Crystal diodes are often represented by "D" plus numbers in the circuit, such as: D5 represents the diode numbered 5.
1. Function: The main characteristic of the diode is unidirectional conductivity, that is, under the action of forward voltage, the on-resistance is very small; while under the action of reverse voltage, the on-resistance is extremely large or infinite. Because diodes have the above-mentioned characteristics, they are often used in rectification, isolation, voltage regulation, polarity protection, coding control, FM modulation and squelch circuits in cordless telephones. The crystal diodes used in telephones can be divided into: rectifier diodes (such as 1N4004), isolation diodes (such as 1N4148), Schottky diodes (such as BAT85), light-emitting diodes, and Zener diodes.
2. Identification method: The identification of diodes is very simple. The N pole (negative pole) of low-power diodes is mostly marked with a color circle on the surface of the diode. Some diodes also use a diode-specific symbol to indicate the P pole (positive pole) or N pole. (Negative), there are also symbols marked "P" and "N" to determine the diode polarity. The positive and negative poles of the light-emitting diode can be identified from the length of the pins. The long pin is positive and the short pin is negative.
3. Test precautions: When using a digital multimeter to measure the diode, the red test lead is connected to the positive electrode of the diode, and the black test lead is connected to the negative electrode of the diode. The resistance value measured at this time is the forward conduction resistance value of the diode. The connection method of the multimeter's test leads is just the opposite.