From: John Woodgate
Subject: Re: Doubling wallwart power rating
Date: Mon, 7 Oct 2002 13:47:12 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 7 Oct 2002 12:52:40 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Gibbo
wrote (in <firstname.lastname@example.org>) about 'Doubling
wallwart power rating', on Mon, 7 Oct 2002:
>Can you explain the relationship between higher current and core saturation ?
The voltage available to produce the induction in the core (equal and
opposite to the back-e.m.f, which may clarify the matter) is equal to
the supply voltage minus the voltage drop across the winding resistance.
A higher output current implies a higher input current, therefore more
voltage drop, therefore less induction.
Someone in this thread said that there is a compensation effect - less
induction implies less magnetizing current. So, over a range of output
currents, the input current magnitude may be nearly constant (but the
phase varies). Except for a very small transformer indeed, (e.g. 1 VA),
the magnetizing current is much less than the total input current at
rated load, so the constant-current range is likely to occur under some
rather light load conditions.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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