Measuring the quality of a transistor with a pointer multimeter
In circuit testing can be divided into on state testing or off state testing. In on state testing, the base voltage can be measured, usually 0.7V for silicon tubes. The voltage of germanium tube is 0.2-0.3V. Indicates normal operation. Otherwise, it is in a cut-off state.
When the power is off, the forward and reverse resistance of PN junction of triode can be measured to see if it is normal. Some triodes can be removed for measurement because of small resistance or inductance in parallel.
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 to know the type and electrode of the transistor:
1. NPN transistor measurement: Set the multimeter's ohm to "R × 100 "or" R × At lk, connect the black probe to the base electrode and the red probe to the other two electrodes in sequence. If the resistance values measured twice are both small, then connect the red probe to the base electrode and the black probe to the other two electrodes in sequence. If the resistance values measured twice are both large, then the transistor is good.
2. Measuring PNP transistor: Set the multimeter's ohm to "R × 100 "or" R × At lk, connect the red probe to the base electrode and the black probe to the other two electrodes in sequence. If the resistance values measured twice are both small, then connect the black probe to the base electrode and the red probe to the other two electrodes in sequence. If the resistance values measured twice are both large, then the transistor is good.
When the markings on the transistor are not clear, a multimeter can be used to preliminarily determine the quality and type of the transistor (NPN type or PNP type), and identify the three electrodes e, b, and c.
The testing method is as follows:
1. Using a pointer multimeter to determine the type of base b and transistor: Set the multimeter's ohm to "R × 100 "or" R × At lk, first assume that a certain pole of the transistor is the "base", and connect the black lead to the assumed base, followed by the red lead to the other two poles. If the resistance values measured twice are very small (or about a few hundred to several thousand ohms), then the assumed base is correct, and the transistor being tested is an NPN type transistor. Similarly, if the resistance values measured twice are both large (about a few thousand ohms to several tens of thousands of ohms) The assumed base is correct, and the tested transistor is a PNP type transistor. If the resistance values measured twice are one large and one small, then the originally assumed base electrode is incorrect. At this point, it is necessary to assume the other electrode as the "base electrode" and repeat the above test.
2. Determine the collector c and emitter e: still set the ohm gear of the pointer multimeter to "R × 100 "or" R × At 1k ", using an NPN tube as an example, connect the black lead to the assumed collector c and the red lead to the assumed emitter e, and pinch the b and c electrodes with your hand (do not allow b and c to come into direct contact) 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 to retest. If the resistance value measured in the first time is smaller than that in the second time, it indicates that the original hypothesis is valid, because the resistance values of c and e indicate that the current passing through the multimeter is high and the bias is normal. Nowadays, pointer multimeters have interfaces for measuring the amplification factor (Hfe) of transistors. You can estimate the amplification of the transistor.
