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    One article to understand: the most complete circuit test process

    Post time: Feb-19-2020

    When a circuit board is soldered, it is usually not to directly supply power to the circuit board when checking whether the circuit board can work normally. Instead, follow the steps below to ensure that there is no problem in each step and then power on is not too late.

    Whether the connection is correct

    It is very important to check the schematic diagram. The first check focuses on whether the chip’s power supply and network nodes are labeled correctly. At the same time, pay attention to whether the network nodes overlap. Another important point is the packaging of the original, the type of the package, and the pin order of the package (remember: the package cannot use the top view, especially for non-pin packages). Check that the wiring is correct, including miswires, fewer wires, and more wires.

    There are usually two ways to check the line:

    1. Check the installed circuits according to the circuit diagram, and check the installed circuits one by one according to the circuit wiring.

    2. According to the actual circuit and the schematic diagram, check the line with the component as the center. Check the wiring of each component pin once and check whether each place exists on the circuit diagram. In order to prevent errors, the wires that have been checked should usually be marked on the circuit diagram. It is best to use a pointer multimeter ohm block buzzer test to directly measure the component pins, so that the bad wiring can be found at the same time.

    Whether the power supply is short-circuited

    Do not power on before debugging, use a multimeter to measure the input impedance of the power supply. This is a necessary step! If the power supply is short-circuited, it will cause the power supply to burn out or more serious consequences. When it comes to the power section, a 0 ohm resistor can be used as a debugging method. Do not solder the resistor before powering on. Check that the voltage of the power supply is normal before soldering the resistor to the PCB to power the unit behind, so as not to cause the chip of the unit behind to be burned because the voltage of the power supply is abnormal. Add protection circuits to the circuit design, such as using recovery fuses and other components.

    Component installation

    Mainly check whether the polar components, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and the pins of the triode are corresponding. For the triode, the pin order of different manufacturers with the same function is also different, it is best to test with a multimeter.

    Open and short test first to ensure that there will be no short circuit after power on. If the test points are set, you can do more with less. The use of 0 ohm resistors is sometimes beneficial for high-speed circuit testing. The power-on test can only be started after the above hardware tests before power-on are completed.

    Power-on detection

    1. Power on to observe:

    Do not rush to measure electrical indicators after power-on, but observe whether there are abnormal phenomena in the circuit, such as whether there is smoke, abnormal odor, touch the outer package of the integrated circuit, whether it is hot, etc. If there is an abnormal phenomenon, turn off the power immediately, and then power on after troubleshooting.

    2. Static debugging:

    Static debugging generally refers to the DC test performed without the input signal or only a fixed level signal. The multimeter can be used to measure the potential of each point in the circuit. By comparing with the theoretical estimate, the circuit principle Analyze and judge whether the DC working status of the circuit is normal, and find out in time that the components in the circuit are damaged or in critical working status. By replacing the device or adjusting the circuit parameters, the DC working status of the circuit meets the design requirements.

    3. Dynamic debugging:

    Dynamic debugging is performed on the basis of static debugging. Appropriate signals are added to the input end of the circuit, and the output signals of each test point are sequentially detected according to the flow of the signals. If abnormal phenomena are found, the reasons should be analyzed and the faults should be eliminated. , And then debug until it meets the requirements.

    During the test, you can’t feel it by yourself. You must always observe with the aid of an instrument. When using an oscilloscope, it is best to set the signal input mode of the oscilloscope to the “DC” block. Through the DC coupling method, you can observe the AC and DC components of the measured signal at the same time. After debugging, finally check whether the various indicators of the function block and the whole machine (such as signal amplitude, waveform shape, phase relationship, gain, input impedance and output impedance, etc.) meet the design requirements. If necessary, further propose circuit parameters Reasonable correction.

    Other tasks in electronic circuit debugging

    1. Determine test points:

    According to the working principle of the system to be adjusted, the commissioning steps and measurement methods are drawn up, the test points are determined, the positions are marked on the drawings and boards, and the commissioning data record forms are made.

    2. Set up a debugging workbench:

    The workbench is equipped with the required debugging instruments, and the equipment should be easy to operate and easy to observe. Special note: When making and debugging, be sure to arrange the workbench clean and tidy.

    3. Select a measuring instrument:

    For the hardware circuit, the measurement system should be the measurement instrument selected, and the accuracy of the measurement instrument should be better than the system under test; for software debugging, a microcomputer and development device should be equipped.

    4. Debugging sequence:

    The debugging sequence of the electronic circuit is generally carried out according to the signal flow direction. The output signal of the previously debugged circuit is used as the input signal of the subsequent stage to create conditions for the final adjustment.

    5. Overall commissioning:

    For digital circuits implemented using programmable logic devices, input, debugging, and download of the source files of the programmable logic devices should be completed, and the programmable logic devices and analog circuits should be connected into a system for overall debugging and result testing.

    Precautions in circuit debugging

    Whether the debugging result is correct is greatly affected by the correctness of the test quantity and the test accuracy. In order to guarantee the test results, it is necessary to reduce the test error and improve the test accuracy. To this end, please pay attention to the following points:

    1. Use the ground terminal of the test instrument correctly. Use the ground-termination case of the electronic instrument for testing. The ground terminal should be connected to the amplifier’s ground end. Otherwise, the interference introduced by the instrument case will not only change the working state of the amplifier, but also cause errors in the test results. . According to this principle, when debugging the emitter bias circuit, if it is necessary to test Vce, the two ends of the instrument should not be directly connected to the collector and the emitter, but Vc and Ve should be measured respectively to ground, and then the two Less. If you use a dry battery-powered multimeter for testing, the two input terminals of the meter are floating, so you can directly connect between the test points.

    2. The input impedance of the instrument used to measure the voltage must be much greater than the equivalent impedance at the location being measured. If the input impedance of the test instrument is small, it will cause a shunt during the measurement, which will cause a large error to the test result.

    3. The bandwidth of the test instrument must be greater than the bandwidth of the circuit under test.

    4. Select test points correctly. When the same test instrument is used for measurement, the error caused by the internal resistance of the instrument will be very different when the measurement points are different.

    5. The measurement method should be convenient and feasible. When it is necessary to measure the current of a circuit, it is generally possible to measure the voltage instead of the current, because it is not necessary to modify the circuit when measuring the voltage. If you need to know the current value of a branch, you can get it by measuring the voltage across the resistance of the branch and converting it.

    6. During the debugging process, not only must be carefully observed and measured, but also be good at recording. The recorded content includes experimental conditions, observed phenomena, measured data, waveforms, and phase relationships. Only by comparing a large number of reliable experimental records with theoretical results, can we find problems in circuit design and improve the design plan.

    Troubleshoot during debugging

    To find the cause of the fault carefully, do not remove the line and reinstall it if the fault cannot be solved. Because if it is a problem in principle, even reinstallation will not solve the problem.

    1. General methods of fault checking

    For a complex system, it is not easy to accurately find faults in a large number of components and circuits. The general fault diagnosis process is based on the failure phenomenon, through repeated testing, analysis and judgment, and gradually find the fault.

    2. Failure phenomena and causes

    ● Common failure phenomenon: There is no input signal in the amplifier circuit, but there is output waveform. The amplifier circuit has an input signal but no output waveform, or the waveform is abnormal. The series regulated power supply has no voltage output, or the output voltage is too high to be adjusted, or the output voltage regulation performance is deteriorated, and the output voltage is unstable. The oscillating circuit does not produce oscillation, the waveform of the counter is unstable and so on.

    ● The reason for the failure: The stereotyped product fails after a period of use. It may be damaged components, short-circuits and open circuits, or changes in conditions.

    Method of checking failure

    1. Direct observation method:

    Check whether the selection and use of the instrument is correct, whether the level and polarity of the power supply voltage meet the requirements; whether the pins of the polar component are connected correctly, and whether there is any connection error, missing connection, or mutual collision. Whether the wiring is reasonable; whether the printed board is short-circuited, whether the resistance and capacitance are burnt and cracked. Check whether the components are hot, smoke, whether the transformer has a coke smell, whether the filament of the electronic tube and oscilloscope tube is on, and whether there is high-voltage ignition.

    2. Use a multimeter to check the static operating point:

    The power supply system of the electronic circuit, the DC working state of the semiconductor triode, the integrated block (including the element, device pins, power supply voltage), and the resistance value in the line can be measured with a multimeter. When the measured value differs greatly from the normal value, the fault can be found after analysis. By the way, the static operating point can also be determined using the oscilloscope “DC” input method. The advantage of using an oscilloscope is that the internal resistance is high, and it can see the DC working state and the signal waveform at the measured point at the same time, as well as the possible interference signals and noise voltage, which is more conducive to analyzing the fault.

    3.Signal tracking method:

    For a variety of more complicated circuits, a certain amplitude and appropriate frequency signal can be connected to the input (for example, for a multi-stage amplifier, a sinusoidal signal of f, 1000 HZ can be connected to its input). From the front stage to the back stage (or vice versa), observe the changes of the waveform and amplitude step by step. If any step is abnormal, the fault is at that level.

    4. Contrast method:

    When there is a problem in a circuit, you can compare the parameters of this circuit with the same normal parameters (or theoretically analyzed current, voltage, waveform, etc.) to find out the abnormal situation in the circuit, and then analyze and analyze Determine the point of failure.

    5. Parts replacement method:

    Sometimes the fault is hidden and cannot be seen at a glance. If you have an instrument of the same model as the faulty instrument at this time, you can replace the components, components, plug-in boards, etc. in the instrument with the corresponding parts of the faulty instrument to facilitate the reduction Fault scope and find the source of the fault.

    6. Bypass method:

    When there is a parasitic oscillation, you can use a capacitor with an appropriate amount of passengers, select an appropriate checkpoint, and temporarily connect the capacitor between the checkpoint and the reference ground point. If the oscillation disappears, it indicates that the oscillation is generated near this or the previous stage In the circuit. Otherwise just behind, move the checkpoint to find it. The bypass capacitor should be appropriate and should not be too large, as long as it can better eliminate harmful signals.

    7. Short circuit method:

    Is to take a short circuit part of the circuit to find the fault. The short-circuit method is most effective for checking open-circuit faults. However, it should be noted that the power supply (circuit) cannot be short-circuited.

    8. Disconnect method:

    The open circuit method is most effective for checking for short circuit faults. The disconnection method is also a method of gradually narrowing down the suspected point of failure. For example, because a regulated power supply is connected to a circuit with a fault and the output current is too large, we take a method of disconnecting one branch of the circuit in order to check the fault. If the current returns to normal after the branch is disconnected, the fault occurs in this branch.



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