1. Fault characteristics and maintenance of capacitance of industrial control circuit board
The failure of capacitance damage is the highest in electronic devices, especially in the case of electrolytic capacitance.
The capacitance failure is shown as follows: 1.The capacity is small;2. Total loss of capacity;3. The leakage;4. A short circuit.
Capacitors play a different role in the circuit, and the failures are also characterized.In the industrial control printed circuit board, the digital circuit board is the majority, the capacitor is used to do the power filter, the signal coupling and the capacitance of the oscillator circuit are less.If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not be vibrating, and there is no voltage output.Or output voltage filtering is not good, the circuit due to voltage instability and logical confusion, the performance of the machine working hours or can not open the machine, if the capacitor and the digital circuit board power supply between the positive and negative, the performance of the above.This is particularly strong near the computer motherboard, many computer use appears sometimes can’t open the machine for many years, the phenomenon of sometimes turns on, open the case, often have electrolytic capacitor beat phenomenon can be seen, if the capacitor down to measure capacity to find is much lower than the actual value.
The life of capacitor is directly related to ambient temperature. The higher the environmental temperature, the shorter the capacitor life.This rule not only applies to electrolytic capacitors but also to other capacitors.Therefore, in the search for the failure capacitance, it is necessary to check the capacitance which is relatively close to the heat source, such as the capacitance by the side of the radiator and the high power assembled components. The closer to it, the more likely it is to be damaged.Have been repaired a X-ray flaw detector power supply, 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 capacitance of China chip, and it also finds that the capacitor is close to the heating parts.Therefore, it should be focused when searching for maintenance.
Some capacitive leakage is very serious, with finger touch can even hot, this kind of capacitor must change.
In the case of good and bad breakdowns, the most common fault is the failure of capacitance damage.So at such failure, capacitance can be key to check, after the change of capacitance is often surprising (of course also should pay attention to the quality of the capacitance, choose better brands, such as ruby, black diamond and the like).
2. Characteristics and discrimination of resistance damage
Resistors are the most numerous components in electrical equipment, but not the most damaged components.Resistance damage is the most common way to open the road, and the resistance is rare and the resistance is very rare.Common carbon film resistance, metal film resistance, wire wound resistance and insurance resistance.The most widely before two kinds of resistance, the characteristic of its damage is a low resistance (below 100 Ω) and high value (more than 100 k Ω) damage rate is higher, intermediate resistance (such as a few hundred to tens of thousands of European) little damage;The second is that the resistance of low resistance is often charred and black, and it is easy to find that there are few marks when the resistance of high resistance is damaged.Wire wound resistance is generally used as a large current limit flow, and the resistance is not large.Some of the cylindrical wires can be blackened or surface blasting, cracks or no marks.Cement resistance is a kind of wire wound resistance, which may break when burnt, otherwise there is no visible trace.When the insurance resistance burns, some surfaces will blow up a piece of skin, some of which has no trace, but it will never be blackened.According to the above characteristics, when checking the resistance, can be focused on, quickly identify 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 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 pcb circuit boards at both ends of the resistance of the resistance, if the amount of resistance is greater than nominal value, is the resistance must be damaged (pay attention to, such as stable resistance showed only after to conclusions, because may shunt capacitance in circuit components, there is a charge and discharge process), if the amount of resistance is smaller than nominal value, you generally don’t have to ignore it.That’s how you measure every resistor on the printed circuit board pcb,even if the “wrong” a thousand, it will not let one.
3.the evaluation method of the operational amplifier
Operational amplifier of discriminant for quite a few electronic maintenance has certain difficulty, not only the degree of cultural relations,hope it will help you to discuss this with you.
The ideal operational amplifier has the characteristics of “virtual short” and “virtual break”. These two characteristics are very useful for analyzing the transport circuit of linear application.In order to ensure linear application, transport must work under closed loop (negative feedback).If there is no negative feedback, the open loop amplification is a comparator.If the device is to be judged, it should be clear whether the device is used as an amplifier or as a comparator in the circuit.
No matter what type of amplifier, there is a feedback resistor Rf, then when we repair can check the feedback resistance on the road from electricity, using a multimeter to check output and reverse the input, the resistance between the if large, such as more than a few M Ω, then we can be sure the device is probably do more for, if the value 0 Ω to dozens of smaller k Ω, is to check whether the resistance between output and reverse the input, amplifier using words will do.
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, the amplifier will no doubt bad!
If the device is used as a comparer, it allows the same to be different from the input terminal and the reverse input terminal. The voltage of the same voltage is opposite to >, and the output voltage is close to the maximum value of the positive.The same voltage is the reverse voltage, and the output voltage is close to 0V or negative maximum (depending on dual power or single power).If the detection voltage does not meet this rule, then the device will be damaged!So you don’t have to use the substitution method, you don’t have to tear down the chip on the printed circuit board to judge the performance of the amplifier.
4. A small trick to test SMT components in a multimeter
Some patch element is very small, using ordinary multimeter pens and test maintenance is not very convenient, it is likely to cause a short circuit, the second is for coated with insulation coating of pin board inconvenience come into contact with the component of metal parts.Here is a simple method to give you a lot of convenience.
Take two pieces of the minimum number of needles, industrial maintenance technology (depth) with multimeter on tight, and then take a more fine copper wire in the cable, with a fine copper wire pens and needles and tied together, reoccupy solder weld.With such small tip pens and to measure the SMT assembling components when no short circuit board, and can pinpoint prick insulation coating, taking the key position, don’t have to bother to scrape the membrane membrane.
5. Repair method of circuit board public power short
Printed circuit board maintenance, if you encounter short-circuit fault of public power supply is often the first large, 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 smt components, could “hoe the earth” hoe to situation will depend on luck.In this recommendation a more effective method, use this method, twice the work, often can quickly find the point of failure.
To have a voltage current all adjustable power supply, voltage 0-30v, current 0-3a, this power supply is not expensive, 300RMB or so.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 smd components, can be removed to further confirm the measurement.Of course, the voltage must not exceed the working voltage of the device, and cannot be connected to the counter, otherwise it will burn other good devices.
6.a small eraser, solve big problem
More and more used in industrial control printed circuit boards pcb, many ways of board with gold finger is inserted into the slot. Because of the bad industrial environment, dust, humidity, corrosive gas environment produces poor contact failure is easy to make the card, a lot of friends may be the way to solve the problem by changing the interface card, but the cost of buying the card is very considerable, especially of some of the imported equipment of the card.In fact, you might as well use an eraser to wipe a few times on the gold finger, clean up the dirt on the gold finger, then try again, maybe solve the problem!The method is simple and practical.
7.The analysis of bad electrical faults in good times
All kinds of good times and bad electrical failures may include the following situations:
1. Poor contact
Bad contact with slot, internal fracture of cable, bad contact of wire plug and terminal, and virtual welding of components, etc.
2. The signal was disturbed
For digital circuits, in the case of certain conditions, the fault is present, may really is too much interference affecting control system make it wrong, also have a circuit board component parameters or individual overall performance parameters changed, the anti-interference ability toward a tipping point, thus a failure;
3. The thermal stability of components is not good
From a large number of maintenance practices, the thermal stability of the first electrolytic capacitor is not good, followed by other capacitors, triode, diodes, IC, resistance, etc.
4. There is moisture and dust on the circuit board
Moisture and dust will conduct electricity, with resistance effect, but also in the process of heat bilges cold shrink resistance will also change, the effect of resistance together with the other elements are in parallel, the effect is stronger when can change the circuit parameters, make failure;
5.Software is also a factor
Many parameters used in the circuit to adjust the software, the allowance of some parameters on the too low, in the critical range, when the machine operation comply with software fault reason, then the alarm will appear.
8. How to quickly find the components information?
Modern electronics is multifarious, the increasingly wide range of components, thousands, in circuit pcb maintenance, especially industrial printed circuit board maintenance areas, many of the components is not seen, or even never heard of, even if some other components of the information is complete, the board in hand but will read the data in the computer analysis, if not a quick search method, the maintenance efficiency is going to sell at a discount greatly, industrial electronic repair field, efficiency is money,so choose a good PCB or PCB Assembly supplier is very important, would like to recommend King-PCB.com for you.