Detection methods of diodes and transistors

I. Diode detection method and experience

1 Detection of low-power crystal diodes

A Identify the positive and negative electrodes

(a) Observe the symbol on the shell. The diode shell is usually marked with the symbol of the diode. The end with the triangular arrow is the positive electrode and the other end is the negative electrode. (b) Observe the

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

(c) Based on the measurement with a smaller resistance value, 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 operating frequency fM. In addition to looking up the operating frequency of the crystal diode from the relevant characteristic table, in practice, it is often distinguished by observing the contact wire inside the diode. For example, the point contact diode is a high-frequency tube, and the surface contact diode is mostly a low-frequency tube. In addition, it can also be tested with the multimeter R×1k block. Generally, the forward resistance is less than 1K, which is mostly a high-frequency tube.

C Detect the highest reverse breakdown voltage VRM. For AC, because it is constantly changing, the highest reverse working voltage is also the AC peak voltage that the diode withstands. It should be pointed out that the highest reverse working voltage is not the breakdown voltage of the diode. In general, the breakdown voltage of the diode is much higher than the highest reverse working voltage (about twice as high).

2 Detecting glass-sealed silicon high-speed switching diodes

The method for detecting silicon high-speed switching diodes is the same as that for detecting ordinary diodes. The difference is that the forward resistance of this tube is larger. Using the R×1k resistor block to measure, the forward resistance value is generally 5K~10K?, and the reverse resistance value is infinite.

3 Detecting fast recovery and ultra-fast recovery diodes

The method of using a multimeter to detect fast recovery and ultra-fast recovery diodes is basically the same as the method of detecting plastic-sealed silicon rectifier diodes. That is, first use the R×1k block to detect its unidirectional conductivity. Generally, the forward resistance is about 45K?, and the reverse resistance is infinite; then use the R×1 block to re-measure once. Generally, the forward resistance is a few?, and the reverse resistance is still infinite.

4 Detection of bidirectional trigger diode

A Set the multimeter to R×1K, and the forward and reverse resistance values ​​of the bidirectional trigger diode should be infinite. If the multimeter pointer swings to the right when the test leads are exchanged for measurement, it means that the tube under test has a leakage fault.

Set the multimeter to the corresponding DC voltage range. The test voltage is provided by the megohmmeter. During the test, shake the megohmmeter, and the voltage value indicated by the multimeter is the VBO value of the tube under test. Then swap the two pins of the tube under test 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 under test.

5 Detection of transient voltage suppressor diode (TVS)

A Use the multimeter R×1K range to measure the quality of the tube.

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

6 Detection of high-frequency variable resistor diodes

A Identify the positive and negative poles

The difference between the high-frequency variable resistor diode and the ordinary diode in appearance is the color code color. The color code color of the ordinary diode is generally black, while the color code color of the high-frequency variable resistor 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.

B Measure the forward and reverse resistance to judge its quality

The specific method is the same as the method of measuring the forward and reverse resistance of ordinary diodes. When using the 500-type multimeter R×1k block for measurement, the forward resistance of a normal high-frequency variable resistor diode is 5K~55K?, and the reverse resistance is infinite.

7 Detection of variable capacitance diodes

Set the multimeter to the R×10k block. No matter how the red and black test leads are swapped for measurement, the resistance value between the two pins of the variable capacitance diode should be infinite. If the multimeter pointer is found to swing slightly to the right or the resistance value is zero during the measurement, it means that the measured variable capacitance diode has a leakage fault or has been broken down and damaged. The multimeter cannot detect and judge the loss of capacitance of the variable capacitance diode or the internal open circuit fault. If necessary, the replacement method can be used for inspection and judgment.

8 Detection of monochrome light-emitting diodes

Attach a 15V dry battery to the outside of the multimeter and set the multimeter to R×10 or R×100. This connection method is equivalent to connecting a 15V voltage in series to the multimeter, 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 contact the two pins of the light-emitting diode. If the performance of the tube is good, 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.

9 Detection of infrared light-emitting diodes

A Determine the positive and negative electrodes of the infrared light-emitting diode. The infrared light-emitting diode has two pins, usually the long pin is the positive pole and the short pin is the negative pole. Because the infrared light-emitting diode is 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 Set the multimeter to the R×1K position and measure the forward and reverse resistance of the infrared light-emitting diode. Usually, the forward resistance should be around 30K and the reverse resistance should be above 500K. Only in this way can the tube be used normally. The higher the reverse resistance, the better.

10 Detection of infrared receiving diodes

A Identify the polarity of the pins

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

(b) Set the multimeter to the R×1K position and check it using the method used to distinguish the positive and negative electrodes of ordinary diodes, that is, swap the red and black test leads twice to measure the resistance value between the two pins of the tube. Under normal circumstances, the resistance value should be one large and one small. The pin with the smaller resistance shall prevail. 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. Detection of performance. Use the resistance block of the multimeter to measure the forward and reverse resistance of the infrared receiving diode. According to the value of the forward and reverse resistance, the quality of the infrared receiving diode can be preliminarily determined.

11. Detection of laser diode

A. Set the multimeter to R×1K and determine the pin arrangement order of the laser diode according to the method of detecting the forward and reverse resistance of ordinary diodes. However, it should be noted during the detection that since the forward voltage drop of the laser diode is larger than that of ordinary diodes, when detecting the forward resistance, the multimeter pointer only slightly deflects to the right, while the reverse resistance is infinite. [page]

Detection methods and experience of transistors

1. Detection of medium and low power transistors

A. The performance of transistors with known models and pin arrangement can be judged according to the following methods

(a) Measure the inter-electrode resistance. Set the multimeter to R×100 or R×1K and test according to six different connection methods of the red and black test leads. Among them, the forward resistance of the emitter junction and the collector junction is relatively low, and the resistance values ​​measured by the other four connection methods are very high, ranging from several hundred kilo-ohms to infinity. However, whether it is low resistance or high resistance, the inter-electrode resistance of silicon material transistors is much larger than that of germanium material transistors.

(b) The value of the transistor's penetration current ICEO is approximately equal to the product of the tube's multiple β and the reverse current ICBO of the collector junction. ICBO increases rapidly with the increase of ambient temperature, and the increase of ICBO will inevitably cause the increase of ICEO. The increase of ICEO will directly affect the stability of the tube's operation, so tubes with small ICEO should be used as much as possible.

By directly measuring the resistance between the e-c pole of the transistor with a multimeter resistor, the size of ICEO can be indirectly estimated. The specific method is as follows:

The multimeter resistance range is generally R×100 or R×1K. For PNP tubes, the black tube is connected to the e pole and the red pen is connected to the c pole. For NPN transistors, the black pen is connected to the c pole and the red pen is connected to the e pole. The larger the measured resistance, the better. The larger the resistance between e and c, the smaller the ICEO of the tube; conversely, the smaller the measured resistance, the larger the ICEO of the tube being measured. Generally speaking, the resistance of medium and low power silicon tubes and germanium low frequency tubes should be hundreds of kilo-ohms, tens of kilo-ohms and more than ten kilo-ohms respectively. If the resistance is very small or the multimeter pointer shakes back and forth during the test, it indicates that the ICEO is very large and the performance of the tube is unstable.

(c) Measuring the amplification capacity (β). At present, some models of multimeters have scale lines for measuring the hFE of transistors and their test sockets, which can easily measure the amplification of transistors. First, turn the multimeter function switch to the ? position, turn the range switch to the ADJ position, short-circuit the red and black test leads, adjust the zero adjustment knob, make the multimeter pointer indicate zero, then turn the range switch to the hFE position, and separate the two short-circuited test leads, insert the transistor to be tested into the test socket, and read the tube amplification from the hFE scale line.

In addition: For medium and low power transistors of this type, the manufacturer directly marks different color dots on the top of the tube shell to indicate the amplification factor β value of the tube. The corresponding relationship between the color and β value is shown in the table, but it should be noted that the color codes used by different manufacturers are not necessarily exactly the same.

B Detection and discrimination electrode

(a) Determine the base. Use the multimeter R×100 or R×1k block to measure the forward and reverse resistance values ​​between each two electrodes of the three electrodes of the transistor. When the first probe is connected to a certain electrode, and the second probe successively touches the other two electrodes and measures a low resistance value, the electrode connected to the first probe is the base b. At this time, pay attention to the polarity of the multimeter probe. If the red probe is connected to the base b. When the black probe is connected to the other two poles, the measured resistance values ​​are all small, then it can be determined that the transistor being tested is a PNP type tube; if the black probe is connected to the base b, and the red probe touches the other two poles, the measured resistance values ​​are small, then the transistor being tested is an NPN type tube.

(b) Determine the collector c and emitter e. (Take PNP as an example) Set the multimeter to R×100 or R×1K, with the red probe on the base b and the black probe on the other two pins. The two resistance values ​​measured will be one larger and one smaller. In a measurement with a small resistance value, the pin connected to the black probe is the collector; in a measurement with a large resistance value, the pin connected to the black probe is the emitter.

C. Distinguishing high-frequency tubes from low-frequency tubes

The cutoff frequency of high-frequency tubes is greater than 3MHz, while the cutoff frequency of low-frequency tubes is less than 3MHz. Generally, the two cannot be interchanged.

D. In-circuit voltage detection judgment method

In actual applications, small-power transistors are mostly directly soldered on printed circuit boards. Due to the high installation density of components, disassembly is more troublesome. Therefore, during detection, the DC voltage block of the multimeter is often used to measure the voltage value of each pin of the transistor being tested to infer whether it is working normally, and then judge whether it is good or bad.

2. Testing of high-power transistors

The various methods of using a multimeter to test the polarity, tube type and performance of medium and low-power transistors are basically applicable to testing high-power transistors. However, since the working current of high-power transistors is relatively large, the area of ​​their PN junctions is also relatively large. The larger the PN junction, the larger the reverse saturation current. Therefore, if the multimeter's R×1k block is used to measure the inter-electrode resistance of medium and low-power transistors, the measured resistance value will inevitably be very small, as if the inter-electrode short circuit is present. Therefore, the R×10 or R×1 block is usually used to test high-power transistors.

3. Testing of ordinary Darlington tubes

The testing of ordinary Darlington tubes with a multimeter includes identifying electrodes, distinguishing between PNP and NPN types, and estimating amplification capabilities. Because there are multiple emitter junctions between the E-B poles of the Darlington tube, the R×10K block of the multimeter, which can provide a higher voltage, should be used for measurement.

4 Detection of high-power Darlington tubes

The method of detecting high-power Darlington tubes is basically the same as that of detecting ordinary Darlington tubes. However, since high-power Darlington tubes are equipped with protection and leakage current discharge components such as V3, R1, and R2, the impact of these components on the measured data should be distinguished in the detection quantity to avoid misjudgment. The specific steps can be followed as follows:

A Use the multimeter R×10K block to measure the PN junction resistance between B and C, and it should be clearly measured that it has unidirectional conductivity. There should be a large difference between the forward and reverse resistance values.

B There are two PN junctions between the high-power Darlington tube B-E, and resistors R1 and R2 are connected. When using the multimeter resistance block for detection, when measuring in the forward direction, the measured resistance value is the result of the forward resistance of the B-E junction in parallel with the resistance values ​​of R1 and R2; when measuring in the reverse direction, the emitter junction is cut off, and the measured resistance is the sum of (R1+R2), which is about several hundred ohms, and the resistance value is fixed and does not change with the change of the resistance block position. However, it should be noted that some high-power Darlington tubes also have diodes on R1 and R2. In this case, the resistance measured is not the sum of (R1+R2), but the parallel resistance of (R1+R2) and the sum of the forward resistances of the two diodes.

5. Testing of damped output transistors

Set the multimeter to R×1 and measure the resistance between the electrodes of the damped output transistor to determine whether it is normal. The specific test principles, methods and steps are as follows:

A. Connect the red test lead to E and the black test lead to B. This is equivalent to measuring the resistance of the equivalent diode of the B-E junction of the high-power tube and the protection resistor R in parallel. Since the forward resistance of the equivalent diode is small and the resistance of the protection resistor R is generally only 20~50?, the resistance of the two in parallel is also small; conversely, swap the test leads, that is, connect the red test lead to B and the black test lead to E, then what is measured is the reverse resistance value of the equivalent diode of the B-E junction of the high-power tube and the parallel resistance value of the protection resistor R. Since the reverse resistance value of the equivalent diode is large, the resistance measured at this time is the value of the protection resistor R, which is still small.

B Connect the red test lead to C and the black test lead to B. This is equivalent to measuring the forward resistance of the B-C junction equivalent diode of the high-power tube in the tube. The resistance value measured is generally small. Swap the red and black test leads, that is, connect the red test lead to B and the black test lead to C. This is equivalent to measuring the reverse resistance of the B-C junction equivalent diode of the high-power tube in the tube. The resistance value measured is usually infinite.

C Connect the red test lead to E and the black test lead to C. This is equivalent to measuring the reverse resistance of the damping diode in the tube. The resistance value measured is generally large, about 300 to ∞. Swap the red and black test leads, that is, connect the red test lead to C and the black test lead to E. This is equivalent to measuring the forward resistance of the damping diode in the tube. The resistance value measured is generally small, about a few ohms to dozens of ohms.