What is a digital multimeter


Digital multimeter, a multi-purpose electronic measuring instrument, generally includes the functions of ammeter, voltmeter, ohmmeter and so on. Sometimes it is also called multimeter, multi-purpose meter, multi-purpose meter or three purpose meter.


Digital multimeter is suitable for portable devices for basic fault diagnosis. There are also devices placed on the workbench, and some resolutions can reach seven or eight bits.


Digital multimeter (DMM) is an electronic instrument used in electrical measurement. It can have many special functions, but 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 measurement fields of physics, electricity, electronics and so on.


Resolution is to judge the measurement results of a meter. By knowing the resolution of a meter, you can know whether you can see small changes in the measured signal. For example, if the DMM has a resolution of 1mV in the 4V range, you can see a small change of 1mV (1 / 1000 volt) when measuring a 1V signal


If you want to measure a length less than 1 / 4 inch (or 1 mm), you certainly won't use a ruler with the smallest unit of inch (or centimeter). If the temperature is 98.6 ° F, it is useless to measure with a thermometer marked with an integer only. You need a thermometer with a resolution of 0.1 ° F.


Digits and words are used to describe the resolution of the table. Digital multimeters are classified by the number of digits and words they can display.


A 3-and-a-half digit table can display three full digits from 0 to 9 and one half digit (only 1 or no display). A 3-and-a-half bit digital watch can reach the resolution of 1999 words. A 4-and-a-half digit digital watch can reach the resolution of 19999 words.


The resolution of a digital table described by words is better than that described by bits. The resolution of a 3-bit and a half digital table has been improved to 3200 or 4000 words.


The 3200 word digital meter provides better resolution for some measurements. For example, a 1999 word meter, when measuring a voltage greater than 200V, you can't display 0.1V. The 3200 word digital meter can still display 0.1V when measuring the voltage of 320 volts. When the measured voltage is higher than 320v and to reach the resolution of 0.1V, we need to use a more expensive 20000 word digital meter.


Measuring current


Current measurement is different from measuring other quantities with a digital multimeter. The direct current measurement method is to connect the digital multimeter directly to the circuit under test, and let the current of the circuit under test flow directly through the internal circuit of the multimeter. Indirect measurement method does not need to open the circuit and connect the multimeter to the circuit under test. The indirect method uses current clamp.


direct measurement


1. Turn off the power of the circuit


2. Disconnect or disassemble the circuit in order to string the meter into the circuit


3. Select the corresponding AC (a ~) and DC (a --) gears


4. Insert the black probe into the com socket and the red probe into the 10 Amp Jack (10a) or 300 Ma Jack (300mA). The selection of that Jack is mainly based on the possible measured values.


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


6. Turn the circuit on


7. Observe the reading and pay attention to the unit.


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


Input protection


A common mistake is to put the test wire in the current Jack and try to test the voltage. A small resistance in a DMM will cause a short circuit to the voltage source. Large current flows through the digital multimeter. If the multimeter does not have sufficient protection, it will not only damage the meter and circuit, but also hurt the operator. If it is a high-voltage circuit (480 volts or higher), it will lead to greater danger.


Therefore, the DMM should have a current input protection fuse with sufficient capacity. Meters without current input fuse cannot be used in high-energy circuits (> 240V AC). The digital multimeter with fuse tube shall have enough capacity


Quantity to remove high-energy faults. The rated voltage of the fuse should be greater than the maximum voltage you expect. For example, one in a multimeter


The fuse tube of 20a and 250V cannot play a protective role when the multimeter measures the circuit of 480v. Only a 20a, 600V fuse tube can play a protective role in measuring 480v circuit with a multimeter.


Current test


Sometimes, the current value you want to measure exceeds the range of the digital multimeter or the field 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 high accuracy. The current clamp covers the wire to measure the current and converts it into a value that can be processed by the digital meter.


There are two basic current probes: the current converter type, which is only used for AC measurement. Hall effect probe, used to measure AC or DC current.


Current converter. Generally, 1 Ma is used to represent 1 ampere. The current value of 100 amps will become 100 Ma, which can be measured safely with a digital multimeter. Connect the wire to the "Ma" and "com" sockets, and turn the function selection knob to AC Ma gear.


Hall effect probe. Use 1 MV to represent the AC or DC value of 1 Amp. For example, a 100 AMP AC current is converted to a 100 mV AC voltage. 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 1 MV to represent 1 amp current.


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


Do not test the voltage when the test wire is inserted into the current hole. It may cause damage to the watch or endanger 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 probe can test high AC or DC current. It converts current into voltage signal.


The voltage display of 1mV represents 1A actual current.


working principle


The function of multimeter to measure voltage, current and resistance is realized through the conversion circuit, 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 digital DC voltmeter. The converter converts the analog voltage that changes continuously with time into digital quantity, and then the electronic counter counts the digital quantity to obtain the measurement result, and then the decoding display circuit displays the measurement result. The coordinated work of logic control circuit completes the whole measurement process in sequence under the action of clock.


Development history


Digital multimeter is slowly developed through history. The early multimeter used a dial with a magnet deflection pointer, which was the same as the classical galvanometer; Modern uses LCD or VFD.


Analog Multimeter is not difficult to find in the second-hand market, but it is not very accurate because it is prone to bias in zeroing and accurate reading from the instrument panel.


Some Analog Multimeter use vacuum tube to amplify the input signal. This kind of designed multimeter is also called vacuum tube voltmeter or vacuum tube multimeter


Modern multimeter has been fully digitized and is specially called digital multimeter. In this device, the measured signal is converted into digital voltage and amplified by digital preamplifier, and then the value is directly displayed by digital display screen; This avoids the deviation caused by parallax when reading.


Similarly, better circuit system and electronics also improve the measurement accuracy. The basic accuracy of the old analog instrument is between 5% and 10%, while the modern portable digital multimeter can reach ± 0.025%, while the accuracy of the workbench equipment is as high as one millionth.




usage method


Digital multimeter is a relatively simple measuring instrument. In this article, the author will teach you the correct use of digital multimeter. Starting from the measurement methods of voltage, resistance, current, diode, triode and MOS FET of digital multimeter, you can better master the measurement methods of multimeter.


Before use, carefully read the relevant operation instructions and be familiar with the functions of power switch, range switch, Jack and special socket


(1) set the on / off switch to the on position and check the 9V battery. If the battery voltage is insufficient, it will be displayed on the display. At this time, the battery needs to be replaced. If the display does not show, follow the steps below.


⑵ the symbol next to the test pen Jack indicates that the input voltage or current should not exceed the indicated value, which is to protect the internal circuit from damage.


(3) before the test. The function switch should be placed in the range you need.


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