How to detect components with a multimeter

　　1. Measure the polarity of each foot of the rectifier bridge.
　　Set the multimeter to R×1k, connect the black test lead to any pin of the bridge rectifier, and use the red test lead to measure the other three feet in turn. If the readings are all infinite, the black test lead is connected to the positive output of the bridge rectifier. If the readings are 4~10kΩ, the pin connected to the black test lead is the negative output of the bridge rectifier, and the other two pins are the AC input of the bridge rectifier.
　　2. Determine the quality of the crystal oscillator .
　　First use a multimeter (R×10k) to measure the resistance value at both ends of the crystal oscillator. If it is infinite, it means that the crystal oscillator has no short circuit or leakage; then insert the test pen into the AC power jack, pinch any pin of the crystal oscillator with your fingers, and touch the other pin to the metal part at the top of the test pen. If the neon bulb of the test pen turns red, it means that the crystal oscillator is good; if the neon bulb does not light up, it means that the crystal oscillator is damaged.
　　3. Unidirectional thyristor detection:
　　You can use the R×1k or R×100 range of the multimeter to measure the forward and reverse resistance between any two poles. If you find that the resistance of a pair of poles is low (100Ω~1kΩ), then the black test lead is connected to the control pole, the red test lead is connected to the cathode, and the other pole is the anode. Thyristors have a total of three PN junctions. We can determine whether it is good or bad by measuring the size of the forward and reverse resistance of the PN junction. When measuring the resistance between the control pole (G) and the cathode [C), if the forward and reverse resistances are both zero or infinite, it indicates that the control pole is short-circuited or open-circuited; when measuring the resistance between the control pole (G) and the anode (A), the forward and reverse resistance readings should be large;
　　when measuring the resistance between the anode (A) and the cathode (C), the forward and reverse resistances should be large.
　　4. Polarity identification of bidirectional thyristor
　　Bidirectional thyristor has main electrode 1, main electrode 2 and control electrode. If the resistance between the two main electrodes is measured with a multimeter at R×1k, the reading should be close to infinity, while the forward and reverse resistance between the control electrode and any main electrode is only tens of ohms. Based on this characteristic, we can easily identify the control electrode of the bidirectional thyristor by measuring the resistance between the electrodes. When the black test lead is connected to the main electrode 1 and the red test lead is connected to the control electrode, the forward resistance measured is always smaller than the reverse resistance. Based on this, we can easily identify the main electrode 1 and the main electrode 2 by measuring the resistance.
　　5. Check the quality of the light-emitting digital tube
　　First set the multimeter to R×10k or R×l00k, then connect the red test lead to the "ground" lead of the digital tube (taking the common cathode digital tube as an example), and the black test lead to the other leads of the digital tube in turn. The seven segments should all light up separately, otherwise it means that the digital tube is damaged.
　　6. Identify the electrodes of junction field effect tubes.
　　Set the multimeter to R×1k, touch the pin assumed to be the gate G with the black test lead, and then touch the other two pins with the red test lead. If the resistance values ​​are all relatively small (5-10 Ω), swap the red and black test leads and measure again. If the resistance values ​​are all large (∞), it means that they are all reverse resistances (PN junction reverse), which belongs to N-channel tubes, and the pin touched by the black test lead is the gate G, and it means that the original assumption is correct. If the resistance values ​​measured again are all very small, it means that it is a forward resistance, which belongs to a P-channel field effect tube, and the black test lead is also connected to the gate G. If the above situation does not occur, you can swap the red and black test leads and test according to the above method until the gate is determined. Generally, the source and drain of the junction field effect tube are symmetrical during manufacturing. Therefore, after the gate G is determined, it is not necessary to identify the source S and drain D, because these two poles can be used interchangeably. The resistance between the source and the drain is several thousand ohms.
　　7. Identification of transistor electrodes
　　For a transistor with unclear model or no mark, if you want to distinguish its three electrodes, you can also use a multimeter to test. First, turn the multimeter range switch to the R×100 or R×1k resistor block. The red test pen touches any electrode of the transistor, and the black test pen touches the other two electrodes in turn, and measure the resistance between them respectively. If the measured resistance is a few hundred ohms, the electrode touched by the red test pen is the base b, and this tube is a PNP tube. If the measured resistance is a few tens to hundreds of kiloohms, the electrode touched by the red test pen is also the base b, and this tube is an NPN tube.
　　On the basis of distinguishing the tube type and base b, the collector is determined by using the principle that the forward current amplification factor of the transistor is larger than the reverse current amplification factor. Arbitrarily assume that one electrode is the c pole and the other electrode is the e pole. Turn the multimeter range switch to the R×1k resistor block. For PNP tube, connect the red test lead to the c pole and the black test lead to the e pole, then pinch the b and c poles of the tube with your hands at the same time, but do not make the b and c poles touch each other directly, and measure a certain resistance value. Then swap the two test leads for a second measurement, and compare the resistances measured twice. For PNP tube, the electrode connected to the red test lead is the collector when the resistance value is small. For NPN tube, the electrode connected to the black test lead is the collector.
　　8. How to judge whether a potentiometer is good or bad
　　First measure the nominal resistance of the potentiometer. Use the ohmmeter of the multimeter to measure the "1" and "3" terminals (set the "2" terminal as the active contact ), and the reading should be the nominal value of the potentiometer. If the pointer of the multimeter does not move, the resistance value does not move, or the resistance value is very different, it indicates that the potentiometer is damaged. Then check whether the active arm of the potentiometer is in good contact with the resistor. Use the ohmmeter of the multimeter to measure the two ends of "1", "2" or "2", "3", and rotate the shaft of the potentiometer counterclockwise to a position close to "off". At this time, the resistance should be as small as possible. Then slowly rotate the shaft handle clockwise, and the resistance should gradually increase. When it is rotated to the extreme position, the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, it means that the movable contact is in poor contact.
　　9. Measuring the leakage resistance of large-capacity capacitors
　　Use a 500-type multimeter to set it to R×10 or R×100. When the pointer points to the maximum value, immediately switch to the R×1k block for measurement. The pointer will stabilize in a short time, so as to read the leakage resistance value.
　　10. Identify the pins of the infrared receiver.
　　Set the multimeter to R×1k, first assume that one pin of the receiver is the ground terminal, connect it with the black test lead, and use the red test lead to measure the resistance of the other two pins respectively. Compare the two measured resistance values ​​(generally in the range of 4~7k Q). The one with smaller resistance is connected to the +5V power pin by the red test lead, and the other with larger resistance is the signal pin. On the contrary, if the red test lead is connected to a known ground pin, and the black test lead is used to measure the known power pin and signal pin respectively, the resistance values ​​are all above 15kΩ, the pin with smaller resistance is the +5V terminal, and the pin with larger resistance is the signal terminal. If the measurement result meets the above resistance value, it can be judged that the receiver is intact.
　　11. Determine the polarity of unsigned electrolytic capacitors
　　First, short-circuit the capacitor to discharge it, then mark the two leads with A and B, set the multimeter to R×100 or R×1k, connect the black test lead to lead A, and the red test lead to lead B, wait for the pointer to stop and read the value, and then short-circuit and discharge it; then connect the black test lead to lead B, and the red test lead to lead A, compare the two readings, and the one with the larger resistance value is connected to the positive electrode with the black test lead, and the red test lead is connected to the negative electrode.
　　12. Test LED
　　Take an electrolytic capacitor with a capacity greater than 100 "F (the larger the capacity, the more obvious the phenomenon), first charge it with the multimeter R × 100 block, connect the black test lead to the positive pole of the capacitor, and the red test lead to the negative pole. After charging is completed, change the black test lead to the negative pole of the capacitor, and connect the LED to be tested between the red test lead and the positive pole of the capacitor. If the LED lights up and then gradually goes out, it indicates that it is good. At this time, the red test lead is connected to the negative pole of the LED, and the positive pole of the capacitor is connected to the positive pole of the LED. If the LED does not light up, swap its two ends and reconnect it for testing. If it still does not light up, it indicates that the LED is damaged.
　　13. Photocoupler detection
　　The multimeter should be set to the resistance R×100 range, and the R×10k range should not be selected to prevent the battery voltage from being too high and breaking down the light-emitting diode. Connect the red and black test leads to the input terminal, and measure the forward and reverse resistance. Normally, the forward resistance is tens of ohms, and the reverse resistance is several thousand ohms to several tens of kiloohms. If the forward and reverse resistances are similar, it indicates that the light-emitting diode is damaged. Select the resistance R×1 range for the multimeter. Connect the red and black test leads to the output terminal, and measure the forward and reverse resistance. Normally, they are close to ∞, otherwise the light-receiving tube will be damaged. Select the resistance R×10 range for the multimeter, and connect the red and black test leads to the input and output terminals respectively to measure the insulation resistance between the light-emitting tube and the light-receiving tube (if conditions permit, use a megohmmeter to measure the insulation resistance. At this time, the output rated voltage of the megohmmeter should be slightly lower than the withstand voltage allowed by the photoelectric coupler being measured). The insulation resistance between the light-emitting tube and the light-receiving tube should normally be ∞.
　　14. Photoresistor detection
　　When testing, set the multimeter to the R×1kΩ position, and keep the light-receiving surface of the photoresistor perpendicular to the incident light. The resistance directly measured on the multimeter is the light resistance. Then place the photoresistor in a completely dark place. The resistance measured by the multimeter is the dark resistance. If the light resistance is several thousand ohms to tens of thousand ohms, and the dark resistance is several to tens of megohms, it means that the photoresistor is good.
　　15. Determination of laser diode damage
　　Remove the laser diode and measure its resistance. Under normal circumstances, the reverse resistance should be infinite and the forward resistance should be between 20kΩ and 40kΩ. If the measured forward resistance exceeds 50kΩ, it means that the performance of the laser diode has declined; if its forward resistance exceeds 90kΩ, it means that the tube is damaged and can no longer be used.