Follow these 5 steps to debug PCB, basically reliable
After debugging PCB, many engineers and friends around me still have these and other problems with board. Today editor summarizes debugging steps. You can follow this method and save yourself a lot of trouble.
When PCB is soldered, when checking whether PCB can work normally, it is usually not to apply power directly to PCB, but following steps are taken to ensure that there are no problems at each step before power-on, it is not too late.
1. Is connection established correctly?
Check circuit diagram. The main points that need to be checked are whether power supply of microcircuit and network nodes are correctly indicated. At same time, it is also necessary to pay attention to whether network nodes overlap. This is focus of verification. Another important point is packaging of original. The type of package, order of pins on package and top view cannot be used. Please note, especially for packages without pins.
Check connection, including wrong line, missing line, and multiple lines. There are usually two ways to test a string:
①Check installed circuit according to circuit diagram, and check installed circuits one by one in a specific order according to circuit connection;
②Check circuit diagram according to actual diagram, and check line with component in center. Check wiring of each pin of component once and check if each location is on circuit diagram. To prevent errors, lines that have been tested should usually be marked on wiring diagram. It is best to test buzzer with a dial multimeter resistance block and measure component's pins directly, so that bad wiring can be found at same time.
2. Component installation
Whether there is a short circuit between contacts or a poor contact in connection, you can use multimeter diode to detect, and test pen slides on PCB to detect.
Whether polarity of diodes, triodes, integrated devices and electrolytic capacitors is connected incorrectly.
Whether there is a short circuit in power interface. If power supply is not turned on before debugging, power supply will short circuit, causing power supply to burn out, and sometimes more serious consequences. Use a multimeter to measure input impedance of power supply, which is a necessary step. Before turning on, unplug power cord and use a multimeter to check for a short circuit between power terminal and ground.
When designing a power supply, you can use 0 ohm resistor as a debugging method. Do not solder resistor before turning on power. After confirming that voltage of power supply is normal, solder resistor on circuit board to supply power to following units so as not to cause power-on. Because voltage of power supply is abnormal, rear unit chip is burnt. Add protection schemes to circuit design, such as using fuses and other components.
It mainly checks polarized components such as light emitting diodes, electrolytic capacitors, rectifier diodes, etc., and checks matching of triode contacts.
First, it is best to test for an open circuit and a shortshort circuit to make sure short circuit will not happen after power on. If breakpoints are set correctly, you can get twice result with half effort. The use of 0 ohm resistors is also sometimes useful for testing high speed circuits.
The power-on test can only be started after completing above test without turning on power.
1. Power-on monitoring
Don't rush to measure electrical readings after turning on power, but observe whether there is any abnormality in circuit, such as whether there is smoke, whether there is an abnormal smell, whether outer package of integrated circuit is touched by hand, whether it is hot, etc. e. If there is an abnormal phenomenon, power should be turned off immediately, and power should be turned on again after troubleshooting.
2. Static Debugging
Static debugging generally refers to a DC test under conditions of no input signal or only a fixed level signal. The potential of each point in circuit can be measured with a multimeter and compared with theoretical estimated value, combined with circuit principle analysis, evaluation of normal working state of DC circuit, and timely detection of damaged or critical working state components in circuit. By replacing components or by adjusting circuit parameters, operating condition of DC circuit can meet design requirements.
3. Dynamic Debugging
Dynamic debugging is based on static debugging. Appropriate signals are added to input end of circuit, and output signals of each test point are detected sequentially according to direction of signal flow. found, causes must be analyzed. And troubleshooting, and then debugging until requirements are met.
In process of testing, one should not rely on sensations and impressions, but always observe with help of instruments. When using an oscilloscope, it is best to set oscilloscope's input mode to "DC" block. With DC coupling mode, you can simultaneously observe AC and DC components of measured signal.
Through debugging, finally check whether function blocks and various indicators of whole machine (such as signal amplitude, waveform, phase relationship, gain, input and output impedance, etc.) meet design requirements, and if necessary , , reasonable corrections to circuit parameters are proposed below.
Design debugging steps and measurement methods according to operating principle of custom system, define control points, mark positions on drawings and boards, draw a debug data record form, etc.
Set up a debugging workbench equipped with necessary debugging tools and locatione tools should be easy to operate and easy to observe. Students often do not pay attention to this problem. When making or setting up a machine tool, workbench is very dirty, and tools, books, clothes, etc. are mixed with tool, which will affect debugging. Special Reminder: When crafting and debugging, workbench must be clean and tidy.
For a hardware circuit, meter should be selected according to system being tuned, and accuracy of meter should be higher than that of system under test.
The debugging sequence of electronic circuits is usually performed according to direction of signal flow, and output signal of previously debugged circuit is used as input signal of next stage to create conditions for final tuning.
Select digital circuits implemented by programmable logic devices, complete input, debugging and loading of programmable logic device source files, and connect programmable logic devices and analog circuits to system for general debugging and verification of results.
While debugging, carefully observe and analyze experimental phenomena, take proper notes, and ensure integrity and reliability of experimental data.
Adequate debug result depends a lot on scope of test and accuracy of test. In order to guarantee test results, it is necessary to reduce test error and improve test accuracy. To do this, we need to pay attention to following points:
1. Use tester's ground terminal correctly
Everything is tested with an electronic instrument, ground terminal of which is connected to chassis. The common ground terminal must be connected to ground terminal of amplifier. The test resulted in an error.
According to this principle, when debugging emitter bias circuit, if Vce needs to be checked, two ends of device should not be directly connected to collector and emitter, but Vc and Ve should be measured with respect to ground respectively. and then two are subtracted. If a dry battery multimeter is being used for testing, since two input leads of ammeter are floating, it can be connected directly between test points.
2. The input impedance of device used to measure voltage must be much greater than equivalent resistance of measuring point
If input impedance of meter is low, it will cause a shunt during measurement, resulting in large errors in test results.
3. The bandwidth of test instrument must be greater than bandwidth of circuit under test
4. Choose right breakpoint
When same meter is used for measurement, measuring point is different, and error caused by meter's internal resistance will vary greatly.
5. The measurement method should be convenient and feasible
When you want to measure current in a particular circuit, you can usually measure voltage instead of current, as far as possible, because you don't need to modify circuit to measure voltage. Easy to check. If it is necessary to know current value of a certain branch, it can be obtained by measuring voltage at both ends of resistor on branch and converting it.
6. In process of debugging, it is necessary not only to carefully observe and measure, but also to record well
Recorded content includes experimental conditions, observed phenomena, measured data, waveform and phase relationship, etc. Only with a large number of reliable experimental recordings and comparison with theoretical results, problems in circuit design can be found and calculation scheme can be improved.
You must carefully investigate cause of fault and do not remove or reinstall line if fault cannot be corrected. Since there may still be various problems in reinstalled schema, if it is a theoretical problem, even reinstalling cannot solve problem.
We should view error detection and error cause analysis as a good learning opportunity through which we can continuously improve our ability to analyze and solve problems.
1. Troubleshooting Tips
For a complex system, it is not easy to accurately find a fault among a large number of components and circuits. The overall process of diagnosing a malfunction starts from phenomenon of a malfunction, analyzes and evaluates through repeated tests, and gradually detects a malfunction.
2. Symptoms and causes of failure
A common failure phenomenon is that amplifier circuit does not have an input signal, but an output signal. The amplifier circuit has an input signal but no output waveform, or waveform is abnormal. The series regulated power supply has no output voltage, or output voltage is too large to adjust, or output voltage stabilization performance is degraded, and output voltage is unstable. The oscillatory circuit does not vibrateXia, shape of oncoming signal is unstable, etc.
The reason for failure is that final product fails after being used for a certain period of time. This may be due to a damaged component, a shorted or disconnected connection, or a change in state.
3. Common troubleshooting methods
Direct observation method. Check correctness of choice and use of device, whether level and polarity of supply voltage meet requirements, whether terminals of polar components are connected correctly, whether there is a wrong connection, lack of connection and mutual conflict. Is wiring good, is there a short circuit on printed circuit board, are resistors and capacitors burned or burst, etc. Turn on power and check if components are hot or smoke, if there is a burning smell from transformer, is filament bright electronic tube and oscilloscope tube, whether there is high voltage sparking, etc.
Check static set point with a multimeter. The power supply system of electronic circuits, DC operating state of semiconductor transistors and integrated circuits (including components, device leads, supply voltage), as well as resistance value in circuit, can all be measured with a multimeter. When measured value is very different from normal value, fault can be found after analysis.
By way, rest point can also be determined from "DC" input of oscilloscope. The advantage of using an oscilloscope is that internal resistance is large, and working state of DC current and waveform at measured point can be seen at same time, as well as possible interference signals and noise voltages, etc., which is more convenient for error analysis.
Signal tracking method. For various complex circuits, a signal of a certain amplitude and corresponding frequency can be input to input (for example, for a multi-stage amplifier, a sinusoidal signal f = 1000 Hz can be input to input), and using an oscilloscope, you can determine transition from previous stage to next (or vice versa) , step-by-step observation of changes in waveform and amplitude If any stage is abnormal, fault is in this stage. Here's how to carefully test your circuit.
Contrast method. When you suspect that there is a problem in some circuit, you can compare parameters of this circuit with parameters of operating state of same normal circuit (or theoretically analyze current, voltage, waveform, etc.) to find out abnormal situation in circuit., and then analyze cause of failure and determine location of failure.
Part replacement method. Sometimes fault is hidden and cannot be seen at a glance. For example, if you have an instrument of same model as faulty instrument at that time, you can replace parts, components, plug-in boards, etc. in an instrument with appropriateparts in faulty device to facilitate reduction of range of fault, then find fault.
A traversal method. When a parasitic oscillation phenomenon occurs, you can use a capacitor of an appropriate capacity, select an appropriate test point, and temporarily connect a capacitor between test point and ground reference point. If oscillation disappears, this indicates that oscillations are generated near this or previous step in circuit. Otherwise, he is in back, then move checkpoint to find him. It should be noted that bypass capacitor should be suitable and should not be too large if it can better eliminate harmful signals.
Short circuit method. This is a method of temporarily shorting a part of a circuit for troubleshooting purposes. The short circuit method is most effective for checking for an open circuit, but it should be noted that short circuit method cannot be used for a power source (circuit).
The method of disconnecting circuit. The open circuit method is most effective for short circuit testing. The open circuit method is also a method of progressively narrowing down range of suspected fault points. For example, if a regulated power supply is connected to a circuit with a fault, output current is too large, we use method of disconnecting a certain branch of circuit to check for a fault. If current returns to normal after a branch is switched off, a fault occurs in that branch.
When debugging. There are many ways to find cause of a malfunction, and above are just some of commonly used methods. The use of these methods can be flexibly controlled depending on condition of equipment and fault conditions. For simple faults, one method can be used to locate fault, but for more complex faults, several methods complementary to each other must be used. and cooperate with each other to figure out point of failure.
General troubleshooting practice is as follows:
First use direct observation method to troubleshoot obvious problems, and then use a multimeter (or oscilloscope) to check static operating point. The signal tracking method is a simple and intuitive method that is universally applicable to various circuits and widely used. used in dynamic debugging.
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