I. Detection before power on
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, but instead to follow the steps below to ensure that there is no problem at each step and then power on is not too late.
1 Is the connection correct
Check the schematic diagram. The main points to be checked are whether the chip’s power supply and network nodes are labeled correctly. At the same time, pay attention to whether the network nodes overlap. This is the focus of inspection. Another focus is the packaging of the originals. The type of the package, the order of the package pins, and the top view of the package cannot be used. Remember, especially for non-pin packages.
Check wiring, including miswires, fewer wires, and more wires. There are usually two ways to check the line:
Check the installed lines according to the circuit diagram, check the installed lines one by one according to the circuit wiring;
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.
2 component installation
Whether there is a short circuit between the pins, and whether there is a poor contact at the connection, you can use the multimeter’s diode to test, and the test pen slides on the circuit board to detect;
Are the diodes, transistors, integrated devices, and electrolytic capacitors connected incorrectly?
Whether there is a short circuit in the power interface. If the power is not turned on before debugging, the power supply will be short-circuited, which will cause the power supply to burn out and sometimes cause 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 a power cord and use a multimeter to check whether there is a short to ground from the power supply.
When designing the power supply, 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 units behind, so as not to cause power-on due to The voltage is abnormal and the chip of the rear unit is burned. Add protection circuits to the circuit design, such as using recovery fuses and other components.
3 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.
It is best to perform open-circuit and short-circuit tests first to ensure that short-circuit does not occur 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.
After the above non-power-on test is completed, the power-on test can be started.
Second, 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.
Static debugging generally refers to the DC test performed without the input signal or only a fixed level signal. The potential of each point in the circuit can be measured with a multimeter, and compared with the theoretical estimate, combined with the principle of the circuit Analyze and determine whether the DC working status of the circuit is normal, and timely find the components in the circuit that are damaged or in critical working status. By replacing the device or adjusting the circuit parameters, the DC working state of the circuit meets the design requirements.
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 an abnormal phenomenon is found, the cause should be analyzed and the fault should be eliminated. , And then debug until it meets the requirements.
During the test, you can’t rely on feelings and impressions. 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.
Third, other work in electronic circuit debugging
1 According to the working principle of the system to be adjusted, draw up debugging steps and measurement methods, determine test points, mark locations on drawings and boards, and draw debugging data record forms.
2 Set up a debugging workbench. The workbench is equipped with the necessary debugging instruments. The equipment should be easy to operate and easy to observe. Students often don’t pay attention to this problem. When making or adjusting the machine, the workbench is very messy, and tools, books, clothing, etc. are mixed with the instrument, which will affect debugging. Special note: When making and debugging, be sure to arrange the workbench clean and tidy.
3 For the hardware circuit, the measuring instrument should be selected according to the system being adjusted, and the accuracy of the measuring instrument should be better than the system under test.
4The 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 next stage to create conditions for the final adjustment.
5 Select the digital circuit implemented by the programmable logic device, complete the input, debugging, and download of the source file of the programmable logic device, and connect the programmable logic device and the analog circuit into a system for overall debugging and result testing.
6 During the debugging process, it is necessary to 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 is greatly affected by the correctness of the test quantity and the 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, please pay attention to the following points:
1Use the ground terminal of the test instrument correctly
For everything, use the ground-terminated electronic instrument for testing. The ground terminal should be connected to the amplifier’s ground terminal, otherwise the interference introduced by the instrument casing will not only change the working state of the amplifier, but also make the test results appear. error. 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 the emitter, but Vc and Ve should be measured respectively to the ground. Less. If you use a 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.
4Select the 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. Easy to test. 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 In the debugging process, not only must be carefully observed and measured, but also good at recording
The recorded content includes experimental conditions, observed phenomena, measured data, waveforms, and phase relationships. Only with a large number of reliable experimental records and comparison with theoretical results can we find problems in circuit design and improve the design scheme.
V. Failure during debugging
To carefully find the cause of the fault, do not remove the line and reinstall it whenever the fault cannot be resolved. Because the reinstalled line may still have various problems, if it is a theoretical problem, even the reinstallation will not solve the problem.
We should regard finding faults and analyzing the causes of faults as a good learning opportunity, and continuously improve our ability to analyze and solve problems through it.
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
A common failure phenomenon is that the amplifier circuit does not have an input signal but has an 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 generate oscillation, the counter waveform is unstable, and so on.
Cause of failure
The stereotyped product fails after a period of use. It may be damaged components, short-circuits and open circuits, or changes in conditions.
3 general methods for checking failures
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 components 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 scorched and cracked. Power on to observe whether the components are hot or smoke, whether the transformer has a coke smell, whether the filaments of the electronic tubes and oscilloscope tubes are on, and whether there are high-voltage sparks.
Check the static operating point with a multimeter. 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 it has high internal resistance, and can simultaneously see the DC working state and the signal waveform at the measured point, as well as possible interference signals and noise voltages, which is more conducive to analyzing the fault.
Signal tracking method. For various 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 with 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 step. This is a way to examine the circuit in depth.
Contrast method. When there is a problem in a circuit, you can compare the parameters of this circuit with the working state and the parameters of the same normal circuit (or theoretically analyzed current, voltage, waveform, etc.) to find out the abnormal situation in the circuit. , And then analyze the cause of the failure and determine the point of failure.
Component 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 parts, components, plug-in boards, etc. of the instrument with the corresponding parts of the faulty instrument to facilitate the reduction Fault scope to find the fault further.
Bypass method. When there is a parasitic oscillation phenomenon, you can use a capacitor with an appropriate amount, 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, you will be behind and 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. 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, but it should be noted that short-circuit methods cannot be used for power supplies (circuits).
Disconnect method. The open circuit method is most effective for checking for short circuit faults. The disconnection method is also a method to gradually narrow the scope of the suspected point of failure. For example, a certain regulated power supply is connected to a circuit with a fault, which causes the output current to be too large. We take the approach 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 will occur in this branch.
During actual debugging. There are many ways to find the cause of the fault. The above only lists a few common methods. The use of these methods can be flexibly grasped according to the equipment conditions and fault conditions. For simple faults, one point can be used to find the fault point, but for more complex faults, multiple methods must be used to complement and cooperate with each other to find The point of failure. In general, the usual way to look for faults is:
Use direct observation method first to eliminate obvious faults
Use a multimeter (or oscilloscope) to check the static operating point
The signal tracking method is a generally applicable and simple and intuitive method for various circuits, and is widely used in dynamic debugging.