Multimeter inspection method of components

1. Measure the polarity of each leg of the rectifier bridge
The multimeter is set to R×1k block, the black test lead is connected to any pin of the bridge stack, and the red test lead measures the remaining three pins successively. If the reading is infinite, then the black test lead is connected to the output positive pole of the bridge stack, if the reading is 4～10kΩ , The pin connected to the black test lead is the output negative terminal of the bridge stack, and the remaining two pins are the AC input terminals of the bridge stack.
  2. Judge the quality of the crystal oscillator
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 hold any pin of the crystal oscillator with your finger The other pin touches the metal part on 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 is not bright, it means the crystal oscillator is damaged.
3. Unidirectional thyristor inspection
The R×1k or R×100 block of a multimeter can be used to measure the forward and reverse resistance between any two poles. If the resistance of a pair of poles is found to have a low resistance value (100Ω～lkΩ), then the black test pen is connected to the control The red test lead is connected to the cathode, and the other is the anode. The thyristor has 3 PN junctions, and we can judge its quality by measuring the forward and reverse resistance of the PN junction. When measuring the resistance between the control electrode (G) and the cathode (C), if the forward and reverse resistances are both zero or infinite, it indicates that the control electrode is short-circuited or open; measuring the resistance between the control electrode (G) and the anode (A) In the case of resistance, both the positive and reverse resistance readings should be large;
When measuring the resistance between the anode (A) and the cathode (C), both the forward and reverse resistance should be large.
4. Polarity recognition of bidirectional thyristor
The bidirectional thyristor has a main electrode 1, a main electrode 2 and a control electrode. If the resistance between the two main electrodes is measured with 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 tens of ohms. According to 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. The forward resistance measured when the red test lead is connected to the control electrode 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 the R×10k or R×100k block, then connect the red test pen to the “ground” terminal of the digital tube (take the common cathode digital tube as an example), and the black test pen sequentially connects to the other terminals of the digital tube. They should emit light separately, otherwise the digital tube is damaged.
6. Identify the electrode of the junction field effect tube
Place the multimeter in the R×1k block, touch the pin assumed to be grid G with the black test pen, and then touch the other two pins with the red test pen respectively. If the resistance values are both relatively small (5-10 Ω), set the red , Black test pen exchange measurement. If the resistance values are all large (∞), it means that they are all reverse resistance (PN junction is reversed), which are N-channel tubes, and the pin contacted by the black meter pen is the grid G, and that the original assumption is correct. If the resistance values measured again are very small, it means that it is a forward resistance, which belongs to the P-channel FET, and the black meter pen is also connected to the grid G. If the above situation does not occur, you can exchange the red and black test leads, and test according to the above method until the grid is judged. Generally, the source and drain of the junction field effect transistor are symmetrical during manufacturing. Therefore, after the gate G is determined, it is not necessary to distinguish the source S and the drain D, because these two electrodes can be used interchangeably . The resistance between the source and drain is several thousand ohms.
7. Discrimination of triode electrode
For a triode with an unclear or unmarked model, if you want to distinguish its three electrodes, you can also use a multimeter to test it. First set the multimeter range switch on the R×100 or R×1k electric barrier. The red test lead randomly touches one electrode of the triode, the black test lead touches the other two electrodes in turn, and the resistance between them is measured respectively. If the measured resistance is a few hundred ohms, the electrode touched by the red test lead is the base b. This tube is a PNP tube. If the measured high resistance is tens to hundreds of kiloohms, the electrode touched by the red test lead 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 the principle that the forward current amplification factor of the transistor 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. Set the multimeter range switch on the R×1k electrical barrier. For: PNP tube, connect the red test lead to the c pole and the black test lead to the e pole, and then pinch the b and c poles of the tube at the same time with your hands, but you can not directly touch the b and c poles to measure a certain resistance value. Then the two test leads are adjusted for the second measurement, and the resistance measured twice is compared. For the PNP type tube, the resistance value is small, and the electrode connected to the red test lead is the collector. For the NPN tube with small resistance, the electrode connected to the black test lead is the collector.
  8. Judgment of the quality of the potentiometer
First measure the nominal resistance of the potentiometer. Use the multimeter's ohm block to measure the "1" and "3" ends (set "2" as the movable contact), and the reading should be the nominal value of the potentiometer, such as the pointer of the multimeter does not move, the resistance value does not move or If the resistance value differs a lot, it indicates that the potentiometer has been damaged. Then check whether the movable arm of the potentiometer is in good contact with the resistance piece. Use the ohm block of a multimeter to measure the ends of "1", "2" or "2", "3", and turn the potentiometer's shaft counterclockwise to a position close to "off". At this time, the resistance should be as small as possible , And then slowly rotate the shaft handle along the clock, the resistance should gradually increase, and when it reaches the extreme position, the resistance should be close to the nominal value of the potentiometer. For example, the pointer of the multimeter is beating during the rotation of the potentiometer's shaft handle, and the contact point of the active touch point is poor.
  9. Measure the leakage resistance of large-capacity capacitors
Use a 500-type multimeter to set the R×10 or R×100 block. When the pointer points to the value, immediately switch to the R×1k block to measure. The pointer will stabilize in a short period of time, so that the leakage resistance value can be read.
10. Identify the pin of the infrared receiver
Set the multimeter to the R×1k block. First assume that a certain leg of the receiving head is grounded. Connect it to the black test lead. Use the red test lead to measure the resistance of the other two pins. Compare the resistance values measured twice (usually 4～7k Q Range), the red test lead with smaller resistance is connected to the +5V power supply pin, and the other with larger resistance is the signal pin. On the contrary, if you use the red test lead to connect to the known ground pin, and the black test lead to measure the known power pin and signal pin respectively, the resistance values are all above 15kΩ, the pin with the smaller resistance value is the +5V terminal, and the pin with the larger resistance value For the signal terminal. If the measurement result meets the above resistance value, it can be judged that the receiving head is intact.
11. Determine the polarity of unsigned electrolytic capacitors
First short-circuit the capacitor and discharge it, then mark the two leads A and B, set the multimeter to R×100 or R×1k, connect the black test lead to the A lead, and the red test lead to the B lead. Read the reading after the pointer is stationary. After the short-circuit discharge; then connect the black test lead to the B lead, and the red test lead to the A lead. Compare the two readings. The black test lead with the larger resistance value is connected to the positive electrode, and the red test lead is connected to the negative electrode.
12. Measuring light-emitting diodes
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×100 block, connect the black test lead to the positive electrode of the capacitor, and the red test lead to the negative electrode. After the charge is completed, the black test lead should be changed. The negative electrode of the capacitor, connect the LED under test between the red test lead and the positive electrode of the capacitor. If the LED turns off gradually after it lights up, it indicates that it is good. At this time, the red test lead is connected to the negative electrode of the light-emitting diode, and the positive electrode of the capacitor is connected to light. The anode of the diode. If the light-emitting diode does not light up, swap its two ends and reconnect the test. If it still does not light up, it indicates that the light-emitting diode is damaged.
13. Photocoupler detection
The multimeter uses the resistance R×100 block instead of the R×10k block to prevent the battery voltage from breaking down the light-emitting diode. Connect the red and black test leads to the input terminals to measure the forward and reverse resistance. Normally, the forward resistance is tens of ohms, and the reverse resistance is a few thousand to tens of thousand ohms. If the forward and reverse resistances are similar, it indicates that the light-emitting diode has been damaged. Multimeter selects resistance R×1 block. Connect the red and black test leads to the output terminals, and measure the positive and reverse resistances. Normally, both are close to ∞, otherwise the light tube will be damaged. The multimeter selects the resistance R×10, the red and black test leads are respectively connected to the input and output ends to measure the insulation resistance between the light-emitting tube and the light-receiving tube (if possible, use a megohmmeter to measure the insulation resistance. At this time, the megohmmeter output rated voltage should be Slightly lower than the allowable withstand voltage value of the photoelectric coupler under test), the insulation resistance between the light-emitting tube and the light-receiving tube should normally be ∞.
14. Photoresistor detection
During detection, set the multimeter to the R×1kΩ block, and keep the light-receiving surface of the photoresistor perpendicular to the incident light, so the resistance directly measured on the multimeter is the bright resistance. Then put the photoresistor in a completely dark place, then the resistance measured by the multimeter is the dark resistance. If the light resistance is several thousand ohms to tens of dry ohms, and the dark resistance is several to tens of megohms, the photoresistor is good.
15. Laser diode damage judgment
Remove the laser diode and measure its resistance. Normally, the reverse resistance should be infinite, and the forward resistance should be 20kΩ～40kΩ. If the measured forward resistance value has exceeded 50kΩ, it means that the performance of the laser diode has been degraded; if its forward resistance value has exceeded 90kΩ, it means that the tube has been damaged and can no longer be used.