AC resistance measurement. Electrical resistance measurement
Determining the operation of any circuit or installation.
Obtaining certain resistance values during the manufacture of electrical machines, apparatus, and instruments during the installation and operation of electrical installations is a necessary prerequisite for ensuring their normal operation.
Some resistances retain their value practically unchanged, while others, on the contrary, are very much subject to change over time, temperature, humidity, mechanical forces, etc. Therefore, both in the production of electrical machines, devices, instruments, and during installation and operation electrical installations inevitably have to measure resistance.
The conditions and requirements for performing resistance measurements are very diverse. In some cases, high accuracy is needed, in others, on the contrary, it is enough to find an approximate resistance value.
Depending on the size they are divided into three groups:
- 1 ohm and less - low resistance,
- from 1 ohm to 0.1 Mohm - average resistance,
- from 0.1 MΩ and above - high resistance.
When measuring small resistances, it is necessary to take measures to eliminate the influence of the resistance of connecting wires, contacts and thermo-EMF on the measurement result.
When measuring average resistances, you can ignore the resistance of connecting wires and contacts, and you can ignore the influence of insulation resistance.
When measuring large resistances, it is necessary to take into account the presence of volume and surface resistances, the influence of temperature, humidity and other factors.
Features of measuring small resistances
The group of low resistances includes: armature windings of electrical machines, resistance of ammeters, shunts, resistance of current transformer windings, resistance of short bus wires, etc.
When measuring small resistances, you always have to take into account the possibility of the influence of the resistance of the connecting wires and transition resistances on the measurement result.
The resistances of the measuring wires have values of 1 x 10 4 - 1 x 10 2 ohms, transition resistances - 1 x 10 5 - 1 x 10 2 ohms.
Transitional resistances or mean the resistances that meet electricity when moving from one conductor to another.
Transition resistances depend on the size of the contact surface, on its nature and condition - smooth or rough, clean or dirty, as well as on the density of contact, pressing force, etc. Let us find out, using an example, the influence of transition resistances and the resistance of connecting wires on the measurement result.
In Fig. 1 shows a diagram for measuring resistance using standard instruments of an ammeter and a voltmeter.
Rice. 1. Incorrect connection diagram for measuring small resistances with an ammeter and voltmeter.
Let's say the required resistance r x is 0.1 ohm, and the resistance of the voltmeter is rv = 500 ohm. Since they are connected in parallel, then r x/rv = Iv/Ix = 0.1/500 = 0.0002, i.e., the current in the voltmeter is 0.02% of the current in the desired resistance. Thus, with an accuracy of 0.02%, the ammeter current can be considered equal to the current in the desired resistance.
Dividing the reading of the voltmeter connected to points 1, 1" by the reading of the ammeter, we obtain: U"v /Ia = r"x = r x + 2r pr + 2r k, where r"x is the found value of the desired resistance; r pr - resistance of the connecting wire; rc - contact resistance.
Counting r pr = r k = 0.01 ohm, we obtain the measurement result r"x = 0.14 ohm, from which the measurement error due to the resistance of the connecting wires and contact resistance is equal to 40% - ((0.14 - 0.1)/0 ,1))x 100%.
It is necessary to pay attention to the fact that as the desired resistance decreases, the measurement error from the above reasons increases.
By connecting the voltmeter to the current terminals - points 2 - 2 in Fig. 1, i.e. to those terminals of resistance rx to which the wires of the current circuit are connected, we obtain the voltmeter reading U"v less than U"v by the amount of voltage drop in the connecting wires and, therefore, the found value of the desired resistance r x"= U" "v /I a = rx + 2 r k will contain an error due only to the resistance at the contacts.
By connecting a voltmeter as shown in Fig. 2, to the potential terminals located between the current ones, we obtain the voltmeter reading U""" v less than U"v by the amount of voltage drop across the contact resistances and, therefore, the found value of the desired resistance r"""x = U""v/Ia = rx
Rice. 2. Correct connection diagram for measuring small resistances with an ammeter and voltmeter
Thus, the found value will be equal to the actual value of the desired resistance, since the voltmeter will measure the actual voltage value at the desired resistance rx between its potential terminals.
The use of two pairs of clamps, current and potential, is the main technique for eliminating the influence of the resistance of connecting wires and transition resistances on the result of small resistance measurements.
Features of measuring high resistances
Poor current conductors and insulators have high resistance. When measuring the resistance of conductors, insulating materials and products made from them, one has to take into account factors that can influence the value of their resistance.
These factors primarily include temperature, for example, the conductivity of electrical cardboard at a temperature of 20°C is 1.64 x 10 -13 1/ohm, and at a temperature of 40°C it is 21.3 x 10 -13 1/ohm. Thus, a change in temperature by 20° C caused a change in resistance (conductivity) by 13 times!
The numbers clearly show how dangerous it is to underestimate the influence of temperature on measurement results. Similarly, a very important factor influencing the value of resistance is the moisture content of both the test material and the air.
In addition, the value of resistance can be influenced by the type of current used to test, the magnitude of the voltage being tested, the duration of the voltage, etc.
When measuring the resistance of insulating materials and products made from them, one must also take into account the possibility of current passing along two paths:
1) through the volume of the test material,
2) along the surface of the test material.
The ability of a material to conduct electric current in one way or another is characterized by the amount of resistance that meets the current at this point.
Accordingly, there are two concepts: volumetric resistance, referred to 1 cm3 of material, and surface resistance, referred to 1 cm2 of the surface of the material.
To illustrate, consider an example.
When measuring cable insulation resistance using a galvanometer, large errors can occur due to the fact that the galvanometer can measure (Fig. 3):
a) current Iv, going from the cable core to its metal sheath through the insulation volume (current Iv, caused by the volumetric resistance of the cable insulation, characterizes the cable insulation resistance),
b) current Is, going from the cable core to its sheath along the surface of the insulating layer (Is, caused by the surface resistance, depends not only on the properties of the insulating material, but also on the condition of its surface).
Rice. 3. Surface and volume current in the cable
To eliminate the influence of conductive surfaces when measuring insulation resistance, a coil of wire (guard ring) is placed on the insulating layer, which is connected as shown in Fig. 4.
Rice. 4. Circuit for measuring cable bulk current
Then the current Is will pass in addition to the galvanometer and will not introduce errors into the measurement results.
In Fig. 5 dan circuit diagram to determine the volumetric resistivity of the insulating material - plate A. Here BB are the electrodes to which voltage U is applied, G is a galvanometer that measures the current due to the volume resistance of plate A, B is the guard ring.
Rice. 5. Measurement of volume resistance of a solid dielectric
In Fig. 6 shows a schematic diagram for determining the surface resistivity of an insulating material (plate A).
Rice. 6. Measurement of surface resistance of a solid dielectric
When measuring large resistances, you should also pay serious attention to the insulation of the measuring installation itself, since otherwise a current will pass through the galvanometer due to the insulation resistance of the installation itself, which will entail a corresponding measurement error.
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 (more often called a resistor) that provides electrical resistance to current. Electrical resistance is due to transformation electrical energy into other types 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 is connected to terminals 1 and 2 (for example, a winding electric machine or apparatus), to terminals 3 and 4 - a galvanometer, and to terminals 5 and 6 - a power source (dry galvanic cell or battery). 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 (with contact B closed).
Exist various designs 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, an unbalanced DC bridge is widely used in various devices for measuring non-electrical quantities by 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 by 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 magnetic field a permanent magnet creates two oppositely directed moments M1 and M2, 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.
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 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 when known meaning current flowing through it. Transistor VT1 creates a constant current value, and maintains its stability 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 setupFirst, 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.
By their physical nature, all substances react differently to the flow of electric current through them. Some bodies transmit it well and are classified as conductors, while others transmit it very poorly. These are dielectrics.
The properties of substances to resist the flow of current are assessed numerical expression- size electrical resistance. The principle of its definition was proposed by Georg Ohm. The unit of measurement of this characteristic is named after him.
The relationship between the electrical resistance of a substance, the voltage applied to it and the flowing electric current is usually called Ohm's law.
Principles of electrical resistance measurement
Based on the dependence of the three most important characteristics of electricity shown in the picture, the resistance value is determined. To do this you need to have:
2. current and voltage measuring instruments.
The voltage source is connected through an ammeter to the area being measured, the resistance of which must be determined, and the voltage drop across the consumer is measured with a voltmeter.
After reading the current I with an ammeter and the voltage U with a voltmeter, calculate the resistance value R according to Ohm’s law. This simple principle allows you to take measurements and make calculations manually. However, it is difficult to use it in this form. For ease of operation, ohmmeters have been created.
Design of a simple ohmmeter
Instrument manufacturers produce resistance measuring devices that operate by:
1. analog;
2. or digital technologies.
The first type of instruments is called pointer instruments due to the method of displaying information - moving the arrow relative to the initial position to the reference point on the scale.
Pointer-type ohmmeters, as resistance measuring instruments, were the first to appear and continue to work successfully to this day. Most electricians have them in their tool arsenal.
The design of these devices:
1. all components of the above circuit are built into the housing;
2. the source produces a stabilized voltage;
3. The ammeter measures current, but its scale is immediately calibrated in units of resistance, which eliminates the need to perform constant mathematical calculations;
4. Wires with ends are connected to the external terminals of the housing terminals, ensuring rapid creation of an electrical connection with the element under test.
Pointer instruments of this class of measurement operate due to their own magnetoelectric system. Inside the measuring head there is a winding of wire into which a conductive spring is connected.
This winding carries a current from the power source through the measured resistance Rx, limited by resistor R to the milliamp level. It creates a magnetic field that begins to interact with the field of a permanent magnet located here, which is shown in the diagram with poles N-S.
The sensitive needle is fixed on the axis of the spring and, under the action of the resultant force generated from the influence of these two magnetic fields, deflects by an angle proportional to the strength of the flowing current or the value of the resistance of the conductor Rx.
The scale of the device is made in resistance divisions - Ohms. Due to this, the position of the arrow on it immediately indicates the desired value.
Working principle of a digital ohmmeter
In their pure form, digital resistance meters are produced to perform complex, special-purpose work. The mass consumer now has access to devices that combine in their design the tasks of an ohmmeter, voltmeter, ammeter and other functions.
To measure resistance, it is necessary to switch the corresponding switches to the required operating mode of the device and connect the measuring ends to the circuit being tested.
When the contacts are open, the display will indicate “I”, as shown in the photo. It corresponds to a greater value than the device can detect in a given sensitivity range. Indeed, in this position, it already measures the resistance of the air section between the contacts of the terminals of the connecting wires.
When the ends are installed on a resistor or conductor, the digital ohmmeter will display the value of its resistance in real numbers.
The principle of measuring electrical resistance with a digital ohmmeter is also based on the application of Ohm's law. But, in its design there are already more modern technologies related to use:
1. appropriate sensors designed to measure current and voltage, which transmit information via digital technology;
2. microprocessor devices that process the information received from sensors and display them on the display in a visual form.
Each type of digital ohmmeter may have its own distinctive user settings that should be learned before use. Otherwise, out of ignorance, you can make gross mistakes, because applying voltage to its input occurs quite often. It manifests itself as burnout internal elements scheme.
Conventional ohmmeters test and measure electrical circuits formed by wires and resistors that have relatively low electrical resistances up to several tens or thousands of ohms.
DC measuring bridges
Electrical resistance measuring instruments in the form of ohmmeters are designed as portable, mobile devices. They are convenient to use for assessing typical, standard circuits or continuity of individual circuits.
In laboratory conditions, where high accuracy and high-quality compliance with metrological characteristics are often needed when performing measurements, other devices work - DC measuring bridges.
Electrical circuits of DC measuring bridges
The operating principle of such devices is based on comparing the resistances of two arms and creating a balance between them. The balanced mode is monitored by a control mil- or microammeter when the current flow in the diagonal of the bridge stops.
When the instrument needle reaches zero, you can calculate the required resistance Rx from the values of the standards R1, R2 and R3.
The measuring bridge circuit can have the ability to smoothly regulate the resistance of the standards in the arms or be performed in steps.
Appearance of measuring bridges
Structurally, such devices are made in a single factory housing with the ability to conveniently assemble a circuit for electrical testing. Standard switching controls allow you to quickly perform resistance measurements.
Ohmmeters and bridges are designed to measure the resistance of electric current conductors that have a resistive resistance of a certain value.
Instruments for measuring ground loop resistance
The need for periodic monitoring of technical condition is caused by the conditions of their presence in the ground, which causes corrosion processes of metals. They worsen the electrical contacts of the electrodes with the soil, conductivity and protective properties against the drainage of emergency discharges.
The operating principle of devices of this type is also based on Ohm's law. The ground loop probe is permanently placed in the ground (point C), due to which its potential is zero.
At equal distances from it, about 20 meters, similar ground electrodes (main and auxiliary) are driven into the ground so that a stationary probe is located between them. A current from a stabilized voltage source is passed through both of these electrodes and its value is measured with an ammeter.
In the section of the electrodes between the potentials of points A and C, a voltmeter is used to measure the voltage drop caused by the flow of current I. Next, the circuit resistance is calculated by dividing U by I, taking into account the correction for current losses in the main ground electrode.
If, instead of an ammeter and voltmeter, you use a ratiometer with current and voltage coils, then its sensitive needle will immediately indicate the final result in ohms, saving the user from routine calculations.
Many brands of pointer instruments operate on this principle, among which the old models MS-0.8, M-416 and F-4103 are popular.
They are successfully complemented by a variety of modern resistance meters created for similar purposes with a large arsenal of additional functions.
Instruments for measuring soil resistivity
Using the class of instruments just discussed, the resistivity of soil and various granular media is also measured. To do this, they are switched on according to a different scheme.
The electrodes of the main and auxiliary grounding electrodes are spaced over a distance of more than 10 meters. Considering that the measurement accuracy can be affected by nearby conductive objects, for example, metal pipelines, steel towers, fittings, it is permissible to approach them no less than 20 meters.
The rest of the measurement rules remain the same.
Instruments for measuring the resistivity of concrete and other solid media operate on the same principle. For them, special electrodes are used and the measurement technology changes slightly.
How do megaohmmeters work?
Conventional ohmmeters operate on the energy of a battery or accumulator - a low-power voltage source. Its energy is enough to create a weak electric current that reliably passes through metals, but it is not enough to create currents in dielectrics.
For this reason, a conventional ohmmeter cannot detect most defects that occur in the insulation layer. For these purposes, another type of resistance measuring device has been specially created, which is usually called “Megaohmmeter” in technical language. The name means:
mega - million, prefix;
Ohm - unit of measurement;
meter is a common abbreviation of the word to measure.
Appearance
Devices of this type are also pointer and digital. As an example, we can demonstrate the M4100/5 megaohmmeter.
Its scale consists of two subranges:
1. MΩ - megaohms;
2. KΩ - kilo-ohms.
Electrical diagram
Comparing it with the circuit diagram of a conventional ohmmeter, it is easy to see that it works on the same principles, based on the application of Ohm's law.
The voltage source is a direct current generator, the handle of which must be rotated evenly at a certain speed of about 120 rpm. The level depends on this high voltage, issued to the circuit. This value should break through the layer of defects with reduced insulation and create a current through it, which will be displayed by moving the arrow on the scale.
The MΩ—KΩ measurement mode switch switches the position of groups of resistors in the circuit, ensuring operation of the device in one of the operating subranges.
The difference between the design of a megohmmeter and a simple ohmmeter is that this device uses not two output terminals connected to the area being measured, but three: G (ground), L (line) and E (screen).
Ground and line terminals are used to measure the insulation resistance of live parts relative to ground or between different phases. The shield terminal is designed to eliminate the impact of leakage currents created through the insulation on the accuracy of the device.
For a large number of megohmmeters of other models, the terminals are designated a little differently: “rx”, “—”, “E”. But this does not change the essence of the device’s operation, and the screen terminal is used for the same purposes.
Digital megohmmeters
Modern instruments for measuring the insulation resistance of equipment operate on the same principles as their pointer counterparts. But they differ significantly big amount functions, ease of measurement, dimensions.
When choosing digital devices for continuous use, you should take into account their feature: operation from autonomous source nutrition. In cold weather, batteries quickly lose their functionality and require replacement. For this reason, working with switch models with a manual generator remains in demand.
Safety rules when working with megohmmeters
The minimum voltage generated by the device at the output terminals is 100 volts. It is used to test the insulation of electronic components and sensitive equipment.
Depending on the complexity and design of the equipment electrical diagram on megohmmeters, other voltage values are used up to 2.5 kV inclusive. The most powerful devices it is possible to evaluate the insulation of high-voltage power line equipment.
All these works require strict compliance with safety rules, and they can only be carried out by trained specialists who have permission to work under voltage.
Typical hazards created by megohmmeters during operation are:
dangerous high voltage at output terminals, test leads, and connected electrical equipment;
the need to prevent the action of the induced potential;
creating a residual charge on the circuit after the measurement is completed.
When measuring the resistance of an insulation layer, a high voltage is applied between the live part and the ground loop or equipment of another phase. On long cables and power lines, it charges the capacitance formed between different potentials. Any incompetent worker with his body can create a path for the discharge of this capacity and receive an electrical injury.
To eliminate such unfortunate situations, before taking measurements with a megohmmeter, check that there is no dangerous potential on the circuit and remove it after working with the device using a special technique.
Ohmmeters, megohmmeters and the meters discussed above operate on direct current and determine only resistive resistance.
Instruments for measuring resistance in alternating current circuits
The presence of a large number of different inductive and capacitive consumers both in household electrical networks and in production, including energy enterprises, creates additional energy losses due to the reactive component of the total electrical resistance. Hence the need to fully take it into account and perform specific measurements.
Instruments for measuring phase-zero loop resistance
When in electrical wiring If a malfunction occurs, leading to a short-circuit of the phase potential to zero, a circuit is formed through which the short circuit current flows. Its value is affected by the resistance of the electrical wiring section from the short circuit to the voltage source. It determines the amount of emergency current that must be switched off by circuit breakers.
Therefore, it is necessary to perform it at the most remote point and, taking it into account, select the ratings of the circuit breakers.
To perform such measurements, several methods have been developed based on:
voltage drop when: the circuit is disconnected and across the load resistance;
short circuit with reduced currents from an external source.
Measuring the load resistance built into the device is accurate and convenient. To perform this, the ends of the device are inserted into the socket farthest from the protection.
It is useful to take measurements in all sockets. Modern meters working using this method immediately show the resistance of the phase-zero loop on their display.
All the devices discussed represent only a part of the devices for measuring resistance. Energy companies operate entire measuring systems that allow them to constantly analyze changing quantities. electrical parameters on complex high-voltage equipment and take emergency measures to eliminate emerging faults.
Science begins with the ability to measure.
D.I.Mendeleev
In the practice of radio amateurs, one encounters the need to measure low-resistance resistances (up to 1 ohm). A simple milliohmmeter is designed to solve this problem. This device can measure resistance from 0.0001 to 1 Ohm with sufficient accuracy for a radio amateur.
When measuring small resistances using digital multimeters in series with the measured resistance, let's call it Rx, the resistance of the connecting wires, the transition resistance of the input terminals or sockets, switch contacts, etc. are inevitably included. This resistance (Rpr.) is in the range of 0.1...0.4 Ohm. Due to the above reasons, the actually measured resistance will be greater than Rx by a certain amount (Rx+Rpr.). The error can reach up to 50% when measuring very small resistances. For large resistances this error is small and can be ignored.
From the above it is clear that it is necessary to exclude the influence of connecting wires, etc. on the result of measuring very small resistances. There is a method for measuring low resistance using a 4-clamp DC circuit. The use of this method completely eliminates the influence of connecting wires on the result of measuring small resistances. This method is used in this milliohmmeter. Let us briefly consider the essence of the 4-clamp measurement method.
Picture 1
Figure 1 (left) shows a diagram for measuring resistance using a 2-clamp circuit. The path of the measuring current is shown in red. As you can see, current flows both through the resistor being measured and through the wire resistance (Rpr) of the multimeter, which introduces an error into the measurement result. The resistance of the voltmeter does not affect the measurement of Rx, since it has a very large (up to 10 MOhm) internal resistance Rin. Figure 1 (right) shows a 4-clamp measurement circuit. From the diagram it is clear that the resistance of the wires does not affect the measurement result, since it is connected in series with the very large internal resistance of the voltmeter. The measuring current flows only through resistor Rx.
Here is a diagram of a milliohmmeter (Fig. 2).
Figure 2
The power source of the circuit is a battery with a voltage of 9 V. Switch SB supplies voltage from the battery to a voltage stabilizer chip type 7806. Capacitor C1 is used to suppress voltage surges. Resistors R1, VR2 are necessary to set the output voltage of the microcircuit within 6 V. Potentiometer VR2 sets the exact value of the output voltage of 6 V. Potentiometer VR3 sets the output current flowing through the measured resistor Rx equal to 100 mA (0.1 A). Since resistor VR3 has a relatively large resistance compared to the measured Rx, the error arising from the presence of resistances Rx (from 1 mOhm to 1 Ohm) will affect the current value of 100 mA within no more than 2%.
Milliohmmeter design
The appearance and installation view of the milliohmmeter parts is shown in photos 1, 2 and 3. The parts were mounted using a hinged method; the microcircuit was not installed on the radiator. Multi-turn resistors are used as potentiometers VR2, VR3 for more precise setting of voltage and current. The device body is plastic, dimensions 11*6*4 cm. Terminals K1 and K2 are metal. Power switch type MT-1.
Photo 1
Photo 2
Photo 3
Preparing to measure resistance
Connect the digital voltmeter probes to terminals K1 and K2. Apply voltage from the power source to the circuit by turning on the SB switch. Use potentiometer VR2 to set the output voltage to 6 V when the Rx resistor is not connected. Next, having turned off SB, switch the multimeter to measuring current (the probes remain in the same place), turn on SB and use potentiometer VR3 to set the output current to 0.1A.
Photo 4
Photo 5
Taking measurements
First, let's take several resistors of known values (0.1; 0.2; 0.5 Ohms) and measure their resistance to make sure the milliohmmeter is working.
Photo 6
Without turning on the power to terminals K1 and K2, we clamp the terminals of the measured resistance. We install the probes of the digital voltmeter into the sockets of terminals K1 and K2, and the measurement limit is at 200 mV. Turn on the power and read the device readings.
Photo 7
Let's say the measured voltage is 22.3 mV. The current was previously set to 100mA. We divide the voltage by the current and get the required resistance. In our case: Rx=22.3: 100= 0.223 Ohm. Of course, it is customary to divide volts by amperes to get Ohms, but this is more convenient; there is no need to convert mV and mA into volts and amperes. We measure other reference resistors in the same way. But still, remember that 1 V is 1000 mV; 100mV-0.1V; 10mV-0.01V; 1mV-0.001V; 1A-1000mA; 100mA-0.1A. In my multimeter, the smallest measurement limit is 200 mV, the division value is 0.1 mV. Input impedance is about 10 MOhm. That is, theoretically it is possible to measure a resistance of 0.001 Ohm (1mOhm). Voltmeters with low input resistance are not suitable for our measurements.
So, we have determined that the measurements taken gave a real result. Now we move on to measuring the unknown resistance. We will use shunts from disassembled avometers as unknown resistances. When measuring the resistance of the largest shunt, the voltage drop was 0.5 mV, the current was 100 mA.
Photo 8
The shunt resistance value, calculated according to Ohm's law, turned out to be 0.005 Ohm. The resistance of the small shunt, measured with a milliohmmeter, is 0.212 Ohms (voltage drop - 21.2 mV).
The milliohmmeter can find practical application in selecting shunts for chargers, measuring resistance in the final stages of low-frequency amplifiers and other devices where it is necessary to measure low resistances (transition resistance of contacts of switches, relays, etc.).
Low-resistance resistance measurements can also be made at currents greater than 0.1 A. To do this, it is necessary to assemble a current stabilizer for the appropriate current. Stabilizer circuits are shown in Fig. 3.
Figure 3
The stabilizer is included in the circuit instead of potentiometer VR3. Of course, this will entail installing the microcircuit and transistor on radiators of the appropriate size, as well as increasing the size of the device.
Resistances less than 1 mOhm (1000 μOhm) are measured using microohmmeters. The measuring current can be up to 150 A. Voltage does not play a big role.
If it is necessary to make a shunt for charger, but there is no nichrome, constantan, or manganin, then you can use a pin of a suitable diameter, as shown in photo 9.
Photo 9
Stud material - steel, bronze, copper, etc. By moving one of the contacts along the pin, the desired shunt resistance is achieved. Calculating the shunt resistance is simple. If there are any questions, we'll discuss them.
