Distinguishing the quality of crystal diodes 4

The emission color of a light-emitting diode is generally the same as its own color, but in recent years, transparent light-emitting tubes have emerged, which can also emit light in colors such as red, yellow, and green, and can only be known when powered on. There are experimental and visual methods for identifying the positive and negative terminals of light-emitting diodes. The experimental method is to turn on the power and see if it can emit light. If it cannot, it is due to polarity mismatch or damage to the light-emitting tube.

Note that light-emitting diodes are current type devices. Although they can emit light when directly connected to a voltage of 3V at both ends, they are prone to damage. In practical use, it is necessary to connect a limiting resistor in series, and the operating current generally ranges from 1mA to 3OmA depending on the model. In addition, since the conduction voltage of light-emitting diodes is generally above 1.7V, a 1.5V battery cannot light up the light-emitting diodes. Similarly, the R × 1 to R × 1K range of a typical multimeter cannot test light-emitting diodes, while the R × 10K range can light up some light-emitting diodes due to the use of a 15V battery.

By observing the light-emitting diode with the eyes, it can be observed that there are two electrodes inside, one large and one small. Generally speaking, the smaller and shorter electrode is the positive electrode of the light-emitting diode, while the larger electrode is its negative electrode. If you buy a new LED, the longer pin is the positive terminal.