Fault characteristics and maintenance of capacitor damage of industrial control circuit board
The failure caused by capacitor damage is the highest in electronic equipment, especially electrolytic capacitor damage is the most common.
Capacitors play different roles in circuits and cause different faults. In the industrial control circuit board, the digital circuit accounts for the vast majority, the capacitor is mostly used for power filter, and the capacitor is less 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.
The power of the once took an X-ray flaw detector, users reflect the smoke came out from the power supply, open the case after arrival a 1000 uF / 350 v large capacitor has the oily stuff out, tear open come down to a capacity of only a few tens of uF, also found that only the capacitance of the rectifier bridge and heat sink nearest, other from far is intact, the capacity is normal. In addition, there is a short circuit in the ceramic capacitor, and it is also found that the capacitor is close to the heating part. 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 encounter of such failure, you can focus on the capacitor check, replace the capacitor is often surprising (of course, also want to pay attention to the quality of the capacitor, to choose a better brand, such as ruby, black gold and so on).
Two, resistance damage characteristics and discrimination
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.
Resistors are the most abundant component in electrical equipment, but not the one with the highest damage rate. Resistance damage with open circuit is most 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 is the most 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 and has small resistance. When the cylindrical wire wound resistance is burned out, some will be black or surface skin, crack, and some have no trace. Cement resistors are a type of wire wound resistors and 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 resistances in the circuit board at both ends of the resistance of the resistance, if the amount of resistance is greater than nominal value, Then the resistance must be damaged (it is necessary to pay attention to the stability of the resistance value before making a 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 value, 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.
Three, the operational amplifier good or bad judgment method
It is difficult for many electronic maintainers to judge the quality of operational amplifiers, not only because of their education level (there are many undergraduates, if they do not teach it, they will certainly not understand it for a long time after teaching it, there is also a graduate student who specialized in learning frequency conversion control with the tutor, even so!) , here with you to discuss together, I hope to help you.
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 the difference of more than 0.5 V, Then the amplifier must be broken! (I use FLUKE179 multimeter).
If the device is used as a comparator, the same input and reverse input are allowed to be unequal.
Covoltage > Reverse voltage, the output voltage is close to the positive maximum value;
Covoltage < Reverse voltage, the output voltage is close to 0V or negative maximum (depending on dual or single power 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.
Four, multimeter test SMT components a tip
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 of the smallest needle, (depth of industrial control maintenance technology column) will be close to the multimeter pen, and then take a multi-strand cable in the fine copper wire, with fine copper wire pen and needle tied together, and then soldering. 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.
Five, the circuit board public power supply short circuit fault maintenance method, circuit board technology
Circuit board maintenance, if public power short circuit fault often head is big, because many devices are sharing the same power supply, each device in the switching power supply, there is the suspicion of short circuit, if the plate element is not much, adopt the way of “hoe the earth” will short-circuit point can be found, if too many components, could “hoe the earth” hoe to situation will depend on luck. 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 power supply is not expensive, about 300 yuan. Will open circuit voltage to the device power supply voltage level, current to minimum first, add the voltage in the circuit of the power supply voltage 5 v points such as the 74 series chips, and 0 v side, depending on the degree of short circuit, the current increases slowly, with the hand touch device, when touch to a certain device heating obviously, this is often damaged components, can be removed to further confirm the measurement. 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.
Six, a small eraser, solve a big problem
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 an 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.
Seven, good and bad electrical fault analysis (circuit board technology_
All kinds of ups and downs of electrical failure in terms of probability, it probably includes the following situations:
- Poor contact
Bad contact between the board card and the slot, broken inside the cable, wire plug and wiring terminal contact is not good, virtual welding of components, etc. - The signal is disturbed
For digital circuit, in specific conditions, the fault will appear, may indeed be too much interference affect the control system to make it wrong, there are circuit board individual component parameters or overall performance parameters have changed, so that the anti-interference ability tends to the critical point, thus failure; - Poor thermal stability of components
From a large number of maintenance practices, the thermal stability of electrolytic capacitor is not good, followed by other capacitors, transistors, diodes, IC, resistance, etc.; - There is moisture and dust on the circuit board.
Moisture and dust will conduct electricity, with resistance effect, and in the process of thermal expansion and contraction resistance value will change, the resistance value and other components have parallel effect, the effect is relatively strong will change circuit parameters, so that the fault occurs; - Software is also a consideration
Many parameters in the circuit are adjusted by software, and the margin of some parameters is too low and in the critical range. When the machine runs under the condition that the software determines the fault, the alarm will appear.