To judge whether a transistor is good or bad, you can proceed as follows:
1. How to identify base and tube types:
Use R×100 gear for measuring germanium tubes and R×1K gear for silicon tubes. 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 the two smaller resistance values. If you can't move the red test lead to the other pins, continue to measure until two small resistances are reached. If you can't find two small resistors by fixing the red test lead, you can fix the black test lead and continue searching. . After finding two small resistors, the pin used by a fixed test lead is the base.
If the fixed test lead is a black test lead, the transistor is NPN type; if the fixed test lead is a red test lead, the transistor is PNP.
2. Distinguish collector:
Because the β is large when the emitter and collector of the transistor are correctly connected (the hands of the watch swing greatly), and the β is much smaller when the transistor emitter and collector are connected in reverse. Therefore, first assume that there is a collector connected with an ohmic gear (for a PNP tube, the emitter is connected to the black test lead and the collector is connected to the red test lead). When measuring, pinch the base and the assumed collector with your hands (or connect it with a 100KΩ resistor). The two poles should not be in contact. If the pointer swings greatly, but the pointer swings small after the two poles are reversed, it means that the assumed collector is correct. , thereby determining the collector and emitter.
If the transistor has the resistance value you mentioned above, either the multimeter is faulty or the transistor is bad. It is better to first find a good triode and 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 transistor must be correctly identified, otherwise, not only will the access circuit not work properly, but the transistor may also be burned out. Knowing the type and electrode of the transistor, the method for judging the quality of the transistor with an analog multimeter is as follows:
①Measure the NPN transistor: Set the ohm block of the multimeter at R × 100 or R × lk, connect the black test lead to the base, and connect the red test lead to the other two poles in succession. If the resistance values measured twice are larger than small, then connect the red test lead to the base, and connect the black test lead to the remaining two poles. If the resistance values measured twice are large, it means that the transistor is good.
②Measure the PNP transistor: Set the ohm block of the multimeter at R × 100 or R × lk, connect the red test lead to the base, and connect the black test lead to the other two poles in succession. If the resistance values measured twice are larger than small, then connect the black test lead to the base, and connect the red test lead to the remaining two poles. If the resistance values measured twice are large, it means that the transistor is good.
When the marking on the transistor is unclear, you can use a multimeter to initially determine the quality and type of the transistor (NPN type or PNP type), and identify the three electrodes e, b, and c. The test method is as follows:
① Use a pointer multimeter to determine the type of base b and triode: Set the ohm block of the multimeter at R × 100 or R × lk, first assume that a certain pole of the triode is the base, and connect the black test lead to the assumed base. Connect the red test lead to the remaining two poles one after another. If the resistance values measured twice are very small (or about a few hundred ohms to several thousand ohms), then the assumed base is correct and the transistor under test is NPN. type tube; same as above, if the resistance values measured twice are very large (about several thousand ohms to tens of kiloohms), then the assumed base is correct and the tested transistor is a PNP type tube. If the resistance values measured twice are one large and one small, the originally assumed base is wrong. At this time, the other electrode must be re-assumed as the base, and the above test must be repeated.
② Determine the collector c and emitter e: Still set the pointer multimeter ohm block at R × 100 or R × 1k. Taking the NPN tube as an example, connect the black test lead to the assumed collector c and the red test lead to the assumed On the emitter e, hold the b and c electrodes with your hands (b and c cannot be in direct contact), pass the human body, connect a bias resistor between b and C, read the resistance value shown on the meter, and then Reverse the connection of the two test leads and retest. If the resistance value measured for the first time is smaller than the second time, it means that the original hypothesis is established, because the small resistance value of c and e means that the current passing through the multimeter is large and the bias is normal. Today's analog multimeters have an interface for measuring the transistor amplification factor (Hfe). You can estimate the amplification factor of the triode.
