pcb debugging steps
Detection before power on
After a circuit board is soldered, when checking whether the circuit board can work normally, you usually do not directly power the circuit board, but follow the following steps. It is not too late to power on after ensuring that there are no problems in each step.
1. Is the connection correct?
Check the schematic diagram. The main things that need to be checked are whether the power supply of the chip and the network node labels are correct. At the same time, you should also pay attention to whether there is overlap in the network nodes. This is the focus of the inspection.
Another important point is the packaging of the original. The model of the package, the pin sequence of the package, the package cannot use a top view, remember, especially for non-pin packages.
Check the connections, including wrong wires, missing wires and multiple wires. There are usually two ways to check the line:
Check the installed circuits according to the circuit diagram, and check the installed circuits one by one in a certain order according to the circuit connections;
Compare the actual wiring to the schematic diagram, and check the wiring centered on the component. Check the wiring of each component pin and check whether each location exists on the circuit diagram.
In order to prevent errors, the checked lines should usually be marked on the circuit diagram, tested with the buzzer of the ohm block of a pointer multimeter, and directly measure the component pins, so that poor wiring can be found at the same time.
2. Component installation status
Whether there is a short circuit between the pins and whether there is poor contact at the connection, you can use the diode of the multimeter to detect and slide the test lead on the circuit board to detect;
Check whether the polarity of diodes, transistors, integrated devices and electrolytic capacitors are connected incorrectly.
Check whether there is a short circuit in the power interface. If the power supply is not powered on before debugging, the power supply will be short-circuited, which will cause the power supply to burn out, sometimes causing more serious consequences. Use a multimeter to measure the input impedance of the power supply. This is a necessary step. Before turning on the power, disconnect one of the power cords and use a multimeter to check whether there is a short circuit between the power terminal and the ground.
When designing the power supply part, you can use a 0 ohm resistor as a debugging method. Do not solder the resistor before powering on. Check that the voltage of the power supply is normal and then solder the resistor on the PCB to supply power to the subsequent units to avoid causing power failure due to the power supply. The voltage is abnormal and the chip of the subsequent unit is burned. Add protection circuits to the circuit design, such as using recovery fuses and other components.
The main thing is to check the polarized components, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and whether the pins of the triodes correspond.
Do open circuit and short circuit tests first to ensure that there will be no short circuit after power on. If the test points are set up well, you can get twice the result with half the effort. The use of 0 ohm resistors is also sometimes beneficial for high speed circuit testing.
After the above non-power detection is completed, the power detection can be started.
Power on detection
1. Power on observation
After powering on, do not rush to measure the electrical indicators, but observe whether there are any abnormal phenomena in the circuit, such as whether there is smoke, whether there is any abnormal smell, whether the outer package of the integrated circuit is hot when you touch it.
If an abnormality occurs, the power supply should be turned off immediately and then turned on again after troubleshooting.
2. Static debugging
Static debugging generally refers to DC testing without adding an input signal, or only adding a fixed level signal. A multimeter can be used to measure the potential of each point in the circuit.
By comparing with the theoretical estimated value and analyzing the circuit principle, we can determine whether the DC working status of the circuit is normal, and timely discover components in the circuit that are damaged or in critical working status. By replacing components or adjusting circuit parameters, the DC working state of the circuit meets 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 cannot rely on feelings and impressions, but always use instruments for observation. When using an oscilloscope, set the signal input mode of the oscilloscope to “DC”. Through the DC coupling method, the AC and DC components of the measured signal can be observed at the same time.
Through debugging, check whether various indicators of the functional 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 reasonable circuit parameters. correction.
Other tasks in electronic circuit debugging
According to the working principle of the system to be adjusted, formulate debugging steps and measurement methods, determine test points, mark the locations on drawings and boards, and draw debugging data recording forms, etc.
Set up a debugging workbench. The workbench should be equipped with the required debugging instruments. The arrangement of the instruments should be convenient for operation and observation. Students often do not pay attention to this problem. When making or debugging machines, the workbench is messy. Tools, books, clothes, etc. are mixed with the instruments, which will affect debugging. Special reminder: During production and debugging, be sure to keep the workbench clean and tidy.
For hardware circuits, the measuring instrument should be selected according to the system being adjusted, and the accuracy of the measuring instrument should be better than that of the system being measured.
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 final input signal to create conditions for unified debugging.
When choosing a digital circuit implemented by a programmable logic device, the input, debugging and debugging of the programmable logic device source file should be completed, and the programmable logic device and analog circuit should be connected into a system for overall debugging and result testing.
During the debugging process, the experimental phenomena must be carefully observed and analyzed, and records must be kept to ensure the integrity and reliability of the experimental data.
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Things to note during circuit debugging
Whether the debugging results are correct is largely affected by the correctness of the test quantities 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. Use the ground terminal of the test instrument correctly:
For any electronic instrument that uses a ground terminal to connect the case for testing, the ground terminal should be connected to the ground terminal of the amplifier. Otherwise, the interference introduced by the instrument case will not only change the working status of the amplifier, but also cause the test results to appear. error.
According to this principle, when debugging the **** bias circuit, if you need to test Vce, you should not connect the two ends of the instrument directly to the collector and ****, but measure Vc and ground respectively. Ve, and then subtract the two.
If a dry cell-powered multimeter is used for testing, since the two input terminals of the meter are floating, they are allowed to be directly connected between the test points.
2. The input impedance of the instrument used to measure voltage must be much greater than the equivalent impedance of the measured location.
If the input impedance of the test instrument is small, shunting will occur during 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. Choose the test point correctly.
When the same test instrument is used for measurement, if the measurement points are different, the errors caused by the internal resistance of the instrument will be greatly different.
5. The measurement method must 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 measuring the voltage does not require changing the circuit. Easy to test. If you need to know the current value of a certain branch, you can get it by measuring the voltage across the resistor on the branch and converting it.
6. During the debugging process, you must not only observe and measure carefully, but also be good at recording:
The recorded content includes experimental conditions, observed phenomena, measured data, waveforms and phase relationships, etc. Only with a large number of reliable experimental records and comparison with theoretical results can problems in circuit design be discovered and the design plan improved.
A glitch occurred while debugging
It is necessary to carefully find the cause of the fault, and never dismantle the line and reinstall it once the fault cannot be solved. Because the reinstalled lines may still have various problems, if it is a principle problem, even reinstallation will not solve the problem.
We should regard finding faults and analyzing the causes of faults as good learning opportunities, through which we can continuously improve our ability to analyze and solve problems.
1. Troubleshooting ideas
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 testing, and gradually finds the fault.
2. Fault phenomena and causes of faults
A common fault phenomenon is that the amplifier circuit has no input signal but an output waveform. The amplifier circuit has an input signal but no output waveform, or the waveform is abnormal. The series-connected regulated power supply has no voltage output, or the output voltage is too high and cannot be adjusted, or the output voltage stabilization performance deteriorates, and the output voltage is unstable, etc. The oscillator circuit does not produce oscillation, the counter waveform is unstable, etc.
The cause of the failure is that the fixed product fails after being used for a period of time. It may be that the components are damaged, the wiring is short-circuited or broken, or the conditions change.
3. General methods for checking faults
direct observation method. Check whether the instrument is selected and used correctly, whether the level and polarity of the power supply voltage meet the requirements; whether the pins of polar components are connected correctly, and whether there are any wrong connections, missing connections, or mutual collisions.
Whether the wiring is reasonable; whether the printed board has short wires and broken wires; whether the resistors and capacitors are burned or exploded, etc. Turn on the power and observe whether the components are hot or smoking, whether the transformer smells burnt, whether the filament of the electron tube and oscilloscope tube is bright, whether there is high-voltage ignition, etc.
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Use a multimeter to check the static operating point.
The power supply system of electronic circuits, the DC working status of semiconductor transistors and integrated blocks (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 is significantly different from the normal value, the fault can be found after analysis.
By the way, it is pointed out that the static operating point can also be measured using the “DC” input mode of the oscilloscope. The advantage of using an oscilloscope is that it has high internal resistance and can simultaneously see the DC working status and the signal waveform at the measured point as well as possible interference signals and noise voltages, etc., which is more conducive to fault analysis.
Signal tracing method.
For various more complex circuits, a signal of a certain amplitude and appropriate frequency can be connected to the input end (for example, for a multi-stage amplifier, a sinusoidal signal of f=1000 HZ can be connected to the input end), and an oscilloscope is used to From the previous stage to the subsequent stage (or vice versa), observe the changes in waveform and amplitude step by step. If there is an abnormality, the fault is at that level. Here’s how to examine a circuit in depth.
Contrast method.
When you suspect that there is a problem with a certain circuit, you can compare the parameters of the circuit with the working status and the parameters of the same normal circuit (or current, voltage, waveform, etc. from theoretical analysis) to find out the abnormal conditions in the circuit. , and then analyze the cause of the fault and determine the fault point.
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, you can replace the parts, components, plug-in boards, etc. in the instrument with the corresponding parts in the faulty instrument to facilitate reduction in size. fault scope to further find the fault.
Bypass method.
When there is parasitic oscillation, you can use an appropriate amount of capacitor, select an appropriate check point, and temporarily connect the capacitor between the check point and the reference ground point. If the oscillation disappears, it means that the oscillation is generated nearby or in the front-end circuit. middle. Otherwise, it’s behind, and then move the checkpoint to find it. It should be pointed out that the bypass capacitor should be appropriate and should not be too large, as long as it can better eliminate harmful signals.
Short circuit method.
The method is to temporarily short-circuit part of the circuit to find the fault. The short circuit method is effective for checking open circuit faults, but it should be noted that the short circuit method cannot be used for the power supply (circuit).
Breaking method.
The circuit breaking method is effective for checking short circuit faults. The circuit breaking method is also a method to gradually narrow down the scope of suspected fault points. For example, if a regulated power supply is connected to a faulty circuit, causing the output current to be too large, we will disconnect certain branches of the circuit in sequence to check for faults. If the current returns to normal after disconnecting the branch, the fault occurred in this branch.
during actual debugging.
There are many ways to find the cause of the fault, and the above only lists a few common methods. The use of these methods can be flexibly controlled according to equipment conditions and fault conditions. For simple faults, one method can be used to find the fault point, but for more complex faults, multiple methods need to be used to complement and cooperate with each other to find out. point of failure.
In general, the common approach to troubleshooting is:
First use direct observation to eliminate obvious faults
Then use a multimeter (or oscilloscope) to check the static operating point
The signal tracing method is a simple and intuitive method that is universally applicable to various circuits and is widely used in dynamic debugging.
