Common methods for fault analysis and location of

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Common methods of circuit fault analysis and location

there are many types of faults in digital circuits, and the causes of faults are different, so the methods of troubleshooting are also different. In case of circuit failure, it is a process to analyze the cause of the failure and determine the position of the failure according to the failure phenomenon through inspection and measurement, so as to find the failed components. Generally, the fault causes of simple circuits are also relatively simple, and the fault analysis and location are easier; However, the faults of more complex circuits are often more complex, so it is relatively difficult to analyze and locate the fault causes. The common methods of circuit fault analysis and location are discussed below

I. direct observation method

the so-called direct observation method refers to the method of directly observing the surface of the circuit to be checked to find problems and find faults without the help of any instruments and equipment. It is generally divided into static observation and power on inspection. The static observation includes the following aspects

1. Observe whether there are scorched marks on the surface of the printed circuit board and components, and whether the wiring and components fall off and break

2. Observe the use of the instrument. Whether the selection of instrument type is appropriate, whether there are errors in the selection of function and range, and whether the common ground connection is properly handled. It is preferred to eliminate external faults before observing the circuit itself

3. Observe the power supply of circuits with high precision, low drift and low noise. Whether the voltage value and polarity of the power supply meet the requirements, whether the power supply has been connected to the circuit, etc

4. Observe the installation of components. Whether the polarity of electrolytic capacitor, lead terminals of diode and triode and lead terminals of integrated circuit are connected incorrectly, missed or collided with each other, whether the installation position is reasonable, and whether there are shielding measures for interference source

5. Observe the wiring. Whether the input and output lines, strong and weak current lines, AC and DC lines violate the wiring principles

power on inspection can be carried out after static observation. After switching on the power supply, observe whether the components are hot and smoke, and whether the transformer is burnt or heated and has abnormal sound

the direct observation method is applicable to the preliminary inspection of the fault, which has a correlation with the cutting method and rolling direction of the sample, and some more detailed faults can be found

II. Instrument test method

1. Power off test method

is a method to use the ohm gear of a multimeter to measure the resistance value of a circuit or components under the condition of circuit power off, so as to judge the fault. For example, check whether the wiring, solder joints and fuses in the circuit are open circuit, measure the resistance value, capacitor leakage, on-off of inductors, and check the quality of semiconductor devices. During the test, in order to avoid the influence of relevant branches, one end of the tested components shall be generally disconnected from the circuit. At the same time, in order to protect the components, do not use high barrier and low barrier to prevent high voltage or large current from damaging the PN junction of semiconductor devices in the circuit

2. Live test method

is a method to measure the static voltage value or voltage waveform of each point in the circuit with the help of instruments, and conduct theoretical analysis to find the fault location under the condition that the circuit is live. For example, check whether the static working point of the transistor is normal, whether the static parameters of the integrated device tensile testing machine suitable for chemical consumption enterprises meet the requirements, and whether the logic relationship of the digital circuit is correct

3. Signal tracing method

is to add a qualified signal to the input end of the circuit as required. According to the signal flow from the front stage to the rear stage, use an oscilloscope or voltmeter and other instruments to check the transmission of the signal in each part of the circuit step by step, analyze whether the function of the ID circuit is normal, and then judge the location of the fault. This method should normally be used when the static operating point of the circuit is under normal conditions

4. Split test method

for some loop circuits with feedback, their working conditions at all levels are related to each other. At this time, the method of split loop can be used to remove the feedback loop, and then check it step by step, so as to find out the fault position faster. For self-excited oscillation 2, the allowable deviation of specified friction coefficient can also be checked by this method

5. Comparison method

when there is a problem in a certain circuit, you can find the same normal circuit for comparison, and compare the States and parameters of the two one by one. You can quickly find the abnormal parameters in the circuit, and then analyze the cause of the fault and find the fault point

6. Substitution method

sometimes the fault is hidden and cannot be found quickly. It needs further inspection. At this time, the unit circuit or component that has been debugged can be used to replace the unit circuit in question, so as to judge whether this unit circuit is found. After the unit circuit with problems is determined, the local substitution method can also be used in the unit circuit to replace the suspected components with well confirmed components, gradually reduce the suspected scope of the fault, and finally find the fault point

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