(1) Detection of ordinary diodes (including detector diodes, rectifier diodes, damping diodes, switching diodes, freewheeling diodes) are semiconductor devices composed of a PN structure and have unidirectional conduction characteristics. By using a multimeter to detect the forward and reverse resistance values, the electrodes of the diode can be identified and whether the diode is damaged can also be estimated.

     1. Determination of polarity: Place the multimeter in the R×100 position or the R×1k position, and connect the two test leads to the two electrodes of the diode. After measuring a result, swap the two test leads and measure another result. Among the two measurement results, one measured a larger resistance value (reverse resistance) and the other measured a smaller resistance value (forward resistance). In a measurement with a small resistance, the black test lead is connected to the anode of the diode, and the red test lead is connected to the cathode of the diode.

     2. Detection and judgment of single negative conductivity performance Generally, the forward resistance value of a germanium material diode is about 1kΩ, and the reverse resistance value is about 300. The resistance value of a silicon material diode is about 5 kΩ, and the reverse resistance value is ∞ (infinity). The smaller the forward resistance, the better, and the larger the reverse resistance, the better. The greater the difference between the forward and reverse resistance values, the better the unidirectional conductive characteristics of the diode.

     If the forward and reverse resistance values of the diode are measured to be close to 0 or have a small resistance value, it means that the diode has an internal breakdown short circuit or is damaged by leakage. If the forward and reverse resistance values of the diode are measured to be infinite, it means that the diode is open circuit and damaged.

     3. Detection of reverse breakdown voltage The reverse breakdown voltage (withstand voltage value) of the diode can be measured with a transistor DC parameter test meter. The method is: when measuring a diode, the "NPN/PNP" selection key of the test meter should be set to the NPN state, then the anode of the diode under test should be connected to the "C" jack of the test meter, and the negative pole should be inserted into the "e" of the test meter. "jack, and then press the "V(BR)" key, the test meter will indicate the reverse breakdown voltage value of the diode.

     You can also use a megohmmeter and a multimeter to measure the reverse breakdown voltage of the diode. When measuring, connect the cathode of the diode under test to the anode of the megohmmeter, connect the anode of the diode to the cathode of the megohmmeter, and use a multimeter (set Monitor the voltage across the diode at a suitable DC voltage range. As shown in Figure 4-71, shake the handle of the megohmmeter (slowly and gradually). When the voltage across the diode stabilizes and stops rising, this voltage value is the reverse breakdown voltage of the diode.

     (2) Detection of Zener Diode

     1. Distinguishing between positive and negative electrodes From the appearance point of view, the positive end of the metal package Zener diode body is flat and the negative end is semicircular. One end with the color mark printed on the plastic Zener diode body is the negative pole, and the other end is the positive pole. For zener diodes with unclear markings, you can also use a multimeter to determine the polarity. The measurement method is the same as for ordinary diodes, that is, use the R×1k range of the multimeter, connect the two test leads to the two electrodes of the zener diode, and measure a After the result is obtained, reverse the two test leads for measurement. In the two measurement results, when the resistance value is smaller, the black test lead is connected to the positive electrode of the Zener diode, and the red test lead is connected to the negative electrode of the Zener diode.

     If the measured forward and reverse resistances of the Zener diode are both very small or infinite, it means that the diode has broken down or is damaged by an open circuit.

     2. The voltage regulator value is measured using a 0~30V continuously adjustable DC power supply. For voltage regulator diodes below 13V, the output voltage of the voltage regulator power supply can be adjusted to 15V. Connect the positive electrode of the power supply to a 1.5kΩ current limiting resistor in series with the voltage to be measured. Connect the negative pole of the measuring Zener diode, the negative pole of the power supply to the positive pole of the Zener diode, and then use a multimeter to measure the voltage value at both ends of the Zener diode. The measured reading is the voltage regulation value of the Zener diode. If the voltage stabilization value of the Zener diode is higher than 15V, the voltage stabilization power supply should be adjusted to above 20V.

     A megger below 1000V can also be used to provide test power for the Zener diode. The method is: connect the positive terminal of the megohmmeter to the negative terminal of the Zener diode, and then connect the negative terminal of the megohmmeter to the positive terminal of the Zener diode, shake the handle of the megohmmeter at a constant speed according to the regulations, and use a multimeter to monitor the voltage regulator at the same time. The voltage value across the diode (the voltage range of the multimeter should depend on the stable voltage value). When the multimeter's indicated voltage indication is stable, this voltage value is the stable voltage value of the zener diode.

     If the measured stable voltage value of the Zener diode fluctuates high and low, it means that the diode is unstable.

     Figure 4-72 is the measurement method of the voltage stabilization value of the Zener diode.

     (3) Detection of bidirectional trigger diodes

     1. Measurement of forward and reverse resistance values Use a multimeter with the R×1k or R×10k range to measure the forward and reverse resistance values of the bidirectional trigger diode. Normally, its forward and reverse resistance values should be infinite. If the measured forward and reverse resistance values are both very small or 0, it means that the diode has broken down and been damaged.

     2. Measuring the Breakover Voltage There are three methods for measuring the breakover voltage of a bidirectional trigger diode.

     The first method is: connect the positive pole (E) and negative pole (L) of the megohmmeter to the two ends of the bidirectional trigger diode, use the megohmmeter to provide the breakdown voltage, and use the DC voltage range of the multimeter to measure the voltage value. Reverse the two poles of the bidirectional trigger diode and measure again. Compare the deviation of the voltage values measured between the two measurements (usually 3~6V). The smaller the deviation value, the better the performance of the diode.

     The second method is: first use a multimeter to measure the mains voltage U, then connect the bidirectional trigger diode under test to the AC voltage measurement circuit of the multimeter, connect it to the mains voltage, read the voltage value U1, and then connect the bidirectional trigger diode to the AC voltage measurement circuit of the multimeter. After the two poles are reversed and connected, the voltage value U2 is read.

     If the voltage values of U1 and U2 are the same but different from the voltage value of U, it means that the conduction performance of the bidirectional trigger diode is symmetrical. If the voltage values of U1 and U2 differ greatly, it means that the conductivity of the bidirectional trigger diode is asymmetrical. If the voltage values of U1 and U2 are the same as the mains U, it means that the bidirectional trigger diode is internally short-circuited and damaged. If the voltage values of U1 and U2 are both 0V, it means that the bidirectional trigger diode is internally open-circuited and damaged.

     The third method is: use a 0~50V continuously adjustable DC power supply, connect the positive pole of the power supply in series with a 20kΩ resistor and then connect it to one end of the bidirectional trigger diode, and connect the negative pole of the power supply in series with the current level of the multimeter (set it to at the 1mA level) and then connect it to the other end of the bidirectional trigger diode. Gradually increase the power supply voltage. When the ammeter pointer swings significantly (above tens of microamps), it means that the bidirectional trigger diode is turned on. At this time, the voltage value of the power supply is the turning voltage of the bidirectional trigger diode.

     Figure 4-73 is the detection method of the transition voltage of a bidirectional trigger diode.

     (4) Detection of light-emitting diodes

     1. Distinguish between positive and negative electrodes: Place the light-emitting diode under a light source and observe the size of the two metal pieces. Usually the larger end of the metal piece is the negative electrode and the smaller end of the metal piece is the positive electrode.

     2. Judgment of performance

     Use the multimeter R×10k range to measure the forward and reverse resistance values of the light-emitting diode. Normally, the forward resistance value (when the black test lead is connected to the positive electrode) is about 10~20kΩ, and the reverse resistance value is 250kΩ~∞ (infinity). For higher-sensitivity light-emitting diodes, the inside of the tube will emit a faint light when measuring the forward resistance value. If you use the multimeter R×1k range to measure the forward and reverse resistance values of the light-emitting diode, you will find that the forward and reverse resistance values are close to ∞ (infinity). This is because the forward voltage drop of the light-emitting diode is greater than 1.6V (high This is because the voltage of the battery in the R×1k range of the multimeter is 1.5V).

     Use the R×10k setting of the multimeter to charge a 220μF/25V electrolytic capacitor (the black test lead is connected to the positive electrode of the capacitor, and the red test lead is connected to the negative electrode of the capacitor), and then the positive electrode of the charged capacitor is connected to the positive electrode of the LED, and the negative electrode of the capacitor is connected to the negative electrode of the LED. If If the LED flashes brightly, it means the LED is in good condition.

     You can also use a 3V DC power supply. Connect a 33Ω resistor in series to the positive electrode of the power supply and then connect it to the positive electrode of the light-emitting diode. Connect the negative electrode of the power supply to the negative electrode of the light-emitting diode (see Figure 4-74). A normal light-emitting diode should glow. Or connect a 1.5V battery in series to the black test lead of the multimeter (set the multimeter to the R×10 or R×100 position, and connect the black test lead to the negative pole of the battery, which is equivalent to being connected in series with the 1.5V battery in the meter), and connect the positive pole of the battery. Connect the positive electrode of the light-emitting diode and the red test lead to the negative electrode of the light-emitting diode. A normal light-emitting diode should glow.

     (5) Detection of infrared light-emitting diodes

     1. Distinguishing positive and negative polarity Infrared light-emitting diodes are mostly encapsulated with transparent resin. There is a shallow plate at the bottom of the tube core. The wide electrode in the tube is the negative electrode, and the narrow electrode is the positive electrode. It can also be judged from the shape of the tube body and the length of the pins. Usually, the electrode close to the side facet of the tube is the negative electrode, and the pin at the other end is the positive electrode. The long pin is positive and the short pin is negative.

     2. Measure the performance of the infrared light-emitting tube using the R×10k range of a multimeter to measure the forward and reverse resistance of the infrared light-emitting tube. Normally, the forward resistance value is about 15~40kΩ (the smaller the value, the better); the reverse resistance is greater than 500kΩ (measured with R×10k range, the reverse resistance is greater than 200kΩ). If the measured forward and reverse resistance values are both close to zero, it means that the infrared light-emitting diode has been broken down internally. If the measured forward and reverse resistance values are both infinite, it means that the diode has been opened and damaged. If the measured reverse resistance value is much less than 500kΩ, it means that the diode has been damaged by leakage.

     (6) Detection of infrared photodiodes

     Set the multimeter to the R×1k position and measure the forward and reverse resistance values of the infrared photodiode. Normally, the forward resistance value (the pin connected to the black test lead is the positive pole) is about 3~10 kΩ, and the reverse resistance value is more than 500 kΩ. If the measured forward and reverse resistance values are both 0 or infinite, it means that the photosensitive diode has broken down or is damaged by an open circuit.

     While measuring the reverse resistance value of the infrared photodiode, point the TV remote control at the receiving window of the infrared photodiode under test (see Figure 4-75). For a normal infrared photodiode, when the button on the remote control is pressed, its reverse resistance value will decrease from more than 500 kΩ to between 50 and 100 kΩ. The more the resistance drops, the higher the sensitivity of the infrared photodiode.

     (7) Detection of other photodiodes

     1. Resistance measurement method: Use black paper or black cloth to cover the light signal receiving window of the photosensitive diode, and then use a multimeter with the R×1k range to measure the forward and reverse resistance values of the photosensitive diode. Normally, the forward resistance value is between 10~20kΩ, and the reverse resistance value is ∞ (infinity). If the measured forward and reverse resistance values are both very small or infinite, then the photodiode is leaking or open circuit damaged.

     Then remove the black paper or black cloth, align the light signal receiving window of the photosensitive diode with the light source, and then observe the changes in its forward and reverse resistance values. Normally, both forward and reverse resistance values should be small. The greater the change in resistance value, the higher the sensitivity of the photodiode.

     2. Voltage measurement method: Place the multimeter in the 1V DC voltage range, connect the black test lead to the negative electrode of the photosensitive diode, and the red test lead to the positive electrode of the photosensitive diode, and align the light signal receiving window of the photosensitive diode with the light source. Normally there should be a voltage of 0.2~0.4V (the voltage is proportional to the light intensity).

     3. Current measurement method: Place the multimeter in the 50μA or 500μA current range. The red test lead is connected to the positive electrode and the black test lead is connected to the negative electrode. Under incandescent light, the current of a normal photosensitive diode increases from a few microamps to several hundred as the light intensity increases. microampere.

     (8) Detection of laser diodes

     1. Resistance measurement method: Remove the laser diode and measure its forward and reverse resistance values with a multimeter in the R×1k or R×10k range. Normally, the forward resistance value is between 20 and 40kΩ, and the reverse resistance value is ∞ (infinity). If the measured forward resistance value exceeds 50kΩ, it means that the performance of the laser diode has declined. If the measured forward resistance value is greater than 90kΩ, it means that the diode has been seriously aged and can no longer be used.

     2. The current measurement method uses a multimeter to measure the voltage drop across the load resistor in the laser diode drive circuit, and then estimates the current value flowing through the tube according to Ohm's law. When the current exceeds 100mA, if the laser power potentiometer is adjusted (see Figure 4-76 ), and there is no obvious change in the current, it can be judged that the laser diode is seriously aging. If the current increases sharply and gets out of control, it means that the optical resonant cavity of the laser diode is damaged.

    

     (9) Detection of varactor diodes

     1. Distinguishing the positive and negative poles: One end of some varactor diodes is painted with a black mark. This end is the negative pole, and the other end is the positive pole. There are also varactor diodes with yellow and red rings painted on both ends of the tube shell. One end of the red ring is the positive electrode and one end of the yellow ring is the negative electrode.

     You can also use the diode setting of a digital multimeter to determine the positive and negative polarity of the varactor diode by measuring its forward and reverse voltage drops. For a normal varactor diode, when measuring its forward voltage drop, the meter reads 0.58~0.65V; when measuring its reverse voltage drop, the meter reads an overflow symbol "1". When measuring the forward voltage drop, the red test lead is connected to the anode of the varactor diode, and the black test lead is connected to the cathode of the varactor diode.

     2. To judge the performance, use the R×10k range of the pointer multimeter to measure the forward and reverse resistance values of the varactor diode. A normal varactor diode has forward and reverse resistance values of ∞ (infinity). If the forward and reverse resistance values of the measured varactor diode have a certain resistance value or both are 0, then the diode is damaged by leakage or breakdown.

     (10) Detection of double base diodes

     1. Identification of electrodes: Set the multimeter to the R×1k range and use two test leads to measure the forward and reverse resistance values between any two of the three electrodes of the double base diode. The forward and reverse resistance values between the two electrodes will be measured. The resistance values are all 2~10kΩ. These two electrodes are the base B1 and the base B2, and the other electrode is the emitter E. Then connect the black test lead to the emitter E, and use the red test lead to touch the other two electrodes in sequence. Generally, two different resistance values will be measured. In a measurement with a small resistance, the red test lead was connected to the base B2, and the other electrode was the base B1.

     2. Judgment of Performance The performance of a double-base diode can be judged by measuring whether the resistance value between its poles is normal. Use the R×1k scale of the multimeter, connect the black test lead to the emitter E, and the red test lead to the two bases (B1 and B2) in sequence. Normally, there should be a resistance value of several thousand ohms to more than ten thousand ohms. Then connect the red test lead to the emitter E, and the black test lead to the two bases in turn. Normally the resistance is infinite.

     The forward and reverse resistance values between the two bases (B1 and B2) of the double-base diode are both in the range of 2~10kΩ. If the measured resistance value between two poles is significantly different from the above-mentioned normal value, then Indicates that the diode is damaged.

(11) Detection of bridge stacks

     1. Detection of full bridge Most rectifier full bridges are marked with "+", "-" and "~" symbols (where "+" is the positive pole of the rectified output voltage, "-" is the negative pole of the output voltage, " ~" is the AC voltage input terminal), it is easy to determine each electrode.

     During detection, you can measure the forward and reverse resistance values of each rectifier diode between the "+" pole and the two "~" poles, and between the "-" pole and the two "~" poles (the same as the measurement method of ordinary diodes). If it is normal, you can judge whether the full bridge is damaged. If the measured forward and reverse resistance values of the whip diodes in the full bridge are both 0 or infinite, it can be judged that the diode has broken down or been damaged by an open circuit.

     2. Detection of half-bridge The half-bridge is composed of two rectifier diodes. By using a multimeter to measure whether the forward and reverse resistance values of the two diodes inside the half-bridge are normal, you can determine whether the half-bridge is normal.

     (12) Detection of high-voltage silicon stack

     The interior of the high-voltage silicon stack is composed of multiple high-voltage rectifier diodes (silicon particles) connected in series. During detection, the forward and reverse resistance values can be measured with the R×10k range of a multimeter. A normal high-voltage silicon stack has a forward resistance value greater than 200kΩ and a reverse resistance value of infinite. If a certain resistance value is measured in both forward and reverse directions, it means that the high-voltage silicon stack has been damaged by soft breakdown.

     (13) Detection of variable resistance diodes

     Use the R×10k range of the multimeter to measure the forward and reverse resistance values of the varistor diode. The forward resistance value of a normal high-frequency varistor diode (when the black test lead is connected to the positive electrode) is 4.5~6kΩ, and the reverse resistance value is infinite. If the measured forward and reverse resistance values are both very small or infinite, it means that the measured varistor diode has been damaged.

     (14) Detection of Schottky diodes

     Two-terminal Schottky diodes can be measured with the R×1 scale of a multimeter. Normally, the forward resistance value (the black test lead is connected to the positive electrode) is 2.5~3.5Ω, and the throw resistance value is infinite. If the measured forward and reverse resistance values are both infinite or close to 0, it means that the diode has been open circuited or broken down.

     For three-terminal Schottky diodes, the common terminal should be measured first to determine whether it is a common cathode pair or a common anode pair, and then measure the forward and reverse resistance values of the two diodes respectively.