How to use a digital multimeter
Digital multimeter is one of the most commonly used tools for measuring current, voltage and resistance. This article introduces five common methods of using digital multimeters in detail!
A digital multimeter is one of the most commonly used tools for measuring current, voltage and resistance. We will often use it in maintenance, but beginners do not know how to use digital multimeters very well. This article introduces the use of digital multimeters. Let's take a look!
How to use a digital multimeter: determine whether the circuit or device is charged or not
The AC voltage gear of the digital multimeter is very sensitive, even if there is a small induced voltage around it, it can be displayed. According to this feature, it can be used as a test pen. The usage is as follows:
Turn the multimeter to the AC20V block, the black test lead is suspended, and the red test lead is in contact with the route or device on the side. At this time, the multimeter will display. If the displayed number is between a few volts and a dozen volts (different multimeters will have different display), indicating that the line or device is charged, if the display is zero or very small, indicating that the line or device is not charged.
How to use the digital multimeter
The first method: You can use the above method to judge. The larger number is the live line, and the smaller number is the neutral line. This method needs to be in contact with the line or device being measured.
Second method: No contact with the line or device being measured is required. Turn the multimeter to the AC2V block, the black test pen is hanging in the air, hold the red test pen and gently slide the pen tip along the line, if the meter shows a few volts at this time, it indicates that the line is the live wire. If the display is only a few tenths of a volt or even less, it means that the line is the zero line. This method of judgment does not directly contact the line, which is not only safe but also convenient and quick.
How to use the digital multimeter three: find the breakpoint of the cable
When there is a breakpoint in the cable, the traditional method is to use a multimeter to find the breakpoint of the cable section by section, which not only wastes time, but also damages the insulation of the cable to a great extent. Using the inductive characteristics of the digital multimeter can quickly find the disconnection point of the cable. First use the resistance gear to determine which cable core wire is broken, and then connect one end of the broken core wire to the AC220V power supply, then turn the multimeter to the position of the AC2V gear, with the black test lead hanging in the air, holding the red test lead Gently slide the pen tip along the line, if the meter shows a voltage of a few volts or tenths of a volt (due to different cables), if it moves to a certain position, the display on the meter suddenly drops a lot, write down At this position, the breakpoint is usually between 10 and 20 cm in front of this position.
This method can also find the breaking point of resistance wires such as faulty electric blankets.
How to use the digital multimeter four: measure the frequency of the UPS power supply
For UPS power supply, the stability of the voltage at the output terminal is an important parameter, and the frequency of its output is also very important, but it cannot be measured directly with the frequency range of the digital multimeter, because the voltage that the frequency range can withstand is very low, only a few Volt. At this time, a 220V/6V or 220V/4V step-down transformer can be connected to the output end of the UPS power supply to reduce the voltage without changing the frequency of the power supply, and then connect the frequency file to the output of the transformer to measure the UPS power supply. Frequency of.
How to use the digital multimeter five: use the digital multimeter to test the quality of the triode
Use the diode gear to measure one pin that is connected to the other two pins, but the two pins are not connected (for digital meters, if the pin common to the other two pins is connected to a red pen, then this is an NPN Tube, if it is connected with a black pen, then this is a PNP tube, if it is a pointer table, it is just the opposite.), this pin is the base set B, and B has been blown to the tube. (Not applicable for internal protection diode)
If the voltage drop measured by the diode gear is greater than 0.5V, it is a silicon tube, and if it is about 0.2V, it is a germanium tube.
Then put the measured triode into the HFE jack according to the pin, change to the HFE file, and the DC magnification can be measured. (It is only valid for ordinary low-power tubes, but invalid for high-power tubes. Guowei high-power tubes require a large base of driving power, which the multimeter cannot provide.) The DC magnification HFE of general low-power tubes is 30-1000 One of the pins, the measured voltage will be a few millivolts larger than the other pin, then this pin is the emitter E, and the remaining voltage drop a few millivolts is the collector C. If the diode is used to measure Any two of the pins are connected, then the tube has been broken down and damaged; if it is not connected, then use the diode test method with a digital multimeter to measure a diode with a digital multimeter. When it is turned on, it will display a few tenths or a point Several. When the meter is exchanged again (that is, when it is cut off), it is displayed as 1, and when 1 is displayed, the black pen is connected to the anode of the diode, and the red pen is connected to the cathode of the diode. If it is different, it must be a bad diode. If the result is 1 when measured twice, the diode is open. If the two measurements are a few tenths or both, the diode has broken down.
The digital multimeter has a gear for measuring diodes. First set the gear switch to this gear, connect the red pen to one foot, and the black pen to touch the other two feet. If both are connected, it is a PNP type, and the red pen is connected is the base.
If neither works, there are two cases:
1. If the test pen is connected to both sides, it is NPN, and the red pen is connected to the base;
2. When one of the test leads is transferred, the other is not. That is PNP, and the base is the one that is transferred.
The red test pen is connected to one foot, and the black pen touches the other two feet. If one is connected and the other is not connected, if the two are not connected, it is PNP. Before the black pen was able to connect to the base.
Detecting transistors (using "diode gear")
Positive measurement: The red pen is connected to the positive pole, and the black pen is connected to the negative pole, which can display the forward voltage drop of the PN junction. Normally, it should display 0.500~0.700 for silicon tube and 0.150~0.300 for germanium tube.
Reverse test: the black pen is connected to the positive pole, the red pen is connected to the negative pole, and it should display "1" normally.
Both positive and negative measurements are 0 or 1, indicating that the tube is damaged.
The triode is detected according to the above-mentioned method for detecting a diode. First, determine the collector C and the emitter E, and use a meter to measure the forward voltage drop of the two PN junctions. The smaller voltage drop is the collector C, the larger voltage drop is the emitter E, and the common electrode used for the two measurements is Red pen, this tube is NPN type, the common pole is black pen, this tube is PNP type.
The above detection methods should be noted when testing: if the two ends of the PN junction are connected in parallel with a resistance of less than 700 ohms, the displayed value will be small. At this time, do not blindly think that the transistor is damaged. You can weld one end of the resistance and test again. This pipe is measured under welding.
The resistance between CE of ordinary triodes should be infinite. For transistors with internal resistance, there is a certain resistance between CE.
Use a digital multimeter to measure the poles of the triode
In addition, digital multimeters generally have a socket for measuring the triode, which is two rows of four small holes. You insert your triode into it. If it is inserted correctly, the magnification of the triode will be displayed. If it is inserted incorrectly, it will not count. So very convenient.
N channel:
1) The red test lead is connected to the first pin, and the black test lead is connected to the other two pins. At this time, there is no reading on the multimeter, that is to say, there is no response, then the red test lead is connected to the G pole.
2) The red test lead is connected to the second pin, the black test lead is connected to the third pin, there is a reading, that is to say, it is connected, the red test lead does not move, the black test lead is connected to the G pole (which has been measured), and then the black test lead is connected again. Connect the third pin and found that it can't work, (this means that the tube 9.9% is good), (the second pin and the third pin are connected because the first step has already charged it, and later the failure is because When the red test lead is connected to the second foot, the black test lead has been discharged when it is connected to the first foot).
3) The red test lead is connected to the third pin, the black test lead is connected to the second pin, and there is a reading, then the red test lead is connected to the S pole, and the black test lead is connected to the D pole.
The test capacitor has a capacitance gear P, the test resistor has a resistance gear R, the test voltage has a voltage gear V, and the test current has a current gear A.
Through the above introduction to the use of digital multimeters, you must have a certain understanding, and you can easily deal with it in the process of using it later!
