Portable AC power source?

Charles Curley charlescurley at charlescurley.com
Wed Jan 27 16:06:02 MST 2010

On Wed, 27 Jan 2010 11:49:49 -0700
Michael Torrie <torriem at gmail.com> wrote:

> Charles Curley wrote:
> >> Anyone know of a good PORTABLE AC power source? Something like a
> >> UPS or something, but smaller. It doesn't have to be big, just
> >> enough to power a couple of external devices to my laptop.
> > 
> > Simplest solution: an inverter. Get one specifically designed for
> > solid state electronics, e.g. laptops, etc. It must produce a sine
> > wave, and if the salesman doesn't know what a sine wave it, it
> > doesn't. For what you are doing, a 140 watt inverter is fine. My
> > Lenovo R51 is rated for 70 watts and actually draws around 40.
> Of course converting DC to AC back to DC is convenient, but not
> efficient.  If his portable hard drive is 12 V (many are these days),
> then maybe he could run it directly off his car electrics with a
> regulator.

Hmmm, it might work. He wants 12V DC, nicely filtered and very quiet.
The car's alternator provides 14.7 volts nominally. So
nominally a regulator should be able to handle the conversion. Depending
on the state of the battery and other factors the voltage could be
anywhere from 8 to 15 volts, and very noisy. I for one would not care
to chance it.

(I once started a marine diesel engine with a nearly exhausted battery.
The ship's voltage went down to 8 volts, but the engine started.)

You are correct that it is inefficient. Buying a new external drive or
laptop every so often would be even less efficient.

> I've used el-cheapo inverters for years (probably square wave instead
> of a nice sine) and they seem to work fine, although they induce some
> noise in my computer's audio.  A ferrite choke helps that though.

OK. How well do the batteries charge? I'd also be concerned with
electrical noise.

I may well be over-protective here, but I've learned to spec out
equipment for applications where the nearest land was over a mile away.
Straight down. I spec conservatively.


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