Re: [TCML] Self ballasting transformers

[Ed Phillips <evp@xxxxxxxxxxx>]

bsneath@xxxxxxxxxxxxxxxx wrote:

> hi all,
>
> I have a question regarding the self-current-limitting transformers that are used in most old appliances. 
> It would seem that they have no shunts at all, and the only way they ballast themselves is with a large number of primary turns.
> To clarify, the transformers i'm talking about are those little <100watt ones that powered stuff like DVD players etc. before switching psu's took 
> over. 
> But how does it work?
> Another thing, when i put a new secondry on one such xfrmr after extending the core (by using 2 E sections back to back instead of the usual E I 
> construction), the current limiting no longer worked as well and i could draw more power out of it than it was rated for.
> so, how do these xfrmrs self-limit, and how does the core extension affect it?
>  
   Are you talking about "wall warts"? What makes you think these 
things had current limiting?  I've never seen one with current limiting 
and the power rating is just that - the maximum output without 
overheating.  That in no way means you can't draw more power.  Most of 
these transformers are really crappy and cheap with higher winding 
resistance than one might want for best efficiency.

Ed

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