SMD diodes are also called crystal diodes, or diodes for short. In addition, there are early vacuum electronic diodes; it is an electronic device that conducts current in one direction. There is a PN junction and two lead terminals inside the semiconductor diode. This kind of electronic device has the conductivity of unidirectional current according to the direction of the applied voltage. Generally speaking, the chip crystal diode is a p-n junction interface formed by sintering a p-type semiconductor and an n-type semiconductor. A space charge layer is formed on both sides of the interface to form a self-built electric field. When the applied voltage is equal to zero, the diffusion current caused by the difference in the carrier concentration on both sides of the p-n junction is equal to the drift current caused by the self-built electric field and is in an electrical equilibrium state. This is also the diode characteristic under normal conditions.

Characteristics of SMD LEDs

①The principle of luminescence is cold luminescence, not by heating or discharging. Therefore, the life of the component is about 50-100 times longer than that of a tungsten bulb, which is about 100,000 hours.

②No warm-up time is required, and the lighting response speed is faster than ordinary lamps (about 3 ~ 400ns).

③ High electro-optical conversion efficiency, low power consumption, saving about 1/3 ~ 1/20 of energy consumption than light bulbs.

④Good shock resistance, high reliability, and low system operation cost.

⑤Easy to be small, thin, lightweight, without shape limitation, and easy to be made into various applications.

Precautions for SMD LEDs

1. Clean, do not use unknown chemical liquids to clean the SMD LEDs: Unknown chemical liquids may damage the SMD LEDs. When cleaning is necessary, immerse the SMD LED in alcohol for less than 1 minute at normal room temperature and let it dry naturally for 15 minutes before starting to use it.

2. Moisture-proof, in order to prevent the product from absorbing moisture during transportation and storage, the packaging of the SMD LED is packaged in a moisture-proof aluminum packaging bag, and the packaging bag contains a desiccant and a humidity card. The desiccant is mainly used to control the content of the packaging bag. Humidity, the humidity card is mainly used to monitor the humidity in the packaging bag.

3. After the storage package is sealed, store it under the condition that the temperature is <40℃.

SMD LED advantages

SMD LEDs are affixed to the surface of the circuit board, suitable for SMT processing, and can be reflowed. SMD LEDs solve the problems of brightness, viewing angle, flatness, reliability, and consistency. Compared with other packaged devices, it It has the advantages of strong anti-vibration ability, low solder joint defect rate, and good high-frequency characteristics. It encapsulates more LED chips in a smaller area, uses lighter PCB and reflective layer materials, and shows that the reflective layer needs to be filled. There is less epoxy resin. By removing the heavier carbon steel material pins and reducing the size, the weight of the product can be easily reduced by half. The volume and weight are only about 1/10 of the traditional plug-in components. After the use of SMD packaging, The volume of electronic products is reduced by 40% to 60%, and the weight is reduced by 60% to 80%, which ultimately makes the application more perfect.

The package of the in-line LED adopts the form of potting. The potting process is to first inject liquid epoxy resin into the LED molding cavity, then insert the pressure-welded LED bracket, put it in an oven to allow the epoxy resin to cure, and then separate the LED from the mold cavity and form it. Due to the relatively simple manufacturing process and low cost, it has a high market share.

Method for judging the positive and negative poles of SMD light-emitting diodes

1. Look at the logo

Large-size LEDs have some marks near the pole piece pins, such as cutting corners, painting, or different pin sizes. Generally, there are signs, small pins, and the short side is the cathode (that is, the negative electrode). Small 0805, 0603 packages have a "T" shape or an inverted triangle symbol "T" on the bottom. One horizontal side is the positive pole; the "side" of the triangle symbol is close to the positive pole, and the "corner" close to the negative pole.

2. Measure with a multimeter

When testing the positive and negative poles of the SMD LEDs with a multimeter, be sure to use the "R*10k" gear for testing. When testing, connect the two test leads to the two leads of the light-emitting diode. If the needle is deflected more than halfway, and there is a bright spot in the light-emitting diode, it means that the patch light-emitting diode is connected in the forward direction. At this time, the positive electrode is connected to the black test lead, and the negative electrode is connected to the red test lead. Then swap the two test leads and connect them to the light-emitting diodes (reverse access type), and the test needles will not move. Regardless of whether it is connected in the forward or reverse direction, the hand does not move or the deflection is excessive, indicating that the SMD LED is damaged.

However, for beginners, they don't know how to use a multimeter. At this time, there is actually a point that everyone knows that light-emitting diodes with higher power are divided into positive and negative electrodes. Each pole represents one end, and one end has an extra pin. If you observe carefully, you will find that there is a small hole on this pin. One end with a small hole is the positive electrode, and the other end is the negative electrode.

3. Observe the electrodes in the tube

Observe the electrodes inside the tube. The smaller one is the positive electrode, and the larger one that resembles a bowl is the negative electrode.

4. Look at the missing corner

In the LED energy-saving lamp lighting industry, 5050 SMD LEDs are often used. This SMD LED is square. One of the four right angles has a small corner, and the end with the small corner is the negative electrode. One end is the positive electrode.

5. Look at the pins

The lead of the light-emitting diode is generally longer at the anode and shorter at the cathode.