement of resistance, many different methods are used, depending on the measurement conditions and the nature of the objects, on the required accuracy and speed of measurements. For example, there are methods for measuring resistance at direct current and at alternating current, measuring high resistances, small and ultra-small resistances, direct and indirect, etc.

The purpose of the work is to identify the main, most common in practice, methods for measuring resistance.

DC Resistance Measurement

The main methods for measuring DC resistance are the indirect method, the direct estimation method, and the bridge method. The choice of measurement method depends on the expected value of the measured resistance and the required measurement accuracy. Of the indirect methods, the most universal is the ammeter-voltmeter method.

Ammeter-voltmeter method

This method is based on measuring the current flowing through the measured resistance and the voltage drop across it. Two measurement schemes are used: measurement of large resistances (a) and measurement of small resistances (b). Based on the results of measuring current and voltage, the desired resistance is determined.

For circuit (a), the desired resistance and relative methodological error can be determined using the formulas:

where Rx is the resistance being measured, and Ra is the resistance of the ammeter.

For circuit (b), the desired resistance and the relative methodological error of measurement are determined by the formulas:

It is clear from the formula that when calculating the desired resistance using an approximate formula, an error arises because when measuring currents and voltages in the second circuit, the ammeter also takes into account the current that passes through the voltmeter, and in the first circuit, the voltmeter measures the voltage in addition to the resistor also on the ammeter .

From the definition of relative methodological errors it follows that measurement according to scheme (a) provides a smaller error when measuring large resistances, and measurement according to scheme (b) - when measuring small resistances. The measurement error using this method is calculated using the expression:

“The instruments used for measurement must have an accuracy class of no more than 0.2. The voltmeter is connected directly to the resistance being measured. The current during measurement should be such that the readings are measured on the second half of the scale. In accordance with this, the shunt used to be able to measure current with a device of class 0.2 is also selected. In order to avoid heating the resistance and, accordingly, reducing the accuracy of measurements, the current in the measurement circuit should not exceed 20% of the nominal one.”

The advantage of the ammeter and voltmeter measurement method circuits is that the same current can be passed through the resistor with the measured resistance as under its operating conditions, which is important when measuring resistances whose values ​​depend on the current.

Direct assessment method.

The direct assessment method involves measuring DC resistance using an ohmmeter. An ohmmeter is a direct reading measuring device for determining electrical active (active resistances are also called ohmic resistances) resistances. Usually the measurement is made using direct current, however, some electronic ohmmeters can use alternating current. Types of ohmmeters: megohmmeters, teraohmmeters, gigaohmmeters, milliohmmeters, microohmmeters, differing in the range of measured resistances.

According to the principle of operation, ohmmeters can be divided into magnetoelectric - with a magnetoelectric meter or magnetoelectric logometer (megaohmmeters) and electronic, which are analog or digital.

“The operation of a magnetoelectric ohmmeter is based on measuring the current flowing through the measured resistance at a constant voltage of the power source. To measure resistances from hundreds of ohms to several megaohms, the meter and the measured resistance rx are connected in series. In this case, the current strength I in the meter and the deviation of the moving part of the device a are proportional: I = U/(r0 + rx), where U is the voltage of the power source; r0 is the resistance of the meter. For small values ​​of rx (up to several ohms), the meter and rx are switched on in parallel.”

Ratiometric megaohmmeters are based on a ratiometer, to the arms of which exemplary internal resistors and the measured resistance are connected in different combinations (depending on the measurement limit), the reading of the ratiometer depends on the ratio of these resistances. As a source of high voltage necessary for carrying out such measurements, such devices usually use a mechanical inductor - a manually driven electric generator; in some megohmmeters, a semiconductor voltage converter is used instead of an inductor.

The operating principle of electronic ohmmeters is based on converting the measured resistance into a voltage proportional to it using an operational amplifier. The resistor being measured is connected to the feedback circuit (linear scale) or to the input of the amplifier. A digital ohmmeter is a measuring bridge with automatic balancing. Balancing is carried out by a digital control device by selecting precision resistors in the bridge arms, after which the measuring information from the control device is supplied to the display unit.

“When measuring small resistances, an additional error may occur due to the influence of transition resistance at the connection points. To avoid this, the so-called four-wire connection method is used. The essence of the method is that two pairs of wires are used - one pair supplies a current of a certain strength to the object being measured, and using the other pair, a voltage drop proportional to the current strength and resistance of the object is supplied from the object to the device. The wires are connected to the terminals of the two-terminal network being measured in such a way that each of the current wires does not directly touch the corresponding voltage wire, and it turns out that the transition resistances at the contact points are not included in the measuring circuit.”

In amateur radio practice, it is sometimes necessary to measure small resistances whose value is below 1 Ohm, for example, in the case of checking transformer windings for short circuits, relay contacts, various shunts. How to measure small resistances of miliohms or microohms? As is known from the electrical engineering course, resistance measurement is based on the effect of converting their value into current or voltage. The circuit of the multimeter attachment is based on this principle.

This simple circuit is used when measuring small resistance values ​​- from 0.001 to 1.999 ohms. We will need a separate battery to power the amateur radio design. The supply voltage is stabilized by the LM317LZ IC. The trimmer must be precisely adjusted to 100 mA to ensure high accuracy and low error.

The printed circuit board is shown in the figure below and is easiest to make using. When assembling the structure, try to reduce the length of the installation wires to a minimum.

A standard D830 digital multimeter will display a value in ohms, ranging from 0.001 to 1.999 ohms. To test the device, determine the value of several parallel-connected one-ohm resistances.

If you want, you can solder not just a console, but a completely finished independent device. This analog milliohmmeter uses two modes for determining resistance. At a stable current of 1A, the scale is 1 division = 0.002 Ohm and at a stable current of 0.1A, the scale is 1 division = 0.02 Ohm. With a current of 0.1A, the device will be able to determine resistance from 0.02 Ohm to one Ohm.

The operating principle of the device is based on determining the voltage drop across the measured resistance when a given stable current passes through it. The resistance of the frame of the pointer measuring device is 1200 Ohms, the total deviation current is 0.0001 A, which means that if we use this indicator as a voltmeter, it is necessary to apply voltage to it U = IxR = 0.0001x1200 = 0.12 V = 120 mV for deflection of the arrow to the last division of the scale. It is this voltage that should drop across a resistance of 1 Ohm at the measuring limit of the device from 0.02 Ohm to 1 Ohm. This means that at this limit we need to pass a stable current I = U/R = 0.12/1 = 0.12A = 120 mA through the measured resistor. By analogy, we calculate the limit for other values.

The operating principle of this circuit is based on the method of measuring the voltage drop across the measured resistance with a previously known value of the current flowing through it. The transistor VT1 creates a constant current value, and its stability is maintained by the operational amplifier, which controls VT1.

DC rating when measuring resistances up to 20 Ohms -10 mA and 100 mA when measuring up to 2 Ohms. For stable operation of the set-top box, the DA1 chip is powered by a 78L05 voltage stabilizer. Toggle switch SA1 selects the measurement limit. We press the SA3 button only at the time of measurements. To protect the voltmeter, a diode VD1 is added to the circuit.

Design setup

First, set the variable resistance knobs R2 and R5 to the middle positions. then a voltage of 8-24 V is applied to the structure. The constant value of the current flowing through the resistance being measured is set using the following method. It is necessary to connect the probes of an accurate ammeter to the terminals of the resistance being measured. Set switch SA1 to the position for measuring resistance up to 2 Ohms, then press SA3 and by changing the variable resistance R5 set the current to 100 mA. Next, set SA1 to a position of up to 20 Ohms, press SA3 and then R2 sets the current to 10 mA. Repeat this method of calibrating the current several times, and then cover the variable resistance motors with varnish or paint.