[ALUG] lappy aboard

Chris Green cl at isbd.net
Tue May 9 10:42:24 BST 2017

On Tue, May 09, 2017 at 09:12:47AM +0100, Mark Rogers wrote:
> On 8 May 2017 at 19:40, Phil Thane <phil at pthane.co.uk> wrote:
> >
> > I have just bought a canal/river boat and want to use my Linux laptop on it.
> > The PSU for it is labelled 19v, 4.7 amps. The boat runs on 12v. The simplest
> > but quite pricy option is to get an inverter to create 220v AC then use the
> > standard PSU but apart from cost it lacks elegance.
> Actually, a simpler option is replacing the laptop... If you have an
> expensive laptop that's not a good choice, but it could otherwise well
> work out cheaper to replace the laptop, maybe with something smaller
> that charges from 5v (as so many things do these days). If it's an
> expensive new laptop the warranty may be void if the power isn't
> supplied by the manufacturer's PSU, for example, and this laptop might
> require 19v but your next one (or your guest's one) might need
> something different...
The power supply I suggested is adjustable.  I use it with both my
Lenovo (20 volts if I remember) and some Acer laptops (19 volts).

> Otherwise as others have said stepping the voltage up is easy enough.
> You'll probably want an inverter anyway at some point, so if the
> laptop is just for occasional use that might still be the best option.
> Inverters are relatively wasteful but universally useful.
They're not *that* inefficient, probably around 80% usually, but they
won't be so good on a small load like a laptop.  The other problem you
may get is that 'modified sine wave' inverters (the cheaper ones)
don't always play nicely with switching power supplies which are
almost universal with laptops (and many wall warts) nowadays.  I had
a tiny (as in for a DECT phone) wall wart switching power supply
*kill* a 3kw inverter.

> Personally I'd want to standardise as far as possible on 12v DC
> (direct from battery), 5v DC (USB devices like phones) and mains. On
> the other hand if it was a decent laptop, critical to my work, and I
> planned on staying on the boat using it for months at a time, then I'd
> be wanting the most efficient option I could find even if it only
> worked for that one device.
That's basically what I have on our boat but with the addition of some
step-up DC<-->DC supplies for laptop (as described here) and step-down
12v to 9v ones for running a router as there is a Beaglebone Black
on the boat running all the time and it connects to the internet
through the router.

> > If I connect the two in series I should get 24v then I should be able to
> > acquire (or even assemble) a DC-DC voltage regulator to bring it down to 19v.
> I think we're fairly universally against that option though :-)
Yes, unless you are fully 24 volts (which many larger boats are) I
think it's a bad idea.

Chris Green

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