Y1 and Y2 Capacitor in SMSP circuit

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are you talking about smps?

Anyway, you put it wherever you put it to get rid of ther noise, ie, get rid of the common mode noise drawn from your input.

but remember, the noise is AC, and AC does not care which is the positive or negative, because AC does not live in the world of DC, and it has no respect for it.

Anyway, when you consider the Y cap that goes from pri to sec of the smps, -its a fact that the secondary ground can and will float up to the peak of the mains supply voltage.............via the Y cap (and partly by the transformer stray capacitance.)

...so you need to put 1 megohm resistors in parallel with your y cap if you want to discharge this pesky voltage...........or just let it float at HV.

But if you touch it, you will get a small snappy electric shock.

I used to work in a power supply dept of a tv factory.......the old engineers would sneak up and snip the 1 megohm discharge resistors and then laugh their heads off when the junior engineers kept getting zapped by the high voltage capacitive discharge.
 
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