This post was last edited by zidonghua01 on 2018-7-3 11:02 In the article "Transistor Inspection and Replacement", you can learn the basic knowledge of transistors, learn how to use pointer multimeters and digital multimeters to detect the quality of transistors, and the key points of transistor replacement, which is very helpful for everyone to improve their electronic and electronic component detection and repair skills. Basic knowledge of transistors The transistor is the most important component in the electronic circuit. Its main function is current amplification and switching. The transistor is a current control device. It uses the special structure of the base region to control the collector current through the diffusion and recombination of carriers. The transistor has three working states: cut-off, amplification, and saturation. The amplification state is mainly used in amplification circuits, and the cut-off and saturation states are mainly used in switching circuits. Transistor replacementyunrun.com.cn/tech/1239.html According to the manufacturing material, it can be divided into silicon transistors and germanium transistors; according to the conductivity type, it can be divided into PNP type and NPN type; according to the working frequency, it can be divided into high-frequency and low-frequency transistors; according to the power consumption, it can be divided into low-power and high-power transistors. The graphic symbols of transistors can be viewed in the "Complete Collection of Electrical Graphic Symbols and Electrical Text Symbols" on this site. The transistor is represented by the text symbol VT. Some high-frequency transistors have four electrode pins, namely b, c, e, and d. Among them, electrode d takes over the shell and is connected to the ground wire of the circuit during use to play a shielding role. Transistor detection 1. Use a pointer multimeter to detect. First determine the three electrodes e, b, and c of the transistor. When judging whether it is good or bad, set the multimeter to "R×100" or "R×1k". When measuring NPN transistors, connect the black pen to the b pole, and the red pen to the c pole and e pole respectively. Measure the forward resistance of the two PN junctions, which should be hundreds of ohms or thousands of ohms. Then swap the test pens and measure the reverse resistance of the two PN junctions, which should be tens of kiloohms or hundreds of kiloohms or more. Then measure the resistance between the c pole or the e pole, and then swap the test pens and measure again. The two resistance values should be more than tens of kiloohms, which means that the transistor is basically good. When measuring PNP transistors, the measurement steps are the same as above, but the red pen should be connected to the b pole. During the measurement, if the forward resistance of the PN junction is infinite, it may be a short circuit in the tube. If the reverse resistance of the PN junction is zero, or the resistance between the C pole and the E pole is zero, the transistor has been broken down or short-circuited. If the forward and reverse resistances of the PN junction are not much different, or the resistance between the C pole and the E pole is very small, the transistor may be bad.
Figure 1 Normal inter-electrode resistance of transistors
From the detection methods introduced above, it can be seen that by swapping the red and black test leads of the multimeter, there will be six different ways to connect a transistor. For a normal transistor, the law of its inter-electrode resistance is shown in Figure 1, which can be used as a reference for detection. The circles in the figure represent the red and black test leads of the multimeter, and the squares represent the high and low resistance values. 2. Use a digital multimeter to test ① Determine the b pole. Set the digital multimeter to the diode block, connect the red test lead to a certain electrode, and use the black test lead to touch the other two electrodes respectively. The two displayed values are basically equal and below 1V, or both display overflows, proving that the red test lead is connected to the b pole. If one of the two displayed values is below 1V and the other overflows, it means that the red test lead is not connected to the b pole. You can replace other electrodes and test again. ② Determine the NPN tube and PNP tube. After determining the b pole, connect the red test lead to the b pole and use the black test lead to touch the other two electrodes respectively. If the display is between 0.5-0.7V, the tube is NPN type. If it overflows twice, the tube is PNP type. 3. Online detection method Online detection is to use the DC voltage block of the multimeter to directly measure the voltage value of each pin of the diode on the circuit board to determine whether it is working normally or damaged. This method is often used to detect medium and low power transistors. When the transistor is in the linear amplification state and works normally, its emitter junction (between e and b poles) should have a forward bias voltage, that is, 0.2-0.3V for germanium tubes and 0.6-0.7V for silicon tubes. The collector junction (between c and b poles) should have a reverse bias voltage, generally above 2V. If the measurement result does not match the above range, the transistor may be faulty. However, when the transistor itself is faulty, the DC voltage value of each electrode to the ground will also change. At this time, it can be judged by measuring the voltage of the c pole and the e pole to the ground. It should be noted that, when peripheral components are damaged, the voltages of the transistors at all poles will also change. Therefore, when the online voltage detection method is used, if the transistor electrode voltage is abnormal, it is necessary to detect and determine whether the relevant peripheral components are normal. If it is difficult to confirm whether the peripheral components are normal, it is better to solder the transistor off the printed circuit board and test it to determine whether the transistor is damaged. Transistor replacement After the transistor is damaged, a transistor of the same model and specification should be used for replacement. If you cannot find a transistor that is exactly the same as the original tube, you can consider replacing it with other transistors. However, the following issues need to be noted: the type of replacement tube should be the same as the original tube type, that is, only PNP tubes can replace PNP tubes, NPN tubes can replace NPN tubes; germanium tubes can replace germanium tubes, and silicon tubes can replace silicon tubes. Parameters such as the maximum allowable collector power consumption, maximum allowable collector current, maximum operating voltage, and frequency characteristics should be the same or higher than the original tube; if there are requirements for installation space, the appearance should also be similar to the original tube.
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