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DC-AC inverter what is a modified sine wave?

Discussion in 'Electronic Design' started by RobC, Apr 9, 2004.

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  1. RobC

    RobC Guest


    Could someone describe the difference between the modified sine wave and
    pure sine wave converters and what equipment would be sensitive to each? I
    have been running my phillips 13" tv on a modified sine wave inverter and it
    works perfect, but worry that I could be doing some damage.

    Perhaps I'm wrong, but is a true sine wave inverter just a large bipolar
    amplifier (like in a stereo) fed from a 60hz sinewave?

  2. scada

    scada Guest

    They have to call it a modified sine wave, because it is not derived from a
    true analog wave. The wave is built from many pulses that are generated from
    a microprocessor. These pulses make up the general waveform shape, then they
    are smoothed out by some filtering. If you were to look at the wave on an
    O-scope, it would look pretty clean. These type inverters are fine for
    running TV's and small electronics. Just be sure that the watts rating is
    above your equipment requirement.

  3. Hey, have you ever really looked at the waveform from these things?

    They are not made by "many" pulse at all, just one positive pulse per
    half-cycle and one negative, with spaces inbetween. The pulses are
    about the peak voltage of a sine wave and the spaces are such that the
    RMS value is about the normal line voltage.

    The true sine wave inverters (modern ones) are probably DC switching
    supplies feeding into a half-bridge class-D amplifier to generate the
    sine wave (with some filtering on the output), but I've never
    dissected one.

    Best regards,
    Spehro Pefhany
  4. The commonest versions, use just three levels (this is what you are
    describing), but some designs use five, or even seven levels. The versions
    that go much better than this are then normally referred to as 'synthesised
    sine wave'.
    The problem is that the term is used on hundreds of different systems for
    anything that is closer to a sine wave, than a simple square wave...
    Best Wishes
  5. scada

    scada Guest

    We use "Exeltech" they are one of the better ones. They do in fact work as
    I described. And the outputs are very clean, but for $1 a watt I guess
    that's why!
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