Identification and fault diagnosis methods for diodes

The main characteristic of a diode is its unidirectional conductivity, which means that under the action of forward voltage, the on resistance is very small; Under the action of reverse voltage, the conduction resistance is extremely high or infinite.

Diodes are divided into crystal diodes, bidirectional trigger diodes, high-frequency variable resistance diodes, variable capacitance diodes, light-emitting diodes, and Schottky diodes according to their applications.

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 diode meter. Some diodes are also marked with diode specific symbols as "P" and "N" to determine the polarity of the diode. The positive and negative poles of light-emitting diodes can be identified by the length of the pins, with the long pin being positive and the short pin being negative.

Fault characteristics

The faults of diodes mainly manifest as open circuits, short circuits, and unstable voltage regulation. Among these three types of faults, the former one shows an increase in power supply voltage; The worry about the future is that the fault manifests as the power supply voltage dropping to zero volts or unstable output.

Crystal diodes (hereinafter referred to as diodes) are generally represented by the letter "D" on circuit boards, and are also identified in the form of "D+number" numbers. Diodes have unidirectional conductivity characteristics, manifested as two extremes: low forward voltage resistance and infinite reverse voltage resistance.

The characteristic of small forward resistance and large reverse resistance of diodes.

Under the reverse characteristic, the positive and negative terminals of the diode are connected to the opposite potential to the forward characteristic, with the positive terminal connected to the low potential end and the negative terminal connected to the high potential end. When the diode is turned on in this state, the current passing through the diode is almost zero, which is what we call "reverse bias", and in the forward characteristic, it is called "forward bias". Under reverse bias, the diode still has a very small current flowing through it. When we increase the voltage to the zero point where the diode flows in reverse, the reverse current of the diode will sharply increase, and the diode will lose its unidirectional conductivity, which is also called "diode breakdown".

The quality of a diode can be determined by its forward and reverse characteristics.

Measure the forward resistance of the diode

Positive characteristic judgment: Connect the black pen of the positive pole in the multimeter to the positive pole, and the red pen of the negative pole in the multimeter to the negative pole. At this time, the multimeter status displays that the pointer is centered on the dial, not 0. Therefore, the dry resistance value of the headphones is a positive resistance value. If the resistance is 0, the diode is damaged, and the resistance of the weak pointer tends towards infinity, then the diode can be judged as short circuited or open circuited, and regardless of which one, the diode cannot be used anymore.

Measure the reverse resistance of the diode

Reverse characteristic judgment: Reverse judgment is simpler than forward judgment. Connect the red pen of the multimeter to the positive terminal of the diode and the black pen to the negative terminal. When the multimeter pointer displays at or near infinity, it can be determined that the diode is not damaged and can be used normally.