Use a digital multimeter to determine the transistor pins (graphic tutorial)

Nowadays, digital multimeters are very popular electrical and electronic measuring tools. They are easy to use and accurate, and are loved by maintenance personnel and electronic enthusiasts. But some friends say that they are not as good as pointer multimeters when measuring some parts, such as transistors. I think digital multimeters are more convenient when measuring transistors. The following is my own experience. I usually judge small transistor devices in this way. You may try it to see if it is easy to use or correct. If you have any comments or questions, you can send me an email.

I have some BC337 transistors on hand, but I don’t know whether they are PNP or NPN transistors.

Figure 1 Transistor
We know that the inside of a transistor is like two diodes combined. Its form is like the figure below. The middle one is the base (B pole).

Figure 2 The internal structure of a transistor

First, we need to find the base and determine whether it is a PNP or NPN tube. As shown in the figure above, the base of the PNP tube is the common point of the two negative poles, and the base of the NPN tube is the common point of the two positive poles. At this time, we can use the diode range of the digital multimeter to measure the base, see Figure 3. For the PNP tube, when the black test lead (connected to the negative pole of the battery in the meter) is on the base, the red test lead is used to measure the other two poles. Generally, the reading is small (usually 0.5-0.8) with little difference. If the test leads are connected in reverse, it will be a larger reading (usually 1). For the NPN meter, the red test lead (connected to the positive pole of the battery in the meter) is connected to the base. From Figures 4 and 5, we can see that the BC337 on hand is an NPN tube, and the middle pin is the base.

Figure 3 Diode measurement mode of the multimeter

Figure 4 Determine the B pole and tube type of BC337

After finding the base and knowing what type of tube it is, you can determine the emitter and collector. If you use a pointer multimeter, you may have to use both hands to get to this step, and some friends may even use their tongues, which can be quite troublesome. It is much more convenient to use the hFE range of the triode on a digital meter (hFE measures the DC amplification factor of the triode) to measure. Of course, you can also skip the above steps and use hFE to measure the pin polarity of the triode directly. I personally think that it is more convenient and accurate to add the above steps. [page]

Set the multimeter to hFE, lower the BC337 to the small hole of the NPN, and align the B pole with the letter B above. Read the reading, then reverse its other two legs and read again. The polarity of the larger reading aligns with the letter marked on the table. At this time, identify the C and E poles of the BC337 according to the letter. Once you learn it, you can do the same for other transistors, which is convenient and fast.

Figure 5 hFE range on a multimeter

Figure 6 Determine the C and E poles

Figure 7 Determine the C and E poles

2. How to read transistor packaging
Commonly used transistor packaging forms include metal packaging and plastic packaging. The arrangement of pins has a certain pattern.

Place the bottom view position so that the three pins form the vertices of an isosceles triangle, and from left to right they are EBC; for small and medium power plastic transistors, make their planes face toward you as shown in the figure, and place the three pins downwards, then from left to right they are EBC.

3. Use a multimeter to detect the transistor

Determination of transistor base: According to the structural diagram of the transistor, we know that the base of the transistor is the common pole of the two PN junctions in the transistor. Therefore, when determining the base of the transistor, as long as the common pole of the two PN junctions is found, it is the base of the transistor. The specific method is to adjust the multimeter to the R×1k block of the resistance block, first put the red test lead on one leg of the transistor, and use the black test lead to touch the other two legs of the transistor. If it is fully connected twice, the leg where the red test lead is placed is the base of the transistor. If it is not found once, the red test lead is changed to another leg of the transistor and measured twice; if it is still not found, the red test lead is changed again and measured twice. If it is still not found, use the black test lead to put it on one leg of the transistor, and use the red test lead to measure twice to see if it is fully connected. If it is not successful once, change it again. In this way, you can always find the base without measuring 12 times at most.

Identification of transistor types: There are only two types of transistors, namely PNP and NPN. When distinguishing, you only need to know whether the base is P-type material or N-type material. When using the multimeter R×1k range, the black test lead represents the positive pole of the power supply. If the black test lead is connected to the base and it is conductive, it means that the base of the transistor is P-type material, and the transistor is NPN type. If the red test lead is connected to the base and it is conductive, it means that the base of the transistor is N-type material, and the transistor is PNP type.