How to debug the new circuit board?

How to debug the new circuit board?

For a newly designed circuit board, debugging is often difficult, especially when the circuit board is large and there are many components. But if you master a set of reasonable debugging methods, it will be much easier.

For the new PCB board that has just been taken back, first of all, it is necessary to roughly observe whether there are problems on the circuit board, such as whether there are obvious openings, short circuits, and few holes. If necessary, check whether the resistance between the power supply and the ground wire is large enough.

Then comes the installation of the components. Independent modules, if you are not sure whether it is guaranteed to work properly, it is best not to install all of them, but install part by part (for relatively small circuits, you can install them all at once), so that it is easy to determine the fault range and avoid problems that cannot be started.

Generally speaking, you can install the power supply first, and then power on to check whether the output voltage of the power supply is normal. If you are not sure when to power on (even if you are sure, it is recommended to add a fuse just in case), you can consider using an adjustable regulated power supply with current limiting function. First preset the overcurrent protection current, and then slowly increase the voltage value of the regulated power supply to monitor the input current, input voltage and output voltage. If there is no overcurrent protection and other problems during upward adjustment, and the output voltage reaches normal, it means that the power supply is normal. Otherwise, disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

Next, install other modules step by step, conduct a power-on test on each installed module, follow the above steps when power-on, to avoid over-current burned components due to design errors or/and installation errors.

Generally, there are several ways to find faults:

Signal injection method.

Add the signal source to the input terminal, and then measure the waveform of each point in turn to see if it is normal, so as to find the fault point. We sometimes use simpler methods, such as holding a pair of tweezers in our hand, to touch the input terminals of all levels, to see if the output terminal responds, this method is commonly used in audio and video amplifier circuits (but note that, This method cannot be used for circuits with heat generated by the bottom plate or circuits with high voltage, otherwise it may cause electric shock). If there is no response at the upper level, but there is a response at the upper level, it means that the problem lies at the upper level and should be checked.

Voltage measurement method.

First of all, confirm whether the power supply pin voltage of each chip 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 an ordinary silicon transistor is turned on, the voltage of the BE junction is about 0.7V, and the voltage of the CE junction is about 0.3V or lower. If the BE junction voltage of the transistor is greater than 0.7V (except for special transistors, such as Darlington tuBE, etc.), the be contact may be open.

Of course, there are many other ways to find fault points, such as listening, seeing, smelling, and touching.

For an experienced electronic maintenance person, he is very sensitive to these smells; "touching" is to test whether the temperature of the device is normal, such as too hot or too cold. Some power devices will heat up when they work. If they are cold to the touch, it can basically be judged that they are not working. But if the place that shouldn't be hot is hot or the place that should be hot is too hot, that won't work either.

"Seeing" is to see if there are obvious mechanical damages to the components, such as cracking, blackening, and deformation. "Listening" refers to whether the working sound is normal, such as things that shouldn't be ringing, where the sound is not, or the sound is abnormal; "odor" is to check whether there is any peculiar smell, such as burning smell, capacitor electrolyte smell, etc.

 For general power transistors, voltage regulator chips, etc., it is completely fine to work below 70 degrees. What is the approximate concept of 70 degrees? If you press your hand on it and you can hold it for more than three seconds, it means that the temperature is below 70 degrees (be careful to touch it tentatively first, and don't burn your hands).