[plug] re: ups

[Leon Brooks]

Alan Howard wrote:
> On Wed, 08 Nov 2000, Leon Brooks wrote:
>> And why do so many laptops (are you listening, Compaq?) require *15* volts DC?
>> 8-16 would be better, 'coz then you could wire it straight into your cigarette
>> lighter or run it from an inverterless micro-RAPS.

> this is not very difficult to acheve with a simple flyback type switch mode
> supply. its possible and easily done , to get 15v from 12v. the TL494 ic from
> motorola ( and others) can very easily do this. there application notes (
> available from motorolas web site) even shows possible circuits that would work
> ( with modification).

This is true, but:

(1) it is probably at least as expensive in real terms as buying a car adaptor;

(2) if you (I) stuff it up, your (my) laptop might not appreciate the 24 volts
it sees;

(3) many laptop owners are smart enough to plug their machine into a cigarette
lighter socket, very few are smart enough (Bill Gates isn't, for example) to
design and build a switchmode power supply, even a four-component jobbie with
manufacturer's tables that you personally could probably assemble blindfolded
and wearing oven mitts;

(4) you lose some extra percentage of your power, exactly how much being
dependent on lots of things but for a TL494 design probably around 20%;

(5) you have to carry an extra piece of electronics around instead of a cable.

Having pulled apart a few laptops, the incoming power gets fed to a switchmode
on its way to the battery anyway, and fed to another on the way out. Modern,
well-designed switchers should be well in excess of 90% efficient, and I imagine
that the people who design the PSUs for $4000 laptops are quite good at
high-efficiency designs.

-- 
Oh, now we're adding details like surrounding solid objects! Next
thing you'll be saying that it's dangerous to use a wireless modem
with a Newton to connect to a machine as root and reconfigure
sendmail while doing 75 in a 45 mph zone. -- Marshall McGowan