john jardine brought next idea :
"Andrew Gabriel" wrote in message
.. .
In article ,
Harry Bloomfield writes:

I would though like to know what method they use to reducing the
voltage, That toroid doesn't look capable of delivering more than about
300w.

If it's an auto-transformer, then for a 30V buck, it will
handle a load of about 8 times its VA rating, but that's
still not much (say, 2,500VA). What it could do is reduce
the buck voltage as the load increases above 2,500VA, e.g.
to 10V max at 7,500VA, but that would somewhat mitigate
the object of having the device.

No idea if this is what it actually does.

-- Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Looks like you're spot on.
http://www.faqs.org/patents/app/20090008997
Probably a Triac feeding that buck winding. Their main worry (rightly!)seems
to be overheating of the core.

...judging by the thermal trip.

Myself, I wouldn't have one given.

I would guess they have based there figures on short term measurements
of the reduction in current demand, rather than any longer term
measurement of actual reduction in consumption.

All resistive loads will use less if the voltage is reduced (Ohms Law),
but they will make up for it by running for increased periods to
produce the same work output.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk