【 Weak current College 】 e-mail technology--fault detection method 】

Use the appropriate method, find, decisions, and identify specific parts of the body and cause trouble, is the key to fault detection. The following describes the various fault detection method, is the long-term practice summary out of proven methods. Specific applications to target specific detection object, cross, flexible, and continually summarize fits your work area, and to achieve fast and accurate effective troubleshooting purposes.

1. the observation method

Observation of law is felt by the body found e-line fault. This is one of the easiest, most secure method, but also various instruments common detection process.
Observation can be divided into static observation and dynamic observation of the law.
1. static observation method
It is also known as not receiving power observation method. In electronic circuit power front mainly through visual inspections to identify certain failure. Practice has proved that accounted for a considerable share of electronic circuit fault of solder joint failures, the wire connector to disconnect, capacitor leakage or exploding, connectors, electrical contacts loose rust, etc, can be found through observation, there is no need to fight for the entire circuit, causing the failure.
"Static" emphasize meditation to contemplate, careful observation, so-so yet often cannot find fault.
Static observation, the first after the progressive outside. Open the enclosure before checking electrical appliance appearance, with and without collision, keystrokes, Jack wire and cable for damage, insurance is blowing. Open a shell, take a look inside all kinds of devices and components, with and without collision, break, burn, etc, and then hand or tool toggle some components, such as wire for further examination. For test circuit or prototype, to control principle check wiring errors, components meet design requirements, IC pins have not inserted in the wrong direction or bend, there is no leakage welding, bridge, etc.
When static observation is not unusual, you can further use dynamic observation of the law.
2. dynamic observation
It also called on observation method, that is, to the line after power-on, the use of human vision, smell, hear, touch check line fault. Power on, especially the larger equipment power should, whenever possible, use the isolation transformer and regulator progressive power, prevent failure. In general you should also use instruments, such as the ammeter, voltmeter, monitoring circuit status.
After power-on, the eye to see circuit has no smoke spark, arm, etc; ears to listen to the circuit has no abnormal sound; nose news has no electrical appliances burning, burning odor; hand to touch some tubes, integrated circuits, such as whether to Perm, (Note: high-voltage, high-current circuit shall be shock, Burns) found exception immediately lost power.
Power on, sometimes you can determine the cause of the failure, but in most cases will not be able to confirm the exact position and the reasons for failure. For example, an integrated circuits fevered might be peripheral circuit failure or power supply voltage is wrong, both might be overloaded or circuit self-excited, of course, does not exclude the IC itself is damaged, it must be combined with other testing methods, analysis, identify the fault lies.

2. measurement method

Measurement is fault detection in the most widely used and most effective method. According to the detection of electrical parameter properties can be divided into resistance, voltage, current law, logical state law and waveform method.
I. resistance method
Resistance to a variety of electronic components and circuits of basic characteristics, using a multimeter measuring electronic component or circuit resistance value between different points to determine the failure of the method known as resistance method.
Measurement of resistance, "online" and "offline" in two fundamental ways.
"Online" measurement, you need to consider to be logging components from other parallel branch, measurements should control schematics of judgment.
"Offline" measurements need to be logging components or circuits from the entire circuit or PCB solder down operation more trouble but the results are accurate and reliable.

Resistance measurement integrated circuit, usually before a table pen grounding, use another table to test the PIN on the pen to resistance values, and then exchange table pen and then measure once, a measured value with normal (some maintenance information given, or their accumulation), the greater the difference between the often failure is located. (Not necessarily the IC bad!)
Resistance method to determine the switches, connectors, cables, printed circuit board and electrically conductive graphic breaking resistor, capacitor short-circuit of metamorphic, inductance coil circuit breakers, etc., and is particularly effective and fast, but on transistors, integrated circuits and circuit unit normally cannot directly determine fault analysis or required the use of other methods, but due to the resistance method instead to circuit power, will detect the risks to a minimum, the first general inspection.
Note
(1) when using resistance method should be in line power, large capacitor discharge, otherwise the result is not accurate, you may damage the multimeter.
(2) in the detection of low voltage power supply IC (5V) avoid using pointer multimeter lOK stalls.
(3) online measurement should be alternating test multimeter table pen, contrast analysis.
2. voltage method
Electronic circuits typical line points has a definite operating voltage, judging by measuring voltage fault called voltage method.
Voltage is the power detection means the most basic, most common methods. According to the power supply properties can be divided into two types of AC and DC voltage measurement.
1. AC voltage measurement
General electronic line AC loop simpler, on 50/60Hz power after step-up or step-down voltage simply select the appropriate using ordinary multimeter AC-voltage measurement range, when you want to pay attention to safety and to develop the habit of using single-handed operation.
On non-50/60 Hz power supply, for example, the converter output voltage measurement must take into account the frequency characteristics using voltmeter, generic pointer for 45 ~ 2000Hz multimeters, digital multimeter 45 ~ 500Hz, out of range or non-sinusoidal wave measurement results are incorrect.
2. DC voltage measurement
Measuring DC voltage is generally divided into three steps:
(1) the measurement voltage circuitOutput correctly.
(2) the cell circuit and the key points of the circuit, "for example" amplifier output, the external parts power end etc voltage is normal.
(3) circuit main components such as transistors and integrated circuits various pin voltage normal, on integrated circuits would first like to measure power supply side.
Perfect product specification should give out the circuit the working voltage, some maintenance information also provides integrated circuits various pin voltage. In addition you can compare working with circuit voltage measured points. Deviation from normal voltage larger parts or components, often the fault lies.
This detection method, requires workers with circuit analysis capabilities as far as possible, to collect relevant information from the data, the circuit in order to achieve a multiplier effect.
Third, the current method
Electronic circuit normal working hours, various parts of the operating current is stable, the deviation from the normal value of the larger sites often fail is located. This is the current method to check the side line array theory.
Current method of direct measurement and indirect measuring methods.
Direct measurement is the ampere meter directly concatenated to detection circuit measured current value of the method. This approach is intuitive, accurate, but often need on line for the "operation", such as broken wires, solder pins, and other components to be measured, and thus not very convenient. For the complete machine total current measurements, usually by two table pen ammeter received a switch on the way to get on line with 220V AC must pay attention to measuring security.
Indirect side method is actually a method of measuring the voltage used to convert the current value. This method is easy, but if the selected side of point of defective components is not easy to accurately judge.
As shown in figure a, you would like to Re by measuring the voltage drop to determine whether the normal transistors operating current, such as Re itself resistance deviation or Ce leakage may cause false positives.

4. wave method
On alternating signal generation and processing circuit, using an oscilloscope on signaling point waveform is the most intuitive and most effective method of fault detection.
Waveform method applied to the following three conditions:
1. There is no wave and shape
In electronic circuits in General on the circuit of the waveform is there and the shape is determined, such as standard TV schematics in the shape of the waveform and amplitude (Figure 2), if measured the point difference waveform shape without or greater, then the fault occurs in the circuit more likely.

When observed should not appear of self-excited oscillation or modulation waveform, although it cannot determine which parts of the body, but can be accessed from the frequency and amplitude size analysis of the cause of the failure.
2. waveform distortion
In the zoom or buffer circuit, circuit parameter mismatch or improper or damaged components selection will cause the waveform distortion, by observing the waveform and analyze circuits you can find out the cause of the failure.
3. waveform parameters
Using the oscilloscope measurement waveforms of the various parameters, such as amplitude, phase and along the cycle, and so on, and typical of a controlled waveform parameters to identify the cause of the failure.
Application note waveform method
(1) on the circuit of high voltage and pulse position must note substantially cannot exceed the allowable voltage range of oscilloscope. If necessary, the use of high-voltage probe or circuit observation points to take partial pressure or sampling.
(2) when the oscilloscope access circuit itself lose people impedance on circuit has a certain impact, especially when measuring pulse circuit, to adopt a compensation of 10: 1 probe, otherwise the actual waveform and observed.
5. logical state method
For the purposes of the digital circuit, only the parts of the judgment in circuit logic State can determine the circuit is working. Digital logic, mainly by the level of two-level State, pulse train and high-impedance state. Thus you can use a logical pen circuit detection.
Logical sum is of small size, easy to use. The function simple logical pen can measure single-circuit (TTL/CMOS) logic, the logic of a full-function pen, in addition to measuring multiple circuit logic State, but also the number of quantitative measuring pulse, some also have a pulse generator function, may issue a single pulsed or continuous pulse for detection circuits.

3. tracking method

Signal transmission circuits, including signal gets (signal generation), signal (signal amplification and conversion, filtering, isolation, etc.) as well as signal perform circuit, in the modern electronic circuits represent a significant proportion. This circuit detection key is tracking signal transmission links. Specific application types according to the circuit will have signal transmission and signal note two human law.
1. signal transmission method
Signal transmission method for signal generation and signal processing circuit for flowed to find signal trace detection methods, specific detection can be divided into forward for searching (who by the distance to the output order lookup), reverse tracing (from output to lose people order lookup) and divided into three kinds of tracing.
Forward for searching is a commonly used measurement method, you can make use of test equipment (Oscilloscopes, frequency meter, Multimeter, etc.) gradual qualitative and quantitative detection signal to determine fault location. Figure 3 Exchange millivoltmeter circuit diagrams and schematic diagram of the testing. We use a fixed sine wave signal to the lost people-millivoltmeter, stage from the beginning of attenuation circuit detection levels, according to the line circuit circuit features and performance you can determine which signals correctly, recursively observations until find fault.

Obviously, the reverse tracing detection only detect a different order.
Searching for cell division more of circuit is an efficient way. We take a certain instrument time base signal generation circuit for example illustrates this approach. The circuit consists of over the thermostat of crystal oscillators produce 5MHz signal, after 9-frequency circuit, the test require 1Hz and 0.OlHz signal, as shown in Figure 4.

Circuit has a total of 10 units, if there is a problem with the 9 cell, using the method you want to find the test 8. equal division is searching method will circuit is divided into two parts: the first part in which the judgment failed, then the failed portion and thenTwo-part test. Still 9 unit failure, for example, with a diagonal measurement of the signal transmission method, find normal 1kHz, finding fault in the second half; and then measuring 1Hz signal, still run fine for failure on 9 and 10 units, the third measuring 0.1Hz signal, you can determine the 9 units. Obviously, by tracing method to be very efficient.
Equal division tracing method for multi-stage tandem structure of circuits and circuit failure rate at all levels in much the same, each test time almost circuit. For a branch, have feedback or unit less circuit shall not apply.
Second, the signal injection
For itself without signal generation circuit or signal generation circuit faulty signal processing circuitry using signal note people method is effective detection method. The so-called signal note, is the signal processing circuit for all levels of transmission and human side lose people known plus test signal, through Terminal indicator (for example indicating instrument, speakers, monitors, etc.) or testing equipment to determine the status of work, so the circuit to identify circuits fault.
Various radio and TV receiving equipment is one method for detecting the signal of note. Figure 5 is a diagram of a typical FM stereo radio. Detection requires two signals: discriminator FM stereo signal before you ask, the decoder after the audio signal. Usually detect radio circuit is the use of reverse signal note, namely a certain frequency and amplitude of the audio signal from the AR, AL started forward goes through speakers or headphones the sound is there and the sound quality and size, so judgment circuit failure. If the audio circuit part properly, use the FM stereo signal source from the G, H. ..... One note, until you find the point of failure.

Using signal when human detection note to note the following points
(1) the order of signal note people based on specific circuits can be forward, backward or in the middle of the note of the order.
(2) Note the nature and human signal ranges according to the circuit and note people point change, as in the above radio signal to the audio portion of the note, the more people close to the speakers, the stronger the signal, the same signal note person B point may be normal, the injection point D may be too strong to amplifiers saturation distortion. You can usually work estimating note people point signal as a signal reference note people.
(3) Note people to choose the right signal to add locations to prevent signal source and the circuit to be measured. In general you can choose to close the add person point injection.
(4) signal and test circuit to select appropriate coupled manner, such as AC signal should be threaded appropriate capacitance, DC signal threaded appropriate resistance signal and impedance matching circuit to be measured.
(5) signal can sometimes injection using a simple means, such as radio detection is available in the human body sensors signal as note one signal (i.e. handheld conductive body touch the appropriate circuit) for identification. Similarly, sometimes you must note the induction signal external signal detection.

4. substitution method

Replace method is the same as with standard properties of normal components, circuits or components, instead of the circuit of the corresponding portion suspected to determine fault is a detection method and circuit debugging, maintenance of the most common, one of the most effective way.
Practice, by replacing the object, you can have three methods.
1. the components replace
Components replace some circuit structure more convenient (for example with the IC connectors, switches, relays, etc.), is generally required desoldering, operation more trouble and can be easily damaged peripheral circuits or components and printed circuit board, so as to replace General only are difficult to other detection method for judging at the resort and avoid on board to be "major surgery." For example, suspect that a two-circuit-wire components, can be welded on a test of the new symbol; suspect a capacitor capacity can be reduced and a capacitor test.
2. replace cell circuit
When you suspect that a cell circuit failure, another one the same model or type of normal circuit, replace the corresponding unit of unknown machine, you can determine this circuit unit circuit is normal. Some circuits are the same number of circuits, such as stereo circuit road left and right channels are exactly the same, can be used to test cross-replace.
When electronic equipment uses cell circuit number plate structure replaces test is more convenient. The on-site maintenance of more sophisticated equipment, can bear with easy replacement of structures, making maintenance of the good.
3. parts replacement
With the integrated circuits and installation technology, electronic product rapidly to more integrated, more functionality, smaller, not only components replace test difficulty level, cell circuit and more inconvenient to replace, in the past ten or so blocks or even dozens of block circuit function, and now can be completed with a integrated circuit, in the area of the PCB can accommodate more circuit units. Circuit testing, repairs to Board-level or even the overall direction. Especially the more complex functionality by if thousands of independent pieces of the system, detection is mainly uses the Replace method of the parts.
Parts replace test must follow the following three points
(1) the replacement parts and original parts must be consistent with the model, specification, or a major performance, functionality, compatibility and working parts.
(2) to replace parts interface is working properly, at least in power and transport, output correctly, does not replace the parts damaged. This point requires replacement front analysis of symptoms and the necessary interface power supply.
(3) replace to separate test, not once for multiple parts.
Finally it should be emphasized that replace method is a commonly used test methods, but not the best method, nor is the preferred method. It was only detected by the other methods on the basis of a part of the selection only when there is doubt.
For the microprocessor system you should be aware of troubleshooting the software first before hardware detection and replacement.

5. comparison

Sometimes used in a variety of detection methods and testing methods will not be able to determine where the fault is not complex, but can be surprise of comparative law. Commonly used the comparative method a total comparison, adjust contrast and comparison, bypassExclude comparing four methods.
1. the complete machine method
Unit comparison method is the fault generator and the same type of machine, find fault. This approach to the lack of information and its more complex equipment, such as a microprocessor-based products is particularly applicable.
The machine is to detect the comparative law method. On the possible failure of circuit parts work point determination and waveform observation, or signal monitoring, compare the difference between good and bad equipment, often find problems. Of course, because each device could not be fully consistent, the test results to analyze and judge, these common-sense issue requires basic theoretical basis and the accumulation of daily work.
2. adjust the comparison method
Adjust the comparison is done by machine equipment adjustable components or change certain status, comparing before and after the change of the circuit adjustment to determine failure for a detection method. This method is especially suitable for a long time, or after handling, drop, and other external conditions change caused failure of equipment.
Normally, testing equipment should not be arbitrarily change adjustable parts. But because the equipment subject to external forces can change the factory setting of failure, thus resulting in testing prior to reset tag under the premise to change certain adjustable capacitor, resistor, inductor, components, and compare before and after adjusting the working conditions of the device. Sometimes you also need to touch the component pins, wires, connectors or pull the plug-ins, or re-plug will doubt the PCB fraction back welding, etc., observe and record state changes before and after the device status, identify failures and troubleshoot.
Use to adjust the comparison method is the most taboo mess-mess with, without marking. Adjust and change the status quo should step by step, to change, at any time, change the status of the before and after comparison and found that adjustment is invalid or bad direction changes should be timely recovery.
3. the bypass method
Bypass method is to use appropriate capacity and voltage of the capacitor to be testing equipment for certain parts of the circuit of a comparative examination of a bypass for power interference, parasitic oscillation, etc.
Because bypass more realistic is a AC short-circuit test, so typically you use a smaller capacitance, temporary crossover in questionable circuit parts and "to" comparison between symptom changes. If the circuit to good direction change, may be appropriate to increase the capacitance and try again until the fault, according to bypass the site can determine the failure of the site.
4. exclusion of comparative law
Some combination of the whole machine or combo system often has a number of identical function and structure of the components found in the debugging functionality of the system is not working properly, you cannot identify the failed component, in this case by excluding the comparative method easy to confirm that the fault lies. Method is to plug the human component, while monitoring the whole machine or system, if the system is functioning properly, you can rule out the suspicion of the component, and then plug another component test person until you find the fault.
For example, a control system with eight card control 8 objects found in the debugging system interference, more exclusion, when inserting one fifth card interference phenomenon appears, confirm that the problem lies in the first five cards, other cards replacing them, exclude interferences.
Note
(1) the above method is excluded, it is clear that the position can also be used for the reverse direction, i.e. reducing exclusion.
(2) the number of units of a system failure is sometimes not a cell component, in this case the comparison should be repeated before it can be excluded.
(3) when using exclude comparison method that each plug or unplug unit components you want to turn off the power to prevent damage to the system plug.

6. computer intelligent automatic detection

Use computer powerful data processing capacity and combined with modern sensor technology can make circuit testing gradually automation and intelligent. This is the current variety of computers and computer equipment for the main and more widely applied, level higher and higher. The following are the common computer detection methods:
I. post
This is a primary detection method. Use computer power to the ROM in curing self test (POST, power-onselftest) on various hardware inside your computer, peripherals and interfaces, and other devices for detection, automatic test equipment hardware and software configuration, when the checkout error (failure), sound, and screen tips.
This post uses software to detect the hardware components of the characteristic parameters, test results and standard values stored in advance a comparative approach to diagnosis, can determine the hardware is good or bad, but under normal circumstances you cannot determine whether the failure of specific sites, or by the operator will make deep human testing.
2. detection Diagnostics
This method is a computer running a specialized detection Diagnostics, it can be set by the operator, and select the test objectives, content, and fault reporting, for most failures can go to chip.
This special program, such as NORTON, PCTOOLS QAPLUS, etc, along with the version upgrade, features and more. In addition the system software in General is also a utility, such as DOS6.X and WIN3.X, WIN.9X have corresponding detection.
Obviously this detection method of the basic premise is that the computer itself. If your computer has a serious fault, this way you can do about it.
Third, intelligent monitoring
This is the latest technology trends, is the most advanced guarantee machine working mode. This method utilizes the installed in the computer's specialized hardware and software on the system for monitoring, for example the CPU temperature, voltage, temperature, etc. within ongoing automated tests, once it is out of scope immediately shows the alarm message, make it easy for users to take measures to ensure the normal operation of the machine. This intelligent monitoring methods in a certain range can also be automatically take measures to eliminate malfunctions, such as the tank temperature is too high, automatic increase fan speed forced cooling, or even force the machine to "hibernate" in the tank temperature is low, reduce the fan speed or stopped to save energy and reduce noise.
Obviously, this kind of proactive and able toAutomatically adjust the running mode is the most ideal method for detection, and now mainstream computer and computer-most of the main device with this advanced feature. As technology evolves, this intelligent monitoring mode will be more on the use of the product, making the electronic products to a higher level of development.