The most complete method of using a multimeter is here!

　　1. The reading accuracy of the pointer meter is poor, but the process of the pointer swinging is more intuitive, and the swing speed and amplitude can sometimes objectively reflect the size of the measurement (such as measuring 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 changes looks very messy and not easy to watch.

　　2. There are usually two batteries in the pointer meter, one low voltage 1.5V, and one high voltage 9V or 15V. The black test lead is the positive terminal relative to the red test lead. A 6V or 9V battery is often used in digital meters. In the resistance range, the output current of the pointer meter lead is much larger than that of the digital meter. Using the R×1Ω range can make the speaker emit a loud "click" sound, and using the R×10kΩ range can even light up the light-emitting diode (LED).

　　3. 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. In some high-voltage and micro-current situations, it is even impossible to measure accurately because its internal resistance will affect the circuit being measured (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 digital meter voltage range is very large, at least in the megohm level, and has little effect on the circuit being measured. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the data measured in some situations with strong electromagnetic interference may be false.

　　4. In short, pointer meters are suitable for measuring analog circuits with relatively large currents and high voltages, such as televisions and audio amplifiers. Digital meters are suitable for measuring digital circuits with low voltages and small currents, such as BP machines and mobile phones. It is not absolute, and pointer meters and digital meters can be used according to the situation.

　　2. Measurement techniques (if no explanation is given, the pointer meter is used):

　　1. Test speakers, headphones, and dynamic microphones: Use the R×1Ω setting, connect one test lead to one end, and touch the other end with the other test lead. Normally, a clear and loud "click" sound will be emitted. If there is no sound, the coil is broken. If the sound is small and sharp, there is a coil rubbing problem and it cannot be used.

　　2. Measure capacitance: Use the resistance range, select the appropriate range according to the capacitance, and pay attention to the black test lead of the electrolytic capacitor to be connected to the positive electrode of the capacitor when measuring. ①. Estimation of the capacity of microwave-level capacitors: You can judge based on experience or by referring to a standard capacitor of the same capacity, based on the maximum swing of the pointer. The reference capacitor does not have to have the same withstand voltage value, as long as the capacity is the same. For example, a 100μF/250V capacitor can be estimated by using a 100μF/25V capacitor as a reference. As long as the maximum swing of their pointers is the same, it can be determined that the capacity is the same. ②. Estimation of the capacity of picofarad-level capacitors: Use the R×10kΩ range, but it can only measure capacitance above 1000pF. For capacitors of 1000pF or slightly larger, as long as the needle swings slightly, it can be considered that the capacity is sufficient. ③. Measure whether the capacitor is leaking: For capacitors above 1,000 microfarads, you can first use the R×10Ω position to quickly charge it and preliminarily estimate the capacitance, then change to the R×1kΩ position and continue measuring for a while. At this time, the pointer should not return, but should stop at or very close to ∞, otherwise there is leakage. For some timing or oscillation capacitors below tens of microfarads (such as the oscillation capacitor of the color TV switching power supply), the leakage characteristics are very high. If there is a slight leakage, it cannot be used. At this time, after charging at the R×1kΩ position, you can switch to the R×10kΩ position to continue measuring. Similarly, the needle should stop at ∞ and should not return.

　　3. Test the quality of diodes, triodes, and voltage regulators in the circuit: Because in the actual circuit, the bias resistor of the triode or the peripheral resistor of the diode and voltage regulator are generally large, mostly hundreds of thousands of ohms or more. In this way, we can use the R×10Ω or R×1Ω range of the multimeter to measure the quality of the PN junction in the circuit. When measuring in the circuit, the PN junction measured with the R×10Ω range should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistances is not obvious, the R×1Ω range can be used instead). Generally, the forward resistance should be measured at about 200Ω when the R×10Ω range is measured, and the forward resistance should be measured at about 30Ω when the R×1Ω range is measured (it may be slightly different depending on the different phenotypes). If the forward resistance value is too large or the reverse resistance value is too small, it means that there is a problem with the PN junction, and the tube is also problematic. This method is particularly effective for maintenance, and can quickly find bad tubes, and can even measure tubes that have not been completely broken but have deteriorated characteristics. For example, when you use a small resistance range to measure the forward resistance of a PN junction and find that it is too large, if you solder it off and measure it again using the commonly used R×1kΩ range, it may still be normal. In fact, the characteristics of this tube have deteriorated and it cannot work normally or is unstable.

　　4. Resistance measurement: It is important to select a good range. When the pointer indicates 1/3 to 2/3 of the full range, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistance range to measure a large resistance of megohm level, you should not pinch your fingers at both ends of the resistor, as the human body resistance will make the measurement result smaller. For common imported high-power plastic-sealed tubes, the C pole is basically in the middle (I have not seen the B pole in the middle). The B pole of some medium and small power tubes may be in the middle. For example, the commonly used 9014 transistors and other models of transistors in its series, 2SC1815, 2N5401, 2N5551 and other transistors, some of their B poles are in the middle. Of course, they also have C poles in the middle. Therefore, when repairing and replacing transistors, especially these low-power transistors, they cannot be installed directly as they are, but must be measured first.

　　Method for testing integrated circuits using only a multimeter as a testing tool

　　Editor's note: Although there are ways to replace integrated circuits, disassembly is troublesome after all. Therefore, before disassembly, you should accurately determine whether the integrated circuit is indeed damaged and the degree of damage to avoid blind disassembly. This article introduces the methods and precautions for off-circuit and on-circuit detection of integrated circuits using only a multimeter as a detection tool. The four methods of on-circuit detection described in the article (measurement of DC resistance, voltage, AC voltage and total current) are practical and commonly used detection methods in amateur maintenance. Here, we also hope that everyone can provide other practical (integrated circuit and component) identification and detection experience.

　　1. Off-Road Detection

　　This method is performed when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values ​​between each pin corresponding to the ground pin, and compared with the intact IC.

　　2. On-road detection

　　This is a method of testing the in-circuit DC resistance of each IC pin (IC in the circuit), AC and DC voltage to ground, and total working current through a multimeter. This method overcomes the limitation of the substitution test method that requires a replaceable IC and the trouble of disassembling the IC, and is the most commonly used and practical method for testing ICs.

　　1. On-line DC resistance detection method

　　This is a method of using the ohmmeter of a multimeter to directly measure the forward and reverse DC resistance values ​​of each IC pin and peripheral components on the circuit board, and compare them with normal data to find and determine the fault. Pay attention to the following three points when measuring:

　　(1) Disconnect the power supply before measuring to avoid damage to the meter and components during testing.

　　(2) The internal voltage of the resistance block of the multimeter should not be greater than 6V, and the best range is R×100 or R×1k.

　　(3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the model of the device being measured, the position of the sliding arm of the potentiometer related to the IC, etc., and also consider the quality of the peripheral circuit components.

　　2. DC working voltage measurement method

　　This is a method of measuring the DC supply voltage and the working voltage of peripheral components with the DC voltage range of the multimeter under power-on conditions; detecting the DC voltage value of each IC pin to ground and comparing it with the normal value, thereby narrowing the fault range and finding the damaged components. Pay attention to the following eight points when measuring:

　　(1) The multimeter should have a sufficiently large internal resistance, at least 10 times greater than the resistance of the circuit being measured, to avoid large measurement errors.

　　(2) Normally, turn each potentiometer to the middle position. If it is a TV, the signal source should use a standard color bar signal generator.

　　(3) Anti-slip measures should be taken for the test leads or probes. Any instantaneous short circuit can easily damage the IC. The following method can be used to prevent the test leads from slipping: Take a section of bicycle valve core and put it on the tip of the test lead, and extend it about 0.5mm beyond the tip of the test lead. This can not only make the tip of the test lead contact well with the tested point, but also effectively prevent slipping, and there will be no short circuit even if it touches the adjacent point.

　　(4) When the measured voltage of a certain pin is inconsistent with the normal value, the quality of the IC can be judged based on whether the pin voltage has a significant impact on the normal operation of the IC and the corresponding changes in other pin voltages.

　　(5) The IC pin voltage will be affected by peripheral components. When the peripheral components leak, short-circuit, open-circuit or change value, or the peripheral circuit is connected to a variable resistance potentiometer, the position of the potentiometer sliding arm will change, which will cause the pin voltage to change.

　　(6) If the voltages of all IC pins are normal, the IC is generally considered to be normal. If the voltages of some IC pins are abnormal, you should start from the point where the voltage deviates the most from the normal value and check whether there are any faults in the peripheral components. If there are no faults, the IC is likely to be damaged.

　　(7) For dynamic receiving devices, such as televisions, the voltages of the IC pins are different when there is a signal or not. If it is found that the pin voltages that should not change change greatly, and the pin voltages that should change with the signal size and the position of the adjustable element do not change, it can be determined that the IC is damaged.

　　(8) For devices with multiple working modes, such as video recorders, the voltages on each IC pin are different in different working modes.

　　3. AC working voltage measurement method

　　In order to understand the changes of IC AC signals, you can use a multimeter with a dB jack to measure the AC working voltage of the IC approximately. When testing, the multimeter is set to the AC voltage range, and the positive probe is inserted into the dB jack; for multimeters without dB jacks, a 0.1-0.5μF DC blocking capacitor needs to be connected in series with the positive probe. This method is suitable for ICs with relatively low operating frequencies, such as the video amplifier stage and field scanning circuit of a TV. Since these circuits have different inherent frequencies and waveforms, the measured data are approximate values ​​and can only be used for reference.

　　4. Total current measurement method

　　This method is to determine whether the IC is good or bad by detecting the total current of the IC power supply line. Since most of the ICs are directly coupled, when the IC is damaged (such as a PN junction breakdown or open circuit), it will cause the subsequent stage to saturate and cut off, causing the total current to change. Therefore, the quality of the IC can be determined by measuring the total current. It is also possible to measure the voltage drop of the resistor in the power supply path and calculate the total current value using Ohm's law.

　　The above detection methods each have their advantages and disadvantages. In practical applications, it is best to combine various methods and use them flexibly.

　　How to detect thyristor with a multimeter

　　There are two types of thyristors: unidirectional thyristors and bidirectional thyristors, both of which have three electrodes. Unidirectional thyristors have a cathode (K), an anode (A), and a control electrode (G). Bidirectional thyristors are equivalent to two unidirectional thyristors connected in reverse parallel. That is, the anode of one unidirectional silicon is connected to the cathode of the other, and its lead end is called the T2 electrode, and the cathode of one unidirectional silicon is connected to the anode of the other, and its lead end is called the T2 electrode, and the rest is the control electrode (G).

　　1. Distinguishing between unidirectional and bidirectional thyristors: First, measure any two poles. If the pointers for both the forward and reverse measurements do not move (R×1), it may be A, K or G, A pole (for unidirectional thyristors) or T2, T1 or T2, G pole (for bidirectional thyristors). If one of the measurements indicates a few dozen to a few hundred ohms, it must be a unidirectional thyristor. The red pen is connected to the K pole, the black pen is connected to the G pole, and the remaining is the A pole. If the forward and reverse measurement indications are both a few dozen to a few hundred ohms, it must be a bidirectional thyristor. Then turn the knob to R×1 or R×10 and re-test. There must be a slightly larger resistance value. The slightly larger red pen is connected to the G pole, the black pen is connected to the T1 pole, and the remaining is the T2 pole.

　　2. Difference in performance: Turn the knob to R×1. For 1~6A unidirectional thyristor, connect the red pen to the K pole, and the black pen to the G and A poles at the same time. Disconnect the G pole while keeping the black pen attached to the A pole. The pointer should indicate several tens of ohms to one hundred ohms. At this time, the thyristor has been triggered, and the trigger voltage is low (or the trigger current is small). Then disconnect the A pole instantaneously and connect it again. The pointer should return to the ∞ position, indicating that the thyristor is good.

　　For 1~6A bidirectional thyristor, connect the red pen to T1 pole, and the black pen to G and T2 poles at the same time. Disconnect G pole while ensuring that the black pen does not separate from T2 pole. The pointer should indicate dozens to more than one hundred ohms (depending on the thyristor current and manufacturer). Then swap the two pens and repeat the above steps. If the pointer indicates a few dozen to dozens of ohms more than the last time, it means that the thyristor is good and the trigger voltage (or current) is small.

　　If the pointer returns to the ∞ position immediately when the A or T2 pole is disconnected and the G pole is disconnected, it means that the thyristor trigger current is too large or damaged. Further measurement can be performed according to the method in Figure 2. For a unidirectional thyristor, the light should be on when the switch K is closed, and the light should not go out when the K is disconnected, otherwise it means that the thyristor is damaged.

　　For a bidirectional thyristor, close switch K, the light should be on, disconnect K, the light should not go out. Then reverse the battery and repeat the above steps. If the result is the same, it means it is good. Otherwise, it means the device is damaged.

　　How to use a multimeter

　　Precautions for using a multimeter

　　(1) Before using the multimeter, you should first perform "mechanical zero adjustment", that is, when there is no electrical quantity to be measured, make the multimeter pointer point to the position of zero voltage or zero current.

　　(2) When using a multimeter, do not touch the metal part of the test leads with your hands. This can ensure accurate measurement and personal safety.

　　(3) When measuring a certain amount of electricity, you cannot change gears while measuring, especially when measuring high voltage or high current. Otherwise, the multimeter will be damaged. If you need to change gears, you should disconnect the test leads first and then measure after changing gears.

　　(4) When using the multimeter, it must be placed horizontally to avoid errors. At the same time, attention should also be paid to avoiding the influence of external magnetic fields on the multimeter.

　　(5) After using the multimeter, the conversion switch should be set to the maximum AC voltage. If the multimeter is not used for a long time, the battery inside the multimeter should be removed to prevent the battery from corroding other components in the meter.

　　Use of Ohm Block

　　1. Choose the appropriate magnification. When measuring resistance with an ohmmeter, you should choose an appropriate magnification so that the pointer indicates near the midpoint. It is best not to use the left third of the scale, as this part of the scale is poorly dense.

　　2. Adjust to zero before use.

　　3. Do not measure while the device is powered on.

　　4. The resistance being measured cannot have any parallel branches.

　　5. When measuring the equivalent resistance of polar components such as transistors and electrolytic capacitors, you must pay attention to the polarity of the two pens.

　　6. When measuring the equivalent resistance of a nonlinear element with different magnifications of the ohmmeter, the measured resistance values ​​are different. This is because the median resistance and full-scale current of each gear are different. In mechanical meters, generally, the smaller the magnification, the smaller the measured resistance value.

　　When the multimeter measures DC

　　1. Perform mechanical zero adjustment.

　　2. Select the appropriate range.

　　3. When using the current range of the multimeter to measure current, the multimeter should be connected in series in the circuit being measured, because only in series can the current flowing through the ammeter be the same as the current of the branch being measured. When measuring, the branch being measured should be disconnected, and the red and black test leads of the multimeter should be connected in series between the two points where the quilt is disconnected. Special attention should be paid to the fact that the current meter can be connected in parallel in the circuit being measured. Doing so is very dangerous and can easily burn the multimeter.

　　4. Pay attention to the polarity of the measured electrical quantity.

　　5. Use the scale and read correctly.

　　6. When the 2.5A range of DC current is selected, the red probe of the multimeter should be inserted into the 2.5A measurement jack, and the range switch can be set to any range of the DC current range.

　　7. If the DC current measured by the quilt is greater than 2.5A, the 2.5A range can be expanded to 5A range. The method is very simple. The user can connect a 0.24 ohm resistor between the "2.5A" jack and the black test lead jack, so that the range becomes a 5A current range. The connected 0.24A resistor should be a wire wound resistor of more than 2W. If the power is too small, it will burn out.

　　1 Use a multimeter to determine the positive and negative poles of the speaker

　　First, set the pointer multimeter to the DC 0-5mA range, and then connect the two test leads to the two welding pieces of the speaker to be tested. Press the speaker cone lightly with your hand and observe the swing direction of the multimeter pointer. If the pointer deflects in the positive direction, the red test lead is connected to the negative pole of the speaker and the black test lead is connected to the positive pole of the speaker. Otherwise, the red test lead is connected to the positive pole and the black test lead is connected to the negative pole.

　　2 Use a multimeter to judge the quality of piezoelectric ceramics

　　Piezoelectric ceramics are a type of artificial piezoelectric material. When subjected to external pressure, electric charges will be generated on both sides, and the amount of charge is proportional to the pressure. This phenomenon is called the piezoelectric effect. Piezoelectric ceramics have the piezoelectric effect, that is, they will deform under the action of an external electric field, so piezoelectric ceramics can be used as sound-generating elements.

　　By utilizing the piezoelectric effect of piezoelectric ceramics, a multimeter can be used to determine whether they are good or bad.

　　Lead two wires from the two poles of the piezoelectric ceramic piece, then place the ceramic piece flat on the table, connect the two leads to the two test leads of the multimeter respectively, set the multimeter to the minimum current range, and then press the ceramic piece lightly with the eraser of a pencil. If the pointer of the multimeter swings obviously, it means that the ceramic piece is intact, otherwise, it is damaged.

　　How to use a multimeter

　　1. Voltages below 36V are safe voltages. When measuring DC higher than 36V or AC 25V, check whether the test leads are in reliable contact, correctly connected, and well insulated to avoid electric shock.

　　2. When changing functions and ranges, the test leads should be away from the test point. Select the correct function and range during testing to avoid misoperation.

　　3. To measure DC voltage, first turn the range switch to the corresponding DCV range, then connect the test leads across the circuit to be tested. The voltage and polarity of the point where the red lead is connected will be displayed on the screen.

　　4. To measure AC voltage, first turn the range switch to the corresponding ACV range, and then connect the test leads across the circuit to be tested.

　　5. To measure DC current, first turn the range switch to the corresponding DCA position, and then connect the meter in series with the circuit to be measured.

　　6. To measure AC current, first turn the range switch to the corresponding ACA position, and then connect the meter in series with the circuit to be measured.

　　7. To measure resistance, turn the range switch to the corresponding resistance range and connect the two test leads across the resistance to be measured.

　　8. To measure capacitance, turn the range switch to the corresponding capacitance range, connect the test leads across the capacitor to be measured and measure, paying attention to the polarity when necessary.

　　9. Diode and continuity test, set the range switch to gear. Connect the red meter to the positive pole of the diode, and the black meter pen to the negative pole of the diode. When testing the continuity of the circuit, connect the meter pen to the two ends of the circuit to be tested. If the buzzer sounds, the circuit is connected, otherwise the circuit is disconnected.

　　10. To measure the tube amplification factor, set the range switch to hFE, determine whether the transistor to be measured is NPN or PNP, and insert the emitter, base, and collector into the corresponding holes respectively.

　　How to Use a Multimeter

　　A multimeter is an essential testing tool in electronic production. It has multiple functions such as measuring current, voltage and resistance.

　　This section will introduce the structure of a multimeter and how to use it. Students should work hard to learn how to use a multimeter.

　　1. Observe and understand the structure of the multimeter.

　　There are many types of multimeters with different appearances, but the basic structure and usage methods are the same. The structure and appearance of commonly used multimeters are shown in the color page.

　　The main components of the multimeter panel are the meter head and the selection switch. There is also an ohmmeter zero adjustment knob and a test lead jack. The following describes the functions of each part:

　　1. Table header

　　The head of the multimeter is a sensitive ammeter. The dial on the head is printed with a variety of symbols, scales and values ​​(as shown in Figure 3-4 (B)). The symbol A-V-Ω indicates that this meter is a multimeter that can measure current, voltage and resistance. There are multiple scales printed on the dial, among which the one marked with "Ω" on the right end 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" indicates direct current, "~" or "AC" indicates alternating current, and "~" indicates a scale line shared by both alternating current and direct current. The lines of numbers below the scale are the scale values ​​corresponding to the different gears of the selector switch.

　　There is also a mechanical zero adjustment knob on the meter head to correct the pointer to point to zero at the left end.

　　(II) Selector switch

　　The selection switch of the multimeter is a multi-position rotary switch. It is used to select the measurement item and range. (As shown in Figure 3-4 (B)). The general measurement items of a multimeter include: "mA"; DC current, "V": DC voltage, "V": AC voltage, "Ω": resistance. Each measurement item is divided into several different ranges for selection.

　　(III) Test leads and test lead jacks

　　The test leads are divided into red and black. When using, the red test lead should be inserted into the jack marked with "+" and the black test lead should be inserted into the jack marked with "-".

　　2. How to use a multimeter

　　1. Before using the multimeter, you should:

　　1. Place the multimeter horizontally.

　　2. Check whether the needle is at the zero position on the left end of the dial. If it deviates, use a small screwdriver to gently turn the mechanical zero adjustment knob on the meter head to make the needle point to zero.

　　3. Insert the test leads into the test lead jack as required above.

　　4. Turn the selection switch to the corresponding item and range. Then it can be used.

　　(II) After using the multimeter, you should do the following:

　　1. Pull out the test leads.

　　2. Turn the selector switch to the "OFF" position. If there is no such position, turn it to the position with the maximum AC voltage range, such as "1000V".

　　3. If the meter is not used for a long time, the battery should be removed to prevent leakage of battery electrolyte and corrosion of the internal circuit.