New Disassembly Activity Starting in August, with prizes including drones, handheld oscilloscopes、computers、vacuum robots、smartwatches、solid-state drives、portable speakers. Come join us for lots of rewards!
Hardware Inspection Before Powering On
After a circuit board is soldered, when checking if the circuit board can operate normally, it is usually not powered directly. Instead, the following steps should be followed to ensure that there are no issues before powering on.
1. Check if the connections are correct. Checking the schematic is crucial. The first focus is on whether the power supply and network node markings of the chip are correct, and also pay attention to whether there are overlapping network nodes. Another focus is on the component packaging, the model of the package, and the pin order of the package; do not use the top view for packaging, especially for non-pin packages. Check if the connections are correct, including wrong, missing, and extra wires.
There are usually two methods for checking connections:1) Check the installed lines according to the circuit diagram, inspecting the installed lines one by one in a certain order based on the circuit connections;2) Compare the actual lines with the schematic, centering on each component to check the connections. Check each component’s pin connections clearly, ensuring each destination exists in the schematic. To avoid mistakes, lines that have been checked should be marked on the schematic, and it is best to use a multimeter in the ohm range with a buzzer to test, directly measuring the component pins to simultaneously discover any poor connections.
2. Check for power supply shorts. Before debugging, do not power on. Use a multimeter to measure the input impedance of the power supply; this is a necessary step! If the power supply is shorted, it may burn out the power supply or cause more serious consequences. For the power supply section, a 0-ohm resistor can be used as a debugging method. Before powering on, do not solder the resistor; check that the power supply voltage is normal before soldering the resistor onto the PCB to supply power to the subsequent units to avoid burning the chips of the subsequent units due to abnormal power supply voltage. Add protection circuits in the circuit design, such as using resettable fuses and other components.
3. Check the installation of components. This mainly involves checking polarized components, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and ensuring that the pins of transistors correspond correctly. For transistors, different manufacturers may have different pin arrangements for the same function, so it is best to test them with a multimeter.
First, perform open-circuit and short-circuit tests to ensure there will be no short circuits after powering on. If the test points are set well, it can be twice as effective. The use of a 0-ohm resistor can sometimes be beneficial for high-speed circuit testing.
After completing the above hardware checks before powering on, you can begin the powered testing.
Powered Testing
1. Power observation: After powering on, do not rush to measure electrical indicators but observe for any abnormal phenomena, such as smoke, unusual odors, or feeling for heat on the integrated circuit package. If any abnormal phenomena occur, immediately turn off the power and only power on again after troubleshooting.
2. Static debugging: Static debugging generally refers to DC testing conducted without input signals or only with fixed-level signals. A multimeter can be used to measure the potential at various points in the circuit, comparing it with theoretical estimates, and analyzing the circuit principles to determine if the DC operating state is normal, promptly discovering any damaged or critically operating components in the circuit. By replacing components or adjusting circuit parameters, ensure that the circuit’s DC operating state meets design requirements.
3. Dynamic debugging: Dynamic debugging is conducted based on static debugging, where suitable signals are added to the input of the circuit. Following the signal flow, test the output signals at each test point in sequence. If any abnormal phenomena are found, analyze the causes, troubleshoot, and then debug until requirements are met.
Throughout the testing process, do not rely on instinct; always use instruments for observation. When using an oscilloscope, it is best to set the signal input mode to “DC”. Through DC coupling, both AC and DC components of the measured signal can be observed simultaneously.Through debugging, finally check whether the various indicators of functional blocks and the entire machine (such as signal amplitude, waveform shape, phase relationship, gain, input impedance, and output impedance) meet design requirements. If necessary, propose reasonable corrections to the circuit parameters.
Other Tasks in Electronic Circuit Debugging
1. Determine test points: Based on the working principle of the system to be debugged, draft debugging steps and measurement methods, determine test points, and mark their locations on the drawings and the board. Create debugging data recording forms, etc.
2. Set up a debugging workstation: The workstation should be equipped with the necessary debugging instruments, and the arrangement of instruments should facilitate operation and observation. Special reminder: During manufacturing and debugging, ensure that the workstation is clean and tidy.
3. Choose measuring instruments: For hardware circuits, select measuring instruments that are more accurate than the system being tested; for software debugging, a computer and development device should be equipped.
4. Debugging sequence: The debugging sequence for electronic circuits is generally conducted according to the signal flow direction, using the output signal of previously debugged circuits as the input signal for the next stage, creating conditions for final overall debugging.
5. Overall debugging: For digital circuits implemented with programmable logic devices, complete the input, debugging, and downloading of the source files for the programmable logic devices, and connect the programmable logic devices with analog circuits to form a system for overall debugging and result testing.
During the debugging process, carefully observe and analyze experimental phenomena and keep detailed records to ensure the completeness and reliability of experimental data.
Precautions in Circuit Debugging
The correctness of debugging results is greatly influenced by the accuracy of the test quantities and the precision of the tests. To ensure the results of testing, it is necessary to minimize testing errors and improve testing accuracy. Therefore, pay attention to the following points:
1. Correctly use the grounding terminals of testing instruments. When testing with electronic instruments grounded to the chassis, the grounding terminals should be connected to the amplifier’s grounding terminals. Otherwise, interference introduced by the instrument’s chassis will not only change the working state of the amplifier but will also cause errors in the test results. Following this principle, when debugging the emitter bias circuit, if you need to test Vce, do not connect the instrument terminals directly to the collector and emitter; instead, measure Vc and Ve to ground separately and then subtract the two. If using a battery-powered multimeter for testing, since the two input terminals of the meter are floating, direct connection to the test points is permitted.
2. The input impedance of the instrument used for measuring voltage must be much greater than the equivalent impedance at the point being measured. If the input impedance of the testing instrument is small, it will cause shunting during measurement, leading to significant errors in the test results.
3. The bandwidth of the testing instrument must exceed the bandwidth of the circuit being measured.
4. Correctly select test points. When measuring with the same testing instrument at different measurement points, the errors induced by the instrument’s internal resistance will vary greatly.
5. The measurement method should be convenient and feasible. When measuring the current of a certain circuit, it is generally preferable to measure the voltage rather than the current, as measuring voltage does not require altering the circuit. If you need to know the current value of a certain branch, you can measure the voltage across the resistor in that branch and calculate it.
6. During debugging, not only should careful observation and measurement be done, but also good record-keeping is essential. The recorded content should include experimental conditions, observed phenomena, measured data, waveforms, and phase relationships, etc. Only by comparing a large amount of reliable experimental records with theoretical results can problems in circuit design be identified and the design scheme improved.
Fault Diagnosis During Debugging
Carefully investigate the causes of faults; do not immediately disassemble the circuit and reinstall it when encountering a fault that cannot be solved. Because if it is a principle issue, reinstalling will not resolve the problem.
1. General methods for fault checking
For a complex system, accurately identifying faults among a large number of components and circuits is not easy. The general fault diagnosis process starts from the fault phenomenon, conducts repeated tests, makes analytical judgments, and gradually identifies the fault.
2. Fault phenomena and causes of faults
1) Common fault phenomena: An 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. Series voltage regulators have no voltage output, or the output voltage is too high and cannot be adjusted, or the output voltage regulation performance deteriorates, or the output voltage is unstable, etc. Oscillation circuits do not oscillate, and counter waveforms are unstable, etc.
2) Causes of faults: For fixed products that have been used for a period, faults may occur due to component damage, wiring short circuits, and disconnections, or changes in conditions, etc.
3. General methods for checking faults
1) Direct observation method: Check whether the selection and use of instruments are correct, whether the power supply voltage levels and polarities meet the requirements; whether the polar components’ pins are connected correctly, and check for incorrect connections, missed connections, and short circuits. Check the wiring for reasonableness; check if the printed circuit board has short lines or broken lines, and whether resistors and capacitors are burned or exploded. When powered on, observe whether components are overheating, smoking, and whether transformers have burnt smells, whether electronic tubes and oscilloscope tubes light up, and whether there is high voltage sparking.
2) Use a multimeter to check the static operating point: The power supply system of electronic circuits, the DC operating state of semiconductor transistors and integrated circuits (including component pins, power supply voltage), and the resistance values in the circuit can all be measured with a multimeter. When the measured values differ significantly from the normal values, analysis can help identify the fault.
It should be noted that the static operating point can also be determined using the oscilloscope in “DC” input mode. The advantage of using an oscilloscope is its high internal resistance, allowing simultaneous observation of the DC operating state and the signal waveform at the measurement point, as well as any interference signals and noise voltage, 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 injected at the input (for example, for multi-stage amplifiers, a sine wave signal of 1000 Hz can be injected at its input), and using an oscilloscope to observe the waveform and amplitude changes stage by stage from front to back (or vice versa). If any stage is abnormal, the fault lies in that stage.
4) Comparison method: If a circuit is suspected of having a problem, compare the parameters of this circuit with the parameters of a normal circuit (or theoretical analysis of current, voltage, waveform, etc.) to identify abnormal conditions in the circuit, and then analyze and determine the fault point.
5) Component replacement method: Sometimes faults are quite hidden and cannot be easily identified. If you have a device of the same model as the faulty instrument, you can replace components, devices, and plug-in boards in the faulty instrument with corresponding parts from the normal instrument to narrow the scope of the fault and find the source.
6) Bypass method: When parasitic oscillation occurs, appropriate capacitors can be used to select suitable test points and temporarily connect the capacitor between the test point and the reference ground. If the oscillation disappears, it indicates that the oscillation originates from this area or the preceding circuit. If not, move the test point to continue searching. The bypass capacitor should be appropriate and not too large; it should only be sufficient to eliminate harmful signals.
7) Short circuit method: This method involves temporarily short-circuiting part of the circuit to locate the fault. The short circuit method is most effective for checking disconnection faults. However, be cautious not to use the short circuit method on the power supply (circuit).
8) Open circuit method: The open circuit method is most effective for checking short circuit faults. It is also a method of gradually narrowing down the suspected fault points. For example, if a voltage regulator has excessive output current due to connection to a faulty circuit, we can check the fault by sequentially disconnecting a branch of the circuit. If the current returns to normal after disconnecting that branch, the fault occurred in that branch.
In practical debugging, there are many methods to find the causes of faults; the above only lists a few commonly used methods. The use of these methods can help identify fault points for simple faults with just one method, but for more complex faults, multiple methods need to be used in conjunction to find the fault point. Generally, the common practice for fault finding is:
1) Use the direct observation method to eliminate obvious faults.
2) Then use a multimeter (or oscilloscope) to check the static operating point.
3) The signal tracing method is universally applicable and simple, widely used in dynamic debugging.
Source: Internet