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"Improving" a Modified Sine Wave

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Posted by Graham Parkinson on December 28, 2003, 7:37 am
Has anyone experimented with RLC filtering the output from a simple modified
sine wave inverter?

I have to run a "computerized" Makita 24V drill "smart" charger from my
"Statpower Prowatt cigarette lighter" style inverter and was wondering if
the charger will not deal well with the stepped output.

The only other option is to find a small inexpensive portable true sine wave

(We are testing the concrete liner inside an 8km long water supply tunnel at
several places - the tunnel is too long for an extension cord.  I have 3
drill batteries but will have a little "shopping cart" with some big 12V
batteries for other testing gear and can run an inverter off them.


Posted by Nothing40 on December 28, 2003, 3:14 pm

I was wondering about some kind of LC,RC,etc filter myself,just a
couple weeks ago. I wonder if those EMF/RFI filters would do anything?
Does anybody have any info on this? I was thinking of something you
could plug into the inverter,and plug "sensitive" stuff into,like a
"filtered" power strip.
My only other thought was to modify the output stage of the inverter
to output a sine wave,erm,it could be done if ya know what you're
doing,but probably not very easy.

Maybe like a 200uf cap,and 3H choke as a starting point? I dunno.
One other consideration is loss in the filter,and (ermm,whats it
called...it's late.) basically the amount of power the filter can let
through to the load,has to do with the impedance(s) and Q and whatnot.

Any info on something to make the output of "modified sine wave"
inverters more like a real sine wave,and filter those nasty harmonics
would be awesome. ;-)

I'm almost completed with my first 12V input PC power supply
*fingers crossed* I hope it works! It started life as a normal 230W AT

Posted by Nelson on December 28, 2003, 4:59 pm


tunnel at

Graham, "Nothing 40"...
    I'm running a Trace DR inverter in a remote cottage location, and have a
lot of trouble with the RF hash.  Here's my experience.

1)    I put a cap, about .1 uf, across the output.  It reduced the hash a
bit, but not a long time later I had to replace the output FETs.  There may
be no cause/effect connection, but still I suspect the FETs were seeing a
short across the cap for at least the leading edge of the square wave, and
it may have toasted them without kicking a breaker.  So I would definitely
use an inductor input filter, trying it again.

2)    I put up a sizeable building using all screws last summer, and the
main tool was a DeWalt rechargeable drill.  It charged quickly and well, and
never faltered.

3)    There's a 486 computer out at that little log cabin running off the
inverter.  Not a hickup.

4)    Finally resorted to setting up a remote antenna with a shielded cable
back to the cabin for the radios.  At least we can listen when the stations
are on daytime power.


Posted by boB on December 28, 2003, 7:37 pm
 On Sun, 28 Dec 2003 10:59:24 -0600, "Nelson"

If you made the DeWalt charger work with a modified square wave, then
you did very good I would say !  Those usually break when plugged into
one of those kind of inverters...

Good luck on filtering though.   You may be able to reduce the
harmonics, but you will significantly decrease the power level of the
inverter as well.   It just doesn't work too well.  We tried doing
just that years ago while at Trace.  Don't want to discourage you
though.   You just might make it work !
Keep us posted !


Posted by Jeff on December 28, 2003, 11:31 pm

The newer (really light weight) ones run off of a triac (as far as I know),
which uses the firing angle with a AC sine wave to regulate voltage and
current. A modified square wave is not a good wave to try and use a firing
angle method of control. Older ones may have been transformer based, which
would have little problem handling a modified sine wave)

Having designed a small 350 W sine wave inverter, I'm also going to say good
luck on filtering a square/modified sine wave. You can reduce the harmonics,
but to filter it to a sine under all conditions at high power levels is a
dream that's not likely to come true. To get a good, high efficiency sine
wave, under changing conditions, you need to synthesize it with PWM (there
are analog ways to do it, but at a large efficiency loss).

It probably prefers a square/modified wave! It is a switch mode power supply
after all!


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