How to measure the quality of a transistor using a multimeter (pointer)
In road testing can be divided into power on state testing or power off state testing. The base voltage can be measured during testing while powered on. The typical voltage for silicon tubes is 0.7V. The voltage of germanium tube is 0.2-0.3V. Indicates normal operation. Otherwise, it is in the cut-off state. When not powered on, measure whether the forward and reverse resistance of the PN junction of the transistor is normal. Some transistors cannot be tested properly due to small resistors or inductors connected in parallel on the circuit and can be removed for measurement.
The pins of the transistor must be correctly identified, otherwise, the connected circuit may not only fail to function properly, but may also burn out the transistor. The method for judging the quality of a transistor using a pointer multimeter is as follows:
① Test NPN transistor: Set the multimeter ohmmeter to "R × 100" or "R × lk", connect the black probe to the base, and connect the red probe to the other two poles in sequence. If the resistance values measured twice are both small, then connect the red probe to the base, and connect the black probe to the other two poles in sequence. If the resistance values measured twice are both large, it indicates that the transistor is good.
② Test PNP transistor: Set the multimeter ohmmeter to "R × 100" or "R × lk", connect the red probe to the base, and connect the black probe to the other two poles in sequence. If the resistance values measured twice are small, connect the black probe to the base, and connect the red probe to the other two poles in sequence. If the resistance values measured twice are both large, it indicates that the transistor is good.
When the markings on the transistor are unclear, a multimeter can be used to preliminarily determine the quality and type of the transistor (NPN type or PNP type), and to distinguish the three electrodes e, b, and c. The testing method is as follows:
① Using a pointer type multimeter to determine the type of base b and transistor: Set the multimeter's ohmmeter to "R × 100" or "R × lk", assume that one pole of the transistor is the "base", connect the black probe to the assumed base, and connect the red probe to the other two poles in sequence. If the resistance values measured twice are both very small (or about a few hundred ohms to several thousand ohms), then the assumed base is correct, and the tested transistor is an NPN type transistor; As above, if the resistance values measured twice are both large (about several thousand to several tens of thousands of ohms), then the assumed base is correct and the tested transistor is a PNP type transistor. If the resistance values measured twice are one big and one small, then the originally assumed base is incorrect. In this case, it is necessary to assume the other electrode as the "base" and repeat the above test.
② To determine the collector c and emitter e: still set the ohmmeter of the pointer type multimeter to "R × 100" or "R × 1k". Taking the NPN tube as an example, connect the black probe to the assumed collector c and the red probe to the assumed emitter e, and pinch the b and c electrodes with your hand (without direct contact between b and c). Through the human body, connect a bias resistor between b and C, read the resistance value shown on the meter head, and then reverse connect the two probes for retesting. If the resistance value measured * * times is smaller than * * times, it indicates that the original hypothesis is true, because the resistance values of c and e indicate that the current passing through the multimeter is large and the bias is normal. Nowadays, pointer multimeters have interfaces for measuring the amplification factor (Hfe) of transistors. Can you estimate the amplification factor of the transistor.
