[Ham-Computers] RE: Running laptop mobile?

[Philip, KO6BB]

Thank you Aaron (and others),

As usual, a very informative answer.  Yes, the 'brick' of this machine puts 
out 19 VDC. What I 'may' do is to try to get a 12VDC cord that fits my 
machine (has a slightly unusual barrel connector) for times I want to run it 
directly on DC. I'm thinking of getting the F150 crewcab truck set up so I 
can go out in the "field" to DX the Longwave beacons I'm so fond of.  Hash 
noise is definitely a consideration in that hobby as many beacons are 
at/below the atmospheric noise floor.  Then perhaps a small 12/110 Volt 
inverter in case I need to recharge the laptop's battery could be added 
later.

IF I can find a plug to fit the laptop I can make my own DC cord, I DO have 
a number of different cords with cigar lighter plugs, and my truck has three 
DC outlets, two in front and one in the back seat consol.

73 de Phil,  KO6BB
http://ko6bb1.multiply.com/
http://members.lycos.co.uk/ko6bb/

DX begins at the noise floor!
RADIO/Antennas: Yaesu FT-2000, Two Modified Mini-whips.
Homebrewed 10Hz Filter, Modified MFJ-1040C Pre-Amplifier.
Merced, Central California, 37.3N 120.48W  CM97sh


----- Original Message ----- 

OK, here's my 2 cents...

Most all laptops use 2 switching power supplies - the power "brick" that 
converts AC to some lower DC voltage (typ between 10v and 21v), and another 
internal DC-DC converter that further drops the voltage as required for the 
various components in the laptop.  Both are subject to causing "hash" in the 
RF spectrum, but the external "brick" (most of which are switching power 
supplies) is usually a worse offender.

A 12v DC to 120v AC inverter itself is a switching power supply, and, as 
such, can (and does) cause RF hash.  Even the "name brand" models can cause 
hash as they are designed for a typical laptop user, not an HF radio 
operator.  To the typical laptop user, they'll never experience the 
fustration of trying to work that "weak one" due to S9+ hash!

So, let's take a closer look at what we can do.  The laptop itself has a 
DC-DC converter which often works with an input as low as 10v DC.  With this 
in mind, it's entirely possible to work directly off a 12v battery or power 
supply.  This has the advantage of removing one source of hash - the 
external power brick!  Will it work?  Most likely - you'll need to check the 
specs of the laptop to be sure.

What's the catch?  Why not just run 12v all the time and forget the "brick"? 
Well, the extra "juice" is usually used as overhead to charge the laptop's 
batteries.  Most laptop battery packs are rated between 10.8v and 14.4v. 
And, if you're familiar with battery charging, you'll know that you actually 
need more voltage than the battery's rating in order to fully charge it. 
So, the caveat of using 12v to power your laptop is...the batteries won't 
charge.  On my HP laptops, the charging circuit drops out of "charge" once 
the input voltage drops below 15 volts.  The "brick" supplies 18.5v @ 3.5A 
(65w) - sound familiar Phil?  Most small to medium laptops are designed 
around a 65w "standard".  Larger laptops will usually have a 90w adapter 
(such as 18.5v @ 5 amps) and the maximum I've seen designed for traveling is 
120w (there are 135+ watt adapters, but they are usually "stationary" and 
power a docking station).

OK, should you get a "factory" 12v DC adapter.  If they have one, it's most 
likely a straight cord with a surge suppressor/noise filter - something you 
could probably builld yourself.  I actually have a "factory" 12v adapter 
cord for an older Compaq laptop - it's a cig lighter plug with a choke and 
cap inside connected to 3 feet of zip cord.  Pretty simple and a steal @ $30 
bucks (really)!  Another option is to get a 3rd party "Auto/Air" adapter 
designed to work with a cigarette lighter or "Air Power" type socket - these 
come with different "tips" to match various laptops.  And, there are 
"universal" bricks that will work with an AC or DC input - I have one of 
these and it works great both overseas (240v) and in the car (12v).  And, 
yes, they are switching supplies and will cause hash.  So if this is of 
primary concern, they see if your laptop will run on 12v.  The 3rd party 
adapters usually run between $30 and $150, depending on brand and quality. 
Targus is the leading 3rd party manufacturer (http://www.targus.com) and 
many places carry them, including office supply stores.

Anyways, hope there's some useful info in here.

73,

  - Aaron, NN6O