Small electronic components seem to be small, but in fact it is one of the important components. Because of the phenomenon of failure of electronic equipment, a large part of the situation is due to the failure or damage of electronic components. As a result, the detection of electronic components has become a very important issue. What are the experiences and techniques for detecting electronic components?

 

1. One-way thyristor detection

 

Can use Rx1k or R×100 of the multimeter to measure the positive and negative resistance of any two poles. If the resistance of a pair of poles is found to be low resistance (100Ω～lkΩ), then the black meter is connected to the control. Polarity, the red lead is connected to the cathode, and the other is the anode. The thyristor has a total of 3 PN junctions. We can determine whether it is good or bad by measuring the size of the PN junction's forward and reverse resistances. When measuring the resistance between the control electrode (G) and the cathode electrode [C], if the positive and negative resistances are zero or infinity, it indicates that the control electrode is short-circuited or open circuited; between the measurement control electrode (G) and the anode electrode (A) In the case of resistance, both the forward and reverse resistance readings should be large; when measuring the resistance between the anode (A) and the cathode (C), both the forward and reverse resistances should be large.

 

2. Check the quality of the luminous digital tube

 

First set the multimeter to R×10k or R×l00k block, and then connect the red meter pen to the “ground” terminal of the digital tube (using the common cathode digital tube as an example). The black pens are connected to the other terminals of the digital tube in turn. Should be respectively light, otherwise the digital tube is damaged.

 

3. Measuring the polarity of each foot of the rectifier bridge

 

The multimeter is set to R×1k block, the black table pen is connected to any pin of the bridge stack, and the red pens measure the remaining three feet. If the readings are infinite, the black meter pen is connected to the positive output of the bridge stack if the reading is 4 to 10 kΩ. Then, the pins connected to the black leads are the negative output of the bridge stack, and the remaining two pins are the AC input of the bridge stack.

 

4. Bipolar thyristor polarity identification

 

The bidirectional thyristor has a main electrode 1, a main electrode 2 and a control electrode. If a multimeter R×1k block is used to measure the resistance between the two main electrodes, the reading should be approximately infinity, and the positive and negative between the control electrode and any one of the main electrodes. The resistance reading is only a few tens of euros. According to this characteristic, we can easily identify the control pole of the bidirectional thyristor by measuring the resistance between the electrodes. When the black pen is connected to the main electrode 1 The forward resistance measured when the red lead is connected to the control electrode is always smaller than the reverse resistance. Therefore, it is easy to identify the main electrode 1 and the main electrode 2 by measuring the size of the resistance.

 

5. Determine the quality of the crystal

 

First use a multimeter (R×10k block) to measure the resistance at both ends of the crystal. If it is infinite, it means that there is no short circuit or leakage of the crystal; insert the test pencil into the power socket and pinch any pin of the crystal with your finger. The other pin touches the metal part of the top of the test pencil. If the test pencil is blistering, it indicates that the crystal is good. If the lamp does not light, it indicates that the crystal is damaged.

 

6. Distinguishing the electrode of junction FET

 

Place the multimeter in the R×1k block, contact the black probe with the pin G assumed to be the gate G, and then contact the other two pins with the red pen. If the resistance is relatively small (5 to 10 Ω), turn the red , black table pen exchange measurement once. If the resistance value is large (∞), it means that the reverse resistance (PN junction reverse) is an N-channel tube, and the contact pin of the black meter pen is the gate G, and the original assumption is correct. If the re-measured resistances are all very small, it means that the resistance is a forward resistance, which belongs to the P-channel FET. The black probes are also connected to the gate G.

 

If the above situation does not occur, red and black meter pens can be exchanged and tested as described above until the gate is judged. The source and drain of the junction FET are generally symmetrical during manufacture. Therefore, when the gate G is determined, it is not necessary to determine the source S and the drain D, since these two electrodes can be used interchangeably. . The resistance between the source and drain is several thousand ohms.

 

7. Triode electrode discrimination

 

For a model with unclear or unmarked triodes, you can use a multimeter to test if you want to identify the three electrodes. First set the multimeter's range switch to R×100 or R×1k. The red meter pen touches one of the triode electrodes, and the black one touches the other two electrodes in order. The resistance between them is measured. If the resistance is a few hundred ohms, the red contact is the base electrode b. This tube is a PNP tube. If the high resistance of several tens to hundreds of kilohms is measured, the electrode contacted by the red meter pen is also the base b, which is an NPN tube.

 

Based on the determination of the tube type and the base b, the collector is determined using the principle that the forward current amplification factor of the triode is larger than the reverse current amplification factor. It is arbitrarily assumed that one electrode is the c-pole and the other electrode is the e-pole. Place the multimeter's range switch on the R×1k barrier. For: PNP tube, the red table pen then c pole, black table pen then e pole, and then pinch the pipe at the same time the b, c pole, but can not make b, c poles directly collide, measure a resistance. Then the second measurement of the two table pens is performed and the resistance of the two measurements is compared. For the PNP type pipe, the resistance value is small, and the electrode connected to the red meter pen is the collector. For the once small NPN tube resistance, the electrode connected to the black meter pen is the collector electrode.

 

8. Potentiometer good and bad

 

The nominal resistance of the potentiometer is first measured. Use the ohmmeter of the multimeter to measure "1" and "3" at both ends (set "2" as the movable contact). The reading should be the nominal value of the potentiometer. If the indicator of the multimeter does not move, the resistance value will not change or A large difference in resistance indicates that the potentiometer is damaged. Check if the contact between the movable arm of the potentiometer and the resistor is good. Use ohmmeter of multimeter to measure “1”, “2” or “2”, “3” at both ends, and turn the potentiometer's rotating shaft counterclockwise to close to the “off” position. The resistance should be as small as possible. Then, slowly rotating the shaft along the clockwise rotation, the resistance should gradually increase. When rotating to the extreme position, the resistance value should be close to the nominal value of the potentiometer. For example, when the pointer of the multimeter has jumped during the rotation of the shaft handle of the potentiometer, the contact point of the kicking activity touch is poor.

 

9. Laser diode damage judgment

 

Remove the laser diode and measure its resistance. Under normal conditions, the reverse resistance should be infinite, and the forward resistance should be 20kΩ~40kΩ. If the measured forward resistance has exceeded 50kΩ, the performance of the laser diode has decreased; if the forward resistance has exceeded 90kΩ, the tube is damaged and can no longer be used.

 

10. Discriminate the infrared receiver head pin

 

Set the multimeter to R×1k block, first suppose that one of the feet of the receiver head is grounded, connect it to the black meter pen, measure the resistance of the other two legs with the red meter pen, and compare the resistance values ​​measured twice (usually at 4 to 7 kΩ Range)) When the resistance is small, the red lead is connected to the +5V power pin, and the other resistance is the signal pin. Conversely, if the red ground leads are connected to the known ground feet, the black pens measure the known power pin and signal foot respectively, the resistance is all above 15kΩ, the pin with small resistance is the +5V end, and the resistance is too large. Signal side. If the measurement result meets the above resistance value, the receiving head can be judged to be in good condition.

 

11. Judging the polarity of unsigned electrolytic capacitors

 

First short-circuit discharge capacitor, and then the two leads to do A, B mark, multimeter set R × 100 or R × 1k block, black table pen to A lead, red table lead to B lead, wait until the pointer is stationary after reading, measuring finished After the short-circuit discharge; then black lead to B lead, red lead to A wire lead, compare the two readings, the resistance of a large black pen is connected to the positive, the red lead is connected to the negative.

 

12. Light-emitting diode

 

Take an electrolytic capacitor with a capacity greater than 100 "F (the larger the capacity, the more obvious the phenomenon), first charge it with a multimeter R x 100 block, the black meter pen is connected to the positive pole of the capacitor, the red meter pen is connected to the negative pole, and after charging, the black meter pen is changed For the negative electrode of the capacitor, connect the LED under test between the red meter and the positive electrode of the capacitor.If the LED is bright and then gradually extinguishes, it indicates that it is good.At this time, the red LED is connected to the negative electrode of the LED, and the positive electrode of the capacitor is connected to the light. The anode of the diode, if the LED does not light, reconnect the two ends of the test, and it will not light, indicating that the LED is damaged.

 

13. Optocoupler Detection

 

The multimeter uses resistor R×100, and must not select R×10k to prevent the battery voltage from overshooting the LED. The red and black pens are connected to the input terminals to measure the positive and negative resistances. The normal 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 LED is damaged. The multimeter chooses resistance R×1 block. The red and black pens are connected to the output terminals to measure the positive and negative resistances. Normal times are close to ∞, otherwise the light pipes are damaged. The multimeter selects resistance R×10, and the red and black pens respectively measure the insulation resistance between the light emitting tube and the light receiving tube at the input and output terminals. (The conditional application Megohmmeter measures the insulation resistance. The megohmmeter output rated voltage should be Slightly lower than the withstand voltage value of the optocoupler under test), the insulation resistance of the light-emitting tube and the light-receiving tube should be ∞.

 

14. Photoresistor detection

 

When testing, set the multimeter to R×1kΩ, and keep the light-receiving surface of the photo-resistor perpendicular to the incident light. Then, the resistance directly measured on the multimeter is the light resistance. Place the photoresistor in a completely dark place. The resistance measured by the multimeter is a dark resistance. If the light resistance is several thousand ohms to several tens of dry ohms, the dark resistance is several to several tens of megaohms, indicating that the photosensitive resistor is good.

 

15. Measure leakage resistance of bulk capacitors

 

With 500 type multimeter placed R × 10 or R × 100 block, when the pointer points to the maximum value, and then immediately use R × 1k block measurement, the pointer will stabilize in a relatively short period of time, which reads out the resistance of the leakage resistor.