Two varieties of modified sine wave

Modern modified sine wave inverters come in two varieties.

1. High frequency conversion units which is typical of the design that you will find in inverters that are manufactured overseas in countries like Taiwan and China. 

High frequency units take the incoming 12 Volts DC and will step up that voltage to approximately 200 volts DC through a high frequency DC to DC converter circuit and then will take the 200 Volts and will wave shape it into a modified sine wave using a using a device called a high voltage H-bridge. The high voltage H-bridge is basically a group of field effect transistors that are arranged in such a way as to form the necessary half cycles that create the modified sine wave at the 60 Hz frequency required for US appliances.

By utilizing high frequency, the need for a large iron core output transformer is eliminated and much smaller transformers can be used. As a result of this, high frequency inverters tend to be much lighter but do have a lower surge capacity because they lack the fly wheel effect found in heavy iron core output transformer based inverters. 

The technology was originally developed for the space program where weight is a major consideration. This type of inverter tends to be less expensive to manufacture and are considered a lighter duty type of product than their US made low frequency counterparts.  

2. Low frequency or 60 Hz based conversion units that are typical of US designed inverters. 

As mentioned above these inverters use large heavy iron core transformers that tend to provide a flywheel effect which yields a much greater surge capacity than do high frequency inverters. 

Low frequency units take the incoming 12 Volts DC (direct current) and converts it into AC (alternating current), using a multi-vibrator or microprocessor based circuit. This 12 volts 60 Hertz AC is fed to an iron core step up transformer which converts the 12 volts AC into 120 volts AC . 

Wave shaping and the increased current that is needed to drive the transformer is performed again by an H-bridge which is a group of field effect transistors that are arranged in such a way as to feed high current pulses to the primary windings of the transformer at precise moments of each wave form half cycle. 

The transformer converts the lower voltage which was fed to its primary windings into 120 Volts AC at its secondary windings using simple transformer step up principles involving a 10 to 1 ratio, converting 12 Volts AC to 120 AC. This type of inverter is considered more durable than their high frequency counterparts and have a much higher surge capacity.

You might be asking why anyone would choose a high frequency inverter when low frequency units are more durable ? The answer is cost. 

Low frequency units tend to cost two to five times more than do high frequency units, another consideration is of course the difference in weight. For example a typical 2000 watt high frequency based inverter may weigh 13 Lbs, where a typical low frequency inverter can weigh as much as 50 Lbs.

A modified sine wave looks more like a square wave that has been time shifted in order to produce the energy content and waveform which is found in power produced by the utility company. It's frequency and voltage is rock solid which prevents the brownouts and spikes typically seen with utility company power. Most appliances such as TVs, Lighting, stereos, computers, inkjet printers and power tools run find on modified sine wave power. 

About the only appliances that you may see a problem with are some Laser printers and some of the cheaper types of battery powered tool chargers. In fact 95% of the inverters in RV's today are of the modified sine wave type.

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