What is a Digital Multimeter?

Digital multimeter, a multi-purpose electronic measuring instrument, generally includes functions such as ammeter, voltmeter, ohmmeter, etc., sometimes also called multimeter, multimeter, multimeter, or tri-meter.

Digital multimeter

Digital multimeter, a multipurpose

Resolution refers to the measurement results of a table. 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, then when measuring a 1V signal, you can see a small change of 1mV (1/1000 volts).

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

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

A 3- and a half-digit table can display three all-digit digits from 0 to 9, and one and a half digits (only 1 or not). A three and a half digit table can reach a resolution of 1999 words. A four and a half digit table can reach a resolution of 19,999 words.

It is better to describe the digital table with words than with bits. The resolution of 3 and a half digital tables has been increased to 3200 or 4000 words.

The 3200-word digital meter provides better resolution for some measurements. For example, for a 1999-word meter, you cannot display 0.1V when measuring voltages greater than 200V. The 3200-digit digital meter can still display 0.1V when measuring 320V. When the measured voltage is higher than 320V and the resolution of 0.1V is required, a more expensive 20,000-word digital meter is used.

Accuracy refers to the maximum allowable error that occurs in a specific use environment. In other words, precision is used to indicate

How close the DMM's measured value is to the actual value of the signal being measured.

For multimeters, accuracy is usually expressed as a percentage of the reading . For example, the meaning of 1% reading accuracy is: when the DMM display is 100.0V, the actual voltage may be between 99.0V and 101.0V.

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

The accuracy of the analog meter is calculated based on the full-scale error, not the displayed reading. The typical accuracy of an analog meter is ± 2% or ± 3% of full scale. The typical basic accuracy of a DMM is between ± (0.7% + 1) and ± (0.1% + 1) of the reading, or even higher.

In terms of accuracy and resolution, the digital display has great advantages. The measured value can be displayed in three or more digits.

Analog pointers are slightly inferior in accuracy and resolution. Because you have to estimate the position of the pointer.

The bar graph displays analog signal changes and trends. But it is more durable and reduces damage.

Measure resistance at electrical blocking. Resistance values vary widely, from a few milliohms (m) to a contact resistance of billions of ohms of insulation resistance. Many digital multimeters measure resistance as small as 0.1 ohms, and some measurements can be as high as 300 megohms (300,000 ohms). Extreme resistance, Fluke multimeter will display "OL", which means that the measured resistance is larger than the range. When the measurement is open, "OL" is displayed.

The resistance must be measured with the circuit power off, otherwise the meter or circuit board will be damaged. Some DMMs provide a protection function when a voltage signal is mistakenly connected in the resistance mode. Different types of DMMs have different protection capabilities.

When making accurate low-resistance measurements, the resistance of the measurement lead must be subtracted from the measurement. Typical test leads have a resistance between 0.2 and 0.5. If the resistance of the test lead is greater than 1, the test lead must be replaced.

If the DMM provides a DC voltage of less than 0.6V for measuring resistance, it can measure the resistance value of the circuit board by a diode or semiconductor. So you can test without removing the resistor.

On-off is to distinguish between a circuit or a short circuit by a fast resistance measurement.

The digital multimeter with on / off beep makes the on / off measurement easier and faster. When a short circuit is detected, the meter beeps, so there is no need to look at the meter during the test. Different types of DMMs have different trigger resistance values.

A diode is like an electronic switch. If the voltage is higher than a certain value, the diode is turned on. The silicon diode's on-voltage is typically 0.6V. And diodes only allow current to flow in one direction.

When checking diodes or junctions, the multimeter will not only give a wide range of readings but also drive currents greater than 50mA. (See Table 1)

When measuring the resistance of a circuit containing a diode, the test voltage of the DMM will be lower than 0.6V to prevent the junction from conducting.

When selecting a diode test, the test voltage is increased to check the function of the diode or semiconductor 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 junction in the forward test should be lower than 0.7V, and the circuit is open during the reverse test.

How to test resistance

1.Turn off the circuit power

2.Select electrical blocking

3. Insert the black test lead into the COM jack. Plug the red test lead into the resistance test jack

4.Connect the test probe probe to both ends of the component or circuit under test.

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

Note: 1,000 = 1K; 1,000,000 = 1M

It is important to note that the power is turned off when testing the resistance.

Measuring current

Current measurement is not the same as measuring other quantities with a digital multimeter. The direct current measurement method is to directly connect the digital multimeter to the circuit under test, so that the current of the circuit under test flows directly to the internal circuit of the meter. The indirect measurement method does not need to open the circuit and string the multimeter to the circuit under test. Indirect methods use current clamps.

Direct current measurement

1. Turn off the circuit power

2. Disconnect or unsolder the circuit so that the meter can be connected to the circuit.

3. Select the corresponding AC (A ~), DC (A--) gear

4. Insert the black test lead into the COM socket, and the red test lead into the 10 Amp jack (10A) or the 300 mA jack (300 mA). The choice of which jack is based on the possible measurements.

5. Connect the test leads in series to the disconnected circuit part.

6. Turn on the circuit power

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 place a test lead in a current jack while trying to test the voltage. A small value in the DMM can cause a short circuit in the voltage source. A large current flows through the digital multimeter. If the multimeter does not have sufficient protection, it will not only damage the meter and the circuit, but also hurt the operator. If it is a high-voltage circuit (480 volts or higher), it will cause greater danger.

Therefore, the DMM should have a sufficiently large current input to protect the fuse. Meters without a current input fuse cannot be used in high energy circuits (> 240V ac). Digital multimeters using fuses,

The multimeter's function of measuring voltage, current and resistance is realized through the conversion circuit part, and the current and resistance measurement are based on the measurement of voltage, which means that the digital multimeter is extended based on the digital DC voltage meter. The converter converts the analog voltage quantity that continuously changes with time into a digital quantity, and then the electronic counter counts the digital quantity to obtain a measurement result, and then the decoding display circuit displays the measurement result. 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.

1. Basic functions

The measurement of current, voltage, and resistance is generally considered the basic function of a multimeter. The brand of the early multimeter manufacturer AVO is the abbreviation of the names of these three units of measurement that the device can measure: Ampere, Volt,

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

DC voltage accuracy ± (0.1% + 8)

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

AC voltage accuracy ± (0.8% + 40)

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

DC current accuracy ± (0.2% + 20)

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

AC current accuracy ± (1% + 15)

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

Resistance accuracy ± (0.5% + 20)

Capacitance range 20nF / 200nF / 2F / 20F / 200F / 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 beep

Really effective plant measurement ·

relative value·

Maximum display 20000

Standard test leads, battery, manual, cloth bag, crocodile clip, warranty card, USB interface transmission cable, software CD, test cable

Digital multimeters have slowly developed through history. Earlier multimeters, using dials with magnet deflected hands, were the same as classic galvanometers; modern ones used LCD or VFD (vacuum fluorescent display) digital displays.

Analog multimeters are not difficult to find in the second-hand market, but they are not very accurate, because zeroing and accurate readings from the instrument panel are prone to deviations.

Some analog multimeters use vacuum tubes to amplify the input signal. This design multimeter is also called a vacuum tube voltmeter (VTVM, Vacuum Tube Volt Meters) or a vacuum tube multimeter (VTMM, Vacuum Tube Multimeters).

Modern multimeters have all been digitized and are known as digital multimeters (DMM, Digital MultiMeter). In this device, the measured signal is converted into a digital voltage and amplified by a digital preamplifier, and then the value is displayed directly on the digital display; this avoids deviations caused by parallax when reading.

Similarly, better circuit systems and electronics also improve measurement accuracy. The basic accuracy of the old analog instruments is between 5% and 10%. Modern portable digital multimeters can reach ± 0.025%, and the workbench equipment has an accuracy of one millionth.

Digital multimeters are relatively simple measuring instruments. In this article, the author teaches you the correct use of digital multimeters. Starting from the measurement methods of voltage, resistance, current, diode, triode, MOS field effect tube of the digital multimeter, let you better master the multimeter measurement method.

Before using it, you should read the relevant instruction manual carefully and be familiar with the functions of the power switch, range switch, jack, and special socket.

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 is not showing, follow the steps below.

The symbol next to the test pen jack indicates that the input voltage or current should not exceed the indicated value. This is to protect the internal circuit from damage.

(3) Before the test. The function switch should be set to the range you need.

First, the measurement of voltage

One of the most basic functions of a DMM is measuring voltage. Testing voltage is usually the first step in solving circuit problems. If there is no voltage or the voltage is too low or too high, the power problem must be resolved before further inspection.

The meter is equipped with an automatic power cut-off circuit. When the meter works for about 30 minutes to 1 hour, the power is automatically cut off and the meter goes to sleep. At this time, the meter consumes about 7A of current.

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

1, pointer table

The reading accuracy of the pointer table is poor, but the process of pointer swing is relatively intuitive, and the amplitude of the swing speed can sometimes objectively reflect the measured size (such as measuring the slight jitter of the TV data bus (SDL) when transmitting data) ); The digital meter reading is intuitive, but the process of digital changes looks messy and not easy to watch.

In the pointer table, there are generally two batteries, one with a low voltage of 1.5V and one with a high voltage of 9V or 15V. The black test lead is the positive end compared to the red test lead. Digital meters usually use a 6V or 9V battery. In the resistance range, the output current of the test lead of the pointer meter is much larger than that of the digital meter. Using the R × 1 range can make the speaker make a loud click sound, and the R × 10k range can even light up the light-emitting diode (LED).

In the voltage range, the internal resistance of the pointer meter is relatively small compared to the digital meter, and the measurement accuracy is relatively poor. Some high-voltage micro-current occasions cannot even be accurate, because its internal resistance will affect the circuit under test (for example, the measured value will be much lower than the actual value when measuring the acceleration voltage of the TV picture tube). The internal resistance of the voltage meter of the digital meter is very large, at least in the megohm level, and it has little effect on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage. The measured data in some places with strong electromagnetic interference may be false.

2, measurement skills

1. Measuring speakers, headphones, dynamic microphones:

With R × 1, one test lead is connected to one end, and the other test lead touches the other end, and a click sound will be emitted when normal. If it does not sound, the coil is broken. If the sound is small and sharp, there is a problem of rubbing the circle, and it cannot be used.

2. Measuring capacitance:

Use the resistance file to select the appropriate range according to the capacitance, and pay attention to the positive electrode of the capacitor for the black test lead of the electrolytic capacitor when measuring.

Estimation of the capacitance of the microwave method grade capacitor: It can be determined based on the maximum amplitude of the pointer swing based on experience or referring to a standard capacitor of the same capacity. The referenced capacitors do not have to withstand the same voltage, as long as they have the same capacity. For example, a 100F / 250V capacitor can be estimated by referring to a 100F / 25V capacitor. As long as their pointers have the same maximum swing amplitude, it can be concluded that the capacity is the same.

Estimating the capacity of picofarad capacitors: R × 10k is used, but only capacitors above 1000pF can be measured. For a capacitor of 1000pF or a little larger, as long as the hand of the meter slightly swings, the capacity can be considered sufficient.

Test whether the capacitor has leakage: For capacitors over one thousand microfarads, you can quickly charge it with R × 10, and initially estimate the capacitance, and then change to R × 1k to continue measuring for a while. Should return, but should stop at or very close to , otherwise there is a leakage phenomenon. For some timing or oscillation capacitors below tens of microfarads (such as the oscillation capacitors of color TV switching power supplies), the leakage characteristics are very high. As long as there is a slight leakage, it cannot be used. At this time, it can be charged in the R × 1k file. Then use R × 10k to continue the measurement. Similarly, the hands should stop at instead of returning.

3. Good or bad diodes, triodes and voltage regulators during road test:

Because in the actual circuit, the bias resistance of the triode or the surrounding resistance of the diode and the zener are generally large, most of which are several hundred or thousands of ohms. In this way, we can use the R × 10 or R × 1 range of the multimeter. Come and measure the quality of the PN junction on the road. When measuring the road, the PN junction measured with R × 10 should have obvious forward and reverse characteristics (if the difference between forward and reverse resistance is not obvious, you can use R × 1 to measure). Generally, the forward resistance is at R The hands in the × 10 range should be indicated at about 200, and the hands in the R × 1 range should be indicated at around 30 (may differ slightly according to different types). If the forward resistance value is too large or the reverse resistance value is too small, it indicates that there is a problem with the PN junction, and the tube is also defective. This method is especially effective for maintenance. It can find bad tubes very quickly, and can even detect tubes that have not completely broken but have deteriorated characteristics. For example, when you use a small resistance file to measure the forward resistance of a certain PN junction, if you solder it down and use a common R × 1k file to measure it, it may still be normal. In fact, the characteristics of this tube have deteriorated. Not working or unstable.

4, measuring resistance:

It is important to select the range so that the reading is the most accurate. It should be noted that when measuring a large resistance value of megohm level with R × 10k resistance file, do not pinch your fingers at both ends of the resistance, so the resistance of the human body will make the measurement result smaller.

5, test voltage regulator diode:

The stabilizing value of the voltage regulator we usually use is generally greater than 1.5V, and the resistance file below R × 1k of the pointer meter is powered by the 1.5V battery in the table. In this way, the resistance file below R × 1k is used. Just like measuring diodes, measuring zener diodes have complete unidirectional conductivity. However, the R × 10k range of the pointer meter is powered by a 9V or 15V battery. When R × 10k is used to measure a voltage regulator with a regulated voltage less than 9V or 15V, the reverse resistance value will not be , but there will be Resistance value, but this resistance value is still much higher than the forward resistance value of the voltage regulator. In this way, we can initially estimate the quality of the voltage regulator. However, a good voltage regulator also needs to have an accurate voltage regulation value. How to estimate this voltage regulation value in amateur conditions? It's not difficult, just find another pointer table. The method is: first put a watch in R × 10k range, and its black and red test leads are respectively connected to the cathode and anode of the zener tube. At this time, the actual working state of the zener tube is simulated, and then another table is placed in At the voltage level V × 10V or V × 50V (depending on the regulated value), connect the red and black test leads to the black and red test leads of the watch just now. The voltage value measured at this time is basically this. The voltage regulation value of the zener tube. "Basic" is because the bias current of the first meter to the zener tube is slightly smaller than the bias current in normal use, so the measured voltage value will be slightly larger, but the basic difference is not large. . This method can only estimate voltage regulators whose voltage regulation value is less than the high voltage battery voltage of the pointer meter. If the voltage stabilizing value of the Zener tube is too high, it can only be measured by the external power supply method (in this way, when we choose the pointer meter, it is more suitable to use a high voltage battery with a voltage of 15V than 9V).

6. Test the triode:

Usually we want to use R × 1k file, no matter it is NPN tube or PNP tube, no matter it is small power, middle power, high power tube, its be junction cb junction should show the same unidirectional conductivity as the diode, reverse The resistance is infinite, and its forward resistance is about 10K. In order to further evaluate the quality of the pipe, if necessary, the resistance level should be changed for multiple measurements. The method is: set the R × 10 level to measure the forward resistance of the PN junction at about 200; set the R × 1 level to measure The forward resistance of the PN junction is about 30 ohms. (The above is the data measured by the 47 type meter. The other types of meters are roughly different. You can test a few more tubes and summarize them to make sure that you have a heart.) If the reading is too large Too much, it can be concluded that the characteristics of the pipe are not good. The meter can also be tested at R × 10k, and the tube with a lower withstand voltage (basically the withstand voltage of the triode is above 30V), its cb junction reverse resistance should also be , but its be junction reverse resistance There may be some, the hand will be slightly deflected (generally does not exceed 1/3 of full scale, depending on the pressure resistance of the tube). However, when measuring the resistance between ce or ec with R × 1k or lower, the indicator on the meter should be infinite, otherwise the tube will be defective. It should be noted that the above measurements are for silicon tubes and not applicable for germanium tubes. In addition, the "reverse direction" refers to the PN junction, and the direction of the NPN tube and the PNP tube is actually different.

To avoid electric shock and personal injury, please read the "Safety Information" and "Warning and Caution Points" in the instruction manual before use. The following specifications are general and general specifications:

a. If the meter is damaged, do not use it. Before using the meter, check the housing and especially the insulation next to the terminals.

b. Check the test leads for damaged insulation or bare metal; check the continuity of the test leads; replace the damaged test leads before use.

c. Do not use the meter after abnormal use. The protection circuit may fail. When in doubt, send the meter for repair.

d. Do not place in explosive gas,

1. Maintenance

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

Do not connect DC voltage higher than 1000V or AC RMS voltage higher than 700V.

Do not connect the voltage source when the function switch is in and position.

Please do not use this watch when the battery is not installed or the back cover is not tight.

2. Repair method

Digital multimeters have high sensitivity and accuracy, and their applications are found in almost all enterprises. However, due to the multiple factors of its failure, and the randomness of the problems encountered, there are not many rules to follow. Some of the repair experience accumulated in the work experience are sorted out for reference by colleagues in this profession.

Finding faults should be done first, then first, then first, and then more difficultly. The methods can be roughly divided into the following:

The sensory method uses the senses to directly judge the cause of the failure. Through visual inspection, it can be found such as disconnection, desoldering, short-circuiting of the wire, broken fuse tube, burned out components, mechanical damage, and copper foil on the printed circuit. And breaks; you can touch the battery, resistor, transistor,

A digital multimeter is the most basic testing tool for electronic enthusiasts. A good digital multimeter can not only measure various electrical parameters, but also is a powerful assistant for electronic enthusiasts to troubleshoot and repair electrical appliances. There are many types of digital multimeter products to choose from. How to choose a digital multimeter that is both economical and meets your needs?

Start from your own actual needs

The classification of digital multimeters is very complicated: according to their structure, they can be divided into desktop digital multimeters and handheld digital multimeters. According to whether they have a microprocessor, they can be divided into traditional A / D conversion digital multimeters and intelligent digital multimeters. According to the number of digits, it can be divided into 3 and a half digital multimeters, 4 and a half digital multimeters, etc.

The accuracy, structure, brand, and intelligence of digital multimeters determine the price difference. The digital multimeters currently on the market have a price range from a dozen dollars to tens of thousands of dollars. Therefore, what do you buy a digital multimeter for? What you need to know before choosing a product: If it is just a general test and the measurement accuracy is not high, then choose a cheap handheld digital multimeter. Enough; if you are engaged in precision measurement, repair, or high stability of measurement data, you should choose a high-end handheld digital multimeter or desktop digital multimeter.

Things to note when buying

A, accuracy

The accuracy of the digital multimeter directly affects the accuracy of the measurement results. When you get a digital multimeter, you should first analyze its accuracy technical indicators: one is to see if the accuracy of the table can meet its own requirements, and the other is to the overall of the table For performance analysis, do not think that a 4-digit multimeter must have higher measurement accuracy than a 3-digit half-digit multimeter. The rough measurement of a 4-digit half-digit multimeter may also catch up with many well-crafted 3-digit half-digit multimeters. The owner has encountered such a table: a DT9103 4-digit digital multimeter generated by an unknown small factory. During the measurement process, the number jumps very much, and it can't meet the technical indicators.

B. matters needing attention

1.Appearance

A good digital multimeter can become a pleasing handicraft. No matter from materials, injection molding processes, circuit boards, protective sleeves and other components, it should be as perfect as possible.

2. Inspection items

After getting a digital multimeter:

First of all, you should check whether the case is intact, whether the switches and buttons are functioning properly and feel good.

Whether the toggle switch (measurement function selection) is flexible in rotation, suitable in damping, comfortable in hand, and whether the corresponding measurement function is normal.

If the LCD screen is clear and there are no strokes, the digital 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 circuit board design, selected components, installation and soldering level, etc. A good digital multimeter, the circuit board design is very important. Domestic watches are generally double-layer boards, and foreign countries Some large manufacturers' watches use multilayer circuit board layouts to provide better shielding and electrical performance. The level of selected components determines the measurement accuracy of digital multimeters. High-end digital multimeters often use chip components, which improves the accuracy and reduces the size of the circuit board. Components in key parts are even screened. There is not much to say about the welding level, and electronics enthusiasts know that (when it comes to welding level, you can open a digital multimeter with more than ten yuan to see it, the product guarantee of the hand workshop will make you feel terrible).

You should also pay attention to some details when purchasing, such as whether the security protection, product warranty, test cables, user manual, etc. are complete.

4, performance testing

When purchasing a digital multimeter, you can confirm the measurement function of the digital multimeter by measuring a known amount (such as resistance, capacitance, battery voltage, AC voltage, etc.). If possible, you can send it to the corresponding agency to check the technical indicators item by item, acceptance.