🚀 go-pugleaf

On Thu, 12 Mar 2009 20:27:25 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>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.


 Seperation with fish paper (transformer paper)of a couple thousandths of
an inch, plus the mag wire insulation, plus impregnation, if any is quite
good enough, and can be a mere few thousandths total, and provide 6kV
isolation.  Transformer tape is also only a couple mils or so, and most
of these super small jobs are usually incorporating cheap elements like
paper and there usually is not large gap.

  Most are NOT made in the style you gave a link to.  Most are one
winding right on top of the other, and it makes no difference which one
is inside, and which is outside.  That has more to do with construction
ease.  That 'ease' changes as the turns ratio grows.  It is far easier to
wind a couple thousand turns of an HV transformer right on the bobbin
base to achieve nice, flat winding layers, and then add the primary onto
that.  Placing the larger wire size primary first makes for a very
'terrain' like winding layer on the small gauge secondary.

 These relatively low ratio devices though, can be found to have been
wound either way.  It is all a matter of maker preference as there is
typically no huge efficiency difference between the choices.If there is,
the most efficient method will always be utilized. With these step down
jobs, you will find most to be primary first.