In article ,
"Dave Plowman (News)" writes:
In article ,
Steve Lupton wrote:
Dave Plowman (News) wrote:
In article et,
Dave Liquorice wrote:
I'd be surprised if a SMPSU would run down to 12v though most have a
lower voltage limit somewhere above 50v.

Eh? Both my current phone and the last one had SMPS. The current one
tiny and weighs less than a normal plugtop.


Input not output Most that I have come across seem to stop working at
around 80V

Ah. But there's no reason why an SMPS can't work from any voltage - I've
got a handy little circuit that produces 12 volts with an input of 3 - 30
volts.

It's a question of the design criteria. At the lower voltage input,
the current drawn will be higher which will be a higher average
current in the switching transformer (or inductor, depending on
switcher design). The windings have to be designed to take this.

This issue became aparent in electronic control gear for fluorescent
lamps. These ballasts are all rated for both AC and DC supplies, so
they can be used on batteries for emergency lighting. As the batteries
die and the voltage drops, the ballast would increase the current to
the point where the transformer overheated and burned out. Nowadays,
ballasts always have under-voltage protection to prevent this, but
early ones didn't.

--
Andrew Gabriel
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