Re: Dumb question re power adapter...

["keithdoxey" <ukha@xxxxxxx>]

Not a dumb question at all.

The wrong PSU can wreck something instantly if the device is=20
unprotected.

The Laptop PSU will be fine as it is a regulated PSU giving out 15V=20
at any current upto its 3A limit.=20

A PSU with a higher current rating is fine so long as 2 conditions=20
are met....
1. Minimum Load - SwitchMode PSU's often have a minimum load required=20
on at least one output to ensure correct operation (PC PSU's etc)

2. It is a REGULATED supply. An unregulated supply is guaranteed to=20
be able to supply its rated Voltage at its maximum current. If the=20
current drawn is substantially lower than that, there can be a much=20
greater voltage on the output. Classic examples are the PSU's used on=20
Answering machines. Often rated at 15V @ 750mA or similar, Off load=20
they will measure upto 22V. So long as the device being connected has=20
its own internal regulation there is no problem unless it is=20
excessively high.=20

This is the approach I took with KAT5. Regulation and Polarity=20
protection are built in meaning that cheaper unregulated supplies=20
could be used whilst at the same time the units are protected against=20
the wrong PSU being connected :-)

HTH

Keith


--- In ukha_d@y..., "Paul Gordon" <paul_gordon@h...> wrote:
> Hi all, - a dumb question if you will indulge me.... (too much=20
sherry!)
>=20
> I picked a HP scanner the other week reduced from =A380 to =A330 as
it=20
had no=20
> power adapter. (which I knew I'd be able to get for about a tenner).
>=20
> However, having had a poke around, it looks like the power adapter=20
>from
> laptop is pretty close to being perfect... (apart from 1 thing...)
>=20
> The Scanner wants 15V, 0.8A, centre positive.
>=20
> My laptop power adapter has exactly the right size plug, and it is=20
centre=20
> positive, and it also outputs 15V. However, it is also rated at=20
3A...
>=20
> So my dumb question is: - is the rated output current stated on the=20
power=20
> brick what it is "capable" of supplying, or what it will
pump out=20
> regardless... i.e. can I plug it into this scanner safely? - is it=20
the case=20
> that the scanner will just draw 0.8A which the adapter will=20
obviously by=20
> happy to supply, or will the adapter always blat 3A into the=20
scanner?
>=20
> Ta...
>=20
> Paul G.
>=20
>=20
>=20
>=20
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