Methods and requirements for measuring with a digital multimeter

　　Measuring Voltage
　　One of the most basic functions of a digital multimeter is to measure voltage. A typical source of DC voltage is a battery, such as a car battery. AC voltage is usually generated by a generator, and the most common source of AC voltage is the wall socket at home. Some devices convert AC to DC, such as televisions, stereos, VCRs, computers and other electronic devices. The multimeter measures capacitance through a rectifier plugged into the wall socket to convert AC to DC; DC voltage is the power supply required by these electronic devices.
　　Testing voltage is usually the first step to solve a circuit problem. If there is no voltage or the voltage is too low or too high, the power supply problem should be solved before further inspection. The waveform of AC voltage may be sinusoidal (sine wave) or non-sinusoidal (sawtooth wave, square wave, etc.). Many digital multimeters (such as OI857 Ⅱ and OI859CF Ⅱ) can display the "rms" (effective value) of AC voltage. The effective value is the value of AC voltage equivalent to DC voltage. However, most digital multimeters (such as BM805) only have the "average responding" function, which can give effective values ​​when a pure sine wave is input. This type of digital multimeter cannot accurately measure the effective value of non-sinusoidal waves. A digital multimeter with a true-rms function can accurately measure the true-rms value of non-sinusoidal waves (see the article "Multimeter Design Principles and Measurement Calculation Methods").
　　The crest factor is the ratio of the peak value of a signal to its effective value. The ability of a digital multimeter to measure AC voltage is limited by the frequency of the signal being measured. Most digital multimeters can accurately measure 50Hz~500Hz AC voltage, but some digital multimeters have an AC measurement bandwidth of up to several hundred kilohertz (for example, the OI859CF Ⅱ digital multimeter can reach 100KHz).
　　Steps for measuring voltage with a digital multimeter: 1. Select the desired gear and range; 2. Insert the black test lead into the COM port and the red test lead into the voltage input jack; 3. Connect the test lead across the load or power supply (and on the circuit); 4. Check the reading and confirm the unit.
　　To correctly read the polarity (±) of the DC voltage when measuring voltage, connect the red probe to the positive terminal of the circuit and the black probe to the negative terminal or circuit ground. If the opposite connection is used, a digital multimeter with an automatic polarity reversal function will display a negative sign to indicate the negative polarity. At this time, if it is an analog pointer multimeter, there is a risk of damaging the multimeter. High-voltage probes are required when measuring high voltages, such as those used in the repair of televisions and video recorders, which can withstand 40kV high voltages. However, these high-voltage probes (such as OIHVP40) cannot be used in high-voltage and high-energy power applications, but can only be used in low-energy applications.
　　Measuring resistance
　　Resistance values ​​generally vary greatly, from a few milliohms (mΩ) of contact resistance to billions of ohms of insulation resistance. Many digital multimeters can measure resistances as low as 0.1 ohms and as high as 300 megohms. For extremely large resistances, the digital multimeter will generally display "OL", indicating that the measured resistance exceeds the range. When measuring an open circuit, it will display "OL". The resistance must be measured with the circuit power turned off, otherwise the meter or circuit board will be damaged. Some digital multimeters provide a protection function when a voltage signal is mistakenly connected in the resistance mode. Different models of digital multimeters have different protection capabilities.
　　When making accurate measurements of low resistance, the resistance of the measuring wire must be subtracted from the measured value. The typical resistance of the test wire is between 0.2Ω and 0.5Ω. If the resistance of the test wire is greater than 1Ω, the test wire must be replaced. If the digital multimeter provides a DC voltage of less than 0.6V for measuring resistance, the resistance value isolated by a diode or semiconductor on the circuit board can be measured. Therefore, the resistance can be tested without removing it. Steps for measuring resistance with a digital multimeter (be sure to turn off the resistance power when testing the resistance):
　　1. Turn off the circuit power; 2. Select the resistance block; 3. Insert the black test lead into the COM jack. 1. Insert the red test lead into the resistance test jack; 2. Connect the test lead probes across the two ends of the component or circuit being tested; 3. Check the reading and pay attention to the unit of ohm (Ω), kiloohm (kΩ), or megaohm (MΩ).
　　Continuity measurement
　　Continuity is to distinguish between open circuit and short circuit by fast resistance measurement. Continuity measurement is simpler and faster when using a digital multimeter with a continuity beep. When a short circuit is detected, the digital multimeter emits a peak beep, so there is no need to look at the digital multimeter during the test. Different models of digital multimeters have different trigger resistance values.
　　Diode test
　　A diode is like an electronic switch. If the voltage is higher than a certain value, the diode will conduct. Usually, the silicon diode conducts at a voltage of 0.6V, and the diode only allows current to flow in one direction. When checking a diode or a junction, the digital multimeter will not only give a wide reading range, but also give a drive current greater than 50mA. When measuring the resistance of a circuit containing a diode, the test voltage of the digital multimeter will be lower than 0.6V to prevent the junction from conducting. When the diode test is selected, the test voltage is increased to check the function of the diode or semiconductor junction. Some digital multimeters have a diode test function, which measures and displays the actual voltage drop across the diode. The voltage drop of the silicon junction should be less than 0.7V during the forward test, and the circuit is open during the reverse test.
　　Measuring current
　　The digital multimeter's current measurement is a very important function, which is different from the digital multimeter's measurement of other quantities. The direct current measurement method is to connect the digital multimeter directly to the circuit under test, so that the current of the circuit under test flows directly through the internal circuit of the digital multimeter. The indirect measurement method does not require opening the circuit and connecting the digital multimeter to the circuit under test. The indirect method requires the use of a current clamp or a current probe. The steps of direct current measurement with a digital multimeter are as follows: 1. Turn off the power supply of the circuit; 2. Disconnect or desolder the circuit so that the digital multimeter can be connected in series with the circuit; 3. Select the corresponding AC (A~) and DC (A--) gears; 4. Insert the black test lead into the COM jack and the red test lead into the 10A jack (or 300 mA jack. The choice of which jack is mainly based on the possible measurement value. 5. Connect the test leads in series to the disconnected circuit part. 6. Turn on the power supply of the circuit. 7. Observe the reading and pay attention to the unit.
　　Digital multimeters are most likely to burn out when measuring current. A common mistake is to put the test lead in the current jack and try to test the voltage, which is a common mistake of inserting the test lead wire incorrectly (see the article "Digital Multimeter User Experience (1)"). At this time, the small value resistor in the digital multimeter will cause the voltage source to short-circuit, and a large current will flow through the digital multimeter. If the digital multimeter does not have enough Protection 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 digital multimeter should have a current input protection fuse with a large enough capacity. Digital multimeters without current input fuses cannot be used in high-energy circuits (>240Vac). For digital multimeters using fuses, the fuses should have a large enough capacity to eliminate high-energy faults. The rated voltage of the fuse should be larger than the maximum voltage you expect. For example, a 20A/250V fuse in a digital multimeter cannot play a protective role when the digital multimeter measures a 480V circuit; a 20A/600V fuse can play a protective role when the digital multimeter measures a 480V circuit. Some digital multimeters have a buzzer alarm function when measuring current when the probe line is plugged in incorrectly, such as OI857 Ⅱ and OI859CF Ⅱ.
　　When measuring current, be sure to pay attention to the following: When disconnecting or desoldering the circuit, 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 lead is inserted into the current hole. This may cause damage to the digital multimeter or endanger personal safety.
　　Improve the range and capability of digital multimeter current testing
　　If you need to measure current values ​​that exceed the range of the digital multimeter or the field conditions do not allow the circuit to be disconnected to measure the current, a current clamp or current probe will be very convenient when testing high currents (usually greater than 2A) but do not require high accuracy. The current clamp is wrapped around the wire to measure the current and convert it to a value that the digital multimeter can handle. There are two basic types of current probes: ① Current transformers, which are only used for AC measurements. ② Hall effect probes, which are used to measure AC or DC currents.
　　Current transformers generally use 1 milliampere to represent 1 ampere, so a current value of 100 amperes will become 100 milliamperes, which can be safely measured with a digital multimeter. Connect the wires to the "mA" and "Com" jacks and turn the function selector knob to the AC milliampere position. The Hall effect probe uses 1 millivolt to represent the AC or DC value of 1 ampere. For example, an AC current of 100 amperes will be converted to an AC voltage of 100 millivolts. Connect the test wires to the "V" and "Com" jacks. Turn the function selector knob to the "V" or "mV" position. At this time, the multimeter uses 1 millivolt voltage to represent 1 ampere current. A typical transformer current probe like K2, the digital multimeter uses 1mA display to represent the actual 1A signal. A typical Hall effect probe like K1 can test very high AC or DC current. It converts current into a voltage signal, and the 1mV voltage display represents 1A actual current.