Traditional ohmmeters with a nonlinear scale do not allow even an approximately accurate reading of the measured resistance, especially at the edges of the scale. It is more convenient to use a device with a linear scale, and when making such an ohmmeter, there is no need to calibrate and draw a scale, since the same dial gauge scale remains.
The operation of a linear scale ohmmeter is based on the principle of an operational amplifier (op-amp), according to which, when feedback is applied to the inverting input of the op-amp, the voltage transfer coefficient is equal to the ratio of the resistances Rx. to R0, where Rx is the resistance between the op-amp output and
inverting input, and R0 is the resistance between the inverting input and the common bus. Due to the fact that a constant voltage U0 is applied to the non-inverting input, the voltage drop across the resistor is U0 Rx/R0, that is, proportional to the measured resistance. The schematic diagram of the ohmmeter is shown in the figure.

Here U0 is the voltage of the zener diode VD1, and R0 is the resistance of one of the included reference resistors R1-R5. In order not to load the op-amp when measuring small resistances, the measuring circuit is connected to the output of the op-amp through an emitter follower assembled on transistor VT1. The voltage drop across the measured resistor Rx is measured by a voltmeter formed by a microammeter PA1 and additional resistors R8 and R9. Thus, at Rx – R0, a voltage equal to U0 and amounting to 3.9 V is supplied to the voltmeter, and its needle should deflect to the full scale. Depending on the internal resistance of the microammeter, when setting up the device, you should reduce the resistance of resistor R9, and use variable resistor R8 to set the arrow exactly to the last division of the scale. In the author's version, the circuit uses a microammeter with a total deviation current of 100 μA. Therefore, the result of reading the measured resistance on the scale should either be divided by two and multiplied by a coefficient corresponding to the established measurement limit, or considered as a percentage of the resistance of the standard resistor. It is more convenient to install a microammeter with a total deviation current of 50 μA, then the readings will not have to be divided by two. But in this case it is necessary to increase the resistance of resistor R9 to 75 kOhm.
The figure shows a printed circuit board of the device with circuit elements installed on it.

Model resistors R1-R5 must be selected quite accurately according to the resistances indicated on the diagram: the accuracy of the measurement depends on their tolerance.

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Among radio amateurs, especially beginners, ohmmeters with a linear scale are very popular, which do not require replacement or calibration of the dial indicator scale. The relatively simple design of such an ohmmeter was developed using an operational amplifier. An ohmmeter allows you to measure resistance from 1 ohm to 1 megohm, which is quite sufficient for many practical purposes.

The principle of operation of an ohmmeter on an operational amplifier is illustrated in Fig. 1. Measuring resistor RX included in the feedback circuit between the output of the amplifier and its inverting input. There is also a reference resistor in the same circuit. R 3 . The non-inverting input is supplied with a reference voltage from the source G1. In this mode, the output voltage of the operational amplifier will depend on the resistance ratio R x And R 3 feedback circuits. It is measured relative to the reference voltage by a voltmeter PV, the readings of which are directly proportional to the resistance R x.

Rice. 1. Functional diagram of an ohmmeter with a linear scale

The schematic diagram of the ohmmeter is shown in Fig. 2. The reference voltage + 2 V at the non-inverting input of the amplifier is created by a resistor divider R10 and a current stabilizer on the transistor VI. The exact value of the reference voltage is selected using a variable resistor R12. Since when measuring small resistances, the current in the measuring circuit, and therefore the output current of the amplifier, may exceed what is permissible for an op-amp, an emitter follower on a transistor is inserted into the ohmmeter V3. To protect the dial indicator from overloads when the amplifier's output voltage accidentally increases due to the incorrect position of switch S1, a diode is connected parallel to the indicator terminals V2,

A voltmeter consists of a milliammeter PA1 and resistors R13, R14. In the position of the button shown in the diagram S2 The voltmeter is designed to measure voltages up to 2 V. When the button contacts are closed, the resistor R14 is shunted and the voltmeter measures the voltage up to 0.2 V.

Reference resistors are connected to the inverting input of the op-amp using a switch S1. The resistance of the reference resistor determines the measurement subrange of the ohmmeter. So, when you turn on the resistor R1 The device can measure resistance from approximately 100 kOhm to 1 MOhm. At the next switch position, the maximum measured resistance can reach 300 kOhm, and at further positions these values ​​will correspond to 100 kOhm, 30 kOhm, 10 kOhm, 3 kOhm, 1 kOhm, 300 Ohm, 100 Ohm. This results in nine measurement subranges.

Thanks to the button S2 the limits of measured resistances can be reduced by 10 times. It is used only on the last two subbands. Thus, more are added to the existing subranges two: up to 30 Ohm and up to 10 Ohm.

Rice. 2. Schematic diagram of an ohmmeter with a linear scale

In order to more economically consume the energy of the power source, it is connected to the device with the S3 button only during measurement.

Rice. 3. Placement of parts on the front panel of the case

The ohmmeter parts are housed in a small housing. An indicator and a subrange switch are mounted on a removable front panel made of getinax measuring 190 X 130 mm (Fig. 3). S1 and push-button switches S2, S3, calibration resistor R12 and terminals for connecting the power source and the resistor being tested (or other part with ohmic resistance).

The reference resistors are soldered directly to the switch blades, and the operational amplifier and transistors are mounted on a fiberglass board (you can getinaks) measuring 35 X 30 mm, which can be attached, for example, to the front panel from the inside.

Resistors R1 - R9 can be MLT-0.125, MLT-0.25 or others, selected with an accuracy of ±1% - the accuracy of measurements largely depends on this. Variable resistor R12 - SPZ-4a or other. Diode V2 It can be, in addition to what is indicated in the diagram, D226 with any letter index or another with a direct voltage of 0.3...0.6 V. Transistors are any of the K.T312, KT315 series. The dial indicator can have a total needle deflection current of 1 mA and an internal resistance of 82 Ohms. Then the resistor R.I.3 must have a resistance of 118 ohms, a R14 - 1.8 kOhm. An M24 microammeter with a full needle deflection current of 100 μA and an internal resistance of 783 Ohms is also suitable. (such an indicator is shown in Fig. 3), it is convenient because it has a scale of 100 divisions, making it easier to read the measured resistances. But in this case, it is necessary to bypass the indicator with a resistor with a resistance of about 92 Ohms so that the indicator needle deviates by the final division at a current of 1 mA. Resistor values R13, R14 for this option remain unchanged. If you use an indicator with a different internal resistance, you will have to recalculate the resistance of the resistors so that with the resistor R14 the indicator arrow deviated by the final scale division at a voltage of 0.2 V, and with resistors connected in series R13, R14 - n.p.And voltage 2 V.

Setting up the device begins with checking the correct installation. Then a 9 V source is connected to the power terminals, for example two 3336L batteries connected in series. The terminals of a precisely measured resistor, for example, with a resistance of 100 kOhm, are connected to the “Rx” terminals. Variable resistor motor R12 set to the middle position, and the switch handle S1 - to position ".300 k." Only then do you press the button S3. The indicator needle should deviate by about a third of the scale. This is achieved with a variable resistor R12 "Caliber". Then the switch sets the subrange "100 k" and a variable resistor achieve precise deflection of the indicator needle by the final scale division. Check calibration on other subranges by connecting to the terminals « Rx» resistors with a resistance of 30 kOhm, 10 kOhm, 3 kOhm and so on. If there are significant discrepancies in the indicator readings and the resistance of the measured resistor, you should select a more accurate reference resistor.

To avoid the indicator needle going off scale when working with an ohmmeter, you should always start measurements in the switch position “1 M", and then, as the indicator arrow deviates, gradually move to other subranges.

A home handyman, when renovating an apartment with his own hands, is faced with the need to connect lamps, sockets and switches according to different circuits. This activity requires electrical measurements and knowledge of basic safety rules when working under voltage.

Our tips will help you choose the best multimeter for these purposes and understand the basic rules for safe work with it both in household electrical wiring and for repairing devices connected to it.

The article compares two types of measuring devices: dial analog and digital. This will allow you to evaluate different measurement technologies, compare their capabilities, and make a choice of a suitable design.

Purpose

The compound word multimeter means its first part is “multi” - many functions that this device performs, and the second “meter” is the measurement of electrical quantities.

It allows you to determine:

• effective voltage value;
• the strength of the flowing current;
• electrical resistance of the connected circuit;
• some other parameters.

Please note that the device may have other names:

1. avometer, an abbreviation for ampere, volt, ohm measurement;
2. or tester assigned to the first analog models.

In technical language it is called a multifunctional measuring device.

Principles of measuring electrical quantities

An explanatory picture from the Internet with little men is intended to explain the relationship between the processes occurring in electrical systems, which allows you to analyze multimeters of any design.

The source voltage in volts tries to push the current in amperes through the resistance provided to it by the resistance in ohms. To analyze these three tasks, the multimeter includes 3 separate measuring instruments:

• ammeter;
• voltmeter;
• ohmmeter

Let's briefly look at their functions.

How does an ammeter work?

The measuring head of the magnetoelectric system is taken as the basis for the operation of analog devices.

When electric current flows through it, a movable frame with an opposing spring and an arrow attached to it rotates, indicating its strength on the scale in microamps - thousandths of an ampere. In this range, currents flow through the measuring head.

However, the ammeter does not measure fractions of an ampere, but whole and even significantly larger values. Such current values ​​can burn out all the conductive lines of the head. To prevent this from happening, they are limited to parallel connection of a calibrated electrical resistance called a shunt.

The principle of shunting with additional resistance reduces the amount of current flowing through the head and makes it proportional to the input value. Due to this, the scale is graduated in amperes, and not in thousandths.

Digital devices use current sensors that operate using microprocessor technology.

Voltmeter device

The same measuring head is connected in series to additional resistances - current-limiting resistors. The instrument scale is graduated in volts.

The mode switch for the ammeter and voltmeter allows you to expand the measurement limits.

A digital voltmeter operates from a voltage sensor.

Ohmmeter design

The principle of resistance measurement is disclosed in the article about.

An ohmmeter also works using a measuring head.

For this, a built-in voltage source is used, which produces a strictly reference value. When preparing the ohmmeter for operation, it must be manually calibrated.

The resistance to be measured is connected to the sockets of the device. A current passes through it, limited depending on the value of the resistor. It deflects the ohmmeter needle by an amount proportional to the value of electrical resistance.

The ohmmeter scale is simply graduated in ohms.

Digital instruments calculate the resistance value based on information received from current and voltage sensors, but also operate from a built-in power supply. They do not require manual calibration.

Types of multimeters

Analog devices

Let's look at the example of the Ts4324 tester.

The multifunctional scale with several rows and mode switches with a large operating range immediately catch your eye.

The factory diagram of internal connections is shown in the photo below.

The purpose of the measuring head scale is shown in more detail in the picture.

For each measurement, it is necessary to analyze the position of the arrow in a certain range corresponding to the type of current and the signal being tested.

The central switch positions are divided into three main sectors (ammeter, voltmeter and ohmmeter) highlighted by red arrows. During operation, it is necessary to determine not only the range of the measured value, but also the shape of the signal.

Digital devices

The internal design of this type of multimeter is much more complex, while the external parts are designed to be simpler for the user. As a sample, we will select one of the standard models with a minimum number of automatic settings.

Instead of a pointer pointer and a complex scale, there is a display, and by positioning the central switch you can select all measurement modes in any sector.

The test leads are connected to two of the three sockets:

• central - general;
• left - used to measure currents more than 10 amperes;
• right - in all other cases.

Methods of electrical measurements

Any multimeter itself does not measure anything. It shows only those values ​​that the user has prepared in the mode he created. Indication errors are most often associated with inattentive human performance.

Let's look at the same type of operations that need to be performed on a dial and digital multimeter.

Measurements with tester Ts4324

Voltage measurement

We select the appropriate mode by pressing the middle button below and set the measurement limit greater than the voltage of the battery being measured - 3 V.

You will need to evaluate the polarity of the wire connections. If you run the current in the opposite direction through the measuring head, the needle will simply hit the stopper to the left of zero. Measuring will not work.

To take a reading, you must select the correct voltage scale with the DC sign on it. You should take into account its multiplicity at the corresponding switch position.

Please note that such an operation is dangerous and requires increased attention.

Press the right button at the bottom with the “~” icon until it locks. We select the appropriate voltmeter mode with the central switch and set it to 300 V. Only after this do we install the ends into the contacts of the socket.

We take readings of 250 V from the scale. The method of using it is the same as in the previous case.

Current measurement

The position of the switches and work with the scale is carried out according to the previous method.

A 1.5 V AA battery supplied a 6.3 V light bulb with a current of 142 mA.

Resistance measurement

In this mode it is important:

• check that the pointer is set to zero using the measuring head spring tension regulator located under the pointer;
• set the calibrated value of the power source with the knob of the “Setup 0” potentiometer located at the very bottom on the front side;
• provide .

To measure, you will need to press the two left buttons simultaneously and set the switch to the ohms icon. The reading on the Ω scale turned out to be 1.5. This is the resistance of the filament when cold.

The resistance measurement mode with a multimeter is designed to test resistors and other elements of radio-electronic devices. It is not intended to evaluate the insulation quality of the dielectric layer. The power supply is not sufficient for this type of measurement.

The assessment of the insulation resistance of cables and wires is carried out with special devices powered from powerful sources: hand generators or a household network 220 or built-in converters with a set of batteries. They are called megohmmeters.

The three experiments presented with a small-sized incandescent light bulb and battery show that the power of the energy source and consumer should be correctly selected according to load and voltage.

1.5 V for a battery and 6.3 for a light bulb is a clear discrepancy. The source operates in emergency mode and cannot cope with the task: the thread barely glows. An overload mode has been artificially created for him.

A similar case can occur in a household network 220, where the power is removed from the equipment with a time delay.

When connecting any consumer to the electrical network, always evaluate its ability to operate reliably and the ability of the protection to eliminate emergency situations.

Digital Multimeter Measurements

Voltage measurement

Working with DC Power Sources

You only need to set the central switch to the position of measuring the voltage at the appropriate limit (= 2 V), insert the wires into the sockets of the device and connect them to the battery being tested. The result is immediately displayed on the board.

If the polarity of connecting the source to the multimeter is reversed, a minus sign will appear on the display. This means that the measurement must be repeated by turning the wires on the battery over.

This technique is used to determine the polarity of the source.

When measurements are performed at a higher limit, the accuracy of the result will be underestimated. It is necessary to observe the correspondence of values.

Working with AC Power Sources

First, set the mode switch to the “~600 V” position, and then check the voltage in the outlet.

We got a result of 231 volts.

Current measurement

The multimeter is inserted into the current circuit, having previously switched it to ammeter mode and set it to the appropriate measurement position. We have a reading of 145 mA at the 200 limit.

A minus sign in front of the current value indicates that the polarity of connecting the device wires to the circuit is reversed. The current flows through it in the opposite direction.

For electricians who often deal with measurements, we recommend purchasing a multimeter with a detachable magnetic circuit of the current transformer - clamps. They are convenient for seamless connection and quick measurements.

Resistance measurement

The central switch of the multimeter is set to the 200 Ω position, and the result of 9.75 is displayed on the display.

The device operates in the same way on the kΩ scale. In the photo shown, the resistance measurement limit is even overestimated. This doesn’t particularly affect the result, although it does.

Dialing mode

A digital multimeter, unlike an analog pointer, has this additional function. It allows you to simply determine the presence of electrical contact within the circuit being tested.

In a closed and open circuit, the indication on the display changes, and for many models of devices an additional sound signal appears.

The continuity mode is designed to analyze small resistances characteristic of current circuits. But they should not be used in voltage circuits. It is especially convenient for testing semiconductor elements.

Another useful function for radio amateurs, called in their slang “beeper”. The multimeter produces high-frequency signals that allow you to check the paths of audio amplifiers and various channels of transmitters or receivers.

Owners of pointer instruments do not have such a function. They are forced to make such a generator with their own hands.

Transistor testing

Another useful function of a digital multimeter, which is also found on more complex designs of pointer models.

To check a bipolar transistor, it is enough to correctly insert its legs into the appropriate socket, taking into account the structure of the p-n-p or n-p-n semiconductor junction. For this purpose, four contact holes are created into which the legs are installed by rotating the body to one side.

For a working transistor, the gain h21 is immediately displayed.

The same function on pointer testers requires taking readings and performing mathematical calculations.

Basic safety rules

The multimeter is designed to measure electrical quantities and allows you to work under voltage. Its body and wires are made with and according to standards.

The quality of protection for digital devices is higher, and their design is more thoughtful. However, even when using them, you should be careful and careful and follow the manufacturer’s recommendations.

Any digital multimeter can be damaged by improper handling, despite its undoubted advantages over a pointer instrument:

• operation of built-in “foolproof” protections, which disconnect the circuit from the penetration of dangerous currents created during all measurement modes;
• increased dielectric strength of insulation.

Old pointer testers require even more attention: if they are incorrectly connected to current or voltage circuits, especially in a household network 220, the elements of their internal circuits burn out. If the calibration resistors can still be replaced, then with the contacts of switches and buttons the repair situation is aggravated.

But more often than not, the conductive spring or winding of the measuring head fails. In this situation, repairs are more expensive than purchasing a new digital multimeter.

A radio amateur often needs to know the resistance of a particular resistor or some section of a circuit, but he may not have a multimeter at hand, but there may be an Arduino nearby, on the basis of which you can independently assemble a simple ohmmeter for measuring resistance.

How to measure resistance using Arduino

It should be noted right away that in addition to the Arduino, you also need one resistor with a known value. The circuit is very simple and is based on a voltage divider, in which one resistor is known and the resistance of the other must be determined. Then we will run a program on the Arduino that will calculate the resistance using Ohm's law. So, the Arduino-based ohmmeter and voltage divider circuit looks like this:

The code (sketch) for creating a simple ohmmeter based on Aduino is presented below:

int analogPin= 0; int raw= 0; int Vin= 5; float Vout= 0; float R1= 1000; float R2= 0; float buffer= 0; void setup() ( Serial.begin(9600); ) void loop() ( raw= analogRead(analogPin); if(raw) ( buffer= raw * Vin; Vout= (buffer)/1024.0; buffer= (Vin/Vout ) -1; R1 * buffer; Serial.print(Vout); )

Enter the value of your known resistor (in ohms) in line 5 of the code above. In this case, a well-known resistor with a value of 1 KOhm (1000 Ohms) is used. So line 5 should look like this: float R1 = 1000. The program sets analog pin A0 to read the voltage between the known resistor and the unknown resistor. You can use any other analog pin, but just change the line number in line 1 and connect the circuit accordingly. When you open the serial monitor, you will see resistance values ​​output once per second. There will be two values: R2 and Vout. R2: The resistance of your unknown resistor in Ohms. Vout: Voltage drop across your unknown resistor.

How accurate will measurements using Arduino be? Below is the serial port screen when measuring a 200 ohm resistor.

The values ​​are quite accurate, the error is only 1.6%. But this is only true for those cases when the unknown resistor is not orders of magnitude different from the known one, so that the voltage is not too small and can be read using the Arduino ADC. But here are the values ​​that can be obtained if you measure the resistance of a resistor with a nominal value of 220 Kom with a reference resistor of 1 Kom.

So different resistance measurement ranges require different reference resistors. In general, this project allows you to make a fairly simple and cheap ohmmeter using Arduino with your own hands.

Homemade measuring instruments

Radio magazine 1 issue 1998
In Sychev. Moscow

In the manufacture of electrical measuring instruments, some difficulties may arise associated with the manufacture of instrument shunts. These shunts are usually low resistance. and they must be selected carefully, since the accuracy of the meter depends on this. To do this, it is proposed to make a simple electronic ohmmeter, which can measure small resistances on a linear scale at four limits: 10, 25.100 and 250 Ohms.

Device diagram

The diagram of the device is shown in the figure. It consists of a stabilized current source on transistor VT1. the operating mode of which is set by the zener diode VD1 and resistors R3. R4, R5, and a voltmeter (microammeter PA1 and resistors R1, R2).

The collector current of transistor VT1 creates a voltage across resistor Rx proportional to its resistance. Therefore, if you calibrate (i.e. set the microammeter pointer to the last scale division) the measuring part using a certain reference resistor Roop. then the measured resistance can be read on the linear scale of the measuring device.

Working with the device is as follows. The resistor being tested (for example, a shunt being manufactured) is connected to the “Rx” terminals, and a standard resistor corresponding to the selected measurement limit is connected to the “Ro6p” terminals. Switch SA2 is moved to the corresponding measurement limit, and switch SA1 is moved to position “K” (calibration). After applying the supply voltage, by pressing the SB1 button, the tuning resistor R4 sets the pointer pointer to the last scale division. Then switch SA1 is switched to the “AND” (measurement) position and the Rx resistance is measured. The accuracy of the measurement will mainly depend on the accuracy of the reference resistors.

If you use a power source with a voltage of 8...9 V or a less sensitive head in an auxiliary device, then the D814A zener diode must be replaced with KS139A or KS147A, and the resistance of resistor R5 must be reduced to 100 Ohms. a R4 - up to 470 - 680 Ohm. In addition, if the resistance of the reference resistor does not correspond exactly to the required measurement limit, then it is permissible to calibrate the meter by setting the reading corresponding to the nominal value of this resistor, if it is at least 80% of the limit.

The device can use standard resistors such as MT, BLP, S2-29V. S2-36. S2-14: MLT resistors (R1. R3. R4. R5): resistor R2 types SPO-0.5, SP3-4b or similar; transistors of the KT814 series. KT816 with a base current transfer coefficient of more than 50. A measuring head that will be installed in the manufactured device (for example, 50 or 250 μA) is applicable as a PA1 microammeter. Switches SA1 and SA2 are TV2-1 type toggle switches. Generally speaking, the SA1 switch can be eliminated, leaving one pair of terminals to which the Rocp resistor must first be connected. and after calibration - the Rx resistor.

In the case of using more common transistors of the p-p-p structure in the device, the polarity of the power supply of the stabilizer and the microammeter should be changed.