Posted by Traveler on December 23, 02 at 21:14:34:
In Reply to: Power Inverters. Do they really work???? posted by Bill in Ma on December 23, 02 at 10:56:35:
Some inverters say modified sine wave and it's really nothing but a square wave with offsets. True costs more but is worth it.
That said I have a cheap mod sine from Sam's 400W. It requires a direct battery connection with less than 4 feet of wire.
High currents! 400W from 120V is only 3.33 A but drawn from 12V it's getting close to 40A when you factor in the efficiency.
About the sine wave... A transformer will respond to a sine wave within a certain range of frequencies, and beyond that it will resist and absorb current. A true sine wave is basically very effective when feeding into a transformer or inductive load.
Any waveform that deviates from a sine wave is composed of a sum of various frequencies of sine waves (Fourier synthesis). A square wave for example is an infinite sum of odd multiples of the fundamental frequency with decreasing amplitudes as the frequency gets higher. Those high frequencies cannot pass through the transformer and cause it to heat. Power supplies without transformers can have trouble handling non-sine as well.