Good points John.

Typical rated power to the AC adapter for a laptop is close to 150W, but
output to computer is in order of 65W. Explains why the damn things get hot.

Direct power may well be worth investigation, rather than using DC to AC to
DC.

surfnturf

--------------------------------------
"John Weston" wrote:

Watts = Volts x Amps snip
If your laptops are so inefficient as to require the full 300W
of available input power from the inverter then yes, that
equates to at least 25A at 12V. If it requires less power,
then the current drawn from the 12V battery drops to that
necessary to provide the power.

It doesn't matter that the laptop is being supplied at
120/240V from the inverter (and so the output current will be
around 2.5/1.25A), you'll still need more than 25A at the
input to the inverter if it needs the full 300W. If that is
the case, you wouldn't be using a 300W inverter, since they
don't like running at their maximum output for too long.

Typically, with computers, the rating of the power supply is
based on the expected maximum power requirement of the
peripherals and circuits, plus a bit for reliability. What is
actually drawn from the power source depends on what you are
doing with the computer. I'd expect a computer on a boat to
draw a *maximum* of around 100W (depending on its design) so
your cigar-lighter socket should be OK at around 8A. However,
if you ever try to power 3 or 4 working laptops, then your
fuse may blow, if the inverter doesn't quit first...

snip John

John Weston wrote:
surfnturf, in article kcxYf.218601$sa3.159619@pd7tw1no,
says...
Good points John.

Typical rated power to the AC adapter for a laptop is close to 150W,
but output to computer is in order of 65W. Explains why the damn
things get hot.

Direct power may well be worth investigation, rather than using DC
to AC to DC.


A good power supply design shouldn't just dump the excess
power as heat. That's what linear regulators do, not the more
usual switching regulators. However, even they loose
something as heat so, in my opinion, just the one converter,
to provide the laptop's DC input needs directly is best

I use a DC-DC converter (12Vdc battery to approx 18Vdc) to
power my laptop since there is only one conversion loss
(+heat). It seems more efficient to me. Others prefer going
all the way up to 120/240Vac (+heat) to power the laptop AC-DC
power brick (+more heat) which goes back down to the 18-24Vdc
needed by the laptop.

I'm looking at replacing this with a 12V battery driven mini-
ITX when I get a round tuit now my laptop is faulty... I'll be
sailing on a boat with a shuttle-based system this summer so
will see how well it works

The laptop's power brick plus an inverter is far better at dealing with the
voltage "spikes" that an alternator can put out, as I found to my cost a
couple of years ago, blowing the system board using one of these DC-DC
converters.

Look at
:http://www.logicsupply.com/product_i...roducts_id/504

The new dc-dc convertor handle things a lot better now.

Hanz

Dennis Pogson wrote:
John Weston wrote:

surfnturf, in article kcxYf.218601$sa3.159619@pd7tw1no,
says...

Good points John.

Typical rated power to the AC adapter for a laptop is close to 150W,
but output to computer is in order of 65W. Explains why the damn
things get hot.

Direct power may well be worth investigation, rather than using DC
to AC to DC.


A good power supply design shouldn't just dump the excess
power as heat. That's what linear regulators do, not the more
usual switching regulators. However, even they loose
something as heat so, in my opinion, just the one converter,
to provide the laptop's DC input needs directly is best

I use a DC-DC converter (12Vdc battery to approx 18Vdc) to
power my laptop since there is only one conversion loss
(+heat). It seems more efficient to me. Others prefer going
all the way up to 120/240Vac (+heat) to power the laptop AC-DC
power brick (+more heat) which goes back down to the 18-24Vdc
needed by the laptop.

I'm looking at replacing this with a 12V battery driven mini-
ITX when I get a round tuit now my laptop is faulty... I'll be
sailing on a boat with a shuttle-based system this summer so
will see how well it works


The laptop's power brick plus an inverter is far better at dealing with the
voltage "spikes" that an alternator can put out, as I found to my cost a
couple of years ago, blowing the system board using one of these DC-DC
converters.

John Weston wrote:
Dennis Pogson, in article HmLYf.12303$NN4.1853@newsfe7-
win.ntli.net, says...


The laptop's power brick plus an inverter is far better at dealing
with the voltage "spikes" that an alternator can put out, as I found
to my cost a couple of years ago, blowing the system board using one
of these DC-DC converters.

Interesting observation, Dennis. I haven't had this
experience in either boat or car so I'll have to "look into
it". It suggests bad design because why didn't a similar
spike kill or ride-through the Inverter? The "buck
converter" design should have a similar transformer isolating
input and output as that in the inverter - unless it is a
cheap transformerless design... Perhaps the solution is to
get rid of the spike before it enters the electronics with an
additional filter before the input?? At least that would be
less power wasting.

There are probably cheap DC-DC converters which can't handle spikes, and
more expensive ones that can. The ones sold here in the UK for around £12
look altogether too simple to me. The one referenced by Hanz looks a
different matter altogether. I don't think the one bad experience I had is
sufficient to condemn all such devices, but a new system board these days is
as expensive as a new laptop, so my caution is understandable.

Running the laptop without the engine/alternator would probably be the
safest bet, but we all know crew members who will, as soon as the speed
drops below 2 knots, press the starter button!

Dennis.

I've been running VIA system (CL10000,SP13000) for serval years. I've
been going with M1-ATX ,PW-200v,PW80 and now the M2-ATX. I use the power
to run the mainboard and also my LCD monitor and my LCD/TV. The monitor
and tv like only 12v. I power supply provide 12 with 10-24v input.

I'm now looking at Commell LV-675D Pentium M Mini-ITX. The Intel cpu
will be about 2.0 G.(Intel Pentium M760 2.0GHz Processor - 533MHz 2MB
Cache). I use only standard HD and DVD-r/w, not NOTEBOOK drive.

Hanz


John Weston wrote:
Dennis Pogson, in article WJRYf.6695$8o.1068@newsfe6-
win.ntli.net, says...


There are probably cheap DC-DC converters which can't handle spikes, and
more expensive ones that can. The ones sold here in the UK for around £12
look altogether too simple to me. The one referenced by Hanz looks a
different matter altogether. I don't think the one bad experience I had is
sufficient to condemn all such devices, but a new system board these days is
as expensive as a new laptop, so my caution is understandable.

Running the laptop without the engine/alternator would probably be the
safest bet, but we all know crew members who will, as soon as the speed
drops below 2 knots, press the starter button!


Hanz's one is the one I have on my mini-ITX experimental rig:

http://mini-itx.com/store/?c=10#p1830

It's a full power supply for a mini-ITX motherboard, rather
than a simple DC-DC converter for a laptop.

It handles the drop down to 6V whilst the starter runs (but
the house battery doesn't see this - now...). It also handles
up to 28V, clamping any excess. It shuts down everything after
a preset delay so you aren't left with the few milliamps of
drain with a standard inverter or converter.

Your comment about laptop repair is why I'm looking at the ITX
route to use more standardized components.

Typical rated power to the AC adapter for a laptop is close to 150W, but
output to computer is in order of 65W. Explains why the damn things get
hot.

So don't bother with DC from the batteries to AC and then back again. Get a
laptop power supply that has a 12V input. The iJuice (aka iGo) adapters
work well. Then it's just a matter of using a cigarette lighter outlet.

Bill Kearney wrote:
Typical rated power to the AC adapter for a laptop is close to 150W,
but output to computer is in order of 65W. Explains why the damn
things get hot.

So don't bother with DC from the batteries to AC and then back again.
Get a laptop power supply that has a 12V input. The iJuice (aka iGo)
adapters work well. Then it's just a matter of using a cigarette
lighter outlet.

All these little adapters work well on a regulated voltage, but with an
alternator running, the voltage can be far from regulated.