First, ordinary diode colored end identifies a very negative electrode;

Two, LED long foot is positive, short foot is negative. If the feet are the same length, the larger dots inside the LEDs are negative and the smaller ones are positive. Some LEDs have a flat surface, and a lead near the flat surface is the negative electrode.

In the multimeter: the red pen is connected with "+", the black pen is connected with "-"; In the measurement of light-emitting diodes, low barrier can not be measured, available RX10K file measurement, two table pen contact diode two levels. If the resistance is small, the black watch pen is connected to the positive pole, the resistance is large, the black watch pen is connected to the negative pole. Light-emitting diodes, if connected with TTL components, generally need to be connected with a 470R step-down resistor, in order to prevent damage to the device.

Three, crystal diode
The crystal diode consists of a PN junction, two electrode leads and a shell. The PN junction on both sides of the lead to be encapsulated, is the crystal diode. The letter symbol of the crystal diode is V. The conduction direction of a PN junction is from a p-type semiconductor to an n-type semiconductor, that is, p-to-n conduction (P is the positive electrode and N is the negative electrode). PN junction forward guide through, up to reverse, has the characteristics of single - phase conduction.