[aprssig] Power For a Lap Top

Dave Baxter dave at emv.co.uk
Tue Aug 30 07:14:55 EDT 2005


Just make sure that you know what the "Inverter" produces, AC or DC
output.  (Seriously!!)
Then compare with what the "device" actualy needs.  Anything with a
transformer, will of course need AC only.  Most (but not all!) AC input
switched mode supplies, can run OK from a DC source.  

However, If it's "Power Factor" corrected device though, stick to an AC
source (and preferabley a sine wave) unless you have a spare unit.  Some
are OK with DC input, some go BANG!  (Don't ask!)

Of course, as others have said, make sure that the invertor (and it's
12/24V DC source) is fully up to the job.

Also of note, most laptop PSU's are not that efficient, and most
invertors are only about 80% or so efficient, some are a little better,
so you may as I once did, find youself unexpectedly needing a set of
jump leads to restart the engine!  (Seroiusly embarrasing, as it was an
emergency exersise!)  Unless, you have two batteries and a split charge

This laptop (A Tosh'M30 actualy needs 15V at up to 4A.  It's external
PSU is quoted at 15V/5A (75W) out.  But..  It's rated at 240V at 0.6A in
(144W!)  It get's very hot in normal use, so I guess it lives up to its
rating plate...

And from personal experience, with the 200W inverter I have in the
vehicle, it does draw some 15A from the 12V source powered that way...
Hence the jump leads needed.....

I now use a "propper" DC power unit (90 watt version) for the laptop if
needed.  That I've measured to be nearly 90% efficient, so the 12V /M or
/P current requirements are down to about 7A. (even less if I remove the
laptop's battery to disable the internal charger, but you then loose the
UPS function, needed when starting the engine!)  I've not needed a jump
start since using that device.  They can be found from many web based
shops too.  Mine has exchangeble DC plugs for the laptop end, so it can
be used with 15V and 19/21V machines, just by selecting the appropriate
plug.  (It's input is rated 10 to 32V DC, so good for the truckers too,
sorry, but no make model details as I've not got it here with me at the

I have also found that the laptop will actualy run OK on 12V (actualy
down to 10V!), but the battery does not get charged at all if the DC
supply to the laptop is below 14V.  However, I doubt if there is much
spike suppression in this thing, so that only happens if desperate, and
when the engine is off!

Many older laptops with dead battery packs (some packs are marked up as
12V) could be adapted to direct 12V use I feel.  Remove the dead
batteries, and fit a noise/spike filter and overvoltage protection
(crowbar/fast-fuse, or big fat zenner+slow-fuse etc, or even a
Buck/Boost DC/DC converter?) into the old battery pack, and get the
longest life from a 12V source you can.

Just an idea....

Dave B. G0WBX

> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org 
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Ron
> Sent: Sunday, August 28, 2005 2:38 PM
> To: TAPR APRS Special Interest Group
> Subject: [aprssig] Power For a Lap Top
> Just wondering of any precaution when operating a Lap Top 
> (Compaq Armada
> 3500) from a 12 VDC to 110 VAC inverter.  I heard of a case 
> where someone tried a cell phone charger only to fry the 
> power supply.  I question that since the cell PS is low 
> current draw and not much iron in the transformer that spikes 
> may have played a roll in this accident or may be it was just 
> that time for the transformer to go?  Thanks for your thoughts.
> Ron 

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