The Modified Sine Wave is usually a sort of square wave where the power isn't on all the time over the 1/60sec. cycle. If you compare the waveform to the true sine wave, you will see that when the sine wave hits about 70% of it's max voltage, the power goes on with the modified sine wave. The power goes off again at the 70% point where the true sine wave is dropping and then turns back on the opposite polarity again for the second half of the cycle at the 70% points again.
This technique gives about the same resistive power and peak voltage (~180V) that a 120V sine wave gives.
FWIW, those inverters aren't that accurate with the 60Hz (and are often quite temp sensitive) but to someone who knows electronics, you can easily add a speed paddle box to control the actual frequency of the output section.
If you have the lightweight ones, there's two sections to the inverter - a 12V - 180V inverter (running at a very high frequency) and then a H bridge switching circuit that drives the output line at the 60Hz. The chip that controls the output driver has a frequency determining circuit in it that can be modified easily. With these designs, I usually go to 10Hz and 120Hz for the two button controls of fast and slow and a pot to control the exact resistance that the chip wants to put out 60Hz.
I might also note that the output is usually fully isolated from the input and the ground pin on the 3 prong connector is tied to the ground of the 12V input.

• Maybe? - walter - February 10, 2002 - 17:07 UTC
  • Inverters - bobmay - February 10, 2002 - 23:52 UTC