Smoothing 'pseudo-sine' wave inverter output

Discussion in 'General Electronics Chat' started by Junkman, Feb 9, 2009.

  1. Junkman

    Thread Starter New Member

    Jan 7, 2009
    My first post, after much reading. If I do it wrong, please forgive a noob.
    I have an inverter, 12v dc (nominal, actual more like 13.8vdc) to 120vac-ish.
    I am aware that a 'stepped' approximation of 120 vac isn't quite the same thing as a real sine wave as I get from the local power company. I am also aware that motors and transformers don't 'like' the stepped square wave output' of this inverter nearly as well as a real sine wave, running slower, (a fan) and hotter, (both) with much lower efficiency. When running on batteries, efficiency is important!
    What I am wondering, as I have an abundance of transformers of various types, is can I rewind one or more to use as an inductor that would smooth out the stepped square wave? If so, what aprox. would I need? I know, too open of a question. I have in mind either a microwave oven transformer, (MOT) or possibly a sodium vapor light transformer that could be rewound. (I also have several older, big transformers intended for florescent lighting, as well as mercury vapor lighting. Most of the florescent units are potted, so I discount them as a source of rewinding stock) I have rewound transformers many times, so this part doesn't scare me. But I need to know a starting point, as to what to shoot for, winding wise, transformer size wise, wire size and type, etc. And, mostly, is it even worth the effort? Has anyone ever tried this, and are the losses in any inductor worth the potential gain, assuming it even works at all? If an inductor is not a viable choice, does anyone know of another method? I also have a lot of high voltage AC caps, like motor start, and microwave rectifier caps. Any of these workable?
    Thanks in advance. I have searched around, and could not find this subject discussed anywhere, so if this has been dealt with, I apologize in advance.
    Last edited by a moderator: Sep 13, 2009
  2. S_lannan

    Active Member

    Jun 20, 2007
    the main problem here is in the spectrum of the square wave you are filtering the fundamental you want (50/60 hz) is at an amplitude of much less than 120v.

    The harmonics make a substantial amount of the energy in this waveform and filtering them out would reject a lot of power in the original signal.
  3. PRS

    Well-Known Member

    Aug 24, 2008
    An inductor in parallel with a capacitor tuned to 50 or 60 Hz might be the answer. Can't hurt to try.
    Last edited by a moderator: Sep 13, 2009
  4. mikeflood

    New Member

    Sep 13, 2009
    A one to one isolation transformer will smooth out almost all the wrinkkles but the "wrinkles" contain power and the transformer will get warm - not likely a probelem except that you are giving up electrical energy for heat. You will lose efficiency.

    Be generous in your sizing of the transformer - a 1000W inverter should have a 1.5kVA transformer to keep it cool enough.

    You may not like the cost of this option, either.
  5. Audioguru


    Dec 20, 2007
    An inductor as a lowpass filter has resistance that will reduce the output voltage of the inverter too much.

    A pure sine-wave inverter does not start with a stepped square-wave then filter it. Instead it creates a pure sine-wave.

    Your inverter with a stepped square-wave is half-way between a very cheap pure square-wave and a very expensive pure sine-wave inverter.