The device most likely to cause circuit failure

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1.head capacitor failure

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The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitor is the most common. Capacitor damage is manifested as: smaller capacity, complete loss of capacity, leakage, short circuit.

The role of capacitors in the circuit is different, and the faults caused by them also have their own characteristics: in the industrial control circuit board, the digital circuit accounts for the vast majority, and the capacitors are mostly used for power supply filtering, and the capacitors are less used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate, and there is no voltage output;

Or the output voltage filter is not good, the circuit due to voltage instability and logic confusion, as the machine works good or bad or can not open the machine, if the capacitor and between the positive and negative terminals of the digital circuit power supply, the fault performance is the same as above.

This is especially obvious on the computer motherboard, many computers with a few years appear sometimes can not open the machine, sometimes can turn on the phenomenon, open the chassis, often can see the phenomenon of electrolytic capacitor bulge, if the capacitor removed to measure the capacity, found that the actual value is much lower.

The life of the capacitor is directly related to the ambient temperature, and the higher the ambient temperature, the shorter the life of the capacitor. This law applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for the fault capacitor, it should focus on checking the capacitor close to the heat source, such as the capacitor near the heat sink and high-power components, the closer it is, the greater the possibility of damage. Therefore, it should be focused on the maintenance and search.

Some capacitors leak more seriously, and even hot hands when touched with fingers, this capacitor must be replaced. In the maintenance of the good and bad fault, excluding the possibility of poor contact, generally most of the fault caused by capacitor damage. So in the event of such a failure, you can focus on checking the capacitor, and replacing the capacitor is often surprising.

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2.Resistance fault

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Often see many beginners in the maintenance of the circuit in the resistance toss, and dismantle and weld, in fact, much more repair, you only need to understand the resistance damage characteristics, you do not have to spend a lot of time.

Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is most common in open circuit, and it is rare for the resistance value to become large, and it is rare for the resistance value to become small. Common carbon film resistor, metal film resistor, wire wound resistor and safety resistor several.

The first two resistors are the most widely used, and their damage is characterized by a high damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ), and very little damage to the intermediate resistance (such as hundreds of euros to tens of thousands of euros); Second, low resistance resistance is often burnt black when damaged, it is easy to find, and high resistance resistance damage is rarely traces.

Wire-wound resistance is generally used as a large current limiting, the resistance value is not large; When the cylindrical wire wound resistance burns out, some will be blackened or the surface explodes, cracks, and some have no traces; Cement resistor is a kind of wire wound resistor, it may break when burned out, otherwise there is no visible trace; When the safety resistor burns out, some surfaces will blow off a piece of skin, and some will not have any traces, but will never burn black. According to the above characteristics, you can focus on checking the resistance and quickly find out the damaged resistance.

According to the characteristics listed above, we can first observe the low resistance on the circuit board there is no trace of burning black, and then according to the resistance damage when the vast majority of open circuit or resistance value becomes larger and high resistance value resistance easy to damage characteristics, we can use a multimeter on the circuit board first directly measure the resistance value of the resistance at both ends of the high resistance value.

If the resistance value is larger than the nominal resistance value, then the resistance must be damaged (pay attention to the resistance value is stable before concluding, because there is a possibility of parallel capacitive elements in the circuit, there is a charge-discharge process), if the resistance value is smaller than the nominal resistance value, then generally ignore it. In this way, every resistance on the circuit board is measured again, even if the "wrong kill" one thousand, it will not miss one.

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3.Operational amplifier fault

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The identification of the operational amplifier is good or bad for a considerable number of electronic maintainers has a certain difficulty, not only the relationship of education, here and we discuss together, I hope to help you.

Ideal operational amplifiers have the characteristics of "virtual short" and "virtual break", which are very useful for analyzing linear operational amplifier circuits. In order to ensure linear operation, the op amp must work in a closed loop (negative feedback). Without negative feedback, the open-loop amplifier becomes a comparator. If you want to judge the quality of the device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit.

According to the principle of the amplifier virtual short, that is, if the operational amplifier works normally, the same input and reverse input voltage must be equal, even if there is a difference is mv level, of course, in some high input impedance circuits, the internal resistance of the multimeter will have a little impact on the voltage test, but generally will not exceed 0.2V, if there is a difference of more than 0.5V, The amplifier is bound to fail.

If the device is used as a comparator, the same input and the reverse input are allowed to differ. If the same voltage is greater than the reverse voltage, the output voltage is close to the positive maximum; If the same voltage is < the reverse voltage, the output voltage is close to the maximum of 0V or negative (depending on the dual power supply or single power supply). If the voltage detected does not meet this rule, the device will be broken! So you don't have to use substitution, you don't have to remove the chip on the circuit board to determine whether the op amp is good or bad.

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4.SMT component failure

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Some patch components are very small, it is very inconvenient to test and repair with ordinary multimeter pen, one is easy to cause short circuit, and the other is inconvenient for the circuit board coated with insulation coating to contact the metal part of the component pin. Here I tell you a simple method that will bring a lot of convenience to the detection.

Take two of the smallest sewing needles and hold them close to the multimeter pen. Then take a thin copper wire from a multi-strand cable and tie the pen and needle together with the thin copper wire. In this way, there is no danger of short circuit when using a stylus with a small tip to measure those SMT components, and the tip can Pierce the insulation coating and go straight to the key parts, and there is no need to bother to scrape the film.

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5.The public power supply is short circuit faulty

In circuit board maintenance, if the fault of short circuit of the public power supply is often large, because many devices share the same power supply, each device that uses this power supply is suspected of short circuit.

If there are not many components on the board, the short circuit point can be found by the way of "hoe earth"; If there are too many components, the "hoe earth" can not hoe the condition depends on luck. In this recommend a more effective method, the use of this method, often can quickly find the fault point.

To have a voltage and current adjustable power supply, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level, first set the current to the minimum, apply this voltage to the circuit power supply voltage point such as 5V and 0V terminals of the 74 series chip, depending on the degree of short circuit, slowly increase the current.

Touch the device by hand, when a device is obviously heated, this is often the damaged component, which can be removed for further measurement and confirmation. Of course, the operating voltage must not exceed the operating voltage of the device, and can not be reversed, otherwise it will burn other good devices.

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6.Board fault

More and more boards are used in industrial control, and many boards are inserted into slots by goldfinger. Due to the harsh environment of the industrial site, the dusty, humid, corrosive gas environment is easy to make the board produce poor contact failure, many friends may solve the problem by replacing the board, but the cost of buying the board is very considerable, especially the board of some imported equipment.

In fact, you may wish to use an eraser to rub several times on the gold finger, clean up the dirt on the gold finger, and then try the machine, maybe it will solve the problem, the method is simple and practical.

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7.electrical failure

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A variety of good and bad electrical failures from the probability of the size probably includes the following situations:

Poor contact: poor contact between the board and the slot, failure to pass when the cable is broken inside, bad contact between the wire plug and the wiring terminal, and virtual welding of components are all such;

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Signal interference: for digital circuits, the fault will be presented under specific conditions, it is possible that the interference is too much to affect the control system to make it wrong, there are also changes in the parameters of individual components of the circuit board or the overall performance parameters, so that the anti-interference ability tends to the critical point of failure;

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The thermal stability of components is not good: from a large number of maintenance practices, the first is the thermal stability of electrolytic capacitors is not good, followed by other capacitors, transistors, diodes, ics, resistors, etc.;

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There is moisture on the circuit board, dust, etc. : moisture and dust will conduct electricity and have a resistance effect, and in the process of thermal expansion and contraction of the resistance value will change, the resistance value will have a parallel effect with other components, this effect is relatively strong when it will change the circuit parameters to make the fault occur;

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Software is also one of the factors to consider: many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low in the critical range. When the operating condition of the machine is in line with the reason for the software to determine the fault, then the alarm will appear.

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