Identification and detection of crystal diodes

A crystal diode, also known as a semiconductor diode or simply a diode, is essentially a PN junction. A lead is drawn from each of the P and N regions, and then packaged in a tube to create a diode. The arrow indicates the direction of current flow. The lead out terminal of zone P is called the positive electrode, and the lead out terminal of zone N is called the negative electrode, with the text symbol VD and graphic symbol.

1. Classification of diodes

According to different manufacturing processes, diodes can be divided into three types: point contact type, surface contact type, and planar type.

According to the material, it can also be divided into silicon tubes and germanium tubes.

According to their purposes, they can be divided into detectors, rectifiers, voltage regulators, light-emitting tubes, and switching tubes.

A crystal diode has low resistance and conducts when subjected to a forward voltage, and high resistance and shuts off when subjected to a reverse voltage, indicating that the crystal diode has unidirectional conductivity.

2. Main parameters of diodes

(1) Maximum rectified current/FNt. The maximum forward average current allowed to pass through a diode during long-term use is called the maximum rectified current, commonly referred to as the rated operating current, which is determined by the PN junction area and heat dissipation conditions.

(2) Maximum reverse operating voltage URM. The maximum reverse voltage specified to ensure the normal operation of a diode without breakdown is often referred to as the rated operating voltage. In general, the maximum reverse operating voltage is about half of the breakdown voltage.

(3) Maximum reverse current/RM. The maximum reverse current is the reverse current at the maximum reverse operating voltage, and the smaller this value, the better the unidirectional conductivity of the diode.

3. Detection of ordinary diodes

Commonly used crystal diodes include 2AP, 2CP, 2CZ, and 1N4000 series.

Generally, diodes have polarity markings on the casing. If there are no markings, the polarity of the diode can be determined by its small forward resistance and large reverse resistance, and this characteristic can also be used to detect the quality of the diode.

(1) Performance discrimination. The testing method is that the greater the difference between the forward and reverse resistance values of the crystal diode, the better. The greater the difference between the two, the better the unidirectional conductivity of the diode. If the forward and reverse resistance values of the diode are very similar, it indicates that the tube is broken. If both the forward and reverse resistances are small or zero, it indicates that the tube has been broken down and the two electrodes have short circuited; If both the forward and reverse resistances are high, it indicates that there is an open circuit inside the tube and it cannot be used.

(2) Polarity discrimination. When testing the forward and reverse resistance, when the measured resistance value is small, the electrode connected to the black probe is the positive terminal of the diode, and the electrode connected to the red probe is the negative terminal of the diode.

The forward and reverse resistance values of the diode are significantly different from the range (R × 100, R × lk) selected for the multimeter ohm range, which is a normal phenomenon.

4. Detection of light-emitting diodes

(1) Discrimination of positive and negative polarity. Most light-emitting diodes are transparent or semi transparent. By observing the size of the two metal plates inside the light-emitting diode, the polarity of the diode can also be determined. Usually, the larger end of the metal sheet is the negative electrode, and the smaller end of the metal sheet is the positive electrode. If the LED is new, it can be judged by the length of the pins, that is, the long pin is the positive pole and the short pin is the negative pole.

The positive and negative polarity can also be determined by measuring the forward and reverse resistance values of the light-emitting diode using the R × lOk range of a multimeter. When the pointer of the multimeter deviates significantly in the positive direction, the black probe is connected to the positive terminal.

(2) Judgment of performance quality. When measuring the forward and reverse resistance of a light-emitting diode using the R × lOk range of a multimeter, it should have the measurement characteristics of a regular diode. When measuring the forward resistance value, a faint light will be emitted inside the tube.

Attention: Do not measure the forward and reverse resistance values of the light-emitting diode using the R × lk range of the multimeter, otherwise it will be found that the forward and reverse resistance values are close to infinity. This is because the forward conduction voltage of the light-emitting diode is greater than 1.8V, which is higher than the voltage value of the 1.5V battery when the multimeter is in the R × lk range and cannot be made conductive. At this point, the performance of the light-emitting diode can be judged by connecting an external battery. Measure the schematic diagram, and if the diode can emit light normally, it indicates that the light-emitting diode is intact.

Two multimeters, both placed in the R × 10 range, can be connected in series to test the light-emitting diode. If the diode can emit light normally, it indicates that the light-emitting diode is intact.

There is also a color changing light-emitting diode that can emit three colors. It consists of two light-emitting diodes connected by a common cathode, with K being the common cathode (negative electrode), R being the positive electrode of the red light-emitting diode LED1, and G being the positive electrode of the green light-emitting diode LED2. The detection method is the same as that of a regular light-emitting diode.