1、 The common diode has a cathode line mark, and one pole is the negative pole;

2、 The long leg of LED is positive, and the short leg is negative. If the feet are the same length, the big point in the LED is the negative pole and the small one is the positive pole. Some light-emitting diodes have a small plane, and a lead near the small plane is the negative pole. SMD light emitting diodes generally have a small bump to distinguish between positive and negative electrodes, and those with special marks are negative electrodes, while those without special marks are positive electrodes.

In the multimeter, the red pen is connected with "+" and the black pen is connected with "-"; When measuring the light-emitting diode, the low barrier cannot be measured. The RX10K file can be used to measure. Two probes contact the two stages of the diode. If the resistance is small, the black probe is connected to the positive pole, while the resistance is large, and the black probe is connected to the negative pole. If the light-emitting diode is connected to the TTL module for use, it is generally necessary to connect a 470R step-down resistor in series to prevent damage to the device.

1. The LED with large identification size has some marks near the pin of the pole piece, such as chamfering, coloring, or different pin sizes. Generally, the cathode (negative pole) is on the small and short side of the marked LED, and the positive pole is on the side of the small 0805 and 0603 package with "T" at the bottom or the inverted triangle symbol "T" at the horizontal side; The "side" of the triangle symbol is close to the positive polarity, and the "angle" is close to the negative polarity.

2. When using a multimeter to measure the positive and negative poles of the chip LED, the "R * 10k" gear must be used for detection. Connect the two probes with the two leads of the LED during detection. If the needle deflects more than half and there is a light spot in the LED, it means that the chip LED is connected in the positive direction. At this time, the black probe is connected to the positive electrode, and the red probe is connected to the negative electrode. Then connect the two probes to the LED (reverse access type), and the probe will not move. Whether it is forward or reverse connection, the meter needle does not move or deflects to the end, indicating that the chip LED is damaged. However, for beginners, they don't know how to use a multimeter at all. In fact, we all know that high-power LED is divided into positive and negative poles. Each pole represents one end, and one end has an extra pin. Careful observation will show that there is a small hole in this pin. One end of the hole is positive, and the other end is negative.

3. The electrode inside the observation tube is used to observe the electrode inside the tube. The smaller one is the positive electrode, and the larger one, similar to the bowl shaped one, is the negative electrode.

4. Look at the missing angle. In the LED energy-saving lamp lighting industry, the 5050 chip LED is often used. This chip led is square. One of the four right angles has a small missing angle. The end with the small missing angle is the negative pole, and the other end is the positive pole. 5. It can be seen that the positive pole of LED pin is generally long and the negative pole is short.

3、 Crystal diode

The crystal diode consists of a PN junction, two electrode leads and a tube shell. On both sides of the PN junction, a crystal diode is led out and packaged with a wire. The letter symbol of the crystal diode is V. The conduction direction of the PN junction is from the P-type semiconductor to the N-type semiconductor, that is, the conduction from P to N (P is the positive pole, N is the negative pole). The PN junction has the characteristics of conducting in the forward direction and closing in the reverse direction.

The diode package and its orientation are shown as follows:

The method of judging the diode direction by screen printing on PCB is summarized as follows:

Generally:

1. The notched end is the negative pole;

2. One end of the bar is the negative pole;

3. One end with white parallel bars is the negative pole;

4. One end in the direction of the triangle arrow is the negative pole;

5. One end of the small circle of the plug-in diode screen is the negative pole, and the big circle is the positive pole.

In the case of vertical welding, the original body is in the positive ring

6. The first pin of the square hole of the plug-in LED is the positive pole.