How to use a multimeter to judge whether the triode is good or bad
Use the unidirectional conductivity of the PN junction in the triode to check the forward and reverse resistance of the PN junction between the electrodes. If the difference is large, the tube is good. If the forward and reverse resistances are both large, it means that there is an open circuit inside the tube or the performance of the PN junction. not good. If the forward and reverse resistances are small, it means that the tubes are short-circuited or broken down.
Use digital meter and pointer meter to judge the quality of NPN transistor
1. The digital multimeter is placed in the pn junction block
Connect the red test lead to B, and the black test lead to E. If the highest bit still shows 1, it means that the emission junction is not connected, and the judgment is bad;
Connect the red test lead to B, and the black test lead to C. If the highest bit still shows 1, it means that the collector junction is not connected, and the judgment is bad.
Connect the red test lead to C, and the black test lead to E. If the display becomes a limited number or even beeps, it means that there is a breakdown between C-E, and the judgment is bad.
2. The pointer multimeter is placed in the X1k electric barrier
Connect the black test lead to B, and the red test lead to E. If the watch needle does not move at all, it means that the emission connection is not connected, and the judgment is bad;
Connect the black test lead to B, and the red test lead to C. If the test needle does not move at all, it means that the collector junction is not connected, and the judgment is bad.
Connect the black test lead to C, and the red test lead to E. If the watch needle swings, it means that there is a breakdown between C-E, and it is judged to be bad.
The judgment of the triode is good or bad, according to the following method:
1. Discrimination method of base and tube type:
Use the R×100 file for the germanium tube to be measured, and the R×1K file for the silicon tube. First, fix the red test lead in contact with any one of the legs, and measure the remaining two legs with the black test lead. See if you can measure two small resistance values. If you can’t move the red test lead to other pins and continue to measure two small resistances, if you can’t find two small resistances with the red test lead fixed, you can fix the black test lead and continue to search. . When two small resistors are found, the pin used by the fixed test lead is the base.
If the fixed test lead is a black test lead, the triode is NPN type, and if the fixed test lead is red, the tube is PNP.
2. Discriminant collector:
Because when the emitter and collector of the triode are correctly connected, the β is large (the swing range of the watch needle is large), and the β is much smaller when the connection is reversed. Therefore, first assume that a collector is connected with an ohm file (for PNP type tubes, the emitter is connected to the black test lead, and the collector is connected to the red test lead). When measuring, pinch (or connect with a 100KΩ resistor) the base and the hypothetical collector. The two poles cannot be in contact. If the pointer swings a lot, and the pointer swings small after the poles are reversed, it means that the assumption is the correct collector. , thus determining the collector and emitter.
The triode with the resistance value you mentioned above is either a problem with the multimeter, or the triode is bad. It is better to find a good triode first, measure it according to the above method, and then compare it, so that you can judge whether it is good or bad and sum up experience.
The pins of the triode must be correctly identified, otherwise, the connected circuit will not only fail to work normally, but may also burn out the transistor. Knowing the type and electrode of the transistor, the method of judging the quality of the transistor by the pointer multimeter is as follows:
①Measure NPN transistor: set the ohm block of the multimeter to R × 100 or R × lk, connect the black test lead to the base, and connect the red test lead to the other two poles successively. If the resistance values measured twice are lower than Then connect the red test lead to the base, and connect the black test lead to the other two electrodes successively. If the resistance values measured twice are very large, it means that the triode is good.
②Measure the PNP transistor: set the ohm block of the multimeter to R × 100 or R × lk, connect the red test lead to the base, and connect the black test lead to the other two poles successively. If the resistance values measured twice are lower than Then connect the black test lead to the base, and connect the red test lead to the other two electrodes successively. If the resistance values measured twice are very large, it means that the triode is good.
When the marks on the triode are not clear, you can use a multimeter to preliminarily determine the quality and type of the triode (NPN or PNP), and identify the three electrodes e, b, and c. The test method is as follows:
①Use a pointer multimeter to determine the type of base b and the transistor: set the ohm of the multimeter to R × 100 or R × lk, first assume that a certain pole of the transistor is the base, and connect the black test lead to the hypothetical base. Connect the red test lead to the other two poles successively. If the resistance values measured twice are very small (or about several hundred ohms to several thousand ohms), then the assumed base is correct, and the triode under test is NPN. Same as above, if the resistance values measured twice are very large (approximately several thousand ohms to tens of thousand ohms), then the assumed base is correct, and the tested triode is a PNP type tube. If the resistance values measured twice are one large and one small, the base assumed originally is wrong. At this time, another electrode must be assumed to be the base again, and the above test must be repeated.
②Judging the collector c and emitter e: still place the ohm block of the pointer multimeter at R × 100 or R × 1k, take the NPN tube as an example, connect the black test lead to the hypothetical collector c, and the red test lead to the hypothetical On the emitter e of the emitter, and pinch the b and c poles by hand (cannot make b and c directly contact), pass through the human body, connect the bias resistor between b and c, read the resistance value shown on the meter, and then connect The two test leads are reversed and retested. If the resistance value measured for the first time is smaller than the second time, it means that the null hypothesis is established, because the resistance values of c and e are small, which means that the current passing through the multimeter is large and the bias is normal. The current pointer multimeter has an interface for measuring the magnification of the triode (Hfe). You can estimate the magnification of the triode.
