The emitter and collector of the transistor are measured by testing

There are many types of transistors at present. It is difficult to determine the pin polarity only from the pin arrangement, so a multimeter is often used to determine the pin polarity. The method of using a multimeter to detect transistors is simple and convenient. The principle of using a multimeter to determine the polarity of the transistor pins is: the transistor is composed of two PN junctions. For the NPN transistor, its base is the common positive electrode of the two PN junctions; for the PNP transistor, its base is the common negative electrode of the two PN junctions, so the base and tube type of the transistor can be determined. According to the characteristics that when the emitter junction voltage applied to the transistor is positive and the collector junction voltage is negative, the transistor works in the amplification state. At this time, the penetration current of the transistor is large, and the emitter and collector of the transistor can be measured.

1 pointer multimeter to detect transistors

1.1 Pointer multimeter to detect ordinary transistors

When using a pointer multimeter to judge the three electrodes, polarity and quality of an ordinary transistor, the measurement and judgment is often carried out in two steps by selecting the R×100 or R×1k gear.

(1) Three inversions, find the base; PN junction, determine the tube type

The internal equivalent diagram of the transistor is shown in Figure 1. When measuring, you should always keep this diagram in mind so that you can become proficient through practice.

Figure 1 Internal equivalent diagram of a transistor

① Three reversals to find the base. Take any electrode and set it as the base (such as electrode 2), connect any probe to this electrode, and use the other probe to measure the remaining two electrodes (such as electrodes 1 and 3), and record the data twice; then, swap the probes, measure again according to the above method, and record the data twice. In these three reversal measurements (not necessarily three times), if the measurement results are that the resistance values ​​are very small twice (forward resistance) and the resistance values ​​are very large twice (reverse resistance), then the assumed base is correct.

②PN junction, determine the tube type. After finding the base of the transistor, the conductivity type of the tube can be determined according to the direction of the PN junction between the base and the other two electrodes. In the above measurement process, if the black test lead is connected to the base and the measured resistance is very small, the tube is NPN type; conversely, if the red test lead is connected to the base and the measured resistance is very small, the tube is PNP type. The measurement diagram for finding the base and determining the tube type is shown in Figure 2.

Figure 2 Schematic diagram of measurement for finding base and determining tube type

(2) Determine the base and collector: Follow the arrow, the deflection is large; if it cannot be measured, move your mouth

After finding the base, determine whether it is PNP or NPN type, and then find the emitter and collector.

①Follow the arrow, the deflection is large. At this time, the collector and emitter can be determined by measuring the penetration current ICEO.

For NPN transistors, use the black and red test leads to measure the forward and reverse resistances RCE and REC between the two poles. Although the deflection angles of the multimeter pointer are very small in both measurements, if you observe carefully, there will always be a slightly larger deflection angle. At this time, the current flow direction must be: black test lead → C pole → B pole → E pole → red test lead. The current flow direction is exactly the same as the arrow direction in the transistor symbol ("follow the arrow"), so the black test lead must be connected to the collector and the red test lead must be connected to the emitter.

For PNP transistors, the principle is similar to that of NPN transistors. The current flow direction must be: black test lead → E pole → B pole → C pole → red test lead. The current flow direction is also consistent with the arrow direction in the transistor symbol, so the black test lead must be connected to the emitter, and the red test lead must be connected to the collector.

② If no measurement is available, move your mouth. If in the measurement process of "follow the arrow, large deflection", the two measurement pointer deflections before and after the reversal are too small to distinguish, you need to "move your mouth". The specific method is: in the two measurements of "follow the arrow, large deflection", pinch the joints of the two test leads and the pins with two fingers respectively, hold the base B with your mouth (or press it with your tongue), and still use the judgment method of "follow the arrow, large deflection" to distinguish the collector from the emitter. The human body plays the role of a DC bias resistor, the purpose is to make the effect more obvious. The measurement diagram for judging the base and collector is shown in Figure 3.

Figure 3 Schematic diagram of base and collector measurement

The normal forward and reverse resistance values ​​between the transistor electrodes are shown in Table 1.

Table 1 Normal transistor inter-electrode forward and reverse resistance values

1.2 Detection of Band-Rejection Transistor

The detection of band-resistance transistors is basically similar to that of ordinary transistors, but because there is a resistor inside, the resistance value detected is slightly different. Take the NPN transistor in Figure 4 as an example, select a pointer multimeter, set the range to R×1k, and if the band-resistance transistor is normal, the following rules apply.

①The forward and reverse resistances between the B and E poles are relatively small (the specific measured value is related to the internal resistance), but the forward resistance between the B and E poles (black test lead connected to B, red test lead connected to E) will be slightly smaller, because the emitter junction will be turned on when measuring the forward resistance.

②The forward resistance between the B and C poles (black test lead connected to B, red test lead connected to C) is small, and the reverse resistance is close to infinity.

③The forward and reverse resistances between the E and C poles are close to infinity.

When the test results do not match the above, it can be determined that the band-stop transistor is damaged.

Figure 4 Detection of band-stop transistor

1.3 Detection of damped transistor

The detection of damped transistors is basically similar to that of ordinary transistors, but because of the damping diode inside, the detected resistance value is slightly different. Take the NPN transistor in Figure 4 as an example, select a pointer multimeter, set the range to R×1k, if the damped transistor is normal, the following rules apply.

①The forward and reverse resistances between the B and E poles are relatively small, but the forward resistance (black test lead connected to B, red test lead connected to E) will be slightly smaller.

②The forward resistance between the B and C poles (black test lead connected to B, red test lead connected to C) is small, and the reverse resistance is close to infinity.

③The forward resistance between the E and C poles (black test lead connected to C, red test lead connected to E) is close to infinity, and the reverse resistance is very small (because the damping diode will be turned on).

When the test results do not match the above, it can be judged that the damped transistor is damaged.

1.4 Detection of Darlington (compound tube) transistor

Taking the NPN Darlington transistor in Figure 5 as an example, select a pointer multimeter and set the range to R×10k. If the Darlington transistor is normal, the following rules apply.

①The forward resistance between the B and E poles (black test lead connected to B, red test lead connected to E) is small, but its reverse resistance is infinite.

②The forward resistance between the B and C poles (black test lead connected to B, red test lead connected to C) is small, and the reverse resistance is close to infinity.

③The forward and reverse resistances between the E and C poles are close to infinity.

When the test results do not match the above, it can be determined that the Darlington transistor is damaged.

Figure 5 Detection of Darlington transistor

2. Digital multimeter to detect transistors

The digital multimeter can not only identify the polarity of transistor pins and measure the common emitter current amplification factor hFE of the tube, but also distinguish silicon tubes from germanium tubes. Since the test current of the digital multimeter's diode block is very small, it is not suitable for testing transistors. The diode block or hFE block should be used for testing.

Set the digital multimeter to the diode position, connect the red test lead to any pin, and touch the other two pins in turn with the black test lead. If the displayed values ​​are both less than 1V or the overflow symbol "0L" or "1" is displayed twice, the pin connected to the red test lead is the base B. If in two tests, one displayed value is less than 1V, and the other displayed the overflow symbol "OL" or "1" (depending on the digital multimeter), it means that the pin connected to the red test lead is not the base B, and other pins should be replaced and measured again until the base B is found.

After the base is determined, connect the red test lead to the base, and the black test lead to the other two pins in turn. If the values ​​on the display screen are all 0.600~0.800V, the transistor being tested is a silicon NPN medium and small power tube. Among them, the pin connected to the black test lead is the emitter when the value displayed is larger. If the values ​​on the display screen are all 0.400~0.600V, the transistor being tested is a silicon NPN high power tube. Among them, the pin connected to the black test lead is the emitter when the value displayed is larger.