Newsgroups: sci.electronics.misc
Subject: Re: Switchmode Supply
References: 
Michael Hofmann  writes:

> I have a problem understanding a certain component in switchmode power 
> supplies. In some of those there is a condensor connecting the primary and 
> secondary side, which may lead to a sensible voltage at the output. Though 
> this is not dangerous while high-impedance, it may be irritating to the 
> user.
> What is the use of this condensor? Anyone there to enlighten me?

It is a part of the high frequency interference filtering circuit 
inside the switched mode power supply. The capacitive coupling 
inside transformer will always generate some high frequency 
noise to the secodary side, and the capacitor here acts
like a "short" for high frequency signals (so the output 
will stay on same potential as input at high frequecies).

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/