AC Male Power Socket

Hi Guys, I am a newbie to electronics but love to get started learning. So after a few google searches and YouTube videos, I have decided build my own variable Power Supply using an old Computer ATX Power supply for my further learning and future projects.

When I take the power supply apart, the first thing I've noticed behind the AC main input male socket was 2 capacitors from L & N parallel to the Ground, a resister across L and N. Also some component packed in white plastic across L and N.

I have learned to purpose of resister and capacitors. But cannot control my curiosity why such configuration needed in an AC input(at least in this case)? Also what is the component in white package? Please excuse if this is a stupid question. I hope someone can explain. Thanks in advance.


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Component in white package is an X-capacitor for EMI suppression or C1 in the schematic above. This has the inadvertent effect of keeping your plug at voltage even after you unplug it, which will cause an electric shock if you touch it. To counter this, you are required to bring the voltage down below acceptable levels (40 V last time I checked in Europe) within a certain amount of time, normally one second. This is the reason for the resistor R1. They are often referred to as X-cap discharge resistors.

The two blue capacitor you see soldered between ground and L and N are the Y-capacitors C2 in the schematic.


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