PCB News - Precautions for installing PCB board layout

Capacitance failure

The failure caused by capacitor damage is common in electronic equipment, especially in electrolytic capacitor damage. Capacitor damage is characterized by small capacity, complete loss of capacity, leakage, and short circuit.

The role of capacitance in the printed circuit board is different, the fault caused by each has its own characteristics: in the industrial control circuit board, digital circuit accounted for the vast majority, capacitor is used for power filter, less capacitor is used for signal coupling and oscillation circuit. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate, no voltage output; Or the output voltage filter is not good, the circuit due to voltage instability and logic chaos, performance for the machine working good or bad or can not open the machine, if the capacitor and in the digital circuit between the positive and negative power supply, fault performance as above.

This is especially obvious on the computer motherboard, many computers used for a few years sometimes can not open the machine, and sometimes can be turned on the phenomenon, open the case, often can see the phenomenon of electrolytic capacitor bulges, if the capacitor is removed to measure the capacity, found that the actual value is much lower than the actual value.

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

Some capacitors have serious leakage and can even become hot when touched by your fingers, so they must be replaced. In the maintenance of good and bad fault, rule out the possibility of poor contact, generally most of the capacitor damage caused by the fault. So in the event of such a failure, you can focus on the capacitor check, after replacing the capacitor is often surprising.

Resistance fault

Often see many beginners in the maintenance of the circuit in the resistance toss, and disassembly and welding, in fact, much repair, as long as you understand the damage characteristics of the resistance, you do not have to spend a lot of time.

Resistance is a component of large quantity in electrical equipment, but it is not the component of damage rate. Resistance damage with open circuit is common, resistance increase is rare, resistance decrease is very rare. Common carbon film resistance, metal film resistance, wire wound resistance and insurance resistance.

The first two kinds of resistance widely used, the characteristics of the damage is a low resistance (below 100 ω) and high resistance (more than 100 K ω) damage rate is high, the middle resistance (such as hundreds of euros to tens of thousands of euros) very little damage; Second, when low resistance is damaged, it is often scorched and black, and it is easy to find, while there are few traces when high resistance is damaged.

Wire wound resistance is generally used for high current limiting, resistance is not large; When the cylindrical wire wound resistance is burned out, some will be black or surface skin, crack, and some have no trace; Cement resistance is a kind of wire wound resistance, may break when burned out, otherwise there is no visible trace; When the resistors burn out, some surfaces will blow off a piece of skin, others will have no marks, but will never burn black. According to the above characteristics, in the inspection of resistance can be focused on, quickly find out the damaged resistance.

According to the characteristics of the listed above, we can first look at the low resistance values for any signs of blackened on printed circuit boards, and then according to the damage to the vast majority of open circuit or value become larger and easy to damage the characteristics of high resistance values, we can use the multimeter direct volume very high resista

nces in the circuit board at both ends of the resistance of the resistance.

If the amount of resistance is larger than the nominal resistance, then the resistance is definitely damaged (it should be noted that the resistance is stable before conclusion, because there may be a parallel capacitor element in the circuit, there is a charge-discharge process), if the amount of resistance is smaller than the nominal resistance, it is generally ignored. So every resistance on the circuit board is measured again, even if the "wrong kill" one thousand, will not let go of a.

Operational amplifier failure

The judgement of operational amplifier is good or bad to quite many electronic maintainers has certain difficulty, not only the relation of education degree, here and everybody discuss together, hope to help everybody.

The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", which are very useful for analyzing the operational amplifier circuit with linear operation. To ensure linear operation, the operational amplifier must operate in a closed loop (negative feedback). If there is no negative feedback, the open-loop amplifier becomes a comparator. If you want to judge the quality of the device, should be clearly divided into devices in the circuit is used for amplifier or comparator.

According to the principle of virtual amplifier short, that is to say, if the operational amplifier working properly, the same input and reverse voltage of the input will be equal, even though there are differences also mv level, of course, in some of the high input impedance circuit multimeters resistance will be a little influence on voltage test, but generally not more than 0.2 V, If there is a difference of more than 0.5V, the amplifier will definitely fail.

If the device is used as a comparator, the coinput and reverse input are allowed to be unequal. Covoltage > Reverse voltage, the output voltage is close to the positive value; Covoltage < Reverse voltage, the output voltage is close to 0V or negative value (depending on dual or single supply). If the voltage detected does not conform to this rule, the device must fail! So you don't have to use substitution, you don't have to take the chip off the circuit board to determine whether the op amp is good or not.

SMT component failure

Some patch components are very small, with ordinary multimeter pen test maintenance is very inconvenient, one is easy to cause short circuit, the second is coated with insulation coating circuit board inconvenient contact element pin metal part. Here to tell you a simple method, will bring a lot of convenience to the detection.

Take two sizes of needle, put them close to the multimeter pen, and then take a thin copper wire in a multi-strand cable, tie the pen and needle together with the fine copper wire, and then solder securely. So with a small tip pen to measure the SMT components when there is no short circuit, and the tip can Pierce the insulation coating, direct to the key part, no longer need to bother to scrape the film.

The public power supply is short-circuited

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

If the board is not many components, the use of "hoe earth" way can find the short circuit point; If there are too many components, it's a matter of luck that the hoe works. In this recommended a more effective method, using this method, get twice the result with half the effort, often can quickly find the fault point.

To have a voltage and current adjustable power supply, voltage 0-30V, current 0-3A, this kind of power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the power supply voltage level of the device. First, adjust the current to. Apply this voltage to the power supply voltage point of the circuit, such as the 5V and 0V ends of the 74 series chip. Touch the device by hand. When you feel a device heating obviously, 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 connected inversely, otherwise it will burn out other good devices.

The board failure

More and more boards are used for industrial control, and many boards use the way of inserting gold fingers into slots. Because the industrial site environment is bad, dust, damp, corrosive gas environment easy to produce bad contact fault board, a lot of friends may solve the problem by replacing the board, but the cost of buying board is very considerable, especially some imported equipment board.

In fact, you might as well use the eraser on the gold finger repeatedly wipe a few times, the gold finger dirt clean, then try the machine, maybe solve the problem, the method is simple and practical.