Digital multimeter

Multi-purpose electronic measuring instrument

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Digital multimeter, a multi-purpose electronic measuring instrument, generally includes Ammeter 、 voltmeter 、 Ohmmeter And other functions, sometimes called multimeter, multimeter Multipurpose meter , or three use electricity meter.

Digital multimeter is applicable to basic fault diagnosis There are also portable devices placed on the workbench, some of which can reach a resolution of seven or eight bits.

Digital multimeter (DMM) is an electronic instrument used in electrical measurement. It can have many Special functions However, its main function is to measure voltage, resistance and current. As a modern multi-purpose electronic measuring instrument, digital multimeter is mainly used in the fields of physics, electricity, electronics and other measurement fields.

Chinese name: Digital multimeter
Classification: Multi-purpose electronic measuring instrument

Contains: Ammeter voltmeter , ohmmeter and other functions
Purpose: Basic fault diagnosis

catalog

nineteen Voltage measurement
twenty Current measurement
twenty-one Resistance measurement
twenty-two Diode measurement
twenty-three Triode measurement
twenty-four measure
twenty-five instructions
twenty-six matters needing attention
twenty-seven Product maintenance
twenty-eight Product maintenance

resolving power

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Resolution is to judge a table measurement result Good or bad. By knowing the resolution of a table, you can know whether you can see the Measurement signal Minor changes. For example, if Digital multimeter The resolution within the range of 4V is 1mV, so when measuring the signal of 1V, you can see the slight change of 1mV (1/1000 volt)

If you want to measure a length less than 1/4 inch (or 1 mm), you will certainly not use the smallest unit of inch (or cm) ruler 。 If the temperature is 98.6 ° F, use the thermometer Measurement is useless. You need a piece with a resolution of 0.1 ° F thermometer 。

Bits and words are used to describe the resolution of a table. DMMs are classified by the number of digits and words they can display.

A three and a half digit meter can display three full digit digits from 0 to 9 and one half digit (only 1 or no display). A three and a half digit table can achieve the resolution of 1999 words. A four and a half digit table can reach the resolution of 19999 words.

The resolution of a digital table described in words is better than that described in bits. The resolution of a three and a half digit table has been increased to 3200 or 4000 words.

The 3200 word digital table provides better resolution for some measurements. For example, a 1999 word meter cannot display 0.1V when measuring a voltage greater than 200V. The 3200 word digital meter can still display 0.1V when measuring the voltage of 320V. When the measured voltage is higher than 320V and the resolution is 0.1V, the expensive 20000 word digital meter is required.

measurement accuracy

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Precision means Use environment Down, appearing Maximum allowable error 。 In other words, precision is used to indicate

The closeness between the measured value of the digital multimeter and the actual value of the measured signal.

For DMMs, the accuracy usually uses percentage express. For example, a reading accuracy of 1% means that when the display of the digital multimeter is 100.0V, the actual voltage may be between 99.0V and 101.0V.

Specific values may be added to the basic precision in the detailed instructions. It means the number of words to be added when changing the rightmost end of the display. In the previous example, the precision may be labeled ± (1%+2). Therefore, if GMM The reading is 100.0V, and the actual voltage will be between 98.8V and 101.2V.

The accuracy of the analog meter is Full range Is calculated based on the error of, rather than the displayed reading. Typical accuracy of analog meter is ± 2% or ± 3% of full range. The typical basic accuracy of a DMM is between ± (0.7%+1) and ± (0.1%+1) of the reading, or even higher.

Digital analog display

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In terms of precision and resolution, digital display has a good advantage. The measured value can be displayed in three or more bits.

The analog pointer is slightly inferior in precision and resolution. Because you have to estimate the position of the pointer.

The bar graph analog pointer displays the change and trend of the signal. But it is more durable and has less damage.

Measure resistance

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Measure the resistance at the resistance block. The resistance varies greatly from a few milliohms (m Ω) contact resistance to billions of ohms insulation resistance 。 Many DMMs measure resistance as low as 0.1 ohms, and some measurements can be as high as 300 megohms (300000 ohms). If the resistance is extremely high, the Fluke multimeter will display "OL", indicating that the measured resistance is too high to exceed the range. When measuring the open circuit, "OL" will be displayed.

The resistance must be measured with the circuit power off, otherwise Check the table Or the circuit board will be damaged. Some DMMs provide the function of protection when the voltage signal is wrongly connected in the resistance mode. Different models of DMMs have different protection capabilities.

When making an accurate measurement of low resistance, the resistance of the measuring wire must be subtracted from the measured value. The resistance of typical test wire is between 0.2 Ω and 0.5 Ω. If the resistance of the test wire is greater than 1 Ω, the test wire will be replaced.

If the DMM provides less than 0.6V DC voltage for measuring resistance, it can measuring circuit The resistance value on the board isolated by diode or semiconductor. Therefore, the resistance can be tested without removing it.

Measure on-off

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Making and breaking is through fast Resistance measurement To distinguish open circuit or short circuit.

When digital multimeter with on-off beep is used, on-off measurement is simpler and faster. When a short circuit is detected, the meter will beep, so it is unnecessary to look at the meter during the test. Different models of DMMs have different trigger resistance values.

Diode test

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The diode is like a Electronic switch 。 If the voltage is higher than a specific value, the diode will turn on. Usually silicon diode On voltage 0.6V. And the diode only allows unidirectional current flow.

When checking the diode or crystal junction, the multimeter will not only give a wide reading range, but also give a driving current greater than 50mA. (See Table 1)

When measuring the resistance of a circuit containing diodes, the test voltage of the DMM will be lower than 0.6V to prevent crystal junction conduction.

When selecting diode test, the test voltage is increased to check the function of diode or semiconductor crystal junction.

Some DMMs have a diode test function. This function measures and displays the actual voltage drop across the diode. The voltage drop of the silicon node in the forward test should be less than 0.7V, and the circuit is open in the reverse test.

How to test resistance

1. Turn off the power supply of the circuit

2. Select resistance block

3. Insert the black probe into the COM jack. Insert the red probe into the resistance test socket

4. Connect the probe to both ends of the tested element or circuit

5. Check the reading and note whether the unit is ohm (Ω), kiloohm (k Ω) or megohm (M Ω).

Note: 1,000Ω=1KΩ； 1,000,000Ω=1MΩ

Be sure to turn off the power supply when testing the resistance.

Measuring current

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Current measurement is different from measuring other quantities with digital multimeter. The direct current measurement method is to connect the digital multimeter directly to the circuit under test, so that the current of the circuit under test flows directly through the internal circuit of the multimeter. Indirect measurement It is not necessary to open the circuit and connect the multimeter to the circuit under test. indirect Legal essentials be used Current clamp 。

direct measurement

1. Turn off the power supply of the circuit

2. Disconnect or disassemble the circuit to connect the meter to the circuit

3. Select corresponding AC (A~) and DC (A -) Gear

4. Insert the black probe into the COM socket, and insert the red probe into the 10 amp socket (10A) or 300 mA socket (300mA). The selection of the socket is mainly based on the possible measurement values.

5. Connect the probes in series to the disconnected circuit part.

6. Turn the circuit on

7. Observe the reading and note the units.

Note: When measuring DC, if the test probe is connected reversely, "-" will appear.

Input protection

A common mistake is to place the test lead in the current socket and try to test the voltage. A small resistance in the DMM will cause a short circuit in the voltage source. large current If the digital multimeter is not adequately protected, it will not only damage the meter and circuit, but also hurt the operator. If it is High voltage circuit (480 volts or more), which can lead to greater danger.

Therefore, the DMM should have a current input protection fuse with sufficient capacity. Meters without current input fuses cannot be used in high energy circuits (>240V ac). Digital multimeter with fuse, Fuse There should be enough capacity

To remove high-energy faults. The rated voltage of the fuse should be greater than the maximum voltage you expect. For example, one of the multimeters

20A, 250V fuse can not play a protective role when measuring 480V circuit with multimeter. Only a 20A, 600V fuse can play a protective role when measuring 480V circuits with a multimeter.

Current test

In some cases, the current value you want to measure exceeds the range of the DMM or site conditions do not allow you to open the circuit to measure the current. The current clamp is very convenient for high current (usually greater than 2A) testing without requiring high accuracy. Current clamp Traverse survey Current and convert it to a value that can be processed by the digital meter.

There are two basic Current probe ： Current converter Type, for AC measurement only. Hall effect Type probe, used to measure AC or DC current.

Current converter type. Generally, 1 is used Milliampere Represents 1 ampere. The current value of 100 amperes will become 100 milliamperes, which can be safely measured with a digital multimeter. Connect the wires to the "mA" and "Com" sockets, and turn the function selection knob to the AC milliampere gear.

Hall effect Type probe. Use 1 millivolt to represent an AC or DC value of 1 ampere. For example, an AC current of 100 amperes would be converted to an AC voltage of 100 millivolts. Connect the test lead to the "V" and "Com" sockets. Turn the function selection knob to "V" or "mV". At this time, the multimeter uses 1mV to represent 1 Ampere current 。

When the circuit is disconnected or desoldered for current measurement, the power supply of the circuit to be tested must be disconnected. Small current It can also cause danger.

Do not test the voltage when the test wire is inserted into the current hole. May cause damage or danger to the meter Personal safety 。

Converter type current probe, like Fluke 80i-400. The digital multimeter uses 1mA display to represent the actual 1A signal.

Fluke I-1010 Hall effect The probe can test very high AC or DC current. It converts the current into a voltage signal.

1mV voltage display represents 1A actual current.

Safety certification

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When you see something like UL CSA 、 TUV When you read such words as "design meets....", you will often think of independent testing certification agencies. Of course, the actual independent testing is not the wishful thinking of instrument design engineers.

How can you prove that your watch meets CAT Ⅲ or CAT Ⅱ? It's not that simple. It is very likely that the manufacturer thinks that his watch meets CAT III or CAT II, but has not passed any independent certification. The International Electrotechnical Commission only develops and recommends its standards, but is not responsible for forcing the implementation of this standard Independent verification Institutional Standard time Only then can the marks of verification organizations such as UL, CSA and TUV be marked on the instrument. For example, UL3111 is a standard formulated according to IEC1010. In the mixed instrument market, independent Test certification It is not the best, but it may be the most direct identification method.

Certification test

Safe use Multimeter First, select the DMM that is suitable for the application and meets the application environment. Once you have selected the appropriate multimeter, you should follow the normal application program To use it. The International Electronical and Electronic Association Power system New standards have been formulated. Confirm that the table you are using meets IEC And its Rated voltage Meet the requirements of field application. For example, if you measure a 480V power panel, your watch should meet the CAT Ⅲ _600V standard. This means that the Input circuit It is designed to withstand transients in this application environment without harming users. Select the label UL, CSA ，CDE， TUV Means that this watch not only meets IEC Standards It has also passed independent certification tests and met its own standards.

Cause of damage

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1. When the probe is still in the current socket, use it to measure AC voltage

2. In resistance mode, measure AC power supply

3. Withstand too high transient

4. Maximum input limits (voltage and current) exceeded

protection circuit

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1、 Automatic recovery Protection. When the circuit system of some meters detects an overload, it will protect the meter until the situation no longer exists. After the overload is removed, the DMM automatically returns to normal running state 。 Usually used Protective resistance Function to avoid voltage overload.

2. There is no automatic recovery function. Some meters will protect the meter after detecting overload, but will not recover automatically until the user takes corresponding measures, such as replacing the resistance.

Advantages of digital watch

1. Fuse current input protection

2. Use high energy fuse off (600V or higher)

3. High voltage protection in resistance mode (500 V or higher)

4. Voltage transient protection (6000V or higher)

5、 safety design Test lead, safety handle, etc

6. Independent Safety certification

Safety checklist

1. Use the table that meets the field application requirements

2. Use a meter with current input fuse and check before measuring current Fuse

3. Check whether the test wire has Physical damage

4. Check the continuity of the test wire with a meter

5. Only use safety handle and insulated test wire

6. Table with recessed socket

7. Select proper function and range during measurement

8. Confirm that the operation of the table is good

9. Follow the product safety manual

10. First disconnect the red Test line

11. Do not work alone

12. Table with overload protection for resistance gear

13. When measuring the current without current clamp, disconnect the power supply of the circuit before connecting the circuit

14. In the case of high voltage and high current, pay attention to the use of appropriate equipment, such as High pressure probe 、 Current clamp

Product accessories

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DMMs can use many accessories. The accessories can expand the range of DMM and make the measurement easier.

High voltage probe and high current probe will reduce the very high voltage and current to the extent that the digital multimeter can measure safely. Temperature probe Turn your DMM into a convenient Digital thermometer 。 High frequency probe can measure voltage with high frequency.

In addition, the optional test line, probe and test clamp can help you easily connect the DMM with the test circuit. Soft bags and Hard package It can not only protect the DMM, but also facilitate you to store attachments and DMMs in it.

Technical Term

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Accuracy

Indicates the difference between the measured value of the DMM and the actual value. It is expressed as a percentage of the reading or a percentage of the full range.

Analog meter:

use Analog pointer To display the measured value. The user judges the reading by the position of the pointer in the stroke.

Annunciator:

Used to indicate the selected range or function error.

Average Response:

Accurate measurement sine wave , but the precision is not enough when measuring non sine wave.

Count:

The last digit of a DMM is often used with the percentage to indicate the accuracy of the DMM.

Current shunt:

The DMM has a low resistance that is used to measure current. Digital multimeter is used to measure the voltage at both ends Ohm's law To calculate the current value.

Digital Multimeter (DMM): Display the value of the measured signal in digital form. The characteristic of digital meter is that its precision, resolution and reliability are higher than those of analog meter.

Non standard sine wave- sinusoidal waveform):

such as pulse sequence 、 square wave 、 Triangular wave 、 Sawtooth wave , peak wave, etc.

Resolution:

The minimum change that can be observed in the measurement.

Valid value (RMS):

Equivalent to DC signal AC signal Measure value of.

Standard sine wave form:

A signal that changes in a sinusoidal pattern without distortion.

True RMS Digital Multimeter (True rms):

Digital multimeter that can accurately measure the effective value of sine wave and non sine wave.

Product features

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The following features and functions can make your DMM more convenient.

·The alarm function will indicate the quantity being measured (voltage, resistance, etc.)

·The contact holding function can keep the display, and you can measure with both hands before reading

· Single bond Operation, convenient selection of measurement function

· overload protection , which can prevent damage to the meter and circuit and protect the operator

·High energy Fuse The user and the meter can be protected during current measurement and overload

·Automatic range selection, available Auto Select Range. Manual range allows you to select a range

·Automatic polarity display will display negative polarity, even if the test probe is connected incorrectly, the meter will not be damaged

·Low battery display

The numerical table mentioned in this application manual basic function , in the case of Fluke 180 and 170. Fluke also has many DMMs with other advantages and functions.

Number of digits; Such equipment, which is very common in the laboratory, is generally used as the reference of voltage or resistance, or used to adjust multi-function Standard apparatus Performance.

working principle

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The function of measuring voltage, current and resistance of the multimeter is realized through the conversion circuit, while the current Measurement of resistance It is based on voltage measurement, that is to say, digital multimeter is based on digital DC voltmeter It is expanded on the basis of. The converter will change continuously over time Analog voltage Quantitative change into Digital quantity , and then Electronic counter Count the digital quantity to get the measurement result, and then decoding The display circuit displays the measurement results. The coordination of logic control circuit completes the whole process in sequence under the action of the clock measurement process 。

Function introduction

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basic function

The measurement of current, voltage and resistance is generally regarded as the basic function of a multimeter. The brand of the early multimeter manufacturer A.V. Ω is the abbreviation of the three measurement units that the equipment can measure: A ampere (Ampere), V Volt, Ω Ohms (Ohm), so the early electricians generally called the multimeter a three-way meter.

New equipment developed can measure more metrics; Some common Additional Features , and its measurement units include:

H Inductance (Henry)

F capacitance（ Farad ）

Conductivity (West Gate) - rarely used

℃/℉ Temperature（ centigrade or Fahrenheit degree ）

Hz frequency（ hertz ）

%Duty cycle（ percentage ）

DWELL closing angle (automobile digital multimeter)

TACH speed (RPM, automotive digital multimeter)

HFE (triode Magnification ）

Function auxiliary symbol or identification:

AC Or~, exchange

DC or=, DC

Common occurrence forms include: DCV (DC voltage), A~(AC current)

advanced function

From Control Circuit to Miniature embedded computer , integrated circuit makes modern Digital instrument It can provide more functions.

Common enhancement measures include:

Current limiting semiconductor junction Voltage drop Measurement, used to determine transistor Type of;

Graphical display of measured quantity, such as Histogram ；

It can make it easier to measure through (go/no go);

Continuous measurement And give an audible alarm when circuit conditions occur;

Low frequency Oscilloscope ；

Telephone test device;

Automatic circuit test, including automatic timing, delay signal, etc;

Simple data detection function, such as recording the maximum and minimum readings in a specified period, or obtaining a certain amount of sample readings at a fixed interval

Sampling and holding can lock the last reading for reading after the equipment is removed from the test circuit;

automatic Conversion test Measuring range, the instrument automatically selects the appropriate measuring range to protect the instrument from damage.

Digital multimeter usually has circuits or software to ensure accurate measurement of AC voltage at any frequency. This type of multimeter uses Root mean square Method Consolidation Input signal In this way, even if the input signal is not an ideal sine wave, the real voltage value can be read correctly.

Some modern multimeters can infrared 、 RS-232 or IEEE -488 device bus, etc. and personal computer to be connected. In these ways, the computer can record the readings during the measurement, or Slave device Upload a set of results to the computer.

Modern equipment and systems become more and more complex, and the multimeter is used by technicians hold-all It is no longer commonly used in China; More sophisticated and specialized equipment is replacing it. For example, when measuring the antenna, the staff may use Ohmmeter Measure its resistance; Modern technicians may use handheld Analyzer Test several parameters to determine the integrity of the antenna cable.

technical parameter

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DC voltage range 200mV/2V/20V/200V/1000V

direct Voltage accuracy ±（0.1%+8）

AC voltage range 2V/20V/200V/1000V

AC voltage accuracy ± (0.8%+40)

DC current range 200 μ A/2000 μ A/20mA/200mA/10A

DC current accuracy ± (0.2%+20)

AC current range 200 μ A/2000 μ A/20mA/200mA/10A

AC current accuracy ± (1%+15)

Resistance range 200 Ω/2k Ω/20k Ω/200k Ω/2M Ω/20M Ω

Resistance precision ± (0.5%+20)

Capacitance range 20nF/200nF/2 μ F/20 μ F/200 μ F/2mF/20mF

Capacitance accuracy ± (1.5%+20)

Frequency range 20Hz/200Hz/2kHz/20kHz/200kHz/2MHz/20MHz/200MHz

Frequency accuracy ± (0.1%+15)

Duty cycle 10%~90%

Diode test·

On-off Buzzer·

True RMS measurement·

Relative value ·

Maximum display 20000

Standard configuration Probe, battery, manual, cloth bag, crocodile clip ， warranty card ， USB interface Transmission line , software CD, test short wiring

Development history

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Digital multimeter is developed slowly through history. Early multimeters used a dial with a magnet to deflect the pointer Galvanometer Same; Modern uses LCD or VFD (Vacuum fluorescent display) digital display 。

Analog multimeter stay secondhand market It is not difficult to find, but it is not very accurate, because zero adjustment and accurate reading from the instrument panel are prone to deviation.

Some analog multimeters use Vacuum tube To amplify the input signal. This kind of multimeter is also called vacuum tube Voltmeter (VTVM, Vacuum Tube Volt Meters) or vacuum tube multimeter (VTMM, Vacuum Tube Multimeters）。

Modern multimeters have all been digitized and are specially called digital multimeters (DMM, Digital MultiMeter）。 In this equipment, the measured signal is converted into a digital voltage and Preamplifier Enlarge, and then display the value directly on the digital display screen; This avoids reading parallax Deviation caused by.

Similarly, better circuit systems and electronics have also improved measurement accuracy 。 old Analog instrument Its basic accuracy is between 5% and 10%, and the modern portable digital multimeter can reach ± 0.025%, while the accuracy of the workbench equipment is more than one millionth of a million.

usage method

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number A multimeter Relatively speaking, it is a relatively simple measuring instrument. In this article, the author will teach you how to use digital multimeter correctly. From the voltage, resistance, current diode , triode MOS FET The first step is to start with the measurement method, so that you can better master the measurement method of the multimeter.

Before use, carefully read the relevant an instruction manual Be familiar with the functions of power switch, range switch, socket and special socket

⑴ Set the ON/OFF switch to the ON position and check 9V battery If the battery voltage is insufficient, it will be displayed on the display, and the battery needs to be replaced. If the display does not show, follow the steps below.

(2) The symbol beside the test pen socket indicates that input voltage Or the current should not exceed Indicated value This is to protect the internal wiring from damage.

⑶ Before the test. The function switch should be set to the range you need.

Voltage measurement

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One of the most basic functions of DMM is to measure voltage. Testing voltage is usually the first step to solve circuit problems. If there is no voltage or Voltage too low If it is too high, first solve the power problem before further inspection.

AC voltage The waveform of may be sine (sine wave) Or not Sine (sawtooth, square, etc.). Many DMMs can display "rms" (effective value) of AC voltage. The effective value is the value of AC voltage equivalent to DC voltage.

Many watches have“ average value ”(average responding), which can give effective values when inputting a pure sine wave. This kind of meter cannot accurately measure the effective value of non sine wave. The digital multimeter with true rms function can accurately measure the true rms of non sinusoidal waves.

The ability of DMM to measure AC voltage is limited by the frequency of the measured signal. Most DMMs can accurately measure AC voltages from 50 Hz to 500 Hz. But the communication of DMM Measurement bandwidth It can reach hundreds of kilohertz. For AC voltage and current frequency range It shall be consistent with the specification of digital multimeter.

DC voltage measurement

① Insert the black probe into the COM jack and the red probe into the V/Ω jack.

② Place the function switch in the DC voltage range V-range, and connect the test probe to the power supply to be tested (test Open circuit voltage ）Or on the load (measuring the load voltage drop), the polarity of the terminal connected by the red probe will be displayed on the display at the same time.

③ Check the reading and confirm the unit

Note:

① If the measured voltage range is unknown, set the function switch to the maximum range and gradually decrease

② If the display only shows "1", it means over range, and the function switch should be set to a higher range

③ "" means not to measure the voltage higher than 1000V. It is possible to display a higher voltage value, but there is a risk of damaging the internal circuit

④ When measuring high voltage, pay special attention to avoid electric shock

AC voltage measurement

① Insert the black probe into the COM jack and the red probe into the V/Ω jack.

② Place the function switch in the AC voltage range V~range, and connect the test pen to the power supply or load to be tested. The test connection diagram is the same as above. When measuring the AC voltage, there is no polarity display.

Current measurement

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DC current measurement

① Insert the black probe into the COM jack, when measuring Maximum When the current is 200mA, the red probe is inserted into the mA jack. When the maximum current is 20A, the red probe is inserted into the 20A jack.

② Place the function switch in the DC current range A - range, connect the test probe in series to the load to be tested, and the polarity of the red probe will be displayed when the current value is displayed

be careful:

1. If the measured current range is unknown before use, set the function switch to the maximum range and gradually decrease

2. It indicates that the maximum input current is 200mA, and the excessive current will burn out Fuse It should be replaced again. The 20A range has no fuse protection, and the measurement should not exceed 15 seconds

AC current measurement

measuring method Same as 1, but the gear should be set to AC gear, Current measurement After that, plug the red pen back into the "V Ω" hole. If you forget this step and measure the voltage directly, the meter or power supply will be scrapped!

Resistance measurement

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Insert the probe into the "COM" and "V Ω" holes, turn the knob to the required range in "Ω", and connect the probe to the metal parts at both ends of the resistance.

be careful:

1. If the measured resistance value exceeds the maximum value of the selected range, the over range "1" will be displayed, and a higher range should be selected. For the resistance greater than 1M Ω or higher, the reading will be stable after several seconds, which is normal

2. When there is no good connection, such as an open circuit, the instrument displays "1"

3. When checking the impedance of the line under test, ensure that all power supplies in the line under test are removed and all capacitors are discharged Energy storage element Will affect the correctness of line impedance test.

4. There are 10 words in the 200M Ω gear of the multimeter when there is a short circuit. When measuring a resistance, these 10 words should be subtracted from the measured reading. If a resistance is displayed as 101.0 when measuring, 10 words should be subtracted from 101.0. The actual resistance value of the tested element is 100.0, i.e. 100M Ω.

5. It can be measured by hand contact resistance However, do not touch both ends of the resistance at the same time -- the human body is a conductor with large but limited resistance.

Diode measurement

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Digital multimeter can measure LED, Rectifier diode ...... When measuring, the position of the probe and Voltage measurement Turn the knob to the "" position; Connect the red probe to the positive pole of the diode and the black probe to the negative pole, and the positive voltage drop of the diode will be displayed. Schottky diode The voltage drop is about 0.2V for ordinary silicon Rectifier tube (1N4000, 1N5400 series, etc.) is about 0.7V, and LED is about 1.8~2.3V. If the probe is replaced and the display shows "1.", it is normal, because the reverse resistance of the diode is very large, otherwise the tube has been broken down.

Triode measurement

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The probe insertion position is the same as above; Its principle is the same as diode. Assume that pin A is Base , connect the black stylus with the foot, and contact the other two feet with the red stylus respectively; If both readings are about 0.7V, then use the red pen to connect the A pin, and the black pen to contact the other two pins. If both display "1", then the A pin is the base, otherwise it needs to be measured again, and this tube is a PNP tube. that collector and Emitter How to judge? A digital watch cannot be judged by the swing of the pointer like a pointer watch. What should we do? We can use the "hFE" gear to judge: first turn the gear to the "hFE" gear, and you can see a row of small jacks beside the gear, which are divided into PNP and NPN tube measurements. It has been judged previously Tubular type Insert the base into the hole "b" of the corresponding tube type, and insert the other two pins into the holes "c" and "e" respectively. At this time, you can read the value, that is, the value of β; Fix the base again, and switch the other two legs; Compare the two readings, and the position of the pin with the larger reading corresponds to the surface "c" and "e".

Tip: The above method can only measure small tubes such as 9000 series directly. If you want to measure large tubes, you can use the wiring method, that is, use small wires to lead out the three pins. This is very convenient.

measure

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N Channel There are domestic 3D01, 4D01 and Nissan 3SK series. G pole（ grid ）Determination of: use the diode gear of the multimeter. If the positive and negative voltage drop between one pin and the other two pins is greater than 2V, "1" is displayed, and this pin is grid G. Then exchange the probe to measure the other two pins. When the voltage drop is small, the black probe is connected to the D electrode (drain electrode), and the red probe is connected to the S electrode（ Source pole ）。

Voltage gear:

During detection or production, it can be used to measure the voltage of each pin of the device, and compare it with the normal voltage to determine whether it is damaged. It can also be used to detect Zener diode The principle is shown in the figure: R is 1K, and the voltage at the power terminal is Manostat Of nominal It depends on the voltage stabilization value, generally Nominal voltage More than 3V, but not more than 15V. Then use a multimeter to detect the voltage value at both ends of D tube, which is the actual stabilized voltage value of D tube.

Current block

Connect the meter into the circuit to measure and monitor the current. If the current deviates far from the normal value (based on experience or original normal parameters), the circuit can be adjusted or maintenance is required when necessary. The 20A gear of the meter can also be used to measure the short-circuit current , that is, two probes are directly connected to both ends of the battery. Remember that the time should never exceed 1 second! Note: This method is only applicable to dry battery , 5, 7 rechargeable battery And beginners should be guided by personnel who are familiar with maintenance, and must not operate on their own! The performance of the battery can be judged according to the short circuit current. When the same battery is fully charged, the greater the short circuit current, the better.

Resistance barrier;

Can be used to judge the resistance, diode, transistor is good or bad one of the methods. The actual resistance value of the resistance deviates Nominal value If it is too much, it will be damaged. about Diode triode If the resistance between any two pins is not very large (more than several hundred K), it can be considered that the performance is degraded or has been broken down. Note that this triode is not resistive. This method can also be used for manifold blocks. It should be noted that the measurement of manifold blocks can only be consistent with the normal parameters Compare 。

4. The pen of an ordinary multimeter has a large resistance value, and interested enthusiasts can make a pen by themselves; Method: prepare about one meter high quality Speaker line Or multi-core copper wire with insulating sleeve Clip One pair (red and black) for loudspeaker wiring Banana Insert a pair (red and black); One end of the wire is firmly welded on the clamp, and the other end is correspondingly connected to the banana plug; A good pair of pens is a success.

Capacitance test

Before connecting the capacitance to be measured, pay attention to the time required for resetting each time the range is changed, and the existence of drift readings will not affect the test accuracy

1. Set the function switch to the capacitance range C (F)

2. Set capacitor Inserting capacitance Test stand in

be careful:

1. The instrument itself has set the protection for the capacitor, so the capacitance Test process Polarity and capacitor charging and discharging are not considered

2. When measuring the capacitance, insert the capacitance into the special capacitance test seat

3. It takes some time to stabilize the reading when measuring large capacitance

4. Unit conversion of capacitance: 1 μ F=106pFl μ F=103nF

On-off test

1. Insert the black probe into the COM jack, and the red probe into the V/Ω jack (the polarity of the red probe is "+"). Place the function switch in the "What" position, and connect the probe to the diode to be tested, and the reading is diode Forward voltage drop Of Approximate value .

2. Connect the probe to both ends of the line to be tested. If the resistance between the two ends is lower than about 70 Ω, the built-in buzzer will sound

instructions

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1. The instrument is equipped with an automatic power cut-off circuit working hours About 30 minutes - 1 hour, power supply Automatic cut-off , instrument entry Sleep state At this time, the instrument consumes about 7 μ A of current

2. When the instrument power is cut off, if you want to turn on the power again, please press the power switch twice

Pointer multimeter

1. The reading accuracy of the pointer meter is poor, but the process of the pointer swing is more intuitive, and the swing speed amplitude can sometimes objectively reflect Measured Size of (such as measuring TV set data bus (SDL) Slight jitter during data transmission); The reading of the digital meter is intuitive, but the process of digital change seems very messy, which is not easy to watch.

2. Generally, there are two batteries in the pointer meter, one is low voltage 1.5V, and the other is high voltage The black stylus is the positive end compared with the red stylus. Digital meters usually use a 6V or 9V battery. In the resistance gear, the probe output current of the pointer meter is much larger than that of the digital meter. The speaker can make a loud "click" sound with the R × 1 Ω gear, and even light up with the R × 10k Ω gear light-emitting diode （LED）。

3. In the voltage gear, the pointer meter internal resistance Compared with digital meters, it is relatively small, and the measurement accuracy is relatively poor. In some situations with high voltage and low current, it is even impossible to measure accurately, because its internal resistance will affect the circuit under test (such as TV under test Kinescope The measured value will be much lower than the actual value at the acceleration stage voltage of). The internal resistance of the voltage range of the digital meter is very large, at least at the megaohm level, and has little impact on the circuit under test. But extremely high Output impedance Make it vulnerable Induced voltage The impact of electromagnetic interference The data measured in strong occasions may be false.

Measuring Skills

1. Speaker, headset, dynamic microphone:

With the R × 1 Ω gear, any probe connects one end and the other touches the other end. Normally, it will make a clear "click" sound. If there is no noise, the coil is broken. If the noise is small and sharp, there is a problem with the friction ring and it cannot be used.

2. Capacitance measurement:

Select the appropriate range according to the capacitance and pay attention to the Electrolytic capacitor The black probe should be connected to the capacitor positive pole.

① . Estimation of capacitance capacity of microwave method: experience or reference Standard capacitance , according to the maximum swing of the pointer. The reference capacitance does not have to have the same withstand voltage value, as long as the capacity is the same. For example, a 100 μ F/250V capacitance can be estimated as a 100 μ F/25V capacitance. As long as the maximum amplitude of their pointer swing is the same, it can be determined that the capacity is the same.

② . Estimation of picoalk capacitance: R × 10k Ω is required, but only the capacitance above 1000pF can be measured. For capacitance of 1000pF or a little larger, as long as Watch needle A slight swing means that the capacity is enough.

③ . Measure whether the capacitance leaks electricity: for 1000 Microfluid For the above capacitance, the R × 10 Ω gear can be used to Fast charging , and preliminarily estimate the capacitance, then change to R × 1k Ω gear to continue measuring for a while, at this time, the pointer should not return And should stop at or very close to ∞, otherwise there is leakage. For some timing or oscillating capacitors (such as the oscillating capacitor of the color TV switching power supply) that are less than tens of micromethods, the requirements for their leakage characteristics are very high. As long as there is a slight leakage, they cannot be used. At this time, the R × 1k Ω gear can be used to continue the measurement after the charge is completed in the R × 1k Ω gear. Similarly, the meter needle should stop at ∞ instead of returning.

3. In the road test, the diode, triode and voltage regulator are good or bad:

Because in Actual circuit Medium, triode bias resistor Or the peripheral resistance of diodes and voltage stabilizing tubes is generally large, mostly hundreds of thousands of ohms. In this way, we can use the R × 10 Ω or R × 1 Ω gear of the multimeter to measure on the road PN junction Good or bad. During the road measurement, the PN junction should have obvious forward and reverse characteristics when measured with the R × 10 Ω gear (if the difference between the forward and reverse resistance is not obvious, the R × 1 Ω gear can be used instead). Generally Forward resistance In R × 10 Ω gear Chronometry The pointer should indicate about 200 Ω, and when R × 1 Ω is measured, the pointer should indicate about 30 Ω (it may vary slightly according to different phenotypes). If the measured forward resistance is too large or the reverse resistance is too small, it indicates that there is a problem with the PN junction and the pipe. This method is particularly effective for maintenance. It can quickly find out the broken pipes, or even measure the pipes that have not been completely broken but have deteriorated characteristics. For example, if you use a small resistance gear to measure the positive resistance of a PN junction, if you weld it down and use a common R × 1k Ω gear to measure it again, it may still be normal. In fact, the characteristics of this pipe have deteriorated, and it cannot work normally or is unstable.

4. Resistance measurement:

It is important to select the measuring range and the most accurate reading. It should be noted that when measuring megohm resistance with R × 10k resistance scale, do not hold your finger on both ends of the resistance Body resistance It will make the measurement result smaller.

5. Measure voltage stabilizing diode:

The voltage stabilizing value of the voltage stabilizing tube we usually use is generally greater than 1.5V, and the resistance block below R × 1k of the pointer meter is powered by the 1.5V battery in the meter. In this way, measuring the voltage stabilizing tube with the resistance block below R × 1k is as complete as measuring the diode Unidirectional conductivity 。 However, the R × 10k gear of the pointer meter is powered by 9V or 15V batteries. When the R × 10k gear is used to measure the voltage stabilizing tube whose voltage stabilizing value is less than 9V or 15V, the reverse resistance value will not be ∞, but there will be a certain resistance value, but this resistance value is still much higher than the forward resistance value of the voltage stabilizing tube. In this way, we can preliminarily estimate the quality of the zener. However, a good voltage regulator should have an accurate voltage stabilizing value. How can you estimate the voltage stabilizing value under amateur conditions? It's not difficult. Just find another pointer table. The method is: first place a meter in the R × 10k position, and connect its black and red probes respectively to the cathode and anode of the voltage stabilizing tube, then simulate the actual situation of the voltage stabilizing tube working condition Then take another meter and place it on the voltage range V × 10V or V × 50V (according to the voltage stabilizing value), and connect the red and black probes to the black and red probes of the meter just now. At this time, the measured voltage value is basically the voltage stabilizing value of this voltage stabilizing tube. "Basically", because the first meter bias current The bias current is slightly smaller than that in normal use, so the measured voltage stabilizing value will be slightly larger, but the difference is basically small. This method can only estimate the voltage stabilizing tube whose voltage stabilizing value is less than the high voltage battery voltage of the pointer meter. If the voltage stabilizing value of the voltage stabilizing tube is too high, we can only use the method of external power supply to measure it (so it seems that when we select the pointer meter, the high-voltage battery voltage of 15V is more suitable than 9V).

6. Measuring triode:

Generally, we need to use R × 1k Ω gear, whether NPN or PNP, small power, medium power or high power, and measure its be junction cb junction to show the same unidirectional conductivity and reverse resistance as the diode Infinity Its forward resistance is about 10K. In order to further estimate the quality of the pipe characteristics, the resistance gear should be changed for multiple measurements when necessary. The method is to set the R × 10 Ω gear to measure the positive direction of the PN junction On resistance About 200 Ω; Set the R × 1 Ω gear to measure the positive conduction resistance of the PN junction at about 30 Ω (the above data are measured by the 47 meter, and other models are generally different, Can be many pre-test If the reading is too large, it can be concluded that the characteristics of the pipe are not good. The meter can also be placed at R × 10k Ω for re measurement. The reverse resistance of cb junction of the tube with the lowest withstand voltage (basically the withstand voltage of the triode is more than 30V) should also be ∞, but the reverse resistance of be junction may be some, and the meter needle will slightly deflect (generally not more than 1/3 of the full range, depending on the different withstand voltage of the tube). However, when measuring the resistance between ce or ec with the gear below R × 1k Ω, the indicator should be infinite, otherwise there is a problem with the pipe. It should be noted that the above measurement is for silicon tubes, not for germanium tubes. In addition, the "reverse" refers to the PN junction, and the direction of the NPN tube and the PNP tube is actually different.

matters needing attention

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To avoid electric shock and personal injury, please read“ security information ”And "Warnings and Precautions", the following specifications are general specifications:

a. If the instrument is damaged, do not use it. Before using the instrument, check the housing, especially Terminal blocks Side insulation.

b. Checklist Whether the pen has damaged insulation or exposed metal; Check whether the probe is on or off; Replace the damaged probe before use.

c. Do not use the instrument after abnormal use protection circuit It may fail. When in doubt, please send the instrument for repair.

d. Please do not Explosive gas 、 Water vapor Or in dusty environment.

e. Do not use the instrument terminal Upper (two input terminals, or any input terminal and ground) input the rated voltage marked on the instrument.

f. Before use, use the instrument to measure a known voltage to confirm that the instrument is normal.

g. When measuring the current, please turn off the power supply of the circuit before connecting the instrument to the circuit.

h. When repairing the instrument, please only use the parts marked or provided by the manufacturer.

i. The instrument must be used according to the method specified in this manual, otherwise the protective measures provided by the instrument may fail.

j. When measuring the AC voltage with an effective value of 30V, the AC voltage with a peak value of 42V or the DC voltage above 60V, please pay special attention, because such voltage may cause the danger of electric shock.

k. Keep your fingers behind the bezel of the pen.

l. When measuring, connect the black probe wire before connecting the red probe wire（ Common terminal ）； Similarly, when disconnecting, disconnect the red lead wire first and then the black lead wire.

m. When opening the battery door, please remove the probe from the instrument first.

n. Do not use the instrument when its housing is open or loose.

o. To avoid electric shock or personal injury caused by incorrect readings, please replace the battery immediately when the meter indicates low voltage ().

p. Do not measure the voltage of Class II above 600V or other higher categories.

q. Overvoltage The device category shall be in accordance with IEC61010-12000: the instrument shall be designed to protect against transient high voltage in the following categories of equipment:

CAT I high voltage low energy circuit, such as electronic circuit Or copier

Equipment powered by CAT Ⅱ fixed device, such as TV personal computer , portable tools and household appliances.

CAT Ⅲ fixed installation equipment, such as distribution board, Feeder And short circuit protection circuits, large buildings lighting 。

Product maintenance

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Digital multimeter is a precision electronic instrument. Do not change the circuit at will, and pay attention to the following points:

1. Do not connect it higher than 1000V DC voltage Or higher than 700V AC Valid values Voltage.

2. Do not set the Voltage source Access

3. Do not use this meter when the battery is not installed properly or the back cover is not tightened

Product maintenance

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Digital multimeter has high sensitivity and accuracy, and its application is almost all over enterprises. However, due to multiple factors and problems Randomness Large, there are not many rules to follow, so the fault should be found outside first and then inside, Easy before difficult Break up the whole into parts and make breakthroughs in key areas.

Sensory method

By virtue of the senses Cause of fault Make a judgment by Appearance inspection , such as disconnection, desoldering, grounding short circuit Fuse tube Broken and burnt components Mechanical damage 、 printed circuit Upwarping and fracture of upper copper foil; You can touch the temperature rise of battery, resistor, transistor and integrated block, refer to Circuit diagram Find out the cause of abnormal temperature rise. In addition, you can also check whether the components are loose and whether the integrated circuit pins are firmly inserted by hand, Change-over switch Whether the tape is jammed; You can hear and smell whether there are abnormal sounds and smells.

Voltage measurement method

Measure whether the working voltage of each key point is normal, and quickly find out Failure point 。 As measured A/D converter Working voltage, reference voltage, etc.

Short circuit method

The short circuit method is generally used in the inspection method of A/D converter mentioned above, which is often used in repairing weak current and micro current instruments.

Open circuit method

Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for the case of short circuit in the circuit.

Measuring element method

When the fault has narrowed down to some place or several elements, it can be measured online or offline. If necessary, replace it with a good component. If the fault disappears, the component is broken.

Repair skills

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For a faulty instrument, first check and judge whether the fault phenomenon is common (all functions cannot be measured) or individual (individual function or individual range), and then solve the problem according to the situation.

1. If all gears fail to work Key inspection Power circuit and A/D converter Circuit. When checking the power supply, remove it Laminated battery Press the power switch, connect the positive probe to the negative power supply of the meter under test, and connect the negative probe to the positive power supply (for digital multimeter). Turn the switch to the diode measurement gear, if the diode is displayed Forward voltage , it indicates that the power supply is good. If the deviation is large, it indicates that the power supply is faulty. In case of open circuit, focus on the power switch and battery lead. If there is a short circuit, it is necessary to use the open circuit method to gradually disconnect the components using the power supply and focus on the inspection Operational amplifier 、 timer And A/D converter, etc. In case of short circuit, more than one integrated element is generally damaged. The A/D converter can be checked simultaneously with the basic meter, which is equivalent to the DC of the analog multimeter Header , specific inspection method:

(1) The range of the meter under test shall be turned to the lowest DC voltage;

⑵ Measuring A/D converter working voltage Whether it is normal. According to the model of A/D converter used in the table, the measured value corresponds to the V+pin and COM pin Typical value Whether the comparison is consistent.

⑶ Measure the A/D converter Reference voltage The reference voltage of commonly used digital multimeter is generally 100mV or 1V, that is to measure the DC voltage between VREF+and COM. If it deviates from 100mV or 1V, it can be connected externally potentiometer Adjust.

⑷ Check the display number input as zero, and turn the A/D converter Positive end IN+vs Negative terminal IN - short circuit, so that the input voltage Vin=0, the instrument displays "00.0" or "00.00".

(5) Check the full bright stroke of the display. Short circuit TEST pin at test end to V+at positive power supply end Logically Become high potential, all digital circuit Stop working. Since DC voltage is applied to each stroke, all strokes are lit and the alignment table displays "1888", and the alignment table displays "18888". If there is missing stroke, check the corresponding output pin of A/D converter and Conductive adhesive (or wiring), whether there is poor contact and disconnection with the display.

2. If there is a problem with a particular gear, indicate A/D converter And the power supply are working normally. Because DC voltage and resistance share one set Voltage divider ； AC and DC current sharing Diverter ； AC voltage and AC current share one set of AC/DC converter; Others such as Cx HFE , F, etc. are composed of independent different converters. Understand the relationship between them, and then according to the power diagram, it is easy to find the fault location. If measuring small signal inaccurate Or if the displayed number jumps greatly, focus on checking whether the contact of the range switch is good.

3. If measured data Unstable, and the value always increases cumulatively, short circuit the input end of A/D converter, Display Data If it is not zero, it is generally caused by poor performance of 0.1 μ F reference capacitor.

According to the above analysis, the basic repair sequence of the digital multimeter should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance (including Buzzer And check the positive pressure drop of the secondary pipe) → Cx → HFE, F, H, T, etc. But we should not be too mechanical. Some obvious problems can be solved first. However, the above procedures must be followed during the adjustment.

In a word, a faulty multimeter, after proper detection, should first analyze the possible parts of the fault, and then Circuit diagram Find the fault location for replacement and repair. As the digital multimeter is a more precise instrument, components with the same parameters must be used to replace the components, especially the A/D converter, which must be strictly screened by the manufacturer Integrated block Otherwise, errors will occur and the required requirements will not be met Accuracy 。 The newly replaced A/D converter also needs to be checked according to the method described above, and must not be trusted because it is new.

Purchase demand

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Digital multimeter is the most basic for electronic enthusiasts test tools A good digital multimeter can not only measure various electrical parameters, but also be a useful assistant for electronic enthusiasts to troubleshoot and repair electrical appliances. At present, there are many kinds of digital multimeter products to choose from. How to choose a digital multimeter that is economical and can meet your needs? Proceeding from your own actual needs.

number A multimeter The classification of is very miscellaneous: according to its structure, it can be divided into Desktop digital multimeter And hand-held digital multimeter, depending on whether it has microprocessor It can also be divided into tradition A/D Conversion digital multimeter and intelligent digital multimeter can also be divided into three and a half digit multimeter, four and a half digit multimeter, etc.

The accuracy, structure, brand and intelligence of digital multimeters determine the price difference. The current price range of digital multimeters sold in the market ranges from tens of dollars to tens of thousands of dollars. Therefore, what is the purpose of buying a digital multimeter? Before choosing a product, be sure to clarify your needs: if only generality If the requirements for measurement accuracy are not high, it is enough to choose a cheap hand-held digital multimeter; If you are engaged in precision measurement , repair or have high requirements for the stability of measurement data, you should choose high-end handheld digital multimeter or desktop digital multimeter.

Purchasing Guide

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Precision index

The accuracy of digital multimeter directly affects the measurement results accuracy To get a digital multimeter, you should first check its accuracy Technical indicators Analysis: first, check whether the accuracy of this watch can meet its own requirements, and second, analyze the overall performance of this watch. Don't assume that the measurement accuracy of a 4-digit digital multimeter is higher than that of a 3-digit and a half digital multimeter. A 4-digit and a half digital multimeter with rough workmanship may have more measurement accuracy than a 3-digit and a half digital multimeter with excellent workmanship, The shopkeeper encountered such a table: generated by an unknown small factory DT The 9103 4-digit and half digital multimeter has a very strong digital jump in the measurement process, which can not meet the requirements of the technical indicators at all.

Product appearance

A good digital multimeter can become a pleasing handicraft. It should be as perfect as possible in terms of materials, injection molding process, circuit boards, protective sleeves and other components.

Inspection items

After getting a digital multimeter:

You should check it first watch case Whether they are in good condition, whether the switches and keys function normally and feel good.

Toggle switch (Measurement Function selection ）Whether the rotation is flexible, the damping is appropriate, the hand feel is comfortable, and the corresponding measurement function is normal.

LCD Whether it is clear, whether there are missing strokes, and the number jump should be as small as possible.

Whether the buzzer can sound.

Reference factors

If you can open the case, you can pay attention to the design of the circuit board, the selected components, installation and welding level. The design of the circuit board is very important for a good digital multimeter. Domestic watches are generally double-layer boards, while some foreign manufacturers will choose Multilayer circuit board Layout to provide better shielding and Electrical performance 。 The level of component selection determines the measurement accuracy of digital multimeter. High end digital multimeter often uses SMD components to improve the accuracy and reduce the size of the circuit board, Key parts The components of are even filtered. I won't say much about the welding level, Electronics enthusiast You know (speaking of the welding level, you can open a digital multimeter worth more than ten yuan to have a look, Handmade workshop Our product guarantee will make you feel miserable).

Pay attention to some details when buying, such as safety protection , product warranty Test cable 、 user 's manual Whether it is complete.

Performance test

When purchasing a digital multimeter, the measurement function of the digital multimeter can be confirmed by measuring known quantities (such as resistance, capacitance, battery voltage, AC voltage, etc.), and if possible, it can be sent to the corresponding organization to check and accept the technical indicators item by item.

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