Discrimination of polarity and quality of transistor pins

The resistance range of a multimeter can be used to distinguish the polarity of a transistor and determine its quality. When testing low-power transistors, R × 100 (Q) or R × lk (Q) gears are generally used to avoid damaging the transistor.

(1) Discrimination between Tube Type and Base

Set the multimeter to the resistance mode and select the range of R × lk (or R × 100). Contact any probe of the multimeter with an assumed common electrode first, and the other probe with the other two electrodes separately. When the resistance measured twice is very small (or both are very large), the electrode connected to the former is the base. If the resistance values measured twice are significantly different, the assumed base of the former is incorrect and should be replaced with another electrode for retesting. According to the above method, the common pole can be identified, which is the base b.

After determining the base b, use a simulated multimeter with the red probe to contact the base b and the black probe to contact the other two pins. If the meter readings are very small (about a few hundred ohms), it can be concluded that this transistor is an NPN transistor, otherwise it is a PNP transistor.

(2) Discrimination between emitter and collector electrodes

After identifying the base and transistor, determine the emitter and collector. The method is: taking an NPN transistor as an example, assuming that one of the other two pins is the collector, connect the black probe (corresponding to the positive terminal of the battery in the meter) to this pin, connect the red probe to the assumed emitter, and cross a resistor of 100 kQ or more between the assumed collector and the measured base (often hold b and c in your hand, replace them with human resistance, but do not touch the two pins), and record the resistance reading at this time; Set the assumed collector as the emitter and the original emitter as the collector, and repeat the test reading. The assumption that the resistance value is smaller (with a larger deflection angle) in the two readings is correct, with the black probe connected to the collector pin and the remaining pin being the emitter. If it is a PNP tube, swap the probes and use the above method to determine.

(3) Identification of transistor performance

① Judging the quality of a transistor: During testing, use a multimeter to test the forward and reverse resistance of the emitter and collector junctions separately. If it is within the normal range, it indicates that the transistor is fine; otherwise, it is damaged.

② Determination of Penetrating Current ICEO: Use a multimeter R × 100 (Q) or R × 1k (Q) to measure the collector emitter resistance (for NPN transistors, connect the black probe to the collector and the red probe to the emitter). The larger the value, the smaller the ICEO. Generally, silicon tubes should be greater than several megaohms, while germanium tubes should be greater than several kiloohms.

When the measured resistance value is infinite, it indicates that there is a broken wire inside the tube; When the measured resistance value approaches zero, it indicates that the tube has been broken down; Sometimes the resistance value keeps decreasing, indicating unstable performance of the pipe.

③ Estimation of current amplification factor: Use a multimeter R × 100 (Q) or R × 1k (Q) to measure the resistance between the collector and emitter of a transistor (for NPN transistors, connect the black probe to the collector and the red probe to the emitter), observe the reading at this time, and then pinch the base and collector with your fingers (the two poles must not touch each other), while observing the oscillation of the pointer. The larger the swing amplitude, the higher the p-value of the tube.

If it is a PNP tube, swap the probes and use the above method to distinguish. Of course, nowadays tens of thousands of meters in rural areas (such as digital multimeters) have a gear endpoint (hFE) for measuring the amplification factor p. When using, first confirm the transistor type, and then insert the three pins e, b, and c of the tested transistor into the corresponding transistor plug L on the digital multimeter panel. The meter displays the approximate value of hFE.

The method introduced above is a relatively simple test. To further accurately test, a transistor diagram instrument can be used, which can clearly display the characteristic curve and current amplification factor of the transistor.

(4) Identification between germanium tube and silicon tube

To determine whether a transistor is a germanium transistor or a silicon transistor, an analog multimeter or a digital multimeter can be used. Generally, the method of measuring the emitter junction voltage is different.

To use an analog multimeter, the battery, resistor, and tube emitter structure need to form a circuit, and then measure the forward voltage of the emitter junction; Using a digital multimeter is more convenient as it can directly measure the forward voltage of the transmitter junction.

(5) Identification between high-frequency and low-frequency tubes

Distinguishing between high-frequency and low-frequency tubes: The cutoff frequency of high-frequency tubes is greater than 3 MHz, while the cutoff frequency of low-frequency tubes is 3 MHz lower. Generally, the two cannot be used interchangeably. Due to the different models of high and low frequency tubes, when their specifications are clear, they can be easily distinguished by referring to relevant manuals. When their logo models are unclear, BVebo can be used to measure the reverse resistance of the emitter junction with a multimeter to distinguish between high and low frequency tubes.

Taking NPN type tube as an example, place the multimeter in the R × lk position, connect the black probe to the emitter junction e of the tube, and connect the red probe to the pole b. At this time, the resistance value is generally above several hundred kiloohms. Next, pull the multimeter to the R × lOk high blocking position, with the red and black gauge strokes unchanged, and re measure the resistance value between e and b. If the measured resistance value does not change significantly from the first measured resistance value, it can be determined that the tested tube is a low-frequency tube; If the resistance changes significantly, exceeding 1/3 of the multimeter's full capacity, it can be basically determined that the tested tube is a high-frequency tube.

(6) Detection of high-power semiconductor transistors

The method of using universal decay to detect the electrode, tube type, and performance of low-power transistors is basically applicable to high-power transistors, because the metal shell is known (collector), so the discrimination method is relatively simple.

It should be pointed out that due to the large volume of high-power transistors, the inter electrode resistance is relatively small. If the R × 1k (Ω) range of a multimeter is used to detect the forward resistance between the poles of low-power transistors, it will inevitably cause the ohm pointer to approach zero. This situation is similar to an inter electrode short circuit, making it difficult for the detector to make a judgment. To prevent misjudgment, the R × 1 (Ω) or R × 10 (Ω) range should be used when detecting the forward resistance of the PN junction of high-power transistors.