How to test if a newly designed PCB board is faulty
Published time: 2019-09-30
For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is large and there are many components, it is often impossible to start. But if you master a reasonable set of debugging methods, debugging will be more effective. For the new PCB board that I just got back, we must first observe whether there are any problems on the PCB board, such as whether there are obvious cracks, short circuit or open circuit. If necessary, check if the resistance between the power supply and the ground is large enough.
Then there is the component installed. Modules that are independent of each other, if you are not sure that they are working properly, it is best not to install them all, but to install some of them (for smaller circuits, you can install them all at once), so that it is easy to determine the fault range. When you have problems when you are not getting it, you can't start. In general, you can install the power supply first, and then power on to check if the power supply output voltage is normal. If you don't have much control when powering up (even if you have great confidence, it is recommended to add a fuse, just in case), consider using an adjustable regulated power supply with current limiting. First, preset the overcurrent protection current, then slowly increase the voltage value of the regulated power supply, and monitor the input current, input voltage, and output voltage. If there is no overcurrent protection and other problems in the process of up-regulation, and the output voltage is also normal, the power supply part is OK. Otherwise, disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.
Next, gradually install other modules. Each time a module is installed, it is powered on and tested. When powering up, follow the above steps to avoid over-current and burn out components due to design errors or/and installation errors.
There are several ways to find faults:
1 measuring voltage method
The first thing to confirm is whether the voltage of each chip's power supply pin is normal, and then check whether the various reference voltages are normal, and whether the working voltage of each point is normal. For example, when a general silicon transistor is turned on, the BE junction voltage is about 0.7V, and the CE junction voltage is about 0.3V or less. If the BE junction voltage of a triode is greater than 0.7V (except for special triodes, such as Darlington), it may be that the BE junction is open.
2 signal injection method
Add the signal source to the input, and then measure the waveform of each point in turn to see if it is normal to find the fault point. Sometimes we will use a simpler method, such as holding a dice by hand, touching the input terminals of each level to see if there is any reaction at the output, which is often used in audio, video and other amplifier circuits (but note that the hot backplane This circuit cannot be used for circuits or circuits with high voltages, otherwise it may cause electric shock. If there is no reaction at the previous level and there is a reaction at the next level, the problem is at the previous level and should be checked.
3 Of course, there are many other ways to find fault points, such as watching, listening, smelling, touching, etc.
"Look" is to see if the component has obvious mechanical damage, such as cracking, blackening, deformation, etc.: "Listening" means listening to the working sound is normal, for example, something that should not ring, the sound is not ringing or sound Unusual, etc.: "Smell" is to check for odor, such as the smell of burning, the taste of the capacitor electrolyte, etc. For an experienced electronic maintenance person, it is very sensitive to these odors: "touch" is used Try to test if the temperature of the device is normal, such as too hot, or too cold. Some power devices will generate heat when they work. If they are cold, they can basically be judged that they are not working. But if the hot place is not hot or the hot place is too hot, it will not work. General power triodes, voltage regulator chips, etc., working below 70 degrees is completely ok. What is the concept of 70 degrees? If you put your hand on it, you can stick to it for more than three seconds, which means that the temperature is about 70 degrees (note that you should try it first, don't burn your hands).
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