switching & ground isolation question

[salviador]

Hi,
Looking various diagrams of swiching power supplies, I noticed that the ground of high-voltage is not completely isolated from the ground of the low voltage! But is coupled with capacitor!
This a problem because when apply the ground of my dso (in "isolated low voltage") I have small earth shock!!!

Why?

Example of power supply

[michaeliv]

Have a look at this:

http://www.edaboard.com/thread207482.html
http://www.arpnjournals.com/jeas/research_papers/rp_2010/jeas_0210_300.pdf

[T3sl4co1l]

The capacitor provides a path for high frequency AC.

It should be a small enough value that you cannot feel the current flow.

Sometimes, the capacitor is placed on the AC input side of the common mode choke.  Sometimes, a common mode choke is also needed on the secondary side.  The exact combination of chokes and capacitors, and their placement, is hard to figure out until testing.

If you have a three-prong AC cord, the neutral wire should be consistent, and a capacitor to neutral should not deliver any ground leakage current.  This would be a better location than across the transformer.

Tim