Multimeter - Knowledge Summary of Detecting Components
1. Identify the electrodes of junction field effect transistors
Put the multimeter in the R×1k range, use the black test lead to touch the pin that is supposed to be the gate, G, and then use the red test lead to touch the other two pins respectively. If the resistance values are relatively small (5~10Ω), then touch the red test lead , The black test lead is exchanged and measured once. If the resistance value is ∞, it means that it is a reverse resistance (PN junction reverse), which is an N-channel tube, and the pin contacted by the black test pen is the gate 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.
2. Discrimination 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 resistance file. 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. Set the range switch of the multimeter to the R×1k resistance position. For the PNP tube, connect the red test lead to the c pole, 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 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 the PNP type tube, the resistance value is smaller once, and 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.
3. Potentiometer good or bad judgment
First measure the nominal resistance of the potentiometer. Use the ohm range 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 range 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, it means that the movable contact is in poor contact.
4. Measuring Leakage Resistance of Bulk Capacitors
Use a 500-type multimeter to place it in the R×10 or R×100 range. When the pointer points to the maximum value, immediately switch to the R×1k range for measurement. The pointer will stabilize in a short period of time, thereby reading the resistance value of the leakage resistance.
5. Identify the pins of the infrared receiver
Set the multimeter to the R×1 k range, 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 legs with the red test lead, and compare the resistance values measured twice (generally 4 ~ 7kΩ range), the one with the smaller resistance is 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 test the known power supply pin and signal pin respectively, then the resistance value is above 15k Ω, the pin with a small resistance value is the +5V terminal, and the pin with a large resistance value is the +5V terminal. Pins are signal terminals. If the measurement results meet the above resistance value, it can be judged that the receiving head is in good condition.
