Circuit and component fault detection and analysis of causes

  Case 1: optocoupler fault

A customer unit of electric welding machine power supply board, above a optocoupler, often broken, before the optocoupler input signal is detected, but through a connector wires connected to another piece of card, in the receiving end can only get distortion after the triangular wave.

Customer engineer confusion: the luminous end optocoupler is broken, or the photosensitive receiver is broken? Where did the stress come from? What is the stress?

Take over the task, the first thing is to provide 1 to the optocoupler, a few bad. Then began the treatment of Chinese medicine.

1, look at the appearance of normal: optocoupler.

2, smell: smell with the nose, did not smell the smell of burnt smell, or that did not smell the taste of a marked coke, I have a mild rhinitis;

3, ask: did not ask, do not bother to ask.

4, cut: VI curve test, is to use the above instrument to test the port characteristic impedance waveform, do good or bad device contrast test.

After the diagnosis and treatment of traditional Chinese medicine, the basic conclusion is that:

1, there may be no heat damage. Thermal damage will have burnt taste, serious in the "look" of, see traces of obvious discoloration, carbonization and cracking, melting, burst. Slight thermal damage, if the device inside the wafer or conductive tape, the appearance and look not out, the taste is very slight, the nose is not sensitive enough to be heard.

2, according to 2-3 of IV curves can be seen, the light emitting diode side of both ends of the curve, good devices and fault device is the same, basically concluded that the normal side emitting; but 6-5 quality device curve comparison, reverse diode characteristics without. 7-5 empathy.

   It can be completely rejected first thermal damage, because this type of device thermal damage performance should be open, but the performance of the fault is still shown as a one-way diode characteristics, not open.

So the damage of large voltage and small current is basically determined. The next step is to find the source voltage, the internal circuit structure of optocoupler datasheet. Pin6 and pin7 in the film is through, so it is the same as the IV curve of the foot of the phenomenon of the failure of a good understanding of the.

The internal circuit diagram for the push, E-C end Tr-2 PN junction damage, and pressure from 5 end. Similar problems, the maximum possible source of several possible high pressure is:

1, Gnd PE and Gnd Pin-5 together, but the PE end is not good, it will be charged above (generally around 110V), which is related to the structure of the circuit design and the way to deal with;

2 and connector to connect if loose connections, work some vibration, loose tight TB when open, disconnect the moment of ignition also have instant high voltage;

3, Pin-6, Pin-7, Pin-5 tube feet, there are directly connected to the inductive device, in the start and stop moments, the inductive load will generate a reverse electromotive force, the role of the CE Tr2 ends, resulting in damage.

   The rest is the design engineers according to this idea to find hidden points. As a solution, because it involves the circuit principle diagram of the customer, you will forgive the inconvenience revealed. In fact, the solution is very simple.

Case 2: +65V no output of a power supply

    According to the preliminary analysis of the functional circuit, the problem can be focused on the field effect transistor BUZ80 as the center of the amplifier section and the IC2 (UC1524A-J) as the center of the oscillating circuit. For each of the circuits of the circuit to do a comparative test of the IV curve, the failure of the board and a good board to do the same with the same. Found that the Pin-10 IC2 exception, a good difference between the board and the board of a good waveform. There is a capacitor C24 in the circuit connected to the pin, and the fault VI curve has no capacitance. The capacitor is removed, measured with a universal meter, without the capacitance value. To find a replacement for the same capacitor, the test machine, the failure to eliminate.


 

(Source: Inside information)

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