How to debug a new PCB circuit board?

For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is relatively large and there are many components, and there is often no way to start. But if you master a reasonable debugging method, debugging will be twice the result with half the effort. For the newly retrieved PCB board, we first need to roughly observe whether there are any problems on the board, such as obvious cracks, short circuits, open circuits, etc. If necessary, you can check if the resistance between the power supply and ground wire is large enough.

Then it's time to install the components. If you are not sure how to ensure the normal operation of mutually independent modules, it is best not to install them all, but to install them in parts (for relatively small circuits, you can install them all at once), which is easy to determine the fault range and avoid having to start when encountering problems.

Generally speaking, the power supply can be installed first, and then powered on to check if the output voltage of the power supply is normal. If you are not very confident when powering on (even if you are confident, it is recommended to add a fuse as a precaution), you can consider using an adjustable voltage regulator with current limiting function. 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 are no overcurrent protection issues during the upward adjustment process, and the output voltage also reaches normal, it indicates that the power supply is OK. On the contrary, it is necessary to disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

Next, gradually install other modules. After installing each module, power on and test it. Follow the above steps when powering on to avoid burning components due to design or/or installation errors that may cause overcurrent.

There are generally several ways to find faults:

① Measure voltage method.

Firstly, it is necessary to confirm whether the voltage of the power pins of each chip is normal. Secondly, check whether various reference voltages are normal, and also whether the working voltage of each point is normal. For example, when a typical silicon transistor is on, 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 that the BE junction is open.

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 at all levels to see if there is any response at the output terminals. This is often 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 circuits, otherwise it may cause electric shock). If there is no response from the previous level, but a response from the subsequent level, it indicates that the problem lies in the previous level and should be carefully examined.

③ Of course, there are many other ways to find the fault point, such as watching, listening, smelling, and touching.

'Look' is to see if there is obvious mechanical damage to the component, such as breakage, blackening, deformation, etc;

Listening "means listening to whether the working sound is normal, such as something that shouldn't be making a sound, something that shouldn't be making a sound, something that doesn't make a sound, or the sound is abnormal;

'Smell' means checking for any odor, such as the smell of burning or the smell of capacitor electrolyte. For an experienced electronic maintenance personnel, these odors are very sensitive;

Touch "is to use your hands to test whether the temperature of the device is normal, such as being too hot or too cold. Some power devices generate heat when working, and if they feel cold, it can be basically judged that they are not working. But if the place that should not be hot is too hot or the place that should be hot is too hot, then it is also not possible. General power transistors, voltage stabilizing chips, and other devices can operate at temperatures below 70 degrees Celsius without any problems. What is the concept of 70 degrees? If you press your hand up and can hold it for more than three seconds, it means that the temperature is probably below 70 degrees Celsius (be sure to try to touch it first, don't burn your hand).

Alright, let's talk about debugging for now. If you want to learn more about debugging methods and experience, you can find some books on home appliance maintenance. A good designer should first be a good repairman.