Re: [A/S] 12-volt outlet in truck

["Rick Kunath" <k9ao@xxxxxxxxxxx>]

> I just talked to one of our electrical guru's here at
> work.  He noted that a 200 watt inverter is a pretty
> healthy unit to power a laptop.  

That depends on the laptop. Some of the desktop-replacement laptops have 
a hefty current draw when used on AC line power. Most aren't 200 watts 
though. And it depends on how far the designer of the inverter stretched 
the truth about it's actual ratings.

> Some cigarette
> lighters are fused as low as 8 amps.

Not any I have ever seen. Lighters are usually fused at 25 amperes. The 
lighter current draw is usually in the 15 to 20 ampere range. A 
power-tap outlet may be fused lower, but I doubt it.

Do pull the lighter fuse and inspect it for heat damage and other types 
of partial failure, but a simple lighter test after that with the 
cigarette lighter element will tell you if the socket can supply power. 
Give it the "glow" test.

> Dependent upon
> the type of inverter you have and the amperage draw,
> your inverter may be producting a full 200 watts even
> though your PC requires less.  In that case the
> inverter will be pulling full amperage, and if its in
> excess of 8, that fuse will fail.

This is completely incorrect information. The inverter current draw will 
be proportional to the load current being drawn from it plus the losses 
associated with the inversion process. In no case will it draw the full 
rated current with a lighter load. (Unless, of course, it has 
electrically failed.)

Using an inverter that again feeds a laptop AC line power adapter is an 
inefficient way to achieve laptop DC powering.

Use the proper DC power adapter for your model laptop if possible. As 
was discussed in earlier posts and by others, this may be as simple as a 
special fused DC plug, or a fused DC plug with additional noise and 
surge/spike filtering, or a voltage regulated (and possibly a built-in 
inverter as ThinkPads use) DC-to-DC converter to supply the proper 
laptop operating voltage. Regardless of the actual circuitry needed to 
power the laptop on 12 volts DC, you can be assured that you are using 
the appropriate device when you purchase the unit designed by your 
laptop's manufacturer.

My advice is to go that route and avoid separate inverters. Inverters 
can be an acceptable method to power laptops on 12 volts DC, but they 
are almost always less efficient than the manufacturer's solution. Also 
keep in mind that the quality of inverters varies, as does the design. 
Some produce square wave output, most produce a stepped-wave quasi-sine 
wave output that can be acceptable depending on the device, some use 
more sophisticated methods to produce relatively clean power. All of 
these types of clean-power inverters are far more expensive than the 
simple manufacturer's DC power adapter for the laptop, and still do a 
double power conversion (this wasteful double-conversion isn't usually 
an issue on DC power in a moving vehicle, but is when stationary and 
using battery power alone.) And the inverter you may be using is 
probably not of this design class.

I suspect your inverter is the culprit if the cigarette lighter works in 
the socket and "lights up" cherry orange. It may be an inexpensive 
design and be unsuited for your use.

As an engineer, I'd recommend getting the DC power adapter for the 
laptop and being done with it, after verifying that the lighter socket 
works by the "glow" test above.

Rick Kunath
WBCCI #3060