First, Hardware detection before power on
After a circuit board is soldered, when checking whether the circuit board can work normally, it is usually not to directly supply power to the circuit board, but to follow the steps below to ensure that there is no problem in each step before it is too late to power on.
1. Whether the line connection is correct.
It is very important to check the schematic diagram. The first inspection focuses on whether the power supply of the chip and the labeling of the network nodes are correct. At the same time, it is also necessary to pay attention to whether the network nodes overlap. Another important point is the package of the component, the model of the package, and the order of the pins of the package; the package cannot use the top view, remember! Especially for non-pin packages. Check whether the connection is correct, including wrong line, few lines and multiple lines.
There are usually two methods of line checking:
1) Check the installed lines according to the circuit diagram, and check the installed lines one by one in a certain order according to the circuit connection;
2) Follow the actual circuit against the schematic diagram, and 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 mistakes, the lines that have been checked should usually be marked on the circuit diagram. It is best to use the buzzer in the ohm range of the pointer multimeter to test, and directly measure the component pins, so that the bad wiring can be found at the same time.
2. 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 have more serious consequences. When it comes to the power supply part, a 0 ohm resistor can be used as a debugging method. Do not solder the resistor before powering on, check the power supply voltage is normal, and then solder the resistor on the PCB to supply power to the unit behind, so as not to burn the chip of the unit behind due to the abnormal voltage of the power supply when powering on. Add protection circuits in the circuit design, such as using recovery fuses and other components.
3. The installation of components.
It is mainly to check polarized components, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and whether the pins of the triode are corresponding. For triodes, the arrangement of the pins of different manufacturers with the same function is also different, it is best to test it with a multimeter.
Do open circuit and short circuit test first to ensure that there will be no short circuit after power on. If the test points are set well, it can get twice the result with half the effort. The use of 0 ohm resistors is also sometimes beneficial for high-speed circuit testing.
The power-on test can only be started after the above hardware test before power-on is completed.
Second, Power detection
1. Power-on observation:
Do not rush to measure electrical indicators after power-on, but observe whether there are any abnormalities in the circuit, such as whether there is smoke, whether there is abnormal smell, whether the outer package of the integrated circuit is touched by hand, whether it is hot, etc. If there is an abnormal phenomenon, the power should be turned off immediately, and the power should be turned on again after troubleshooting.
2. Static debugging:
Static debugging generally refers to the DC test under the condition of no input signal or only a fixed level signal. The potential of each point in the circuit can be measured by a multimeter, and compared with the theoretical estimated value, combined with the circuit principle Analyze and judge whether the DC working state of the circuit is normal, and find damaged or critical components in the circuit in time. By replacing components or adjusting circuit parameters, the DC working state of the circuit can meet the design requirements.
3. Dynamic debugging:
Dynamic debugging is carried out 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 direction of the signals. If abnormal phenomena are found, the reasons should be analyzed and the faults should be eliminated , and then debug until the requirements are met.
During the test, you can't rely on your feelings, but always use instruments to observe. When using an oscilloscope, it is best to set the signal input mode of the oscilloscope to the "DC" block. Through the DC coupling mode, the AC and DC components of the measured signal can be observed at the same time. Through 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, and if necessary, further propose circuit parameters reasonable correction.
Third, Other work in electronic circuit debugging
1. Set up the debugging workbench: the workbench is equipped with the required debugging instruments, and the arrangement of the instruments should be easy to operate and easy to observe. Special reminder: During production and debugging, the workbench must be arranged clean and tidy.
2. Select the measuring instrument: For the hardware circuit, the system to be adjusted should choose the measuring instrument, and the accuracy of the measuring instrument should be better than the system under test; for software debugging, it should be equipped with a microcomputer and a development device.
3. Determine the test point: draw up the debugging steps and measurement methods according to the working principle of the system to be adjusted, determine the test point, mark the position on the drawing and the board, and make a debugging data record form etc.
4. Debugging sequence: The debugging sequence of electronic circuits is generally carried out according to the signal flow direction, and the output signal of the previously debugged circuit is used as the input signal of the next stage to create conditions for the final adjustment.
5. Overall debugging: For digital circuits implemented by programmable logic devices, the input, debugging and download of source files of 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.
During the debugging process, carefully observe and analyze the experimental phenomena, and make records to ensure the integrity and reliability of the experimental data.
Fourth, Matters needing attention in circuit debugging
Whether the debugging result is correct or not is largely affected by the correctness of the test volume and test accuracy. In order to ensure the test results, it is necessary to reduce the test error and improve the test accuracy. To this end, we need to pay attention to the following points:
1. Correctly use the ground terminal of the test instrument. Use the electronic instrument whose ground terminal is connected to the chassis for testing, and the common ground terminal should be connected to the ground terminal of the amplifier, otherwise the interference introduced by the instrument chassis 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 Vce needs to be tested, the two ends of the instrument should not be directly connected to the collector and emitter, but Vc and Ve should be measured to the ground respectively, and then the two phases reduce. If a multimeter powered by a dry battery is used for testing, since the two input terminals of the ammeter are floating, it is allowed to be connected directly between the test points.
2. The input impedance of the instrument used to measure the voltage must be much greater than the equivalent impedance of the measured place. If the input impedance of the test instrument is small, it will cause a shunt during the measurement, which will bring great errors to the test results.
3. The bandwidth of the test instrument must be greater than the bandwidth of the circuit under test.
4. Select the test point correctly. When the same test instrument is used for measurement, the measurement point is different, and the error caused by the internal resistance of the instrument will be very different.
5. The measurement method should be convenient and feasible. When it is necessary to measure the current of a certain circuit, it is generally possible to measure the voltage instead of the current as much as possible, because the circuit does not need to be changed to measure the voltage. If it is necessary to know the current value of a certain branch, it can be obtained by measuring the voltage at both ends of the resistor on the branch and converting it.
6. During the debugging process, not only must observe and measure carefully, but also be good at recording. The recorded content includes experimental conditions, observed phenomena, measured data, waveform and phase relationship, etc. Only by comparing a large number of reliable experimental records with theoretical results can we find problems in circuit design and improve the design scheme.
Fifth, Troubleshooting during debugging
It is necessary to carefully find out the cause of the fault, and must not remove the line and reinstall it once the fault cannot be solved. Because if it is a problem in principle, even reinstalling will not solve the problem.
1. General method of troubleshooting
For a complex system, it is not easy to accurately find faults among a large number of components and circuits. The general fault diagnosis process starts from the fault phenomenon, makes analysis and judgment through repeated tests, and finds out the fault step by step.
1) Direct observation method: Check whether the selection and use of the instrument are correct, whether the level and polarity of the power supply voltage meet the requirements; whether the pins of the polar components are connected correctly, whether there are any wrong connections, missing connections, and mutual collisions. Whether the wiring is reasonable; whether the printed board is short-circuited, whether the resistors and capacitors are burnt and burst, etc. Power on and observe whether the components are hot and smoke, whether the transformer has a burnt smell, whether the filament of the electronic tube and oscilloscope tube is bright, whether there is high voltage sparking, etc.
2) Use a multimeter to check the static working point: the power supply system of the electronic circuit, the DC working state of the semiconductor transistor and the integrated block (including components, device pins, power supply voltage), and the resistance value in the line can all be measured with a multimeter. When the measured value differs greatly from the normal value, the fault can be found after analysis.
Incidentally, the quiescent operating point can also be determined with the "DC" input of the oscilloscope. The advantage of using an oscilloscope is that the internal resistance is high, and the DC working state and the signal waveform on the measured point can be seen at the same time, as well as possible interference signals and noise voltages, etc., which is more conducive to analyzing faults.
3）Signal tracing method: For various complex circuits, a signal of a certain amplitude and appropriate frequency can be connected to the input terminal (for example, for a multi-stage amplifier, a f,1000 HZ signal can be connected to the input terminal Sinusoidal signal), use an oscilloscope from the front stage to the back stage (or vice versa), and observe the changes in the waveform and amplitude step by step. If any stage is abnormal, the fault is at that stage.
4）Comparison method: When you suspect that there is a problem with a certain 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 abnormality in the circuit. The situation, and then analyze and judge the fault point.
5) Parts replacement method: Sometimes the fault is hidden and cannot be seen at a glance. For example, if you have an instrument of the same model as the faulty instrument at this time, you can replace the parts, components, plug-in boards, etc. in the instrument with those in the faulty instrument. Corresponding parts, in order to narrow down the scope of the fault and find the source of the fault.
6) Bypass method: When there is a parasitic oscillation phenomenon, you can use a capacitor with an appropriate capacity, select an appropriate check point, and temporarily connect the capacitor between the check point and the reference ground point. If the oscillation disappears, it indicates that the oscillation is generated at near here or in the previous stage circuit. Otherwise, look for it in the back, and then move the checkpoint. 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: It is a method to temporarily short-circuit a 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 short circuit method cannot be used for the power supply (circuit).
8) Open circuit method: The open circuit method is most effective for checking short circuit faults. The open circuit method is also a method to gradually narrow the range of suspected fault points. For example, if a regulated power supply is connected to a circuit with a fault, the output current is too large. We take the method of disconnecting a certain 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.
In actual debugging, there are many ways to find the cause of the fault, and the above only lists a few commonly used methods. The use of these methods can find the point of failure with one method for simple faults, but for more complex faults, multiple methods need to be used to complement each other and cooperate with each other to find the point of failure.
In general, the general practice for troubleshooting is to:
1) Use direct observation to eliminate obvious faults.
2) Check the static working point with a multimeter (or oscilloscope).
3) The signal tracing method is a simple and intuitive method that is universally applicable to various circuits and is widely used in dynamic debugging.
2. Fault phenomenon and the cause of the fault
1) Common failure phenomenon: the amplifying circuit has no input signal, but has 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 stabilization performance is deteriorated, and the output voltage is unstable. The oscillating circuit does not oscillate, the counter waveform is unstable and so on.
2) The cause of the failure: The fixed product fails after a period of use, which may be due to component damage, short circuit or open circuit in the connection, or changes in conditions, etc.