In the maintenance of various electronic equipment circuit boards and other hardware, the detection of various commonly used components on the board is a required course for electronic maintainers. In this paper, engineers Long Ren summarized the detection methods and practical skills of various electronic components in circuit board maintenance from the basic components such as resistors, capacitors, diodes, transistors, FETs, etc.

In the long-term research on reverse analysis and reference design of electronics and electronics, Longren engineers have accumulated rich knowledge of circuit detection and maintenance and experience in product imitation development. From the perspective of circuit board maintenance, Longren engineers believe that it is very important to accurately and effectively detect the relevant parameters of components, judge whether components are normal, and find fault points.

1、 Inspection method and experience of resistor:

1. Detection of fixed resistor.

A Connect two probes (regardless of positive and negative) to the pins at both ends of the resistance to measure the actual resistance value. In order to improve the measurement accuracy, the measuring range should be selected according to the nominal value of the measured resistance. Due to the non-linear relationship of the ohm scale, the middle section of the scale has a fine graduation, so the pointer indicating value should fall as far as possible to the middle section of the scale, that is, 20%~80% radians from the beginning of the full scale, to make the measurement more accurate. Depending on the resistance error level. An error of ± 5%, ± 10% or ± 20% is allowed between the reading and the nominal resistance. If not, it means that the resistance value has changed.

B. Attention: during the test, especially when measuring the resistance of more than tens of k Ω, do not touch the probe and the conductive part of the resistance; When the measured resistance is soldered off from the circuit, at least one head must be soldered off to avoid the impact of other components in the circuit on the test, resulting in measurement errors; Although the resistance value of the color ring resistance can be determined by the color ring mark, it is better to test the actual resistance value with a multimeter when using it.

2. Detection of cement resistance. The method and precautions for testing cement resistance are identical to those for testing ordinary fixed resistance.

3. Detection of fusing resistor. In the circuit, when the fusing resistor is fused and open, it can be judged according to experience: if the surface of the fusing resistor is found to be blackened or burnt, it can be concluded that it is due to its overload and the current passing through it is many times more than the rated value; If there is no trace on its surface and the circuit is open, it indicates that the current flowing through is just equal to or slightly greater than its rated fuse value. The fuse resistor without any trace on the surface can be measured with the help of the multimeter R × 1. To ensure accurate measurement, one end of the fuse resistor should be welded off the circuit. If the measured resistance value is infinite, it indicates that the fuse resistor has failed and opened. If the measured resistance value is far from the nominal value, it indicates that the resistance has changed and should not be used again. In the maintenance practice, it is found that a few fuse resistors are broken down and short circuited in the circuit, so attention should also be paid during the detection.

4. Detection of potentiometer. When checking the potentiometer, first turn the handle to see whether it rotates smoothly, whether the switch is flexible, whether the "click" sound is clear when the switch is on or off, and listen to the sound of friction between the contact point inside the potentiometer and the resistance body. If there is a "rustle" sound, the quality is poor. When testing with a multimeter, first select the appropriate resistance gear of the multimeter according to the resistance value of the potentiometer to be tested, and then test according to the following methods.

A. Measure both ends of "1" and "2" with the ohm gear of the multimeter. The reading should be the nominal resistance value of the potentiometer. If the pointer of the multimeter does not move or the resistance value differs a lot, the potentiometer is damaged.

B. Check whether the contact between the movable arm of the potentiometer and the resistor is good. Measure both ends of "1", "2" (or "2", "3") with the ohm gear of the multimeter, rotate the potentiometer shaft counterclockwise to the position close to "off", and the smaller the resistance, the better. Then slowly rotate the shaft handle clockwise, the resistance value should gradually increase, and the pointer in the meter head should move smoothly. When the shaft handle is rotated to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, it indicates that the movable contact has contact * * fault.

5. Detection of positive temperature coefficient thermistor (PTC). When testing, use the multimeter R × 1 gear, which can be operated in two steps

: A. Normal temperature detection (indoor temperature is close to 25 ℃); Measure the actual resistance of two pins of the PTC thermistor with two probes and compare it with the nominal resistance. If the difference between the two is within ± 2 Ω, it is normal. If the difference between the actual resistance value and the nominal resistance value is too large, it means that its performance * * or has been damaged.

B. Heating detection; On the basis of normal normal temperature test, the * * step test heating test can be carried out. A heat source (such as electric soldering iron) is heated close to the PTC thermistor. At the same time, a multimeter is used to monitor whether the resistance value increases with the temperature. If yes, the thermistor is normal. If the resistance value does not change, the performance of the thermistor is deteriorated and it cannot be used any more. Be careful not to make the heat source too close to the PTC thermistor or directly contact the thermistor to prevent it from being burned.

6. Detection of negative temperature coefficient thermistor (NTC).

(1) Measure the nominal resistance Rt

The method of measuring NTC thermistor with a multimeter is the same as that of measuring ordinary fixed resistance, that is, the actual value of Rt can be directly measured by selecting a suitable resistance block according to the nominal resistance value of NTC thermistor. However, since NTC thermistor is very sensitive to temperature, the following points should be noted during the test: A Rt is measured by the manufacturer when the ambient temperature is 25 ℃, so when measuring Rt with a multimeter, it should also be measured when the ambient temperature is close to 25 ℃ to ensure the reliability of the test. B. The measured power shall not exceed the specified value to avoid measurement error caused by current thermal effect. C. Pay attention to correct operation. During the test, do not hold the thermistor by hand to prevent the body temperature from affecting the test.

(2) Estimated temperature coefficient α t

First, measure the resistance value Rt1 at room temperature t1, then use the electric soldering iron as the heat source, close to the thermistor Rt, and measure the resistance value RT2. At the same time, use a thermometer to measure the average temperature t2 of the thermistor RT surface at this time, and then calculate.

7. Detection of varistor. Use the R × 1k gear of the multimeter to measure the positive and reverse insulation resistance between the two pins of the varistor, which is infinite. Otherwise, it means that the leakage current is large. If the measured resistance is very small, it indicates that the varistor is damaged and cannot be used.

8. Detection of photosensitive resistance.

A. Use a piece of black paper to cover the light transmission window of the photosensitive resistor. At this time, the pointer of the multimeter basically remains still, and the resistance value is close to infinity. The higher this value is, the better the performance of the photoresistor is. If the value is very small or close to zero, it indicates that the photoresist has been burned through and damaged, and can no longer be used.

B. Align a light source with the light transmission window of the photosensitive resistor. At this time, the pointer of the multimeter should swing significantly, and the resistance value should be significantly reduced. The more obvious this value is, the better the performance of the photoresist will be. If the value is large or even infinite, it indicates that the open circuit inside the photoresist is damaged and can no longer be used.

C. Turn the light transmitting window of the photosensitive resistor to the incoming light, and use a small black paper to shake on the upper part of the light shielding window of the photosensitive resistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the paper, it indicates that the photosensitive material of the photosensitive resistor has been damaged.

2、 Capacitor detection methods and experience

1. Detection of fixed capacitor

A Detection of small capacitance below 10pF

Since the capacity of the fixed capacitor below 10pF is too small, use a multimeter to measure, and only qualitatively check whether there is leakage, internal short circuit or breakdown. During measurement, the R × 10k gear of the multimeter can be selected, and two probes can be used to connect two pins of the capacitance at random, and the resistance value should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it indicates that the capacitor is damaged by leakage or internal breakdown. B. Check whether 10PF ～ 0.01 μ F fixed capacitor is charged, and then judge whether it is good or bad. R × 1k gear is selected for the multimeter. The β value of both triodes is more than 100, and the through current is small. 3DG6 and other silicon triodes can be selected to form composite tubes. The red and black probes of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the compound triode, the charging and discharging process of the measured capacitance is amplified, so that the pointer swing of the multimeter is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring the capacitance of smaller capacity, the pin of the measured capacitance should be repeatedly replaced to contact A and B, so that the pointer of the multimeter can be clearly seen swinging. C For the fixed capacitance above 0.01 μ F, the R × 10k gear of the multimeter can be used to directly test whether the capacitor has charging process, internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.

2. Detection of electrolytic capacitor

A Because the capacity of electrolytic capacitor is much larger than that of general fixed capacitor, appropriate measuring range shall be selected for different capacities during measurement. According to experience, in general, capacitance between 1~47 μ F can be measured in R × 1k gear, and capacitance greater than 47 μ F can be measured in R × 100 gear.

B. Connect the red probe of the multimeter to the negative electrode and the black probe to the positive electrode. At the moment of contact, the pointer of the multimeter will deflect to the right with a larger deflection (for the same electrical barrier, the larger the capacity, the larger the swing), and then gradually rotate to the left until it stops at a certain position. At this time, the resistance is the forward leakage resistance of the electrolytic capacitor, which is slightly greater than the reverse leakage resistance. The practical use experience shows that the leakage resistance of electrolytic capacitor should generally be more than several hundred k Ω, otherwise, it will not work normally. In the test, if there is no charging phenomenon in both the forward and reverse directions, that is, the meter needle does not move, it means that the capacity disappears or the internal circuit is broken; If the measured resistance value is very small or zero, it means that the capacitor has large leakage or has been damaged by breakdown and cannot be used again.

C. For electrolytic capacitors whose positive and negative signs are unknown, the above method of measuring leakage resistance can be used for discrimination. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the probe to measure a resistance value. The one with the greatest resistance in the two measurements is the positive connection method, that is, the black probe is connected to the positive pole, and the red probe is connected to the negative pole. D. The capacity of the electrolytic capacitor can be estimated and measured according to the amplitude of the pointer swinging to the right by using the resistance gear of the multimeter and the method of charging the electrolytic capacitor forward and backward.

3 Detection of variable capacitor

A. Rotate the shaft gently by hand, and it should feel very smooth. It should not feel that it is sometimes loose, tight or even stuck. When the load shaft is pushed forward, backward, up, down, left, right and other directions, the rotating shaft shall not be loose.

B. Rotate the shaft with one hand and gently touch the outer edge of the moving disc set with the other hand, without any looseness. The variable capacitor that contacts * * between the rotating shaft and the moving blade can no longer be used.

C Place the multimeter in R × 10k gear, connect two probes to the leading out ends of the moving piece and the fixed piece of the variable capacitor respectively with one hand, and slowly rotate the rotating shaft with the other hand for several times. The pointer of the multimeter should remain at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it indicates that there is a short circuit point between the moving piece and the fixed piece; If a certain angle is encountered, the reading of the multimeter is not infinite, but a certain resistance value appears, indicating that there is leakage between the movable and fixed pieces of the variable capacitor.

3、 Inspection methods and experience of inductor and transformer

1. Detection of color code inductor

Place the multimeter at R × 1, and connect the red and black probes to any leading out end of the color code inductor. At this time, the pointer should swing to the right. According to the measured resistance value, the following three cases can be distinguished:

A The resistance value of the measured color code inductor is zero, and there is a short circuit fault inside it. B. The DC resistance value of the measured color code inductor is directly related to the diameter of the enameled wire used for winding the inductor coil and the number of winding coils. As long as the resistance value can be measured, it can be considered that the measured color code inductor is normal.

2. Detection of mid cycle transformer

A Turn the multimeter to R × 1 gear, check the on/off condition of each winding one by one according to the pin arrangement rule of each winding of the mid cycle transformer, and then judge whether it is normal.

B. Test the insulation performance, place the multimeter in R × 10k gear, and conduct the following status tests:

(1) Resistance value between primary winding and secondary winding;

(2) The resistance between the primary winding and the shell;

(3) The resistance between the secondary winding and the housing.

The above test results fall into three categories:

(1) Resistance value is infinite: normal;

(2) Resistance value is zero: there is a short circuit fault;

(3) The resistance value is less than infinity but greater than zero: there is leakage fault.

3. Detection of power transformer

A Check whether there are obvious abnormalities by observing the appearance of the transformer. For example, whether the coil lead wire is broken, desoldered, whether the insulation material is scorched, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.

B. Insulation test. Use a multimeter R × 10k gear to measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and vent secondary, and secondary windings. The pointer of the multimeter should be at infinity. Otherwise, it indicates the insulation performance of the transformer * *.

C. Detection of coil on-off. Put the multimeter in R × 1 gear. During the test, if the resistance of a winding is infinite, it means that the winding has an open circuit fault.

D. Identify primary and secondary coils. The primary and secondary pins of the power transformer are generally led out from both sides, and the primary winding is mostly marked with 220V, while the secondary winding is marked with rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

E Detection of no-load current.

(a) Direct measurement method. Open all the secondary windings, place the multimeter in the AC current block (500mA, connected in series to the primary winding. When the plug of the primary winding is plugged into the 220V AC mains power supply, the multimeter indicates the no-load current value. This value should not be greater than 10%~20% of the transformer's full load current. Generally, the normal no-load current of the power transformer of electronic equipment should be about 100mA. If it exceeds too much, it indicates that the transformer has a short circuit fault.

(b) Indirect measurement. A 10 °/5W resistor is connected in series in the primary winding of the transformer, and the secondary windings are still all no-load. Set the multimeter to AC voltage. After power on, use two probes to measure the voltage drop U at both ends of resistance R, and then use Ohm's law to calculate the no-load current I null, that is, I null=U/R.

F Detection of no-load voltage. Connect the primary of the power transformer to the 220V mains power supply, and measure the no-load voltage of each winding (U21, U22, U23, U24) with the AC voltage of the multimeter in turn, which should meet the requirements. The allowable error range is generally: HV winding ≤± 10%, LV winding ≤± 5%, and the voltage difference between two groups of symmetrical windings with central tap should be ≤± 2%.

G Generally, the allowable temperature rise of small power transformer is 40 ℃~50 ℃. If the quality of insulation material used is good, the allowable temperature rise can also be increased.

H. Detect and judge the same end of each winding. When using power transformers, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When the power transformer is used in series, the same name end of each winding participating in series connection must be correctly connected without any mistake. Otherwise, the transformer cannot work normally.

1. Comprehensive detection and judgment of short circuit fault of power transformer. The main symptoms of short circuit fault of power transformer are severe heating and abnormal output voltage of secondary winding. Generally, the more inter turn short circuit points in the coil, the greater the short circuit current, and the more serious the transformer heating. A simple way to detect and judge whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced previously). The no-load current value of the transformer with short circuit fault will be far greater than 10% of the full load current. When the short circuit is serious, the transformer will quickly heat up within tens of seconds after no-load power on, and it will be hot to touch the iron core with your hand. At this time, it can be concluded that there is a short circuit point in the transformer without measuring the no-load current.

4、 Detection method and experience of diode

1 Detection of small power crystal diode

A Discrimination of positive and negative electrodes

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

(b) Observe the color spots on the shell. The housing of the point contact diode is usually marked with a polar color dot (white or red). Generally, the end marked with color dots is the positive pole. Other diodes are marked with colored rings, and the end with colored rings is negative.

(c) One measurement with smaller resistance shall prevail. One end of the black probe is positive, and the other end of the red probe is negative.

B. Test * high operating frequency fM. In addition to the operating frequency of crystal diode, which can be seen from the relevant characteristic table, it is often used to distinguish the contact wire inside the diode with eyes in practice. For example, the point contact diode belongs to high-frequency tube, and the area contact diode is mostly low-frequency tube. In addition, the multimeter R × 1k gear can also be used for testing. Generally, high-frequency tubes are used when the forward resistance is less than 1k.

C Detect * high reverse breakdown voltage VRM. For AC power, because it is constantly changing, the high reverse operating voltage is the AC peak voltage borne by the diode. It should be noted that * the high reverse operating voltage is not the breakdown voltage of the diode. In general, the breakdown voltage of the diode is much higher (about twice as high) than the high reverse operating voltage.

2. Detection of glass sealed silicon high-speed switching diode

The method of detecting silicon high-speed switching diode is the same as that of detecting common diode. The difference is that the forward resistance of this kind of tube is large. Measure with R × 1k resistance gear. Generally, the forward resistance is 5k ～ 10k, and the reverse resistance is infinite.

3. Detection of fast recovery and ultrafast recovery diodes

The method of detecting fast recovery diode and ultrafast recovery diode with multimeter is basically the same as that of detecting plastic encapsulated silicon rectifier diode. That is, first check its unidirectional conductivity with R × 1k gear. Generally, the forward resistance is about 4.5 k, and the reverse resistance is infinite; Re measure once with gear R × 1. Generally, the forward resistance is several revolutions, and the reverse resistance is still infinite.

4 Detection of two-way trigger diode

A Place the multimeter in R × 1k gear, and measure the positive and reverse resistance of the bidirectional trigger diode to infinity. If the probe is exchanged for measurement, the pointer of the multimeter swings to the right, indicating that the tube under test has leakage fault.

Place the multimeter in the corresponding DC voltage gear. The test voltage is supplied by the megger. During the test, shake the megger, and the voltage indicated by the multimeter is the VBO value of the tested pipe. Then replace the two pins of the pipe to be tested and measure the VBR value with the same method* Then compare VBO with VBR. The smaller the difference between the two is, the better the symmetry of the tested bidirectional trigger diode is.

5 Detection of transient voltage suppression diode (TVS)

A Use a multimeter R × 1k to measure the quality of the pipe

For unipolar TVS, the forward resistance and reverse resistance can be measured according to the method of measuring ordinary diodes. Generally, the forward resistance is about 4k Ω, and the reverse resistance is infinite.

For two-way pole TVS, the resistance between the two pins measured by arbitrarily changing the red and black probes should be infinite, otherwise, the tube performance * * or has been damaged.

6 Detection of high frequency rheostat diode

A Identification of positive and negative electrodes

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

B Measure the forward and reverse resistance to judge its quality

The specific method is the same as that of measuring the forward and reverse resistance of ordinary diodes. When the 500 multimeter R × 1k gear is used for measurement, the forward resistance of normal high-frequency varistor diodes is 5k ～ 5.5k, and the reverse resistance is infinite.

7 Detection of varactor diode

Place the multimeter in R × 10k gear. No matter how the red and black probe are exchanged for measurement, the resistance between the two pins of the varactor diode should be infinite. If the pointer of the multimeter swings slightly to the right or the resistance value is zero during the measurement, it means that the measured varactor diode has leakage fault or has been broken down and damaged. The capacity of varactor diode disappears or the internal open circuit fault cannot be detected and judged with a multimeter. If necessary, the replacement method can be used for inspection and judgment.

8 Detection of monochrome LED

Attach a 1.5V dry battery outside the multimeter, and set the multimeter to R × 10 or R × 100. This connection is equivalent to connecting 1.5V voltage to the multimeter in series to increase the detection voltage to 3V (the starting voltage of LED is 2V). During detection, use the two probes of the multimeter to contact the two pins of the light-emitting diode in turn. If the tube is in good performance, 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. Distinguish the positive and negative electrodes of infrared light-emitting diodes. The infrared light-emitting diode has two pins, usually the long pin is the positive pole, and the short pin is the negative pole. Since the infrared light-emitting diode is transparent, the electrode in the shell is clearly visible. The wider and larger inner electrode is the negative electrode, while the narrower and smaller inner electrode is the positive electrode.

B. Place the multimeter in R × 1k gear and measure the forward and reverse resistance of the infrared light-emitting diode. Generally, the forward resistance should be about 30k and the reverse resistance should be more than 500k, so that such pipes can be used normally. The greater the reverse resistance, the better.

10 Detection of infrared receiving diode

A. Identify pin polarity

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

(b) Put the multimeter in R × 1k gear to check the method of distinguishing the positive and negative electrodes of ordinary diodes, that is, exchange red and black probes to measure the resistance between two pins of the pipe twice. When normal, the resistance value obtained should be one big and one small. The pin connected to the red probe is the negative pole, and the pin connected to the black probe is the positive pole.

B. Test performance. Measure the forward and reverse resistance of the infrared receiving diode with the resistance block of the multimeter. According to the value of the forward and reverse resistance, the infrared receiving diode can be preliminarily judged.

11 Detection of laser diode

A Place the multimeter in R × 1k gear, and determine the pin arrangement order of the laser diode according to the method of detecting the forward and reverse resistance of the common diode. However, it should be noted that the forward voltage drop of the laser diode is greater than that of the ordinary diode, so when detecting the forward resistance, the pointer of the multimeter only slightly deflects to the right, while the reverse resistance is infinite.

5、 Test method and experience of triode

1. Detection of medium and small power triodes

A The performance of triode with known model and pin arrangement can be judged according to the following methods

(a) Measure the resistance between electrodes. Put the multimeter in R × 100 or R × 1k gear, and test according to six different connection methods of red and black probes. Among them, the forward resistance values of the emitter junction and collector junction are relatively low, and the resistance values measured by the other four connection methods are very high, about hundreds of kiloohms to infinity. However, whether it is low resistance or high resistance, the interelectrode resistance of silicon triode is much greater than that of germanium triode.

(b) The value of the through current ICEO of the triode is approximately equal to the product of the multiple β of the tube 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 lead to the increase of ICEO. The increase of ICEO will directly affect the stability of pipe work, so pipes with small ICEO should be used as far as possible.

The ICEO can be indirectly estimated by directly measuring the resistance between the e-c poles of the triode with the resistance of the multimeter. The specific methods are as follows:

The resistance range of the multimeter is generally R × 100 or R × 1k. For PNP tube, the black meter tube is connected to e pole, and the red probe is connected to c pole. For NPN type triode, the black probe is connected to c pole, and the red probe is connected to e pole. The greater the resistance measured, the better. The greater the resistance between e-c, the smaller the ICEO of the pipe; On the contrary, the smaller the measured resistance is, the larger the ICEO of the tested tube is. Generally speaking, the resistance values of medium and small power silicon tubes and germanium low-frequency tubes should be several hundred kiloohms, tens of kiloohms and more than ten kiloohms respectively. If the resistance value is very small or the pointer of the multimeter shakes back and forth during the test, it indicates that the ICEO is large and the performance of the tubes is unstable.

(c) Measure the amplification capacity (β). At present, some models of multimeters have the scale line and test socket for measuring hFE of triode, which can easily measure the magnification of triode. First, turn the function switch of the multimeter to the gear, turn the range switch to the ADJ position, short connect the red and black probes, adjust the zero adjustment knob so that the pointer of the multimeter indicates zero, then turn the range switch to the hFE position, separate the two short connected probes, insert the tested triode into the test socket, and read the tube's magnification from the hFE scale line.

In addition, for the medium and small power triodes with this model, the manufacturer directly marks different color spots on the top of its shell to indicate the magnification 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 code used by each manufacturer is not necessarily the same.

B. Detection and discrimination electrode

(a) Determine the base. Use a multimeter R × 100 or R × 1k gear to measure the positive and negative resistance values between each two of the three electrodes of the triode. When the * * probe is connected to an electrode, and the * * probe contacts the other two electrodes successively to measure the low resistance value, the electrode connected to the * * probe is the base electrode b. At this time, pay attention to the polarity of the multimeter probe. If the red probe is connected to base b. When the black probe is connected to the other two poles respectively, the measured resistance is small, then the triode under test can be judged as PNP type tube; If the black probe is connected to base b and the red probe contacts the other two poles respectively, the measured resistance is small, then the triode under test is NPN type.

(b) Determine collector c and emitter e. (Taking PNP as an example) Place the multimeter in R × 100 or R × 1k gear, red probe base b, and use the black probe to contact the other two pins respectively, the two measured resistance values will be larger and smaller. In a small resistance measurement, the connecting pin of the black probe is a collector; In a measurement with large resistance, the connecting pin of the black probe is the emitter.

C. Distinguish high-frequency tube and low-frequency tube

The cut-off frequency of high-frequency tube is greater than 3MHz, while that of low-frequency tube is less than 3MHz. Generally, the two cannot be interchanged.

D In circuit voltage detection and judgment method

In practical applications, small power transistors are mostly directly welded to the printed circuit board. Because of the high installation density of components and the troublesome disassembly, it is often used to measure the voltage value of each pin of the tested triode by using the DC voltage block of a multimeter to infer whether its work is normal and then judge whether it is good or bad.

2. Detection of high-power crystal triode

Various methods of detecting the polarity, tube type and performance of medium and small power triodes with a multimeter are basically applicable to detecting high-power triodes. However, because the working current of high-power triode is relatively large, its PN junction area is also large. When the PN junction is large, its reverse saturation current will inevitably increase. Therefore, if the R × 1k gear of the multimeter is used to measure the inter electrode resistance of the medium and small power triodes, the measured resistance value must be very small, as if there is a short circuit between the electrodes. Therefore, the R × 10 or R × 1 gear is usually used to detect the high-power triodes.

3. Detection of ordinary Darlington tube

The detection of ordinary Darlington tubes with a multimeter includes identifying electrodes, distinguishing PNP and NPN types, and estimating amplification capacity. Since the E-B poles of Darlington tube contain multiple emission junctions, the R × 10k gear with higher voltage provided by the multimeter should be used for measurement.

4 Detection of high-power Darlington tube

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

A. Measure the resistance value of PN junction between B and C with a multimeter R × 10k gear, and it should be obviously 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 the resistors R1 and R2 are connected. When measuring with the resistance block of the multimeter, the measured resistance value is the result of the parallel connection of the positive resistance of the B-E junction with the resistance values of R1 and R2 when measuring in the positive direction; When the emission junction is cut off in reverse measurement, the measured resistance is the sum of (R1+R2) resistance, which is about several hundred Ohms, and the resistance value is fixed, which does not change with the change of resistance gear. However, it should be noted that some high-power Darlington transistors also have diodes on R1, R2, and the measured resistance value is not the sum of (R1+R2), but the parallel resistance value of (R1+R2) and the sum of the forward resistance of the two diodes.

5 Detection of damped line output triode

Place the multimeter in R × 1 gear, and judge whether it is normal by measuring the resistance between the electrodes of the damped line output triode separately. The specific test principle, method and steps are as follows:

A Connect the red probe to E and the black probe to B. At this time, it is equivalent to measuring the resistance of the parallel connection of the equivalent diode of the high-power tube B-E junction and the protection resistance R. Since the forward resistance of the equivalent diode is small, and the resistance of the protection resistance R is generally only 20 ° to 50 °, the resistance of the parallel connection of the two is also small; On the contrary, when the probe is switched, that is, the red probe is connected to B, and the black probe is connected to E, the measured value is the parallel resistance between the reverse resistance of the equivalent diode of the high-power tube B-E junction and the protection resistance R. Since the reverse resistance of the equivalent diode is large, the measured resistance is the value of the protection resistance R, which is still small.

B. Connect the red probe to C and the black probe to B. At this time, it is equivalent to measuring the forward resistance of the equivalent diode of the high-power tube B-C junction in the tube. Generally, the measured resistance is also small; Swap the red and black probe, that is, connect the red probe to B and the black probe to C, which is equivalent to measuring the reverse resistance of the equivalent diode of the high-power tube B-C junction in the tube, and the measured resistance is usually infinite.

C. Connect the red probe to E and the black probe to C, which is equivalent to measuring the reverse resistance of the damping diode in the tube. The measured resistance is generally large, about 300 °~∞; Swap the red and black probes, that is, connect the red probe to C and the black probe to E, which is equivalent to measuring the positive resistance of the damping diode in the tube. The measured resistance is generally small, ranging from several to several tens of inches.

6、 Common sense of integrated circuit detection

Understand the working principle of integrated circuits and related circuits before testing.

Before checking and repairing the integrated circuit, it is necessary to be familiar with the function of the used integrated circuit, the internal circuit, the main electrical parameters, the role of each pin, the normal voltage of the pin, the working principle of the circuit composed of the waveform and the peripheral components. If the above conditions are met, the analysis and inspection will be much easier.

The test shall not cause short circuit between pins.

A Method for Determining the Quality of General IC Integrated Circuits

1、 Off road detection

This method is carried out when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values between each pin corresponding to the grounding pin, and compare with the intact IC.

2、 On road detection

This is a detection method to detect the DC resistance, AC/DC voltage to ground and total working current of each pin of IC in the circuit (IC in the circuit) through a multimeter. This method overcomes the limitation of substitution test method requiring replaceable IC and the trouble of dismantling IC, and is a common and practical method for detecting IC *.

2. DC working voltage measurement

This is a way to measure the DC power supply voltage and the working voltage of peripheral components with the DC voltage block of the multimeter when the power is on; Detect the DC voltage value of each pin of the IC to the ground and compare it with the normal value, so as to compress the fault range and find out the damaged components. Pay attention to the following eight points when measuring:

(1) The internal resistance of the multimeter shall be large enough to be less than 10 times greater than the resistance of the circuit to be measured, so as to avoid large measurement errors.

(2) Usually, turn the potentiometers to the middle position. If it is a television, the signal source should use a standard color bar signal generator.

3) Anti slip measures shall be taken for the probe or probe. Because any instant short circuit will easily damage the ic. The following measures can be taken to prevent the stylus from sliding: take a section of bicycle valve core and put it on the stylus tip, and grow the stylus tip about 0.5mm. This can not only make the stylus tip contact with the tested point well, but also effectively prevent slippage. Even if it touches the adjacent point, it will not short circuit.

(4) When the measured voltage of a pin is not consistent with the normal value, it should be analyzed according to whether the voltage of the pin has an important impact on the normal operation of the IC and the corresponding changes in the voltage of other pins to judge whether the IC is good or bad.

(5) IC pin voltage will be affected by peripheral components. When leakage, short circuit, open circuit or value change occurs to the peripheral components, or the peripheral circuit is connected to a potentiometer with variable resistance, the pin voltage will change due to the different positions of the sliding arm of the potentiometer.

(6) If the voltage of each pin of the IC is normal, it is generally considered that the IC is normal; If the pin voltage of the IC part is abnormal, start with the deviation from the normal value * and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be damaged.

(7) For dynamic receiving devices, such as TV, the voltage of each pin of IC is different when there is signal or not. If it is found that the pin voltage does not change but changes greatly, and does not change with the signal size and different positions of adjustable components, it can be determined that the IC is damaged.

(8) For devices with multiple working modes, such as video recorders, the voltage of each IC pin is different under different working modes.

There are also two additions:

3. AC working voltage measurement method

In order to master the change of the AC signal of the IC, you can use a multimeter with a db jack to approximately measure the AC working voltage of the IC. When testing, the multimeter is placed in the AC voltage gear, and the positive probe is inserted into the db jack; For a multimeter without db jack, it is necessary to connect a 0.1~0.5 μ f isolating capacitor in series with the positive probe. This method is suitable for ic with low operating frequency, such as video amplification stage of tv, field scanning circuit, etc. Since the natural frequencies and waveforms of these circuits are different, the measured data are approximate and can only be used for reference.

4. Total current measurement method

This method is a way to judge whether an ic is good or bad by detecting the total current of the incoming line of the ic power supply. Since most of the internal ICs are directly coupled, when the IC is damaged (such as a breakdown or open circuit of a pn junction), the subsequent stage saturation and cut-off will be caused, and the total current will change. So we can judge the quality of ic by measuring the total current. You can also measure the voltage drop of the resistance in the power supply path and calculate the total current value with Ohm's law.

When measuring the voltage or testing the waveform with the oscilloscope probe, the probe or probe should not slide and cause a short circuit between the pins of the integrated circuit. * It is better to measure on the peripheral printed circuit directly connected to the pins. Any momentary short circuit is easy to damage the integrated circuit, so you should be more careful when testing CMOS integrated circuits in flat package.

It is forbidden to use grounded test equipment to contact live TV, audio, video and other equipment on the bottom plate without isolation transformer. It is strictly prohibited to directly test the TV, audio, video and other equipment without power isolation transformer with the instrument and equipment whose shell has been grounded. Although general radio recorders have power transformers, when touching special TV or audio equipment, especially those with large output power or little understanding of the nature of the power supply used, it is necessary to first find out whether the chassis of the radio recorder is charged, otherwise it is very easy to cause power short circuit with live TV, audio and other equipment on the backplane, affecting the integrated circuit, and causing further expansion of the fault.? Pay attention to the insulating property of electric soldering iron.

It is not allowed to use soldering iron for welding with electricity. It is necessary to confirm that the soldering iron is not electrified, * so as to ground the shell of the soldering iron, and be more careful with MOS circuits, especially if the low-voltage circuit iron of 6~8V can be used * *.? The welding quality shall be guaranteed. When welding, it is sure to weld firmly. The accumulation of solder and air holes are easy to cause faulty soldering. The welding time generally does not exceed 3 seconds, and the power of the soldering iron should be about 25W of internal heating type. Check the soldered integrated circuit carefully, * use an ohmmeter to measure whether there is a short circuit between the pins, and then turn on the power after confirming that there is no solder adhesion.? Do not judge the damage of integrated circuit easily. Do not easily judge that the integrated circuit is damaged. Because the vast majority of integrated circuits are directly coupled, once a circuit is abnormal, it may lead to multiple voltage changes, and these changes are not necessarily caused by the damage of the integrated circuit. In addition, in some cases, when the measured voltage of each pin is consistent with or close to the normal value, it does not necessarily mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.

The internal resistance of the test instrument shall be large. When measuring the DC voltage of integrated circuit pins, a multimeter with internal resistance greater than 20K Ω/V on the meter head should be selected, otherwise there will be large measurement errors for some pin voltages.? Pay attention to the heat dissipation of power integrated circuits. The power integrated circuit should have good heat dissipation, and it is not allowed to work in a high-power state without a radiator.? The lead wire shall be reasonable. If it is necessary to add peripheral components to replace the damaged parts inside the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the grounding terminal between the audio power amplifier integrated circuit and the preamplifier circuit.

7、 Test Methods and Experience of FET

A large number of various types of electronic components are used in electronic equipment, and the failure of equipment is mostly caused by the failure or damage of electronic components. Therefore, how to detect electronic components correctly is particularly important, which is also a skill that electronic maintenance personnel must master. I have accumulated some experience and skills in testing common electronic components in electrical maintenance for your reference.

1. Measure the polarity of each pin of rectifier bridge

The multimeter is in R × 1k gear, the black probe is connected to any pin of the bridge stack, and the red probe measures the other three pins successively. If the readings are infinite, the black probe is connected to the output positive pole of the bridge stack. If the readings are 4 ～ 10k Ω, the black probe is connected to the output negative pole of the bridge stack, and the other two pins are the AC input terminals of the bridge stack.

2. Judge the quality of crystal oscillator

First, use a multimeter (R × 10k gear) to measure the resistance at both ends of the crystal oscillator. If it is infinite, it means that the crystal oscillator has no short circuit or leakage; Then insert the test pen into the mains socket, hold any pin of the crystal oscillator with your finger, and touch the other pin to the metal part at the top of the test pen. If the neon bubble of the test pen turns red, the crystal oscillator is good; If the neon bubble is not bright, the crystal oscillator is damaged.

3. Unidirectional thyristor detection

The R × 1k or R × 100 gear of the multimeter can be used to measure the positive and reverse resistance between any two poles. If the resistance of one pair of poles is found to be low resistance (100 Ω~lk Ω), then the black probe is connected to the control pole, the red probe is connected to the cathode, and the other pole is the anode. There are three PN junctions in the thyristor. We can judge whether it is good or bad by measuring the positive and reverse resistance of the PN junction. When measuring the resistance between control electrode (G) and cathode [C], if the forward and reverse resistance are both zero or infinite, it indicates that the control electrode is short circuited or open circuited; when measuring the resistance between control electrode (G) and anode (A), the forward and reverse resistance readings should be large;

When measuring the resistance between anode (A) and cathode (C), the forward and reverse resistance should be large.

4. Polarity identification of bidirectional thyristor

The bidirectional thyristor has main electrode 1, main electrode 2 and control electrode. If the resistance between the two main electrodes is measured with a multimeter R × 1k gear, the reading should be approximately infinite, while the positive and negative resistance readings between the control electrode and any main electrode are only tens of ohms. According to this characteristic, we can easily identify the control electrode of bidirectional thyristor by measuring the resistance between electrodes. The black probe is connected to the main electrode 1. The positive resistance measured when the red probe is connected to the control electrode is always smaller than the reverse resistance, so we can easily identify the main electrode 1 and main electrode 2 by measuring the resistance.

5. Check the quality of LED

First place the multimeter in R × 10k or R × 100k gear, then connect the red probe to the "ground" lead out terminal of the nixie tube (taking the common negative nixie tube as an example), and connect the black probe to other lead out terminals of the nixie tube in turn. The seven segments should light up separately, otherwise it means that the nixie tube is damaged.

6. Identify the electrode of junction type field effect transistor

Put the multimeter in R × 1k gear, use the black probe to contact the pin assumed to be grid G, and then use the red probe to contact the other two pins respectively. If the resistance values are relatively small (5~10 Ω), exchange the red and black probes for measurement. If the resistance values are large (∞), it means that they are reverse resistance (PN junction reverse), belong to N-channel tube, and the pin contacted by the black probe is grid G, and it means that the original assumption is correct. If the resistance value measured again is very small, it indicates that it is a positive resistance, belonging to the P-channel field-effect transistor, and the black probe is also connected to grid G. If the above situation does not occur, the red and black probes can be replaced and tested according to the above method until the grid is determined. Generally, the source and drain of JFET are symmetrical during manufacturing. Therefore, when the grid G is determined, it is not necessary to distinguish the source S and drain D, because these two electrodes can be used interchangeably. The resistance between the source and drain is several thousand ohms.

7. Discrimination of triode electrode

For a triode with unclear or unmarked model, if you want to distinguish its three electrodes, you can also use a multimeter to test. First, turn the multimeter range switch to R × 100 or R × 1k resistance gear. The red stylus contacts one electrode of the triode at random, and the black stylus contacts the other two electrodes in turn to measure the resistance between them. If the measured resistance is several hundred Ohms low, the electrode contacted by the red stylus is the base electrode b, and this tube is a PNP tube. If the measured high resistance is tens to hundreds of kiloohms, the electrode contacted by the red probe is also the base electrode b, and this tube is an NPN tube.

On the basis of distinguishing the tube type and the base b, the collector is determined by the principle that the forward current amplification coefficient of the triode is greater than the reverse current amplification coefficient. Arbitrarily assume that one electrode is c-electrode and the other electrode is e-electrode. Turn the multimeter range switch to R × 1k resistance gear. For the PNP tube, connect the red probe to the c electrode and the black probe to the e electrode, and then pinch the b and c electrodes of the tube at the same time, but do not make the b and c poles directly touch each other to measure a resistance value. Then the two probes are exchanged for * * times of measurement, and the resistance of the two measurements is compared. For the PNP type tube, the resistance value is small once, and the electrode connected to the red probe is the collector. For the NPN type tube with small resistance, the electrode connected to the black probe is the collector.

8. Judgment of potentiometer

First measure the nominal resistance of the potentiometer. Measure the two ends of "1" and "3" with the ohm gear of the multimeter (set the end of "2" as the movable contact), and the reading should be the nominal value of the potentiometer. If the pointer of the multimeter does not move, the resistance value does not move, or the resistance value differs a lot, it indicates that the potentiometer is damaged. Then check whether the movable arm of the potentiometer is in good contact with the resistor. Measure the two ends of "1", "2" or "2", "3" with the ohm gear of the multimeter. Turn the potentiometer shaft counterclockwise to the position close to "off". At this time, the resistance should be as small as possible. Then slowly rotate the shaft handle clockwise. The resistance should gradually increase. When turning to the extreme position, the resistance should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, trace and kick the moving contact point * *.

9. Measure the leakage resistance of large capacitance

Use a 500 type multimeter to place it in the R × 10 or R × 100 gear. When the pointer points to the value of *, immediately use the R × 1k gear for measurement. The pointer will be stable in a short time, so as to read the leakage resistance.

10. Identify infrared receiver pin

Put the multimeter in R × 1k gear, first assume that one pin of the receiver is the grounding terminal, connect it with the black probe, measure the resistance of the other two pins with the red probe, and compare the measured resistance values twice (generally in the range of 4~7k Q). The red probe with a smaller resistance is connected to the+5V power supply pin, and the other with a larger resistance is the signal pin. On the contrary, if the red probe is used to connect the known ground pin and the black probe is used to measure the known power pin and signal pin respectively, the resistance value is above 15k Ω. The pin with low resistance value is the+5V terminal, and the pin with high resistance value is the signal terminal. If the measurement results meet the above resistance values, the receiver can be judged to be intact.

11. Judge the polarity of unsigned electrolytic capacitor

First, short circuit the capacitor and discharge it, then mark the two leads with A and B. Put the multimeter in R × 100 or R × 1k gear, connect the black probe to lead A, and connect the red probe to lead B. When the pointer is still, read it, and then perform short circuit discharge after measurement; Then connect the black probe to lead B and the red probe to lead A. Compare the readings twice. The black probe with a larger resistance is connected to the positive pole and the red probe to the negative pole.

12. Measuring LED

Take an electrolytic capacitor with a capacity greater than 100 "F (the larger the capacity, the more obvious the phenomenon), and charge it with a multimeter R × 100 gear. The black probe is connected to the positive electrode of the capacitance, and the red probe is connected to the negative electrode of the capacitance. After charging, the black probe is connected to the negative electrode of the capacitance, and the measured LED is connected between the red probe and the positive electrode of the capacitance. If the LED lights up and then goes out gradually, it is good. At this time, the red probe is connected to the negative pole of the LED, and the capacitor positive pole is connected to the positive pole of the LED. If the LED does not light up, switch its two ends and reconnect them for testing. If it does not light up, it indicates that the LED is damaged.

13。 Photocoupler detection

The resistance R × 100 gear shall be selected for the multimeter, and the R × 10k gear shall not be selected to prevent the LED from breakdown due to excessive battery voltage. The red and black probes are connected to the input terminals, and the forward and reverse resistance are measured. Normally, the forward resistance is tens of ohms, and the reverse resistance is thousands to tens of ohms. If the forward and reverse resistances are similar, the LED is damaged. Select the resistance R × 1 gear on the multimeter. The red and black probe are connected to the output terminal, and the forward and reverse resistance are measured. Normally, they are close to ∞, otherwise the light tube is damaged. Select the resistance R × 10 gear of the multimeter, connect the red and black probe to the input and output terminals respectively to measure the insulation resistance between the light-emitting tube and the light receiving tube (if possible, use a megger to measure its insulation resistance, at this time, the rated output voltage of the megger should be slightly lower than the allowable withstand voltage value of the measured photoelectric coupler), and the insulation resistance between the light-emitting tube and the light receiving tube should be ∞ normally.

14. Damage identification of laser diode

Remove the laser diode and measure its resistance. Under normal circumstances, the reverse resistance should be infinite, and the forward resistance should be 20k Ω~40k Ω. If the measured forward resistance has exceeded 50k Ω, the performance of the laser diode has declined; If the forward resistance has exceeded 90k Ω, the tube is damaged and can no longer be used.

15. Detection of photoresistor

During detection, set the multimeter to R × 1k Ω, keep the light receiving surface of the photoresistor perpendicular to the incident light, and the resistance directly measured on the multimeter is the bright resistance. Then place the photoresistor in a completely dark place, and the resistance measured by the multimeter is the dark resistance. If the light resistance is several thousand ohm to dozens of dry ohm, and the dark resistance is several to dozens of megaohm, it indicates that the photosensitive resistance is good.