So this question is a follow up to my previous question. Here is the story. Few days ago, while installing a 12v LED strip, I found out that the 12v AC-DC power adapter actually carries a high voltage AC in it. My question was marked as duplicated, and the answer from there states that the high voltage AC is from the capacitance between two inductors in the transformer. It really doesn't satisfy my curiosity because I have many AC-DC power adapters -- who don't? -- , and not all of them have this problem. So yesterday, I finally managed to open up that problematic 12v power adapter, and found out that there is a 222 2kv capacitor connected between the two inductors in the transformer. This is probably the reason to the HIGH voltage AC, but my question will be what is its purpose?

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0, what are the purpose to the 222 2kv bridge capacitor?

Other than that, I have few more questions about this circuit design.

1, what is the unmarked IC used in this unit?

2, what are the purpose of the 3 parts: 100k, 222j, and diode on the high voltage side? ( red in the paper)

3, Why the IC is not connected to V+? Pin 8, 7 should be the drain to switch the transformer, and Pin 6 should be the feedback, and Pin 3 is the ground, Pin 1, 4, 6 are not connected to anything.

4, how reliable is this circuit design? any faulty?


That blue 222 2kV capacitor should be "Y-rated" for safety reasons and it is often called the "Y-capacitor".

It is there for EMI purposes (to lower EMI emissions which can disturb other equipment) and closes the path for the high-frequency signals generated by the switching chip or transistor at the primary side. Without this "Y-capacitor" many supplies would likely fail FCC and other regulations.

1) What the unmarked IC is, is any one's guess. There are many of these chips available and they're all slightly different but do operate in roughly the same way. Feel free to compare all application diagrams off all these ICs with your schematic but personally I do not see the point of that exercise.

2) Very likely this is needed for drawing a small amount of power from the transformer which is then used to provide power to the main IC. Your drawing of the transformer is not accurate enough to properly display how this works.

3) the IC is not connected to V+ as there could be 400 V on it and that could fry the chip. What makes you think that the chip needs the 400 V anyway ?

4) Reliability is a tricky subject. How would you distinguish between a reliable circuit and one that is not ? No circuit lasts for ever. If used properly most ICs are guaranteed a 10 years lifetime. If you mistreat the circuit, keep it too hot for example, it will break sooner.

AC power adapter design is a topic for experienced EEs. I fail to see why you would need to fully understand this circuit. If it is a very cheap one then you cannot expect it to last very long. As always, you get what you pay for.

I suggest watching some of the videos from Bigclive on Youtube, in many of these he takes small power supplies like this one to bits while he comments on their design.

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