Triode and field effect tube measurement

Set the multimeter to the R×100 range, connect the red test lead to one leg, and connect the black test lead to the other leg, leaving the third pin floating. If you find that the hands of the watch have a slight swing, it proves that the third pin is the grid. To obtain a more obvious observation effect, you can also use the human body to approach or touch the dangling foot with your fingers. As long as you see the hand making a large deflection, it means that the dangling foot is the grid, and the remaining two feet are the source and drain respectively.
Reasons for judgment: The input resistance of JFET is greater than 100MΩ, and the transconductance is very high. When the grid is open, the spatial electromagnetic field can easily induce a voltage signal on the grid, which makes the tube tend to be cut off or turned on. If the human body induced voltage is directly applied to the grid, the above phenomenon will be more obvious due to the strong input interference signal. If the watch needle deflects to the left by a large margin, it means that the tube tends to be cut off, the resistance RDS between the drain and the source increases, and the current between the drain and the source decreases IDS. On the contrary, if the hand of the watch deflects to the right side by a large margin, it means that the tube tends to conduct, RDS↓, IDS↑. However, the direction in which the hands of the watch deflects depends on the polarity of the induced voltage (forward voltage or reverse voltage) and the operating point of the tube.
2. Transistor measurement
1. Find the base and determine the tube type (NPN or PNP)
For the PNP transistor, the C and E poles are the positive poles of the two PN junctions inside, and the B pole is their common negative pole. For the NPN transistor, the opposite is true: the C and E poles are the two PN junctions. The negative pole, and the B pole is their shared positive pole. According to the characteristics of the PN junction with small forward resistance and large reverse resistance, it is easy to determine the type of base and tube. The specific method is as follows:
Set the multimeter to the R×100 or R×1K gear. The red pen touches a certain pin, and the black test pen is used to connect the other two pins respectively, so that three sets of readings (two times in each set) can be obtained, when one set of secondary measurements is a low resistance value of several hundred ohms When the common pin is a red test lead, the contact is the base, and the transistor type is PNP; if the common pin is a black test lead, the contact is also the base, and the transistor has an NPN type. .
2. Discrimination of emitter and collector
Since the doping concentration of the two P regions or the two N regions is different when the triode is made, if the emitter and collector are used correctly, the triode has a strong amplification ability. On the contrary, if the emitter and collector are used interchangeably, Then the amplification ability is very weak, so the emitter and collector of the tube can be distinguished.
After distinguishing the tube type and base b, the following methods can be used to distinguish the collector and emitter.
Place the multimeter on the R×1K position. Pinch the base electrode and the other pin together by hand (be careful not to directly touch the electrode). To make the measurement obvious, moisten your finger and connect the red test lead to the pin that is pinched together with the base electrode. , Connect the black test lead to the other pin, and observe the amplitude of the multimeter pointer swinging to the right. Then swap the two pins and repeat the above measurement steps. Compare the amplitude of the watch hand swinging to the right in the two measurements, and find the one with the larger swing amplitude. For the PNP type transistor, connect the black test lead to the pin pinched together with the base electrode, repeat the above experiment to find out the large swing range of the test needle. For the NPN type, the black test lead is connected to the collector and the red test lead is connected. It is the emitter. For the PNP type, the red test lead is connected to the collector, and the black test lead is connected to the emitter.
The principle of this distinguishing electrode method is to use the battery inside the multimeter to apply voltage to the collector and emitter of the triode, so that it has the ability to amplify. When there is a hand to pinch its base and collector, it is equivalent to adding a forward bias current to the triode through the resistance of the hand to turn it on. At this time, the amplitude of the hand swing to the right reflects the magnitude of its amplification ability, so it can be correctly judged Come out the emitter and collector.
The following is the structure of the triode and the various identification methods in the circuit diagram

Use a multimeter to test the transistor
as shown in Figure 12. The Iceo of the triode can be checked. Generally, the measured resistance of the germanium medium and small power tubes should be above 10-20kΩ, and the silicon tube should be above 100kΩ. In fact, most of the tubes can't see the movement of the hands, that is, the indication value is infinite. If the measured resistance value is too small, it indicates that the Iceo is too large and the tube cannot be used; if the resistance value is close to zero, it indicates that the tubes C and E have broken down. Use the same method to check Icbo. Just change the test leads to the B and C poles. And pay attention to measure its reverse resistance.
On the basis of measuring IcEo, connect a 10kΩ (silicon tube) or 20kΩ (germanium tube) resistor, as shown by the dotted line in Figure 12. Then you can check the tube magnification factor Hfe. After the resistance is connected, the smaller the indicator value of the meter needle becomes, the larger the hFE of the tube is; if the resistance value remains unchanged or the change is small. It shows that the tube is damaged or has poor amplification ability. In the actual test, you can also use your hands (wet to be wet) to pinch the B and C poles to replace the resistance. This is a method of using human body resistance. Now many multimeters are equipped with a triode test block (hole), which can directly measure the hFE of the tube and judge the B, E, and c poles. This test method should be used as much as possible in practice.
3. Rough inspection of junction field effect tube
Use a multimeter to R*100, the measured forward resistance of the G pole of the 3DJ6 tube to the other two electrodes should be 4-10K, and the reverse resistance should be infinite; the measured resistance between the D and S electrodes should be both positive and negative. K. If the measured value does not meet the requirements, it indicates that the tube is broken or has poor performance.

When turning the multimeter to the diode position (buzzer position) to measure the triode, first we must determine which pin is the b pole. Touch any one of the feet with the red pen without moving, and touch the other two feet with the black pen. If two sets of numbers that are similar and less than 1, can be measured, it means that the red pen is touching the b pole at this time. If the two sets of numbers are not similar, it means that the red pen is not touching the b pole at this time. You should replace the red pen with one foot and the black pen to test the other two feet until you find the b pole. If we know which foot is the b pole, how can we determine which of the other two feet is the c pole and the e pole? As shown below:

The red pen in the picture is pole b. The black pen measured two sets of similar data on the other two feet. One set of data will be slightly larger. This leg is the e pole, and the smaller leg is the c pole. And we know that this tube is an NPN transistor, because the red pen is at the b pole! The measurement method for the PNP transistor is the same, except that the black meter pen is at the b pole and the red pen touches the other two legs to measure two sets of similar data, as shown in the figure below:

The following is the measurement method of the field effect tube.
The English abbreviation of field effect tube is FET, which can be divided into junction field effect tube (JFET) and insulated gate field effect tube (MOSFET). We usually referred to as MOS tube for short, and MOS tube can be divided into enhancement type and depletion type. And what we usually use in the motherboard is the enhanced MOS tube.
The picture below shows the logo of the MOS tube:

The three GDS pins of the MOS tube commonly used in our motherboards are fixed, whether it is N-channel or P-channel, the chip is placed from left to right as G pole, D pole and S pole! As shown below:

The measurement sequence is:
1. To measure the MOS tube with a diode file, first short-circuit the three pins to discharge the tube, then connect the red test lead to the S pole and the black test lead to the D pole. If the measured value is more than 500, it means that the tube is N-channel.

2. The black pen does not move, and the red pen touches the G pole and the measured value is 1.

3. Move the red pen back to the S pole, and the tube should be on at this time.
4. Then the red pen measures the D pole and the black pen measures the S pole. The value should be 1. (Pay attention to this step. Because the voltage of the G pole 2.5V multimeter was given in the previous measurement, the DS is still connected. However, it will return to normal after about 10 seconds. It is recommended to short-circuit the three first during this step. The pin discharges the tube).
5. Then the red pen does not move, and the black pen is used to measure the G pole. The value should be 1.
At this point, we can determine that this N-channel field tube is normal. Some people say that the next two steps can be omitted. However, I habitually use all five steps. Personal habit is a problem! ! ! Of course, the measurement procedure for the P channel is the same, except that the step is to measure the S pole with the black probe and the D pole with the red probe, and more than 500 values can be measured.

My understanding of the junction type is that there is conduction between the D pole and the S pole during measurement. If the red test lead is connected to the G pole and the black test lead is connected to the DS pole respectively, two sets of similar values should be measured. This tube is the N channel. Tao. If the black test lead is connected to the G pole and the red test lead is touched to the DS pole respectively, two sets of similar values should be measured. This tube is a P channel.