Inverter Question Please
Bob Newman
& charge my laptop in a car? Is there anything else I should know about it?
--
Thanks in advance... Bob
;-p
no problem.
JEP
"Bob Newman" <bobn...@att.net> wrote in message
news:hUpMc.8792$_K2.2455@lakeread02...
Barry Watzman
laptop power pack is rated 15 volts at 6 amps, that's 90 watts). Then
double it to get a conservative inverter requirement, e.g. 180 watts in
this example. Neither the inverter nor the laptop power pack is 100%
efficient, the 90 watts is the power pack's OUTPUT power, it's input
power requirement will be significantly higher (the typical efficiency
is 60% to 80%, but will vary).
However, personally, I'd get a 300 watt inverter, which is about the
limit of what you can draw from a car cigarette lighter. You may want
to use it for other things, but you can't get much more than about 300
watts (if that) without direct connection to the car battery.
Jason Cothran
"Barry Watzman" <Watzma...@neo.rr.com> wrote in message
news:410276FD...@neo.rr.com...
| Take the power required by the laptop (volt x amps; that is, if the
| laptop power pack is rated 15 volts at 6 amps, that's 90 watts). Then
| double it to get a conservative inverter requirement, e.g. 180 watts in
| this example. Neither the inverter nor the laptop power pack is 100%
| efficient, the 90 watts is the power pack's OUTPUT power, it's input
| power requirement will be significantly higher (the typical efficiency
| is 60% to 80%, but will vary).
|
| However, personally, I'd get a 300 watt inverter, which is about the
| limit of what you can draw from a car cigarette lighter. You may want
| to use it for other things, but you can't get much more than about 300
| watts (if that) without direct connection to the car battery.
|
Most car lighter plugs have 15amp fuses. At 12 volts (car off) that would be
about 180 watts. At 14 volts (car on) that would be about 210. At any rate,
if OP gets an inverter rated equal to that of his power brick (continuous,
not peak), he will be fine. FWIW, Walmart has one that is 150+/- continuous
for less than $40. I can vouch for it's ability to power and charge an
m6809.
Bob Newman
cheap (under $25.00) would I be correct in assuming there is a catch
someplace?
Bob
"Jason Cothran" <re...@board.nomail> wrote in message
news:LgvMc.12019$%S4....@bignews1.bellsouth.net...
Jason Cothran
| I don't normally shop there but I see several new inverters on ebay very
| cheap (under $25.00) would I be correct in assuming there is a catch
| someplace?
|
Probably not since they have the good one for cheap at walmart. Just make
sure to check the manufacturers site for continuous power and not peak.
Bob Newman
"Jason Cothran" <re...@board.nomail> wrote in message
news:i2xMc.22319$yF.1...@bignews2.bellsouth.net...
Barry Watzman
But, in any case, this part of your post is incorrect: "if OP gets an
inverter rated equal to that of his power brick (continuous, not peak),
he will be fine"
Going to my example, a 15 volt @ 6 amp inverter, that is the output
power of the inverter (90 watts). However, if the inverter is 75%
efficient, then it would require 120 watts of INPUT power, so a 90 watt
inverter would not, necessarily, be adequte (you might get away with it,
but that's another matter entirely).
Note, a power supply (or inverter) that was 100% efficient would not get
"warm" at all. The heat that you feel when you hold an inverter is an
indirect indication of it's "inefficiency".
Barry Watzman
waveform. The best output wave form is a sine-wave, the worst is a
square wave, and most actual products use a stepped approximation to a
sine wave. Truth is, most laptops (and other devices that use switching
power supplies) don't care much. But there's a lot of unknowns in all
of this (including the fact that even the owners of most inverters don't
have any idea what the waveform is, and would not understand the
implications even if they were told).
Barry Watzman
Going to my example, a 15 volt @ 6 amp laptop power supply, that is the
output power of the laptop supply (90 watts). However, if the laptop
supply is 75% efficient, then it would require 120 watts of INPUT power,
so a 90 watt inverter would not, necessarily, be adequte (you might get
away with it, but that's another matter entirely).
Jason Cothran
| My car has 30 amp fuse for the cig. ligher.
|
| But, in any case, this part of your post is incorrect: "if OP gets an
| inverter rated equal to that of his power brick (continuous, not peak),
| he will be fine"
|
| Going to my example, a 15 volt @ 6 amp inverter, that is the output
| power of the inverter (90 watts). However, if the inverter is 75%
| efficient, then it would require 120 watts of INPUT power, so a 90 watt
| inverter would not, necessarily, be adequte (you might get away with it,
| but that's another matter entirely).
|
| Note, a power supply (or inverter) that was 100% efficient would not get
| "warm" at all. The heat that you feel when you hold an inverter is an
| indirect indication of it's "inefficiency".
|
|
The continuous rating on the inverters are not given "at 100% efficiency".
Thats a good chunk of what the Peak rating is. A 120w inverter is capable
of putting out 120w continuous power assumming you are providing the assumed
input power. A power inverter designed and rated at 120w continuous is what
it will provide in real world circumstances. "Peak" is what it is capable of
in short bursts, not sustained largely due to inefficiencies.
Jason Cothran
| Let me correct that:
|
| Going to my example, a 15 volt @ 6 amp laptop power supply, that is the
| output power of the laptop supply (90 watts). However, if the laptop
| supply is 75% efficient, then it would require 120 watts of INPUT power,
| so a 90 watt inverter would not, necessarily, be adequte (you might get
| away with it, but that's another matter entirely).
|
Exactly, but if it required 120w of input power, then your power brick
should already be that to start with. That is why I recomended OP to buy one
rated the same as his power brick. The case you are arguing would be good if
you knew the consumption of the laptop, but needed to know how much input
power you need. Given the supplied power brick, you already know how much
input power is sufficient for running while charging.
Barry Watzman
Efficiency is the % of input power that appears as output power, e.g.
the inverter consumes 120 watts from it's source but supplies only 90
watts to it's load (the other 30 watts is disappated as heat).
Peak vs. continuous is another matter entirely, and usually has to do
with how much heat the inverter can dissipate before something fails.
However, for an inverter with a 75-watt continuous rating and a 100 watt
peak rating, the efficiency won't necessarily vary much (and is probably
a bit higher at the peak load than at the continuous load).
Barry Watzman
make this point clear: by "laptop power supply", I meant the power
brick, NOT the power supply WITHIN the laptop.
The rating of the "power brick" that I was talking about is the OUTPUT
of the "Power brick" (supplied to the laptop), but the input to the
power brick is HIGHER than that (e.g. it's output divided by it's
efficiency (expressed as a decimal fraction less than 1.00).
jimbo
But another question. Aren't there devices specific to a laptop, that
simply takes auto DC and conditions it to what ever the laptop
requires? Doesn't that essentially eliminate the efficiency issue?
Maybe doesn't apply here if there aren't any such devices for that
specific laptop.
jimbo
Barry Watzman
supply), be it an inverter (auto cigarette lighter to something like 110
volt AC wall power), the external laptop power supply ("power brick") or
the internal switching power supply within the laptop -- they all take
in more power from their "source" than they provide to their load. The
ratio of total output power to total input power, always less than 1.00
(or 100%) is the "efficiency" of the power converer. Typically it's 60%
to 80%, but I've seen 40% and low 90%'s exist also (rare, but they do
exist). The power that is lost is converted to heat, and it's usually
more than enough to be felt (cerainly on the devices inside, if not on
the external case).
As to model specificity, well, yes and no.
Inverters -- that make "wall power" (or an acceptable facsimile) from a
DC power source (the car battery) are not specific.
The external laptop power supplies ("power bricks") vary in how model
specific they are. If they are "just" power converters with a "2-wire"
output, they are only specific to the following extent:
-Voltage
-Maximum available curren (and, by implication, power)
-connector type
-connector polarity
That leaves a LOT of ground, especially since the connectors can be
changed ("adapt-a-plug"), and on the "universal" models, even the
voltage can be changed (and in at least some models, the current
limiting can be changed). So the degree of specificity varies. An in
some cases it's almost totally generic, for example the Toshiba models
that use 15 volt 2-wire power supplies with standard coaxial plugs.
However, some of these (external laptop power supplies, "Power Bricks")
have part of the charging circuit in them, or they supply not one
voltage but several, and they do it through highly proprietary multi-pin
connectors. Now we have gotten far more specific. However, the laptop
makers don't want a different power adapter for every model any more
than we do (it gets very expensive for them, too), so typically even a
highly specific external "power brick" will work with at least several
models of laptops (from the same manufacturer).
And, in any case, there is usually both an AC-powered external supply
(plugs into the wall) and a DC-powered one (for auto/air use). On top
of which, you have the option of using the AC-powered supply with an
inverter.
Lots of choices.
None of which is 100% efficient, and when you start stacking 60%
efficient devices (inverter to laptop "power brick" to the switching
supply inside the laptop) you can find that only 20% of the total power
being used is actually being used by the device being powered.
Jason Cothran
| Efficiency and peak vs. continuos ratings are not related.
|
| Efficiency is the % of input power that appears as output power, e.g.
| the inverter consumes 120 watts from it's source but supplies only 90
| watts to it's load (the other 30 watts is disappated as heat).
Then that would be a rated as a 90watt inverter.
|
| Peak vs. continuous is another matter entirely, and usually has to do
| with how much heat the inverter can dissipate before something fails.
| However, for an inverter with a 75-watt continuous rating and a 100 watt
| peak rating, the efficiency won't necessarily vary much (and is probably
| a bit higher at the peak load than at the continuous load).
|
|
Correct.
Jason Cothran
| No, it's the POWER BRICK that we are talking about. Perhaps I did not
| make this point clear: by "laptop power supply", I meant the power
| brick, NOT the power supply WITHIN the laptop.
|
| The rating of the "power brick" that I was talking about is the OUTPUT
| of the "Power brick" (supplied to the laptop), but the input to the
| power brick is HIGHER than that (e.g. it's output divided by it's
| efficiency (expressed as a decimal fraction less than 1.00).
|
|
Now you are making sense. thought when you said power supply you meant
power supply. My stupidity.
no_spam_here
So the answer is that a 150 watt inverter will run his laptop well.
jimbo
the "power brick" can take DC input doesn't that eliminate the loss in
an inverter?
jimbo
Barry Watzman
No.
Consider an "Auto/Air adapter", it's DC-to-DC, taking in DC power from
the vehicle electrical system (nominally 12 to 15 volts) and producing
whatever the laptop needs (usually 15 to 24 volts). The total power
drawn from the vehicle (volts times amps) will be greater than the power
delivered to the load (the laptop), and is disipated as heat (which can
usually be felt). It's efficiency is in the same range as that of an AC
powered "power brick".
There is, in fact, very little difference between an AC and a DC powered
power brick. They are both just switch-mode, electronically regulated
power supplies. In fact, the AC powered device simply converts the
incomming AC line to DC anyway (full-wave rectifier and filter
capacitor). It's then effectively a DC powered device, although it's DC
voltage is "high" (well over 100 volts) while the DC voltage of an
"auto/air adapter" is low. The only consequence of the difference in
the input voltage is that the transformer turns ratios will be quite
different. The principles and method of operations are identical, but
the "auto/air" adapter doesn't need the incoming AC-to-DC conversion
(which is just a full-wave bridge rectifier and capacitor).
Dorothy Bradbury
o Not all car "lighter sockets" are high amperage
o Do not go by the fuse rating - that's against lighter element shorting out
o Check on the cable size - relative to your full-beam lighting cable
Very often the lighter-socket cable is undersized:
o It is only supposed to light a cigarette lighter for a duty cycle of seconds
o You may wish to run an invertor on the thing for thousands of seconds
Car makers will cut pennies off & weight off anywhere they can.
Then sell you a separate "accessory socket & harness" for silly money.
If in doubt:
o Buy a decent cigarette lighter socket - they vary in quality & amperage
o Run a 30A rated red cable back to a FUSE near the battery
o Run a 30A rated black cable to the nearest low-resistance ground
Electrical accessories connected to cigarette lighters have caused many fires.
A car body is grounded, as the cable overheats so it either a) shorts to a high
resistance ground (heat) or b) melts other cables nearby which short out also.
No, the fuse will not necessarily blow - the load can be over several fuses.
Enough TSBs on car electrical systems as it is, from unfused fuel pumps to
abrasion causing fires - German or Japanese, doesn't matter if it's 100k$.
Cars with dedicated "accessory points" have higher duty wiring for it.
--
Dorothy Bradbury