I am considering, among other things, a 12VDC to 110VAC inverter. My
bare minimum need for this is around 1 kilowatt with occasional peak use
of at least 1.5 kW. For the purpose of this item, you can assume that I
have the means to feed the device. I'm interested here in the 12VDC -->
110VAC step itself.

    A couple of options present themselves:

* BUY.
    Inverters are, currently, readily available at a few places. Target
has a 350 watt, 600 watt surge unit on sale for $48.88, which works out
to around $140 a kilowatt - competetive even with the cheeeeper gasoline
generators. I'd have bought three of them if I thought it would do me any
good. But there was only one unit left, and I need that kilowatt on one
plug.

    I found a "1000 watt" unit at a marine supply place, that was really
only 800 watts; they wanted $300 for it. Also, Murray's Auto has 1 kW/2kW
surge units for, I think, $399.99. I think you have to special order those,
though.

    Given, say, the 300W version, is it possible to modify one for more
power? What about combining several of them? The big problem as far as I
can see is getting the waves in sync.

* BUILD.
    I found plans for one on the web - a dual-side oscillator with identical
circuits going to either half of a center-tapped transformer. The schematic
is easy to follow; and most of the parts look common enough to buy off the
shelf. However, for my purposes, the NPN transistors would have to be capable
of carrying 100 amps for extended periods, as would the primary transformer
coil.

    The web page recommends getting the transformer out of an old microwave
oven, as these are rated at around a kilowatt. You'd remove the 2000V
secondary wiring and wrap a few turns of thick wire around what's left,
putting in a loop for a center tap.

    But what about the transistors? Radio Shack only has them up to around
15 amps or so. What would a 100 amp transistor look like? What would they
cost? And who would sell them? I don't suppose paralleling ten 10-amp
transistors to serve as a 100 amp one would work...?


    Any help here?

13 responses total.

Inverters are getting very cheap these days.  If you build, use a FET
not a transistor.  Finding a FET capable of 100 A is no big deal, plus
they can be paralleled, without series current limiting resistors, to
spread the load among several devices.  Microwave transformer?  Possible.
Are those 12 volts and center tapped? Your other problem will be pushing
a square wave through a transformer designed for a sine wave.  They
get very lossy at frequencies much higher than 100 Hz.  60 Hz square
waves contain lots of frequencies above 60 Hz (Else they wouldn't be
square.)  Forget about building a sine wave oscillator.  You would
loose lots of power in the transistors / FETs if they aren't ON or OFF
aw they would be in a square wave oscillator.  (So-called "modified sine
wave inverters are really just square wave inveters.  True sine wave
inverters use a transformers with many taps so the output has steps and
is more of a synthesized sine wave, though very low THD when done right.

I suggest you buy.  A home-brew 1KW inverter is no trivial thing.  What
kind you get depends on the load you want to drive.  (i.e. modified sine
wave inverters have a documented history of blowing out laser printers.)
If you wind up needing a real sine wave, the price about doubles.

I think I have a working 1 to 1.5 KW modified sine wave inverter that
you can have.  It's 48 vdc in, however.

That's a bit much on the input - unless there's a way to quadruple an input
voltage? (I am not likely to have *four* separate 12 volt sources to chain
together.)

I'd just as soon buy myself. But where's a good place to look for a good deal?

(Again, I'd buy a bunch of 300W units if I thought it'd do me any good.)

Maybe I can pump 12 volts into your inverter, get 30 VAC out, and run that
through a 4 to 1 step-up?

Heh.

Linking together a bunch of 300W inverters would be difficult for a couple
of reasons:  First would be power distribution.  If one inverter puts out
slightly more voltage than the others, it will take on the lions share of the
load.  Secondly you would have to sync all of the units to a master oscillator
or they will drift with respect to one another and that will cause trouble.

You need to feed my inverter 48 volts.  1000W input from 48 volts is only
about 21 amps while 1000 W input from 12 volts is about 84 amps.  Wire size
and component selection / cost has a lot to do with your input voltage.

Found a 500 watt unit at Walmart for $105 - coming in at $210 per kilowatt,
comparable to the piss-poor ones at Tar (Honda generators, when I checked
recently, weighed in at around $300/kW.) Still too weak though - I really
*should* have at least 1.5 kW; I'm figuring I can squeak by on 1.0.

Places eliminated as a source: Target, Meijer, Best Buy, Pep Boy's, Auto Zone.
Murray's has 'em, a bit on the expensive side; as does a boat supply
(watercraft) store - almost. So, where best to look?

I'll analyse the circuit I got off the web with Spice to see how it works
out, if I can get Spice figured out.

Trace makes some really nice inverters.  If you can get hold of a copy of
_Home_Power_ magazine you will find ads for other inverter manufactures
too.  You can also download their most recent magazine from http://www.
homepower.com/  (This is the best source of information for anyone 
interested in solar, wind, water power and related topics on a private
scale.)

I now have possession of the following:

* Whistler PP1500AC Inverter from Murray's on Van Born, which for some
  reason had it priced at $330 instead of the usual $400, weighing in
  at 22 cents a watt, not much more than Target's price.

  Effectively Whistler's most powerful model, it's rated at 1500 watts
  continuous, 1800 watts for 20 minutes, and 3000 watts surge. That's
  2 horsepower, 2.4 HP for 20min, and 4 HP surge, which should be enough
  to run any single piece of equipment that I have. (Whistler makes a
  model PP2500AC, but the outlets are circuit-breakered for 15 amps max.)

* Duralast 95 amp-hour deep-cycle marine battery, soon to be fully charged.

* Three feet of #4 guage wire, per the instructions (two conductors, red
  and black insulation).

* battery cable connectors.


Preliminary tests having shown promising results, it's time to move to
the next step: feeding the inverter on an extended basis. What I would like
to do is equip one or more vehicles with an appropriate socket, and the
inverter-battery combination with a matching plug, that I may quickly and
easily plug it in to provide continuous power and recharging, and unplug it
easily when I'm done using it.

Therefore I'm now looking for a place where I can get plug and socket type
connectors appropriate for 12 volts and around 200 to 300 amps, along with
#0 guage (more often called "one-ought") wire, that I may use longer wires
as well as have increased margin and wire-ends that better fit the input
sockets on the inverter.

I've called a handful of electrical supply places around my area. While I
can get the wire at most of them, sometimes at a reasonable price, none of
them carry connectors of the capacity that I need. For that matter, they
barely seem to even know what I'm talking about.

What exactly would a 12 volt 300 amp plug/socket look like? Who sells such
a thing? Is there another way to do this? Also, I really would prefer to
get the wire in at least two colors of insulation, so as to more easily
keep track of which is positive. (I suppose I could rig something with
multiple 220V plugs and sockets; but I'd rather do this right if at all
feasible.)

So, where should I shop?

Truck/fleet stores?  12v stuff there.  I once talked to a concert sound guy
who swore by semi-truck connectors for speakers.

Or try marine supply places.  Boats do a lot of 12v stuff as well.

Solar power catalogs?  RV stores?

Welding supplies - there is a store for these on Carpenter.

Good one, Rane!

I've seen such connectors.  They are rectangular affairs and both ends
are identical.  They are very compact and simple in design.  They are
common in electric fork lifts, electric golf carts, etc.

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