On 2011-05-11 19:38, Robert Rolf wrote:
>
> Why would DC draw be bad? Full wave rectifier transformers draw DC on
> each half of the secondary, so that can't be an issue.

But that doesn't lead to a DC bias on the line side, does it?


> Some simple minded lamp dimmers just use a power diode to cut out half
> the power line cycles to get 50% dimming.

But I suspect those would be frowned upon as well.


> Some power engineer care to enlighten us?
>
> R
>
I found this page that mentions it:=20
http://sound.westhost.com/articles/xfmr-dc.htm

    There are a few older household appliances that can create a DC
    offset, although most are (probably) no longer permitted due to
    increasing problems caused by the DC component.
    ...
    Most of the time, the DC offset is transient - it appears for a
    short while, then goes away again. When it is there, toroidal
    transformers may complain loudly, by making growling or buzzing
    noises. It is important to understand just how this happens, and
    what can be done about it if it causes problems.
    ...
    With larger transformers (500VA and above), the DC resistance is
    usually so low that even a very small offset will cause mechanical
    noise due to saturation.





Joe Koberg



--=20
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