How to debug a newly designed circuit board?

- Nov 29, 2018-

For a new design of the circuit board, debugging will often encounter some difficulties, especially when the board is larger, more components, often do not know how to start.But if you master a set of reasonable debugging methods, debugging will get twice the result with half the effort.For the new PCB board we just got back, we should first roughly observe whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuit, open circuit and other phenomena.If necessary, check that the resistance between the power supply and the ground wire is high enough.

Then you install the components.Independent modules, if you are not sure to ensure that they work properly, it is best not to install all, but part of a part of the installation (for relatively small circuit, can be installed all at one time), so that it is easy to determine the fault range, so as not to encounter problems when there is no way to start.Generally speaking, the power part can be installed first, and then on the power supply to detect whether the output voltage is normal.If you are not too sure when powering up (even if you are, it is recommended that you add a fuse, just in case), consider using an adjustable voltage regulator with a current limiting function.Preset the overcurrent protection current, then slowly increase the voltage value of the stabilized power supply, and monitor the input current, input voltage and output voltage.If there is no over-current protection and other problems in the process of up-regulation, and the output voltage reaches normal, the power supply part is OK.Instead, disconnect the power, look for trouble spots, and repeat the above steps until the power is normal.

Then gradually install other modules, each module installed, on the power test, power is also in accordance with the above steps, to avoid design errors or/and installation errors caused by over-current and burn components.

There are generally several ways to find fault:

Measurement voltage method.Firstly, it is necessary to confirm whether the voltage of power pins of each chip is normal. Secondly, it is necessary to check whether various reference voltages are normal and whether the operating voltages of each point are normal.For example, the general silicon transistor on, BE junction voltage at 0.About 7V, and the CE junction voltage is 0.3V or less.If the BE junction voltage of a transistor is greater than 0.7V (except for special triode, such as darlington tube, etc.), may BE the BE junction on the open circuit.

(2) signal injection method.Add the signal source to the input end, 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 tweezer with the hand, to touch the input end at all levels to see whether the output end is responsive, which is often used in audio, video and other amplification circuits (but be careful, hot substrate circuit or high voltage circuit, cannot use this method, otherwise it may cause electric shock).If there is no reaction at the front level and reaction at the rear level, it indicates that the problem is at the first level and should be checked emphatically.

(3) of course, there are many other ways to find fault points, such as look, listen, smell, touch and so on."See" is to see whether the components have obvious mechanical damage, such as cracking, burning black, deformation;"Listen" is to listen to whether the working sound is normal, such as some things should not ring in the ring, the place should not ring or abnormal sound;"Smell" is to check whether there is any odor, such as the smell of burning, capacitor electrolyte smell, etc., for an experienced electronic maintenance personnel, is very sensitive to these smells;To "touch" is to use your hand to check whether the temperature of the device is normal, such as too hot or too cold.Some power devices, when working, will be hot, if it is cold to the touch, it can basically be judged that it does not work.But if it's too hot in the wrong place or too hot in the right place, it's not going to work.General power transistor, voltage stabilizer chip, working under 70 degrees is completely no problem.What is 70 degrees roughly?If you press your hand up and hold it for more than three seconds, it means the temperature is about 70 degrees or less.