A multimeter to handle all component detection
The digital multimeter is a relatively simple measuring instrument and an essential tool for electronic engineers. This article will teach you how to use a digital multimeter to check whether the components are normal. Digital multimeters can be used to detect the characteristics of components such as resistance, capacitance, current, diodes, transistors, and MOS field effect transistors. Digital multimeter function introduction:
1. Measuring resistance value
a. First adjust the multimeter to the ohm block (ohm is the unit of resistance value), and select a suitable range (generally choose 10K or 20K).
b. Put the red and black test leads of the multimeter on both ends of the resistance (resistance is not divided into positive and negative), and then observe the reading of the multimeter. If there is no reading, it may be because the range is too small. Select a large range and re-measure. .
2. Detection of photoresistor quality
When testing, turn the multimeter to the R×1kΩ block, and keep the light-receiving surface of the photoresistor perpendicular to the incident light, so the resistance measured directly on the multimeter is the light 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, it means that the photoresistor is good.
3. Measure the capacitance value
a. First adjust the multimeter to the capacitance gear, generally only one range is used to measure capacitance.
b. Put the red and black test leads of the multimeter on the two ends of the capacitor respectively, and then observe the reading of the multimeter. Note that some capacitors have positive and negative poles (such as electrolytic capacitors, generally the long leg is positive and the short leg is negative), so when measuring a capacitor with positive and negative poles, connect the red test lead to the positive and the black test lead to the negative.
4. Judging whether the crystal oscillator is good or bad
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 pen into the mains jack, pinch any pin of the crystal oscillator with your fingers, The other pin touches the metal part at the top of the test pen. If the neon bubble of the test pen is red, it means that the crystal oscillator is good; if the neon bulb is not bright, it means that the crystal oscillator is damaged.
5. Measure the polarity of each leg of the rectifier bridge
Set the multimeter to R×1k block, connect the black test lead to any pin of the bridge stack, and measure the remaining three pins successively with the red test lead. If the readings are all infinite, then the black test lead is connected to the output positive pole of the bridge stack. If the reading is 4~10kΩ , then the pin connected to the black test lead is the output negative pole of the bridge stack, and the other two pins are the AC input terminals of the bridge stack.
6. Detect line breakpoints
First adjust the multimeter to the AC 2V gear.
7. One-way thyristor detection
The R×1k or R×100 block of the 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 be low resistance (100Ω～lkΩ), 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. The thyristor has 3 PN junctions in total, and we can judge whether it is good or bad by measuring 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 disconnected; measure the resistance between the control pole (G) and the anode (A) When measuring the resistance, the forward and reverse resistance readings should be very large; when measuring the resistance between the anode (A) and the cathode (C), the forward and reverse resistance should be very large.
8. Polarity identification of bidirectional thyristor
The bidirectional thyristor has the main electrode 1, the main electrode 2 and the control pole. If the resistance between the two main electrodes is measured with a multimeter R×1k, the reading should be approximately infinite, and the positive and negative resistance between the control pole 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 pole of the bidirectional thyristor by measuring the resistance between the electrodes. And when the black test lead is connected to the main electrode 1. The forward resistance measured when the red test pen is connected to the control electrode is always smaller than the reverse resistance, so we can easily identify the main electrode 1 and the main electrode 2 by measuring the resistance.
9. Identification of triode electrodes
For a triode with unclear or unmarked models, if you want to distinguish their three electrodes, you can also use a multimeter to test them. First turn the range switch of the multimeter on the R×100 or R×1k resistor. The red test lead randomly touches one electrode of the triode, the black test lead touches the other two electrodes in turn, and measure the resistance value between them respectively. If the measured resistance is a few hundred ohms, the electrode contacted by the red test lead is the base b. This tube is a PNP tube. If the high resistance of tens to hundreds of kiloohms is measured, the electrode contacted 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 triode is larger than the reverse current amplification factor. Arbitrarily assume that one electrode is c-pole and the other electrode is e-pole. Turn the multimeter range switch on the R×1k resistor. 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 at the same time with your hand, but do not make the b and c poles directly touch each other to measure a certain resistance value. Then the two test leads are reversed for the second measurement, and the two measured resistances are compared. For: PNP type tube, the one with the smaller resistance value, the electrode connected to the red test lead is the collector. For the NPN type tube with a small resistance, the electrode connected to the black test lead is the collector.
10. Measuring the leakage resistance of bulk capacitors
Use a 500-type multimeter to place R×10 or R×100, and when the pointer points to the maximum value, immediately switch to R×1k to measure, the pointer will stabilize in a short period of time, so as to read the resistance value of the leakage resistance.
11. Check whether the light-emitting digital tube is good or bad
First set the multimeter to the R×10k or R×l00k gear, then connect the red test lead to the “ground” terminal of the digital tube (take the common cathode digital tube as an example), and connect the black test lead to the other terminals of the digital tube in turn. They should be illuminated separately, otherwise the digital tube is damaged.
12. Identify the electrodes of the junction field effect transistor
Put the multimeter in the R×1k block, touch the pin assumed to be grid G with a black test lead, and then touch the other two pins with a red test lead, if the resistance values are relatively small (5-10 Ω), then touch the red test lead , The black test lead is exchanged and measured once. If the resistance values are all large (∞), it means that they are all reverse resistances (the PN junction is reversed), and they are N-channel tubes, and the pin contacted by the black test pen is the grid G, and it shows that the original assumption is correct. If the resistance value measured again is very small, it means that it is a forward resistance, which belongs to the P-channel field effect transistor, and the black test lead is also connected to the gate 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 junction field effect transistors are symmetrical during manufacture, so when the gate G is determined, it is not necessary to distinguish the source S and drain D, because these two poles can be used interchangeably . The resistance between source and drain is several thousand ohms.
13. Judging the polarity of unsigned electrolytic capacitors
First short-circuit and discharge the capacitor, then mark the two leads as A and B, set the multimeter to R×100 or R×1k gear, connect the black test lead to A lead, and the red test lead to B lead, read after the pointer is still, and finish the measurement Then 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 the positive pole, and the red test lead is the negative pole.
14. Judgment of the quality of the potentiometer
First measure the nominal resistance of the potentiometer. Use the ohm block of the multimeter to measure both ends of "1" and "3" (set "2" as a 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 does not move or A large difference in resistance value indicates that the potentiometer is damaged. Then check whether the movable arm of the potentiometer is in good contact with the resistor sheet. Use the ohm block of the multimeter to measure the two ends of "1", "2" or "2", "3", and turn the shaft of the potentiometer counterclockwise to the position close to "off". At this time, the resistance should be as small as possible. , and then slowly rotate the handle clockwise, the resistance should gradually increase, and 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 movable contact is in poor contact.
15. Identify the pins of the infrared receiver
Set the multimeter to the R×1k block, first assume that a certain foot of the receiving head is the ground terminal, connect it to the black test lead, measure the resistance of the other two feet with the red test lead, and compare the resistance values measured twice (usually between 4 ~ 7k Q range), the one with the smaller resistance is connected to the 5V power supply pin, and the one with the larger resistance is the signal pin. Conversely, if the red test pen is used to connect the known ground pin, and the black test pen is used to measure the known power supply pin and signal pin respectively, then the resistance value is above 15kΩ, the pin with a small resistance value is the 5V terminal, and the pin with a large resistance value is signal end. If the measurement results meet the above resistance value, it can be judged that the receiving head is in good condition.
16. 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 with R×100 gear, connect the black test lead to the positive pole of the capacitor, and the red test lead to the negative pole. After charging, change the black test lead to For the negative pole of the capacitor, connect the measured light-emitting diode between the red test lead and the positive pole of the capacitor. If the light-emitting diode lights up and then goes out gradually, it indicates that it is good. At this time, the red test lead is connected to the negative pole of the light-emitting diode, and the positive pole of the capacitor is connected to the light-emitting diode. The anode of the diode. If the light-emitting diode does not light up, reverse its two ends and reconnect it for testing. If it still does not light up, it means that the light-emitting diode is damaged.
