Component detection skills on the circuit board of the third party detection center

Date：2023-02-02 16:00:00Views：359

The core of electronic equipment is the circuit board, which is welded and assembled by various types of electronic components. If the equipment fails or has a short circuit, most of the reasons will be caused by the failure or damage of electronic components. Therefore, how to correctly detect electronic components is particularly important, which is also a skill that electronic maintenance personnel must master. Here are some common electronic component detection techniques for your reference.

1. Measure the polarity of each pin of the rectifier bridge

Multimeter set R × At 1k gear, the black lead is connected to any pin of the bridge, and the red lead is connected to the other three pins. If the reading is infinite, the black lead is connected to the output positive pole of the bridge. If the reading is 4~10k Ω, the black lead is connected to the output negative pole of the bridge, and the other two pins are AC input terminals of the bridge.

2. Judge the quality of crystal oscillator

First use a multimeter (R × 10k) 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 mains socket, hold any pin of the crystal oscillator with your finger, and touch the other pin with the metal part at the top of the test pen. If the neon bubble of the test pen turns red, it means that the crystal oscillator is good; If the neon bubble is not bright, the crystal oscillator is damaged.

3. Unidirectional thyristor detection

R of available multimeter × 1k or R × Measure the forward and reverse resistance between any two poles at 100 gear. If the resistance of a pair of poles is found to be low (100 Ω - lk Ω), then the black lead is connected to the control pole, the red lead is connected to the cathode, and the other pole is anode. There are three PN junctions in the thyristor. We can judge whether it is good or bad by measuring the positive and negative resistance of the PN junction. When measuring the resistance between the control electrode (G) and the cathode (C), if the forward and reverse resistance are both zero or infinite, it indicates that the control electrode is short circuited or open circuited; when measuring the resistance between the control electrode (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 resistance should be large.

4. Polarity identification of bidirectional thyristor

The bidirectional thyristor has main electrode 1, main electrode 2 and control electrode. If the multimeter R is used × Measure the resistance between two main electrodes at 1k gear, and the reading should be approximately infinite, while the positive and negative resistance readings between the control electrode and any main electrode are only tens of ohms. According to this characteristic, we can easily identify the control pole of the thyristor by measuring the resistance between the electrodes. When the black probe is connected to the main electrode 1. The forward resistance measured when the red meter pen is connected to the control electrode is always smaller than the reverse resistance. Therefore, we can easily identify the main electrode 1 and the main electrode 2 by measuring the resistance.

5. Check the quality of LED

Set the multimeter to R first × 10k or R × L00k, and then connect the red lead to the "ground" lead of the nixie tube (taking the common cathode nixie tube as an example). The black lead is connected to the other lead of the nixie tube in turn. The seven segments should glow separately, otherwise the nixie tube is damaged.

6. Identify the electrode of junction type field effect tube

Place the multimeter in R × At 1k gear, use the black probe to contact the pin assumed to be grid G, and then use the red probe to contact the other two pins respectively. If the resistance values are relatively small (5~10 Ω), exchange the red and black probes for measurement once. If the resistance values are all large (∞), it means that they are all reverse resistance (PN junction reverse), belong to N-channel tube, and the pin contacted by the black probe is grid G, which means that the original assumption is correct. If the resistance values measured again are very small, it indicates that the resistance is positive, belonging to P-channel field-effect transistor, and the black probe is also connected to grid G. If the above situation does not occur, the red and black probes can be replaced and tested according to the above method until the grid is determined. Generally, the source and drain of the junction field-effect transistor are symmetrical during manufacturing, so when the grid G is determined, it is not necessary to distinguish the source S and drain D, because the two electrodes can be interchanged. The resistance between source and drain is several thousand Ohms.

7. Discrimination of triode electrode

For a triode with unclear or unmarked model, if you want to distinguish their three electrodes, you can also use a multimeter to test. First set the range switch of the multimeter to R × 100 or R × 1k resistor. The red probe contacts one electrode of the triode at will, and the black probe contacts the other two electrodes in turn. Measure the resistance value between them respectively. If the measured low resistance is hundreds of Ohms, the electrode contacted by the red probe is base b, and this tube is PNP tube. If a high resistance of tens to hundreds of kiloohms is measured, the electrode contacted by the red probe is also the base b, and this tube is NPN tube. On the basis of distinguishing the tube type and the base b, the collector is determined by using the principle that the positive current amplification coefficient of the triode is larger than the reverse current amplification coefficient. It is arbitrarily assumed that one electrode is c-pole and the other electrode is e-pole. Turn the multimeter range switch to R × 1k resistor. For PNP tube, connect the red lead to the c pole and the black lead to the e pole, and then pinch the b and c poles of the tube with your hand at the same time, but do not make the b and c poles directly collide, and measure a certain resistance value. Then the two probes are exchanged for the second measurement, and the resistance measured in the two times is compared. For PNP type tube, the electrode connected by the red probe is the collector when the resistance value is small. For NPN type tubes with low resistance, the electrode connected to the black probe is the collector.

8. Determination of potentiometer

First measure the nominal resistance of the potentiometer. Measure the two ends of "1" and "3" with the ohm gear of the multimeter (set the "2" end as the movable 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 differs a lot, it indicates that the potentiometer is damaged. Then check whether the contact between the movable arm of the potentiometer and the resistance piece is good. Measure the two ends of "1", "2" or "2" and "3" with the ohm gear of the multimeter, and turn the potentiometer shaft counterclockwise to the position close to "off". At this time, the resistance should be as small as possible, and then slowly rotate the shaft handle clockwise. The resistance should gradually increase. When it is turned 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, the contact of the movable contact point is poor. 9. Measure the leakage resistance of large-capacity capacitor with a 500 type multimeter at R × 10 or R × 100 gear, when the pointer points to the maximum value, immediately change to R × 1k gear measurement, the pointer will be stable in a short time, so as to read the leakage resistance value.

10. Identify infrared receiver pin

Multimeter set R × In 1k gear, first assume that one pin of the receiver head is the ground terminal, connect it with the black lead, measure the resistance of the other two pins with the red lead, and compare the resistance values measured twice (generally in the range of 4~7k Q). The red lead of the lower resistance is connected to the+5V power pin, and the other with the higher resistance is the signal pin. On the contrary, if the red lead is used to connect the known ground pin and the black lead is used to measure the known power pin and signal pin respectively, the resistance value is above 15k Ω, the pin with low resistance value is+5V terminal, and the pin with high resistance value is signal terminal. If the measurement results meet the above resistance values, the receiver can be judged to be in good condition.

11. Judge the polarity of unsigned electrolytic capacitor

First discharge the capacitor in short circuit, then mark the two leads with A and B, and set the multimeter to R × 100 or R × 1k gear, the black lead is connected with A lead, and the red lead is connected with B lead. Read after the pointer is stationary, and discharge after short circuit; Then connect the black lead to the B lead and the red lead to the A lead. Compare the two readings. The black lead with a larger resistance is connected to the positive pole and the red lead to the negative pole.

12. Measuring LED

Take an electrolytic capacitor with a capacity greater than 100 "F (the larger the capacity, the more obvious the phenomenon), and first use a multimeter R × The black lead is connected to the positive pole of the capacitor and the red lead to the negative pole. After charging, the black lead is connected to the negative pole of the capacitor, and the measured LED is connected between the red lead and the positive pole of the capacitor. If the LED lights up and goes out gradually, it indicates that it is good. At this time, the red lead is connected to the negative pole of the LED, and the capacitor positive pole is connected to the positive pole of the LED. If the LED does not light up, switch its two ends and connect it again for test. If it does not light up, it indicates that the LED is damaged.

13. Photocoupler detection

Resistance R selected for multimeter × Gear 100, R is not allowed × 10k gear to prevent the LED from being broken down due to high battery voltage. The red and black probes are connected to the input terminals to measure the forward and reverse resistance. Under normal conditions, the forward resistance is tens of Ohms, and the reverse resistance is thousands of Ohms to tens of thousands of Ohms. If the forward and reverse resistances are similar, the LED is damaged. Multimeter selection resistance R × First gear. The red and black probes are connected to the output terminal, and the forward and reverse resistance are measured, which are close to ∞ under normal conditions, otherwise the light-receiving tube is damaged. Multimeter selection resistance R × At the 10th gear, the red and black probes are connected to the input and output terminals respectively to measure the insulation resistance between the light-emitting tube and the light-receiving tube (if possible, use a megohmmeter to measure its insulation resistance, at this time, the rated output voltage of the megohmmeter should be slightly lower than the allowable withstand voltage value of the measured optocoupler), and the insulation resistance between the light-emitting tube and the light-receiving tube should be ∞ normally.

14. Detection of photosensitive resistor

Set the multimeter to R × 1k Ω block, keep the light-receiving surface of the photosensitive resistor perpendicular to the incident light, so the resistance measured directly on the multimeter is the light resistance. Then put the photosensitive resistor in a completely dark place, and the resistance measured by the multimeter is the dark resistance. If the bright resistance is several thousand ohm to tens of dry ohm, and the dark resistance is several to tens of megaohm, it indicates that the photosensitive resistance is good.

15. Laser diode damage identification

Remove the laser diode and measure its resistance value. Under normal conditions, the reverse resistance value should be infinite, and the forward resistance value should be between 20k Ω and 40k Ω. If the measured forward resistance value has exceeded 50k Ω, it indicates that the performance of the laser diode has declined; If its positive resistance value has exceeded 90k Ω, it means that the tube has been damaged and can no longer be used.

The above is the relevant content of the component detection techniques on the circuit board organized by the core detection team. I hope it can help you. Chuangxin Testing is a professional testing agency for electronic components. At present, it mainly provides integrated circuit testing services such as capacitance, resistance, connector, MCU, CPLD, FPGA, DSP, etc. Specializing in functional testing of electronic components, appearance testing of incoming materials of electronic components, anatomical testing of electronic components, acetone testing, X-ray scanning testing of electronic components, ROHS component analysis and testing. Welcome to call, we will serve you wholeheartedly!