🚀 go-pugleaf

FatBytestard wrote:
> On Thu, 12 Mar 2009 06:33:08 +0000, Eeyore
> <rabbitsfriendsandrelations@hotmail.com> wrote:
> 
>>
>> Robert Baer wrote:
>>
>>> Eeyore wrote:
>>>> Robert Baer wrote:
>>>>
>>>>>   RFI? Think not; that voltage is about half the line, and is from
>>>>> capacitave coupling from primary to secondary (if transformer type).
>>>>
>>>> I think you need to read up on modern wall wart type switchers.
>>>    Err...read what is inside the parenthesis....
>> How much capacitance would you expect from say a 25VA transformer ?
>>
>> Graham
>>
>   It varies with the type of transformer, and how the windings get laid
> onto it, and the level of separation between them.
> 
>   Same rules for ANY capacitor, donkey.  Two "plates", which make two
> area sizes, and one gap between them consisting of either polymers or
> paper, or impregnated paper. A layer of windings looks like a cylindrical
> plate.  Basic cap formula, physically speaking.
> 
>   This one reason why most transformers do NOT directly 'nest' the two
> windings together.  Good magnetic coupling, but too much inter-winding
> capacitance.
> 
>  Seperate them with transformer tape or paper, and the cap value drops
> with near no loss in coupling.  Also, an isolation benefit is gained.
> 
>   And yes, you already knew this, you just are losing brain cells at too
> high a rate.

The transformers I've seen that are intended to provide isolation have 
windings that are completely separate, such as this

<http://www.dse.com.au/cgi-bin/dse.storefront/49b8d2b804be08842740c0a87e0106ae/Product/View/M6672>

where the top winding is the secondary, and the bottom winding is the 
primary. The intent clearly being that there is no way that the two 
windings will become electrically connected.

I would not expect there to be much inter-winding capacitance.

Sylvia.