Diode identification skills and detection methods

1. Function and identification method of diode

1.1 Function

The main characteristic of a diode is unidirectional conductivity, that is, under the action of a forward voltage, the on-resistance is very small; under the action of a reverse voltage, the on-resistance is extremely large or infinite.

Diodes are divided into crystal diodes, bidirectional trigger diodes, high-frequency varistor diodes, varactor diodes, diodes, and Schottky diodes according to their uses.

1.2 Identification Method

It is very simple to identify a diode. The N pole (negative pole) of a low-power diode is mostly marked with a color circle in the diode table. Some diodes are also marked with diode-specific symbols such as "P" and "N" to determine the polarity of the diode. The positive and negative poles of the light-emitting diode can be identified by the length of the pins, the long pin is positive and the short pin is negative.

1.3 Test Notes

When using a digital multimeter to test a diode, the red test lead is connected to the positive pole of the diode and the black test lead is connected to the negative pole of the diode. The resistance value tested at this time is the forward conduction resistance value of the diode, which is exactly the opposite of the test lead connection method of a pointer multimeter.

1.4 Fault Characteristics

The main faults of diodes are open circuit, short circuit and unstable voltage. Among these three faults, the former fault is manifested by the increase of power supply voltage; the latter fault is manifested by the power supply voltage dropping to zero volts or the output being unstable.

2 Diode test method

2.1 Detection of low power crystal diodes

A. Identify positive and negative electrodes

(a) Observe the symbol on the shell. Usually, the diode symbol is marked on the shell of the diode. The end with a triangular arrow is the positive pole and the other end is the negative pole.

(b) Observe the color dots on the shell. The shell of the point contact diode is usually marked with a polarity color dot (white or red). Generally, the end marked with the color dot is the positive pole. Some diodes are marked with a color ring, and the end with the color ring is the negative pole.

(c) Taking the measurement with smaller resistance as the standard, the end connected to the black test lead is the positive electrode, and the end connected to the red test lead is the negative electrode.

B. Detect the highest reverse breakdown voltage. For AC, because it is constantly changing, the highest reverse working voltage is the AC peak voltage that the diode can withstand.

2.2 Detecting bidirectional trigger diodes

Set the multimeter to the corresponding DC voltage range. The test voltage is provided by the megohmmeter. During the test, shake the megohmmeter and measure the VBR value in the same way. Finally, compare VBO with VBR. The smaller the difference between the absolute values ​​of the two, the better the symmetry of the bidirectional trigger diode being tested.

2.3 Transient Voltage Suppressor (TVS) Detection

A. Use a multimeter to measure the quality of the tube. For single-pole TVS, its forward and reverse resistance can be measured according to the method of measuring ordinary diodes. Generally, the forward resistance is about 4kΩ and the reverse resistance is infinite.

For a bidirectional TVS, the resistance value between the two pins measured by arbitrarily swapping the red and black test leads should be infinite. Otherwise, it means that the tube has poor performance or is damaged.

2.4 Detection of high-frequency varistor diodes

Identify the positive and negative poles. The difference between the high-frequency varistor diode and the ordinary diode in appearance is the color code. The color code of the ordinary diode is generally black, while the color code of the high-frequency varistor diode is light. Its polarity rule is similar to that of the ordinary diode, that is, the end with the green ring is the negative pole, and the end without the green ring is the positive pole.

2.5 Detection of varactor diode

If the red and black test leads of the multimeter are swapped, the resistance between the two pins of the varactor diode should be infinite. If the multimeter pointer swings slightly to the right or the resistance is zero during the measurement, it means that the varactor diode under test has leakage fault or has been broken down.

2.6 Detection of Monochrome LEDs

Attach an energy-saving 1.5V dry battery to the outside of the multimeter, and set the multimeter to R×10 or R×100. This connection method is equivalent to giving the multimeter a 1.5V voltage in series, so that the detection voltage increases to 3V (the turn-on voltage of the light-emitting diode is 2V). When testing, use the two probes of the multimeter to alternately touch the two pins of the light-emitting diode. If the tube performs well, it must be able to emit light normally once. At this time, the black probe is connected to the positive pole and the red probe is connected to the negative pole.

2.7 Detection of infrared light emitting diodes

A. Identify the positive and negative electrodes of infrared light-emitting diodes. Infrared light-emitting diodes have two pins, usually the longer pin is the positive electrode and the shorter pin is the negative electrode. Because infrared light-emitting diodes are transparent, the electrodes inside the tube shell are clearly visible. The wider and larger internal electrode is the negative electrode, while the narrower and smaller one is the positive electrode.

B. First measure the forward and reverse resistance of the red light emitting diode. Usually the forward resistance should be around 30k and the reverse resistance should be above 500k. Only such tubes can be used normally.

2.8 Detection of infrared receiving diode

A. Identify the pin polarity

(a) Identify from the appearance. Common infrared receiving diodes are black in color. When identifying the pins, facing the light receiving window, from left to right, they are positive and negative. In addition, there is a small chamfered plane on the top of the infrared receiving diode. Usually, the pin with this chamfered plane is the negative pole, and the other end is the positive pole.

(b) First, use a multimeter to identify the positive and negative electrodes of an ordinary diode. That is, swap the red and black test leads and measure the resistance between the two pins of the diode twice. Under normal circumstances, the resistance should be one large and one small. The smaller resistance should be used as the reference. The pin connected to the red test lead is the negative pole, and the pin connected to the black test lead is the positive pole.

B. Check the performance. Use the resistance block of a multimeter to measure the forward and reverse resistance of the infrared receiving diode. Based on the values ​​of the forward and reverse resistances, you can preliminarily determine whether the infrared receiving diode is good or bad.

2.9 Laser Diode Detection

A. According to the method of testing the forward and reverse resistance of ordinary diodes, the pin arrangement order of the laser diode can be determined. However, it should be noted that since the forward voltage drop of the laser diode is larger than that of ordinary diodes, the multimeter pointer will only slightly deflect to the right when testing the forward resistance.