On Thu, 12 Jul 2007, Rick (W-A-one-R-K-T) wrote:


I have a so-called "modified sine wave" inverter that is connected to 200+
amp hours of deep discharge battery located in about the center of the
house in the basement. About a 30-foot length of #14 wire goes to an
outlet box in the shack, down on one end of the house, and provides 110VAC
(of a sort) when the power is off and I don't feel like hauling out the
generator. :-)

Trouble is, whenever I turn on the inverter it spews harmonics all up and
down the HF band and makes it difficult to impossible to hear weak signals.

I realize that a pure sine wave inverter would mostly alleviate this but
that's not gonna happen any time soon... meanwhile is there a filter I can
apply to the output of the inverter to round off the corners of the
modified sine wave (which is really nothing but a stairstep square wave)
and eliminate the worst of the harmonic noise?

Thanks...



I have several solid state inverters (12-24 vdc to 110 vac) and they all
generate serious hash. Doesn't matter what I run; radio, TV, anything
audio, sounded terrible. Been like this for years and years. Inverters
are nice and higher efficiency and for things like incandescent bulbs,
drills, etc. I even ran a small air conditioner off a large inverter
and 24 vdc out in a shed on land where there was no utility power.

I would like to hear from guys who actually built "filters" and got "nice"
AC, as determined by actual performance and not "the fundamentals say you
can do this."

FWIW, FYI....

What I got recently was an old dynamotor from Fair Radio Sales (Lima,
Ohio) that goes from 24 vdc to 110 vac at 60 cps (yes). Its in their
catalog. Its only 50% efficient and puts out about 300 watts of AC at
about 600 watts DC (24 v at about 24-27 amps) input, but the output is
very clean. I tried several radios and no hash at all !!! (I did not
listen on ham bands with RF gain up, but AM radios, on broadcast band even
between stations with volume turned up were clean).

Running full speed, these things sound like an electric drill. For me, its
going to sit in the garage and I'll run an extension cord into the house
if I need this kind of clean AC. Running no load, the dynamotor ddraws the
full 10-12 amps at 24 volts, and as you increase the load, the DC amps go
up in proportion to the load.

The commutators on these units had oxide on the copper and so you need to
rotate the armateur to start scraping off the oxide, then the thing will
take off under DC and the heavy carbon brushes will completely bring a
shine to the copper.

These dynamotors will turn on 12 volts, too, and put out about maybe 50-65
volts AC at a little slower frequency. 60 watt light bulb lights up to a
dim level.

IIRC, they weigh about 50 lbs, cost about $65-70 now.

Out at the end of my driveway i need to use an electric weed eater and so
I load the dynamotor into the trunk of the car, with two deep cycle marine
batteries, and buzz away. Too far for extension cords, no matter how I do
it. Don't like gas engines, noise, flamables, etc.