How to debug a new PCB circuit board?
2024-06-11
How to debug a new PCB circuit board?
During the debugging process of a new PCB board, one often encounters some challenges, especially when the board is large and has many elements, and often feels helpless. However, as long as one can proficiently apply an appropriate debugging strategy, the efficiency of debugging work can be improved. For the PCB board we just received, we need to first roughly inspect it to see if there are any problems on the board, such as obvious cracks, or if there are any short circuits, open circuits, or other situations. If necessary, we can test whether the resistance between the power supply and the ground wire meets the standard.
Then it's time to install the components. Independent modules, if you cannot guarantee their normal operation, it is best not to install them all, but to install them part by part (for smaller circuits, you can install them all at once). This makes it easy to determine the scope of the fault, so that you can avoid starting when you encounter problems.
In general, the power supply can be installed first, and then the power output voltage can be checked to see if it is normal. If you are unsure when to turn on the power (even if you are certain, it is recommended to add a fuse just in case), consider using an adjustable current limiting and stabilizing power supply. Firstly, preset the overcurrent protection current, then gradually increase the voltage value of the stabilized power supply, and monitor the input current, input voltage, and output voltage. If there are no issues such as overcurrent protection during the upward adjustment process and the output voltage reaches normal, it indicates that there is no problem with the power supply. Otherwise, please disconnect the power supply, locate the fault point, and repeat the above steps until the power supply returns to normal.
Next, gradually install other modules. After installing each module, power it on and test it. Follow the above steps when powering on to avoid component burnout caused by overcurrent due to design or installation errors.
There are usually several ways to find faults:
(1) Voltage measurement method.
The first thing to confirm is whether the voltage of the power pins of each chip is normal. Secondly, check whether various reference voltages are normal. In addition, check whether the working voltage of each point is normal. For example, when a typical silicon transistor conducts, the BE junction voltage is around 0.7V, while the CE junction voltage is around 0.3V or less. If the BE junction voltage of a transistor is greater than 0.7V (except for special transistors such as Darlington transistors), it may be an open circuit at the BE junction.
(2) Signal injection method.
Add the signal source to the input terminal, and then measure the waveform of each point in sequence to see if it is normal, in order to find the fault point. Sometimes we also use simpler methods, such as holding a pair of tweezers and touching the input terminals of each level to see if there is a response at the output terminals. This is commonly used in audio, video, and other amplification circuits (but it should be noted that this method cannot be used for circuits with hot substrates or high voltage, otherwise it may cause electric shock). If there is no response in the previous level but there is a response in the next level, it indicates that the problem lies in the previous level and should be checked carefully.
(3) Observation, listening, sniffing, and touch sensory methods.
Observation "refers to checking for obvious mechanical damage to components, such as cracking, blackening, deformation, etc;
Listening "means listening to whether the work sound is normal, such as when something should not be making noise, when it should be making noise, or when the sound is not normal;
Sniffing "refers to checking for any unusual odors, such as burnt or capacitor electrolyte odors. For an experienced electronic repairman, they are very sensitive to these odors;
Touch "means using your hands to test whether the temperature of the device is normal, such as too hot or too cold. Some power devices generate heat when working, and if they feel cool to the touch, it can basically be judged that they are not working. But if the area that shouldn't be hot gets too hot or the area that should be hot gets too hot, then it's also not acceptable. Ordinary power transistors, voltage regulators, and other devices can operate at temperatures below 70 degrees without any problems. What is the concept of 70 degrees? If you press your hand on it and can hold it for more than three seconds, it means the temperature is probably below 70 degrees (be sure to try to touch it first, don't burn your hand).
Alright, that's all for today's discussion on debugging. If you want to learn more about debugging methods and experience, you can look for some books on home appliance maintenance. A good designer should first be a good repairman.
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