Multimeter is also called multiplex meterMultimeter, three meter, complex meter, etc., are indispensable measuring instruments for power electronics and other departments. Generally, the main purpose is to measure voltage, current and resistance.The multimeter is divided into pointer multimeter and digital multimeter according to the display mode.It is a multi-functional and multi range measuring instrument. Generally, a multimeter can measure DC current, DC voltage, AC current, AC voltage, resistance, audio level, etc. Some can also measure AC current, capacitance, inductance, and some semiconductor parameters (such as β).
Multimeter is a magnetoelectric instrument with rectifier, which can measure AC and DC current, voltage, resistance and other electrical parameters.Generally, there are several ranges for each electrical quantity.It is also called multi-purpose meter or multi-purpose meter for short.The multimeter is composed of magnetoelectric ammeter (meter head), measuring circuit and selector switch.Through the transformation of the selector switch, various electrical parameters can be easily measured.The circuit calculation is mainly based on Ohm's law of closed circuit.There are many kinds of multimeters, which should be selected according to different requirements.[1]
primary uses
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Multimeter (Figure 1)
The multimeter can not only be used to measure theresistance, AC and DCVoltageIt can also measure DC voltage.Even some multimeters can measuretransistorThe main parameters and capacitance of the capacitor.One of the most basic skills of electronic technology is to fully master the use of multimeter.Common multimeter includes pointer multimeter and digital multimeter.Pointer type multimeter is a multi-functional measuring instrument with one meter head as the core component, and the measured value is read by the pointer of the meter head.The measured value of the digital multimeter is directly displayed in digital form by the LCD, which is convenient to read, and some of them also have voice prompt function.A multimeter is an instrument that shares a common head and integrates a voltmeter, an ammeter and an ohmmeter.
The DC current range of the multimeter is a multi range DC voltmeter.The voltage range can be expanded by paralleling the meter head with a closed circuit voltage divider.The DC voltage range of the multimeter is a multi range DC voltmeter.The voltage range can be expanded by connecting the meter head in series with a voltage divider.The measuring range varies with the difference of voltage divider.The head of the multimeter is a magnetoelectric measurement mechanism, which can only use DC to change AC into DC by diode, thus realizing AC measurement.
Structure composition
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Multimeter (Figure 2)
The multimeter is composed ofHeader, measuring circuit and transfer switch.Multimeter is the most basic tool in the field of electronic testing, and it is also a widely used testing instrument.Multimeters are also called multimeter, three-way meter (A, V, Ω, that is, current, voltage, resistance), multiplexer meter, and universal meter. Multimeters are divided into pointer multimeter and digital multimeter. There is also an oscilloscope multimeter, which is a multi-functional and multi range measuring instrument.General multimeter can measure DC current, DC voltage, AC voltage, resistance and audio frequencylevelWait, some can be measuredAC current, capacitance, inductance, temperature and some parameters of semiconductor (diode, triode).Digital multimeter has become the mainstream and replaced analog instrument.Compared with analog instruments,Digital instrumentHigh sensitivity, high accuracy, clear display, strong overload capacity, easy to carry, and easy to use.
Header
The head of the multimeter isSensitive galvanometer。The dial on the head is printed with a variety of symbols, scale lines and values.The symbol A - V - Ω indicates that the meter is a multimeter that can measure current, voltage and resistance.The dial is printed with multiple scale lines, of which the resistance scale line marked with "Ω" at the right end is zero at the right end and ∞ at the left end, and the scale value distribution is uneven.The symbol "-" or "DC" represents DC, "～" or "AC" represents AC, and "～" represents the scale line shared by AC and DC.The numbers below the scale line are the scale values corresponding to different gears of the selector switch.
The head is also equipped with a mechanical zero adjustment knob to correct the zero position of the pointer at the left end.
Selector switch
Multimeter (Figure 3)
The selector switch of the multimeter is a multi gear rotary switch.Used to select measurement items and ranges.General multimeter measurement items include: "mA";DC current, "V (-)": DC voltage, "V (～)": AC voltage, "Ω": resistance. Each measurement item is divided into several different ranges for selection.
Probe and probe socket
The pens are red and black.When using, insert the red probe marked with“+”The black probe is inserted into the jack marked with“－”No.
Meter head (pointer type)
It is a highly sensitive magnetoelectric DCammeterThe main performance indicators of the multimeter basically depend on the performance of the meter head.HeadingsensitivityIt refers to the meter headPointerFlow through the meter head during full scale deflectionDC currentThe smaller the value, the higher the sensitivity of the meter.measureVoltageThe greater the internal resistance, the better the performance.There are four lines on the headerTick markTheir functions are as follows: the first item (from top to bottom) is marked with R or Ω, indicatingresistanceValue, change-over switch atOhmsThis tick mark is read when the gear is in gear.The second item is marked with "I" and "VA", indicating the deliveryDC voltageAnd DC current value, when the transfer switch is in AC, DC voltage or DC current gear, and the range is in other positions except AC 10V, read this scale line.Article 3 is marked with 10V, indicating 10VAC voltageWhen the change-over switch is in the AC and DC voltage range and the range is AC 10V, this scale line will be read.The fourth one is marked with dB, indicating audiolevel。
Meter head (digital)
Multimeter (Figure 4)
The head of a digital multimeter usually consists of oneA/DThe (analog/digital) conversion chip+peripheral components+LCD display is composed. The accuracy of the multimeter is affected by the meter head. The digits converted from the A/D chip are generally called 3 1/2 digit digital multimeter, 4 1/2 digit digital multimeter, etc.The most commonly used chips are ICL7106 (three half LCD manual range classic chips, subsequent versions are 7106A, 7106B, 72067240, etc.), ICL7129 (107（
Measuring line
The measuring circuit is used to convert all kinds of measured DC to micro DC suitable for meter measurementelectric currentIt is composed of resistor, semiconductor element and battery
It can measure variouselectric current、Voltage、resistanceDifferent measuring ranges, after a series of processing (such as rectification, shunt, voltage division, etc.), are unified into a small DC current with a certain limit, which is sent to the meter head for measurement.
Change-over switch
Its function is to select different measuring circuits to meet the measurement requirements of different types and ranges.The change-over switch is generally a circular dial with functions and ranges marked around it.
working principle
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Multimeter (Figure 5)
The basic principle of the multimeter is to use a sensitive magnetoelectric DC ammeter (microammeter) as the meter head.When a small current passes through the meter head, there will be a current indication.However, the meter head cannot pass large current, so it must be connected in parallel with some resistors in series on the meter head for shunt or step-down, so as to measure the current, voltage and resistance in the circuit.
design principle
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Multimeter (Figure 6)
During the measurement process of digital multimeter, the measured voltage is converted into DC voltage signal by the conversion circuit, and then the analog/digital (A/D) converter converts the voltage analog quantity into digital quantity, and then counts through the electronic counter. Finally, the measurement result is directly displayed on the display screen with digital.
The function of multimeter to measure voltage, current and resistance is realized through the conversion circuit, while the measurement of current and resistance is based on the measurement of voltage, that is to say, the digital multimeter is expanded on the basis of the digital DC voltmeter.
The digital DC voltmeter A/D converter converts the analog voltage that changes continuously with time into a digital quantity, and then counts the digital quantity by an electronic counter to obtain the measurement result, which is then displayed by the decoding display circuit.The logic control circuit controls the coordination of the circuit and completes the whole measurement process in sequence under the action of the clock.
Digital multimeter
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Multimeter (Figure 7)
Digital multimeter is the most commonly used digital instrument at present.Its main characteristics are high accuracy, strong resolution, perfect test function, fast measurement speed, intuitive display, strong filtering ability, low power consumption, and easy to carry.Since the 1990s, digital multimeter has been rapidly popularized and widely used in China, and has become a necessary instrument for modern electronic measurement and maintenance work, and is gradually replacing the traditional analog (i.e. pointer) multimeter.
Digital multimeter, also known as digital multimeter (DMM), has a wide variety of models.Every electronic worker hopes to have an ideal digital multimeter.There are many principles for selecting a digital multimeter, sometimes different from person to person.However, the hand-held (pocket) digital multimeter should generally have the following characteristics: clear display, high accuracy, strong resolution, wide test range, complete test functions, strong anti-interference ability, complete protection circuit, beautiful appearance, generous, simple operation, flexible, reliable, low power consumption, easy to carry, moderate price, etc.
Main indicators, display digits and display characteristics of digital multimeter
The display digits of digital multimeter are usually 3 1/2 digits to 8 1/2 digits.There are two principles for judging the display digits of digital instruments: one is that the digits that can display all digits from 0 to 9 are integer digits;The second is that the value of the fractional digit takes the highest digit in the maximum displayed value as the numerator, and the full scale counting value is 2000, which indicates that the instrument has three integer digits, while the numerator of the fractional digit is 1 and the denominator is 2, so it is called 3 1/2 digits, read as "three and a half digits", and its highest digit can only display 0 or 1 (0 is usually not displayed).3 2/3 digits (read as "three and two thirds digits") The highest digit of the digital multimeter can only display 0~2 digits, so the maximum display value is ± 2999.In the same case, it is 50% higher than the limit of the 3 1/2 digit digital multimeter, especially when measuring 380V AC voltage.
Multimeter (Figure 8)
Universal digital multimeter generally belongs to handheld multimeter with 3 1/2 digit display, and digital multimeter with 4 1/2 and 5 1/2 digits (less than 6 digits) can be divided into handheld and desktop.6 1/2 digits or more are mostly desktop digital multimeters.
Digital multimeter adopts advanced digital display technology, with clear and intuitive display and accurate reading.It not only ensures the objectivity of readings, but also conforms to people's reading habits, and can shorten the reading or recording time.These advantages are not possessed by the traditional analog (i.e. pointer) multimeter.
Accuracy
The accuracy of digital multimeter is the synthesis of systematic error and random error in measurement results.It indicates the consistency between the measured value and the true value, and also reflects the size of the measurement error.Generally speaking, the higher the accuracy, the smaller the measurement error, and vice versa.
The accuracy of digital multimeter is much better than that of analog pointer multimeter.The accuracy of the multimeter is a very important indicator, which reflects the quality and process capability of the multimeter. The multimeter with poor accuracy is difficult to express the true value, which is easy to cause misjudgment in measurement.
Resolution
The voltage value corresponding to the last word of the lowest voltage range of the digital multimeter is called resolution, which reflects the sensitivity of the instrument.The resolution of digital instruments increases with the increase of display digits.Digital multimeters with different digits can achieve different highest resolution indicators.
The resolution index of digital multimeter can also be displayed by resolution.Resolution refers to the percentage between the minimum number (except zero) and the maximum number that can be displayed by the instrument.
Multimeter (Figure 9)
It should be pointed out that resolution and accuracy belong to two different concepts.The former represents the "sensitivity" of the instrument, that is, the "recognition" ability of small voltage;The latter reflects the "accuracy" of the measurement, that is, the consistency between the measurement results and the true value.The two are not necessarily related, so they cannot be confused, and the resolution (or resolution) should not be mistaken for that similar to the accuracy, which depends on the comprehensive error of the internal A/D converter, the function converter and the quantization error of the instrument.From the perspective of measurement, resolution is a "virtual" indicator (independent of measurement error), while accuracy is a "real" indicator (it determines the size of measurement error).Therefore, it is impossible to increase the display digits arbitrarily to improve the resolution of the instrument.
measuring range
In the multi-functional digital multimeter, different functions have their corresponding maximum and minimum values that can be measured.
Measurement rate
The number of times a digital multimeter measures the measured electricity per second is called the measurement rate, and its unit is "times/s.".It mainly depends on the conversion rate of A/D converter.Some hand-held digital multimeters use measurement cycles to indicate the speed of measurement.The time required to complete a measurement process is called the measurement cycle.
Multimeter (Figure 10)
There is a contradiction between the measurement rate and the accuracy index. Generally, the higher the accuracy is, the lower the measurement rate is. It is difficult to take both into account.This contradiction can be solved by setting different display digits or measuring speed conversion switch on the same multimeter: add a fast measuring gear, which is used for the A/D converter with fast measuring speed;The measurement speed is greatly improved by reducing the number of display digits. This method is relatively common and can meet the needs of different users for measurement speed.
Input impedance
When measuring the voltage, the instrument should have a high input impedance, so that the current drawn from the circuit under test during the measurement process is very small, will not affect the working state of the circuit under test or the signal source, and can reduce the measurement error.
When measuring the current, the instrument should have a very low input impedance, so that the impact of the instrument on the circuit to be measured can be minimized after being connected to the circuit to be measured. However, when using the current gear of the multimeter, the input impedance is small, so it is easy to burn the instrument. Please pay attention when using the multimeter.
Classification of digital multimeter
Digital multimeters are classified according to the range conversion mode, and can be divided into three types: manual range (MAN RANGZ), automatic range (AUTO RANGZ), and automatic/manual range (AUTO/MAN RANGZ).
Multimeter (Figure 11)
According to different functions, uses and prices, digital multimeters can be roughly divided into nine categories: low-end digital multimeters (also known as universal digital multimeters), mid-range digital multimeters, mid-range/high-end digital multimeters, digital/analog hybrid instruments, digital/analog dual display instruments, and multimeter (integrating digital multimeters, digital storage oscilloscopes and other kinetic energy).
Test function of digital multimeter
Digital multimeter can measure not only DC voltage (DCV), AC voltage (ACV), DC current (DCA), AC current (ACA), resistance (Ω), diode forward voltage drop (VF), transistor emitter current amplification factor (hrg), but also capacitance (C), conductance (ns), temperature (T), frequency (f),The buzzer gear (BZ) and the resistance gear (L0 Ω) measured by low-power method are added to check whether the circuit is on or off.Some instruments also have the function of automatic conversion of inductance, signal, AC/DC and capacitance.
Most digital multimeters add the following new and practical test functions: HOLD, LOGIC, TRMS, REL Δ, AUTO OFF POWER, etc.
Anti interference ability of digital multimeter
Multimeter (Figure 12)
Simple digital multimeters generally use the integral A/D conversion principle. As long as the forward integration time is exactly equal to the integral multiple of the series frame interference signal period, the series frame interference can be effectively suppressed.This is because the series frame interference signal is averaged out in the forward integration phase.The common frame rejection ratio (CMRR) of middle and low grade digital multimeters can reach 86 ～ 120dB.
Development trend of digital multimeter
Integration: The handheld digital multimeter uses a single A/D converter, and the peripheral circuit is relatively simple, requiring only a few auxiliary chips and components.With the advent of a single digital multimeter chip, a fully functional automatic range digital multimeter can be constructed with one IC, creating favorable conditions for simplifying design and reducing cost.
Low power consumption: New digital multimeters generally use CMOS large-scale integrated circuit A/D converters, and the power consumption of the whole machine is very low.
Advantages and disadvantages of ordinary multimeter and digital multimeter:
Pointer type and digital multimeter have their own advantages and disadvantages.
The pointer multimeter is an average value instrument, which has an intuitive and visual reading indication.(Generally, the reading value is closely related to the pointer swing angle, so it is very intuitive).
Multimeter (Figure 13)
Digital multimeter is an instantaneous sampling instrument.It takes samples every 0.3 seconds to display the measurement results. Sometimes, the results of each sampling are very similar, not identical, which is not as convenient as the pointer type for reading results.Pointer multimeter generally has no internal amplifier, so the internal resistance is small.
The internal resistance of digital multimeter can be very large, usually 1M Ω or more, because the internal amplifier circuit is used.(i.e. higher sensitivity can be obtained).This makes the influence on the measured circuit smaller and the measurement accuracy higher.
Due to the small internal resistance, the pointer multimeter mostly uses discrete components to form a shunt and voltage divider circuit.thereforefrequency characteristicIt is uneven (relatively digital), and the frequency characteristic of digital multimeter is relatively good.The internal structure of the pointer multimeter is simple, so the cost is low, the function is less, the maintenance is simple, and the over-current and over-voltage capacity is strong.
The digital multimeter uses a variety of internal oscillation, amplification, frequency division protection and other circuits, so it has many functions.For example, it can measure temperature, frequency (in a lower range), capacitance, inductanceSignal generatorwait.
The digital multimeter has poor overload capacity due to its internal structure and multi-purpose integrated circuit, and is generally difficult to repair after damage.The output voltage of the digital multimeter is low (usually no more than 1 V).It is inconvenient to test some components with special voltage characteristics (such asSilicon controlled rectifier, LED, etc.).The output voltage of the pointer multimeter is high.The current is also large, which can easily test thyristor, LED, etc.
Pointer multimeter should be used for beginners, and two kinds of meters should be used for non beginners.
Selection principle
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1. The reading accuracy of the pointer meter is poor, but the process of the pointer swing is relatively intuitive, and the swing speed amplitude can sometimes objectively reflect the measured size (such as the slight jitter of the SDL when transmitting data);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 9V or 15V. The black probe is the positive end of the red probe.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 R × 1 Ω gear can makespeakerIt makes a loud "click" sound, and even lights up the light-emitting diode (LED) with the R × 10k Ω gear.
3. In the voltage range, the internal resistance of the pointer meter is smaller than that of the digital meter, and the measurement accuracy is 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 testKinescopeThe 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 megohm level, and has little impact on the circuit under test.But the extremely high output impedance makes it vulnerable toInduced voltageIn some occasions with strong electromagnetic interference, the measured data may be false.
4. In short, in relatively high current and high voltageAnalog circuitPointer gauge is applicable to measurement, such as TV and stereoPower amplifier。At low voltageSmall currentDigital meters are suitable for digital circuit measurement, such as BP machine, mobile phone, etc.It is not absolute. Pointer meter and digital meter can be selected according to the situation.
Operating Procedures
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Multimeter (Figure 15)
1. Be familiar with various functions of the multimeter before use, and correctly select the gear, range and probe socket according to the measured object.
2. When the size of the measured data is unknown, first set the range switch to the maximum value, and then switch from a large range to a small range, so thatGauge pointerIndication is fullscaleMore than 1/2 of.
3. MeasurementresistanceWhen the appropriate magnification gear is selected, touch the two probes to make the pointer point to zero. If the pointer deviates from zero, adjust the "zero adjustment" knob to make the pointer return to zero to ensure accurate measurement results.If zero adjustment is not possible or the digital display meter sends a low voltage alarm, it should be checked in time.
4. When measuring the resistance of a circuit, it is necessary to cut off thePower Supply, no live measurement is allowed.
5. Pay attention to personal andmeterFor the safety of the equipment, it is not allowed to touch the metal part of the probe with hands during the test, and it is not allowed to switch the gear switch with electricity to ensure accurate measurement and avoid accidents such as electric shock and burning of the instrument.[2]
Precautions
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Multimeter (Figure 16)
1. Before using the multimeter, carry out "mechanical zeroing", that is to say, when there is no measured electricity, use the multimeterPointerRefers to zeroVoltageOr zeroelectric currentPosition.
2. When using the multimeter, do not touch the metal part of the probe with your hand, which can ensure the accuracy of measurement on the one hand and personal safety on the other hand.
3. When measuring a certain amount of electricity, do not shift gears at the same time, especially when measuring high voltage or large current.Otherwise, the multimeter will be damaged.If shifting is required, disconnect the probe first, and then measure it after shifting.
4. When using the multimeter, it must be placed horizontally to avoid errors.At the same time, we should also pay attention to avoiding the outside worldmagnetic fieldImpact on the multimeter.
5. After using the multimeter, set the change-over switch toAC voltageMaximum gear of.If it is not used for a long time, the battery inside the multimeter should also be taken out to prevent the battery from corroding other components in the meter.[2]
Troubleshooting
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Multimeter (Figure 17)
Digital multimeter, also known as multimeter, multimeter, multi-purpose electricity meter or three purpose electricity meter, is a multi-purpose electronic measuring instrument, generally including ammeter, voltmeter, ohmmeter and other functions.Compared with pointer multimeter, digital multimeter is widely used because of its high precision, fast speed, large input impedance, digital display, accurate reading, strong anti-interference ability, and high degree of measurement automation.However, if it is used improperly, it is easy to cause failure.
Troubleshooting of digital multimeter generally starts from power supply.Troubleshooting of digital multimeter can be carried out as follows.
1. Visual inspection.
You can touch whether the temperature rise of batteries, resistors, transistors and integrated blocks is too high.If the newly installed battery is hot, the circuit may be short circuited.In addition, observe whether the circuit is broken, desoldered, mechanically damaged, etc.
2. Waveform analysis.
Multimeter (Figure 18)
Observe the voltage waveform, amplitude, period (frequency), etc. of each key point of the circuit with an electronic oscilloscope.For example, if the clock oscillator is tested to see if it is on, if the oscillator has no output, it means that the internal inverter is damaged, or the external component is open circuited.
3. Measure element parameters.
For online or offline measurement of components within the fault range, the parameter value shall be analyzed.When measuring the resistance online, the influence of elements connected in parallel should be considered.
4. Tacit troubleshooting.
Hidden fault refers to the fault that appears and disappears from time to time, and the instrument is good and bad from time to time.This kind of fault is relatively complex. Common reasons include faulty solder joints, loose solder joints, loose connectors, poor contact of transfer switches, unstable component performance, and broken leads.In addition, it also includes some external factors.For example, the ambient temperature is too high, the humidity is too high, or there are intermittent strong interference signals nearby, etc.
5. Test the working voltage at all levels.
Multimeter (Figure 19)
To detect the working voltage at each point and compare it with the normal value, first ensure the accuracy of the reference voltage. It is better to use a digital multimeter of the same model or similar model for measurement and comparison.
In addition to the above possible causes, the damage of the digital multimeter may also be caused by the error of the measuring gear. For example, when measuring AC mains power, the measuring gear is selected in the resistance gear. In this case, once the probe contacts the mains power, the internal components of the multimeter will be damaged instantly.Therefore, before measuring with a multimeter, check whether the measuring gear is correct.After use, place the measurement option at AC 750V or DC 1000V, so that no matter what parameters are measured incorrectly in the next measurement, the digital multimeter will not be damaged.
