Multimeters, also known as multimeters, multimeters, three-meters, multimeters, etc., are indispensable measuring instruments in power electronics and other departments. Generally, the main purpose is to measure voltage, current and resistance. Multimeters function are divided into pointer multimeters and digital multimeters according to the display method. It is a multi-functional and multi-range measuring instrument. Generally, the multimeter function can measure DC current, DC voltage, AC current, AC voltage, resistance and audio level, etc., and some can also measure AC current, capacitance, inductance and semiconductor. Some parameters (such as β) and so on.
1. Basic definition of multimeter
The multimeter function is a magnetoelectric instrument with a rectifier that can measure various electrical parameters such as AC and DC current, voltage and resistance. For each electrical quantity, there are generally several ranges. Also known as multimeter or multimeter for short. The multimeter is composed of a magnetoelectric ammeter (meter head), a measuring circuit and a selection switch. Various electrical parameters can be measured conveniently by changing the selection switch. The main basis of its circuit calculation is the closed circuit Ohm's law. There are many types of multimeters, and they should be selected according to different requirements when using them.
2. Basic functions of the multimeter
The multimeter functions can be used not only to measure the resistance of the measured object, but also to measure the DC voltage. Some multimeters function can even measure the main parameters of transistors and the capacitance of capacitors. It is one of the most basic skills of electronic technology to be fully proficient in the use of multimeters. Common multimeters function include pointer multimeters and digital multimeters. The pointer multimeter is a multi-functional measuring instrument with the 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 liquid crystal display, which is easy to read, and some also have a voice prompt function. A multimeter is a meter that combines a voltmeter, an ammeter and an ohmmeter in one head.
The DC current file of the multimeter function is a multi-range DC voltmeter. The voltage range can be expanded by connecting the closed-circuit voltage dividing resistor in parallel with the meter head. The DC voltage file of the multimeter function is a multi-range DC voltmeter. Connecting the voltage dividing resistor in series with the meter head can expand its voltage range. Different voltage dividing resistors have different corresponding measuring ranges. The head of the multimeter function is a magnetoelectric system measuring mechanism, which can only pass through direct current, and use diodes to change alternating current into direct current, thereby realizing the measurement of alternating current.
3. The structure of the multimeter
The multimeter is composed of three main parts: meter head, measuring circuit and changeover switch. The multimeter function is the most basic tool in the field of electronic testing, and it is also a widely used testing instrument. Multimeters are also called multimeters, three-purpose meters (A, V, Ω are current, voltage, and resistance), multimeters, and multimeters. Multimeters are divided into pointer multimeters and digital multimeters. There is also a multimeter with oscilloscope function. The oscilloscope multimeter is a multi-functional, multi-range measuring instrument. General multimeters can measure DC current, DC voltage, AC voltage, resistance and audio level, etc., and some can also measure AC current, capacitance, inductance, temperature and some parameters of semiconductors (diodes, triodes). Digital multimeters have become mainstream and have replaced analog meters. Compared with analog instruments, digital instruments have high sensitivity, high precision, clear display, strong overload capacity, easy to carry, and more convenient and simple to use.
The head of the multimeter is a sensitive galvanometer. The dial on the head is printed with various symbols, scale marks and values. The symbol A-V-Ω indicates that the ammeter is a multimeter that can measure current, voltage and resistance. There are multiple scale lines printed on the dial, among which the one marked with "Ω" on the right is the resistance scale line, the right end is zero, the left end is ∞, and the scale value distribution is uneven. The symbol "-" or "DC" means direct current, "~" or "AC" means alternating current, and "~" means the scale line shared by AC and DC. Several rows of numbers under the scale line are the scale values ??corresponding to the different positions of the selector switch.
There is also a mechanical zero adjustment knob on the meter head to correct the zero position of the pointer at the left end.
3.2 Selection switch
The selector switch of the multimeter is a multi-position 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.
3.3 Test lead and test lead socket
The test leads are divided into red and black. When in use, insert the red test lead into the jack marked "+", and insert the black test lead into the jack marked "-".
3.4 Header (pointer type)
It is a high-sensitivity magnetoelectric DC ammeter, and the main performance indicators of the multimeter basically depend on the performance of the meter head. The sensitivity of the meter head refers to the DC current value flowing through the meter head when the pointer of the meter head is deflected at full scale. The smaller the value, the higher the sensitivity of the meter head. The greater the internal resistance when measuring voltage, the better its performance. There are four scale lines on the meter head, and their functions are as follows: the first line (from top to bottom) is marked with R or Ω, which indicates the resistance value. When the switch is in the ohm block, read this scale line. The second bar is marked with ∽ and VA, indicating the AC, DC voltage and DC current value. When the transfer switch is in the AC, DC voltage or DC current gear, and the range is in a position other than AC 10V, read this scale Wire. The third line is marked with 10V, which indicates the AC voltage value of 10V. When the switch is in the AC and DC voltage range and the range is at AC 10V, read this scale line. The fourth bar, labeled dB, indicates the audio level.
3.5 Header (digital)
The head of a digital multimeter is generally composed of an A/D (analog/digital) conversion chip + peripheral components + liquid crystal display. The accuracy of the multimeter is affected by the head. The number converted by the A/D chip is generally also called For 3 1/2 digit multimeter, 4 1/2 digit multimeter and so on. The most commonly used chips are ICL7106 (3 and a half LCD manual range classic chip, the subsequent version is 7106A, 7106B, 7206, 7240, etc.), ICL7129 (4 and a half LCD manual range classic chip), ICL7107 (3 and a half LED manual range Classic chip).
3.6 Measuring circuit
The measurement circuit is a circuit used to convert various measured objects into a small DC current suitable for meter measurement. It is composed of resistors, semiconductor components and batteries.
It can convert various measured objects (such as current, voltage, resistance, etc.) and different ranges into a certain amount of tiny DC current through a series of processing (such as rectification, shunting, voltage division, etc.) gauge to measure.
3.7 Transfer switch
Its function is to select a variety of different measurement lines to meet the measurement requirements of different types and ranges. The transfer switch is generally a circular dial with function and range marked around it.
4. How the multimeter works
The basic principle of the multimeter is to use a sensitive magnetoelectric DC ammeter (microampere meter) 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 a large current, so some resistors must be connected in parallel or in series on the meter head to shunt or lower the voltage, so as to measure the current, voltage and resistance in the circuit.
5. Design principle of multimeter
The measurement process of the digital multimeter converts the measured value into a DC voltage signal by the conversion circuit, and then converts the voltage analog quantity into a digital quantity by the analog/digital (A/D) converter, then counts through the electronic counter, and finally uses the digital measurement result displayed directly on the display.
The function of the multimeter to measure voltage, current and resistance is realized through the conversion circuit part, and 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 A/D converter of the digital DC voltmeter converts the analog voltage quantity that changes continuously with time into a digital quantity, and then the digital quantity is counted by the electronic counter to obtain the measurement result, and then the measurement result is displayed by the decoding display circuit. The logic control circuit controls the coordinated work of the circuit, and completes the entire measurement process in sequence under the action of the clock.
6. Digital Multimeter
Digital multimeter is currently the most commonly used digital instrument. Its main features 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 multimeters have been rapidly popularized and widely used in my country, and have become necessary instruments for modern electronic measurement and maintenance work, and are gradually replacing traditional analog (ie, pointer) multimeters.
Digital multimeters are also known as digital multimeters (DMMs), and there are many types and models. Every electronic worker hopes to have an ideal digital multimeter. There are many principles for choosing a digital multimeter, and sometimes they even vary from person to person. However, for a handheld (pocket) digital multimeter, it should generally have the following characteristics: clear display, high accuracy, strong resolution, wide test range, complete test functions, strong anti-interference ability, relatively complete protection circuit, and beautiful appearance , generous, easy to operate, flexible, good reliability, low power consumption, easy to carry, moderate price and so on.
6.1 The main indicators, display digits and display characteristics of the digital multimeter
The display digits of a digital multimeter are usually 3 1/2 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 numbers from 0 to 9 are integer digits; The numerator is the numerator, and the count value is 2000 when the full scale is used, which indicates that the instrument has 3 integer digits, and 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 ", the highest bit can only display 0 or 1 (0 is usually not displayed). 3 2/3 digits (pronounced "three and two-thirds digit"), the highest digit of the digital multimeter can only display numbers from 0 to 2, so the maximum display value is ±2999. Under the same conditions, it is 50% higher than the limit of a 3 1/2 digit digital multimeter, which is especially valuable when measuring 380V AC voltage.
Popular digital multimeters generally belong to handheld multimeters with 3 1/2 digits display, and 4 1/2, 5 1/2 digits (less than 6 digits) digital multimeters are divided into handheld and desktop. More than 6 1/2 digits mostly belong to desktop digital multimeters.
The digital multimeter adopts advanced digital display technology, with clear and intuitive display and accurate reading. It not only ensures the objectivity of the reading, but also conforms to people's reading habits, and can shorten the reading or recording time. These advantages are not available in traditional analog (ie pointer) multimeters.
6.2 Accuracy (Precision)
The accuracy of a digital multimeter is a combination of systematic and random errors in the measurement results. It indicates the degree of agreement 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.
Digital multimeters are far more accurate than analog analog multimeters. The accuracy of the multimeter is a very important indicator. It reflects the quality and process capability of the multimeter. It is difficult for a multimeter with poor accuracy to express the real value, which may easily cause misjudgment in measurement.
6.3 Resolution (resolution)
The voltage value corresponding to the last digit of the digital multimeter on the lowest voltage range is called resolution, which reflects the sensitivity of the meter. The resolution of digital digital instruments increases with the increase of display digits. The highest resolution indicators that digital multimeters with different digits can achieve are different.
The resolution index of the digital multimeter can also be displayed by resolution. Resolution is the percentage of the smallest number (other than zero) that the meter can display to the largest number.
It should be pointed out that resolution and accuracy belong to two different concepts. The former characterizes the "sensitivity" of the instrument, that is, the ability to "recognize" tiny voltages; the latter reflects the "accuracy" of measurement, that is, the degree of consistency between the measurement result and the true value. There is no necessary connection between the two, so they cannot be confused, and the resolution (or resolution) should not be mistaken for similarity. Accuracy depends on the comprehensive error and quantization error of the internal A/D converter and functional converter of the instrument. From the perspective of measurement, resolution is a "virtual" indicator (which has nothing to do with measurement error), and accuracy is a "real" indicator (it determines the size of measurement error). Therefore, it is not possible to arbitrarily increase the number of display digits to improve the resolution of the instrument.
6.4 Measuring range
In a multi-function digital multimeter, different functions have their corresponding maximum and minimum values that can be measured.
6.5 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 the A/D converter. Some handheld digital multimeters use the measurement period to indicate the speed of measurement. The time required to complete a measurement process is called the measurement cycle.
There is a contradiction between the measurement rate and the accuracy index. Usually, the higher the accuracy is, the lower the measurement rate is, and it is difficult to balance the two. To solve this contradiction, you can set different display digits or set the measurement speed conversion switch in the same multimeter: add a fast measurement file, which is used for the A/D converter with a faster measurement rate; To increase the measurement rate, this method is relatively common and can meet the needs of different users for the measurement rate.
6.6 Input Impedance
When measuring voltage, the instrument should have a high input impedance, so that the current drawn from the circuit under test is very small during the measurement process, which will not affect the working status of the circuit under test or the signal source, and can reduce measurement errors.
When measuring current, the instrument should have a very low input impedance, so that the influence of the instrument on the circuit under test can be reduced as much as possible after being connected to the circuit under test. Burn out the meter, please pay attention when using it.
6.7 Classification of digital multimeters
Digital multimeters are classified according to the range conversion method and can be divided into three types: manual range (MAN RANGZ), automatic range (AUTO RANGZ), and automatic/manual range (AUTO/MAN RANGZ).
According to different functions, uses and prices, digital multimeters can be roughly divided into 9 categories: low-end digital multimeters (also known as popular digital multimeters), mid-range digital multimeters, medium/high-end digital multimeters, digital/analog hybrid instruments, digital Instrument with dual display of /analog diagram, multi-purpose oscilloscope (integrating digital multi-meter, digital storage oscilloscope and other kinetic energy into one body).
6.8 Test function of digital multimeter
The digital multimeter can not only measure 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), can also measure capacitance (C), conductance (ns), temperature (T), frequency (f), and added a buzzer file (BZ) for checking the continuity of the line, low power method to measure resistance file (L0Ω). Some instruments also have inductance gear, signal gear, AC/DC automatic conversion function, and capacitance gear automatic range conversion function.
Most digital multimeters have added the following novel and practical test functions: reading hold (HOLD), logic test (LOGIC), true effective value (TRMS), relative value measurement (RELΔ), automatic shutdown (AUTO OFF POWER), etc.
6.9 Anti-interference ability of digital multimeter
Simple digital multimeters generally adopt the principle of integral A/D conversion. As long as the positive integration time is selected to be exactly equal to the integral multiple of the period of the serial interference signal, the serial interference can be effectively suppressed. This is because the cross-frame interference signal is averaged out in the forward integration stage. The common frame rejection ratio (CMRR) of middle and low-end digital multimeters can reach 86-120dB.
6.10 Development trend of digital multimeter
Integration: The handheld digital multimeter uses a single-chip A/D converter, and the peripheral circuit is relatively simple, requiring only a few auxiliary chips and components. With the advent of dedicated chips for single-chip digital multimeters, a fully functional automatic range digital multimeter can be formed using a single IC, which creates favorable conditions for simplifying design and reducing costs.
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.
6.11 Comparison of the advantages and disadvantages of ordinary multimeters and digital multimeters:
Both analog and digital multimeters have advantages and disadvantages.
The pointer multimeter is an average meter, which has an intuitive and vivid reading indication. (The general reading value is closely related to the swing angle of the pointer, so it is very intuitive).
A digital multimeter is an instantaneous meter. It uses 0.3 seconds to take a sample to display the measurement results, sometimes the results of each sampling are very similar, not exactly the same, which is not as convenient as the pointer type for reading the results. The pointer multimeter generally does not have an amplifier inside, so the internal resistance is small.
Due to the internal use of the operational amplifier circuit in the digital multimeter, the internal resistance can be made very large, often 1M ohms or greater. (ie higher sensitivity can be obtained). This makes the impact on the circuit under test can be smaller, and the measurement accuracy is higher.
Due to the small internal resistance of the pointer multimeter, discrete components are often used to form a shunt and voltage divider circuit. Therefore, the frequency characteristics are uneven (compared to the digital type), and the frequency characteristics of the digital multimeter are relatively better. The internal structure of the pointer multimeter is simple, so the cost is lower, the function is less, the maintenance is simple, and the overcurrent and overvoltage ability is strong.
The digital multimeter uses a variety of oscillation, amplification, frequency division protection and other circuits inside, so it has many functions. For example, you can measure temperature, frequency (in a lower range), capacitance, inductance, make a signal generator, and so on.
Since the internal structure of the digital multimeter uses integrated circuits, the overload capacity is poor, and it is generally not easy to repair after damage. DMMs have low output voltages (usually no more than 1 volt). It is inconvenient to test some components with special voltage characteristics (such as thyristors, light-emitting diodes, etc.). The pointer multimeter has a higher output voltage. The current is also large, and it is convenient to test thyristors, light-emitting diodes, etc.
A pointer multimeter should be used for beginners, and two meters should be used for non-beginners.
7. Selection principle of multimeter
7.1 The reading accuracy of the pointer table is poor, but the process of the pointer swing is more intuitive, and its swing speed amplitude can sometimes reflect the size of the measured objectively (such as the slight jitter of the TV data bus (SDL) when transmitting data ); the reading of the digital meter is intuitive, but the process of digital change looks messy and not easy to watch.
7.2 Generally, there are two batteries in the pointer meter, one is low-voltage 1.5V, the other is high-voltage 9V or 15V, and the black test lead is the positive terminal relative to the red test lead. Digital meters usually use a 6V or 9V battery. In the resistance mode, the output current of the test pen of the pointer meter is much larger than that of the digital meter. The loudspeaker can make a loud "da" sound with the R×1Ω gear, and the light-emitting diode (LED) can even be lit with the R×10kΩ gear.
7.3 In the voltage range, the internal resistance of the pointer meter is relatively small compared with the digital meter, and the measurement accuracy is relatively poor. Some occasions with high voltage and micro current cannot even be measured accurately, because its internal resistance will affect the circuit under test (for example, when measuring the acceleration stage voltage of a TV picture tube, the measured value will be much lower than the actual value). The internal resistance of the voltage range of the digital meter is very large, at least in the megohm level, and has little effect on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the measured data may be false in some occasions with strong electromagnetic interference.
7.4 In short, pointer meters are suitable for the measurement of analog circuits with relatively high current and high voltage, such as TV sets and audio amplifiers. It is suitable for digital meters in the measurement of low-voltage and low-current digital circuits, such as BP machines, mobile phones, etc. It is not absolute, and pointer tables and digital tables can be selected according to the situation.
8. Operating procedures of the multimeter
8.1 Before use, you should be familiar with the functions of the multimeter, and correctly select the gear, range and test lead jack according to the object to be measured.
8.2 When the size of the measured data is unknown, the range switch should be set to the maximum value first, and then switch from the large range to the small range, so that the indicator pointer of the instrument is above 1/2 of the full scale.
8.3 When measuring resistance, after selecting the appropriate magnification, touch the two test leads so that the pointer points to the zero position. If the pointer deviates from the zero position, adjust the "zero adjustment" knob to make the pointer return to zero to ensure accurate measurement results. If it cannot be adjusted to zero or the digital display meter sends out a low voltage alarm, it should be checked in time.
8.4 When measuring the resistance of a certain circuit, the power supply of the circuit under test must be cut off, and live measurement is not allowed.
8.5 When using a multimeter to measure, pay attention to the safety of the person and the instrument. Do not touch the metal part of the test pen with your hands during the test. It is not allowed to switch the gear switch with power on to ensure accurate measurement and avoid accidents such as electric shock and burning the instrument.
9. Precautions for using the multimeter
9.1 Before using the multimeter, perform "mechanical zero adjustment", that is, make the pointer of the multimeter point to the position of zero voltage or zero current when there is no electricity to be measured.
9.2 In the process of using the multimeter, do not touch the metal part of the test lead with your hands, so that on the one hand, the accuracy of the measurement can be guaranteed, and on the other hand, personal safety can also be guaranteed.
9.3 When measuring a certain amount of electricity, it is not possible to change gears while measuring, especially when measuring high voltage or large current, more attention should be paid. Otherwise, the multimeter will be destroyed. If you need to change gears, you should disconnect the test leads first, and then measure after changing gears.
9.4 When using the multimeter, it must be placed horizontally to avoid errors. At the same time, attention should be paid to avoid the influence of the external magnetic field on the multimeter.
9.5 After using the multimeter, the changeover switch should be placed at the maximum level of AC voltage. 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.
10. Troubleshooting the multimeter
Digital multimeter, also known as multimeter, multimeter, multimeter or three-purpose meter, is a multi-purpose electronic measuring instrument, generally including ammeter, voltmeter, ohmmeter and other functions. Compared with the pointer multimeter, the digital multimeter has the advantages of high precision, fast speed, large input impedance, digital display, accurate reading, strong anti-interference ability, and high degree of measurement automation, so it is widely used. However, if used improperly, it is easy to cause failure.
Digital multimeter troubleshooting should generally start with the power supply. Digital multimeter troubleshooting can be roughly carried out as follows.
10.1 Visual inspection.
You can touch the battery, resistors, transistors, and integrated blocks to see if the temperature rise is too high. If the newly installed battery heats up, the circuit may be short-circuited. In addition, the circuit should also be observed for disconnection, desoldering, mechanical damage, etc.
10.2 Waveform Analysis.
Use an electronic oscilloscope to observe the voltage waveform, amplitude, period (frequency), etc. of each key point of the circuit. For example, if the clock oscillator is oscillating, if there is no output from the oscillator, it means that the internal inverter is damaged, or the external components may be open.
10.3 Measuring element parameters.
For components within the fault range, conduct online or offline measurements, and analyze parameter values. When measuring resistance online, the influence of components connected in parallel with it should be considered.
10.4 Hidden Troubleshooting.
Hidden faults refer to faults that appear and disappear from time to time, and the instrument is good and bad. This kind of failure is more complicated, and the common reasons include weak welding of solder joints, looseness, looseness of connectors, poor contact of transfer switches, unstable performance of components, and continuous breakage of 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.
10.5 Check the working voltage at all levels.
Detect the working voltage of each point and compare it with the normal value. First, ensure the accuracy of the reference voltage. It is best to use a digital multimeter of the same model or similar to measure and compare.
In addition to the above possible reasons, the damage of the digital multimeter may also be caused by the wrong measurement position. For example, when measuring the AC mains, the measurement position is selected to be placed in the electric block. In this case, once the test pen touches the mains, it will instantly Can cause damage to the internal components of the multimeter. Therefore, before using the multimeter to measure, be sure to check whether the measurement gear is correct. After use, set the measurement option to AC 750V or DC 1000V, so that no matter what parameter is mismeasured in the next measurement, it will not cause damage to the digital multimeter.