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15 major experience sharing of component testing

  • Author:o-leading.
  • Source:o-leading.com
  • Release on :2019-02-13
A large number of various types of electronic components are used in electronic devices, and equipment failures are mostly caused by failure or damage of electronic components. Therefore, how to correctly detect electronic components is particularly important, which is also the skills that electronic maintenance personnel must master.



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First, measure the polarity of each leg of the rectifier bridge

The multimeter is set to R×1k block, the black pen is connected to any pin of the bridge stack, and the red test pen measures the remaining three legs. If the readings are infinite, the black test pen is connected to the output positive pole of the bridge stack. If the reading is 4~10kΩ The pin connected to the black pen is the output negative of the bridge stack, and the remaining two pins are the AC input terminals of the bridge stack.


Second, judge the quality of the crystal

First use a multimeter (R × 10k block) 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 pencil into the mains socket and pinch any pin of the crystal with your finger. The other pin touches the metal part at the top of the test pencil. If the test pencil is red, the crystal is good. If the bubble is not bright, the crystal is damaged.


Third, one-way thyristor detection

The R×1k or R×100 block of the multimeter can be used to measure the positive and negative resistances of any two poles. If the resistance of a pair of poles is found to be low resistance (100Ω~lkΩ), then the black test pen is connected to control. The pole is connected to the cathode and the other is the anode. There are three PN junctions in the thyristor. We can judge the quality of the PN junction 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 positive and negative resistances are both zero or infinite, it indicates that the control electrode is short-circuited or open; the measurement between the control electrode (G) and the anode (A) For resistance, both positive and negative resistance readings should be large; when measuring the resistance between anode (A) and cathode (C), the forward and reverse resistance should be large.


Fourth, the polarity identification of the triac

The triac has a main electrode 1, a main electrode 2 and a control electrode. If the resistance between the two main electrodes is measured by a multimeter R×1k block, the reading should be approximately infinite, 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 ohms. According to this characteristic, it is easy to identify the control pole of the triac by measuring the resistance between the electrodes. And when the black pen is connected to the main electrode 1. The forward resistance measured when the red meter is connected to the gate 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 resistance.



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Fifth, check the quality of the LED

First set the multimeter to R×10k or R×l00k, then connect the red test lead to the “ground” lead of the digital tube (for example, the common digital tube), and the black test lead is connected to the other terminals of the digital tube. Should be separately illuminated, otherwise the digital tube is damaged.

six, Identify the electrode of the junction field effect transistor

Place the multimeter in the R×1k block, touch the pin assumed to be the gate G with the black pen, and then touch the other two pins with the red pen. If the resistance is small (5~10Ω), then red, The black test leads are exchanged once. If the resistance is large (∞), it means that the reverse resistance (PN junction reverse) is an N-channel tube, and the pin touched by the black test lead is the gate G, which indicates that the original assumption is correct. If the resistance value measured again is small, it means that it is a forward resistance, belonging to the P-channel field effect tube, and the black meter pen is also connected to the gate G. If the above situation does not occur, you can change the red and black test leads and test according to the above method until the gate is judged. Generally, the source and drain of the junction field effect transistor are symmetrical at the time of manufacture. Therefore, after the gate G is determined, the source S and the drain D do not have to be discriminated because the two poles can be used interchangeably. . The resistance between the source and the drain is several thousand ohms.

Seven, Discrimination of Triode Electrode

For a triode with an unclear or unmarked model, you can use a multimeter to test out the three electrodes. First, turn the multimeter's range switch on the R × 100 or R × 1k electric block. The red test pen arbitrarily contacts one electrode of the triode, and the black test pen contacts the other two electrodes in turn, and respectively measures the resistance value between them. If the measured resistance is several hundred ohms and low resistance, the electrode touched by the red test pen is the base b, This tube is a PNP tube. If the high resistance is measured from tens to hundreds of kiloohms, the electrode contacted by the red pen is also the base b, and the tube is an NPN tube.

On the basis of discriminating the tube type and the base b, the collector is determined by 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 c pole and the other electrode is e pole. Turn the multimeter's range switch on the R × 1k electrical block.

For: PNP tube, make the red meter pen connected to the c pole, the black meter pen to the e pole, and then pinch the b and c poles of the tube at the same time, but can not make the b and c poles directly touch each other and measure a certain resistance value. Then, the two test pens perform the second measurement, and compare the resistances measured twice. For the PNP type tube, the resistance is small once, and the electrode connected to the red test pen is the collector. For the NPN type tube resistance value, the electrode connected to the black test pen is the collector.





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Eight, the quality of the potentiometer

First measure the nominal resistance of the potentiometer. Use the ohmmeter of the multimeter to measure the "1" and "3" ends (the "2" end is the active contact). The reading should be the nominal value of the potentiometer. If the pointer of the multimeter does not move, the resistance does not move or A large difference in resistance indicates that the potentiometer is damaged. Then check if the movable arm of the potentiometer is in good contact with the resistor.

Use the ohmmeter of the multimeter to measure the "1", "2" or "2", "3" ends, and turn the potentiometer's shaft counterclockwise to the position close to "off". At this time, the resistance should be as small as possible. Then, slowly rotate the shaft handle clockwise, the resistance should be gradually increased. When it is rotated to the extreme position, the resistance should be close to the nominal value of the potentiometer. For example, during the rotation of the shaft handle of the potentiometer, the pointer of the multimeter has a beating image, and the touch of the kicking activity is poor.


Nine, measuring the leakage resistance of large capacity capacitor

Use a 500-type multimeter to place R × 10 or R × 100 gears. When the pointer points to the maximum value, immediately use R × 1k block measurement, the pointer will stabilize in a short time, thus reading the leakage resistance resistance.


Ten, distinguish the infrared receiver head pin

The multimeter is set to R×1k, first assume that a certain foot of the receiving head is the grounding end, connect it with the black test pen, and measure the resistance of the other two feet with the red test pen, and compare the resistance values ​​measured twice (generally in the range of 4-7 kQ) ), when the resistance is small, the red test lead is connected to the +5V power supply pin, and the other resistance value is the signal pin.

On the other hand, if the red pen is used to connect the known foot, and the black pen measures the known power pin and signal pin respectively, the resistance is above 15kΩ, the pin with small resistance is +5V, and the pin with large resistance is too large. For the signal side. If the measurement result meets the above resistance value, it can be judged that the receiving head is intact.


Eleven, Judging the polarity of unsigned electrolytic capacitor

First, short-circuit the capacitor, then mark the two leads with A and B. The multimeter is set to R×100 or R×1k. The black meter is connected to the A lead, and the red meter is connected to the B lead. After the pointer is stationary, the reading is completed. After short-circuit discharge; then connect the black test lead to the B lead, the red test lead to the A lead, compare the two readings, the black pen with a larger resistance value is connected to the positive pole, and the red test lead is connected to the negative pole.






Twelve, measuring 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 the multimeter R × 100 block, the black pen is connected to the positive electrode, the red test pen is connected to the negative pole, after the charging is completed, the black test pen is changed. The negative pole of the capacitor connects the LED to be tested between the red test lead and the positive pole of the capacitor.

If the LED is lit up and then goes out, it is good. At this time, the red pen is connected to the negative pole of the LED, and the positive pole of the capacitor is connected to the anode of the LED. If the LED is not lit, reconnect the two ends of the test and it is not lit, indicating that the LED is damaged.


Thirteen, Photocoupler detection

The multimeter should use the resistor R×100 block, and R×10k block should not be selected to prevent the battery voltage from being too high to break through the LED. The red and black test leads are connected to the input terminal, and the positive and negative resistances are measured. The normal forward resistance is several tens of ohms, and the reverse resistance is several thousand ohms to several tens of kilo ohms.

If the positive and negative resistances are similar, the LED is damaged. The multimeter selects the resistor R × 1 block. The red and black test leads are connected to the output end, and the positive and negative resistances are measured. When normal, they are close to ∞, otherwise the light pipe is damaged. The multimeter selects the resistor R×10 block, and the red and black test pens respectively connect the input and output terminals to measure the insulation resistance between the light-emitting tube and the light-receiving tube. (The conditional application of the megohmmeter is used to measure the insulation resistance. At this time, the megohmmeter output rated voltage should be Slightly lower than the allowable voltage value of the optocoupler under test), the insulation resistance of the light-emitting tube and the light-receiving tube should be normal.


Fourteen, the detection of photoresistor

During the test, the multimeter is turned to the R×1kΩ block, and the light-receiving surface of the photoresistor is kept perpendicular to the incident light, so that the resistance directly measured on the multimeter is a bright resistance. The photoresistor is placed in a completely dark place, and 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 photoresistor is good.


Fifteen, laser diode damage discrimination

Remove the laser diode and measure its resistance. Under normal conditions, the reverse resistance should be infinite, and the forward resistance should be between 20kΩ and 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.