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By theropod
Hey all,

This is a simple alert to provide a heads up to all the forum members. A week ago Friday our Cobra modified sine wave inverter died after 10+ years of faithful service. We immediately ordered an Aims 4000 watt pure sine wave inverter/charger (we knew the Cobra was going to go sooner or later and we're actively researching the issue), but needed a stop gap inverter to provide a small amount of AC power until the Aims was delivered and I could install it. So, off to Home Depot I ran and bought a CAT 1000 watt modified sine wave inverter. Within minutes of powering up the little POS our LG HDTV died. It still has sound but the picture is terminally gone. I suspect the illumination LEDs and or the driver is fried. I have contacted CAT, Home Depot and the importer Baccus Global / and informed them of this issue. I do not expect any of them to take responsibility for the little POS but at least they all know the product has a serious flaw. My intent in posting here is to warn the forum users that these CAT inverters can cost you many multiples of the unit cost in destroyed equipment. Do not waste your money on these!

Oh how I wished I had an oscilloscope to know for sure what the wave form of this POS really is. I suspect a nasty/noisy modified square wave is a more apt description of the output than a modified sine wave. I don't, so I will probably harvest the Fets and fans and chalk this all up to an expensive learning experience. Living off grid can be a real pain sometimes.
By jremington
No surprises there! "Modified sine wave" inverters have been around for a couple of decades, and the waveform is anything but a sine wave. It can damage lots of equipment, even motors! Usually OK for hotplates and other resistance heaters.

It is a cheap way to make an AC waveform that has approximately the same integrated power as a sine wave.

Here is what the waveform looks like, and how it is generated by two cheap MOSFET switches driving a center-tapped transformer.
By theropod
Yeah, I understand the stepped wave form. The problem is not all of these things have the same number of steps in the wave form. The Cobra apparently had a greater number of steps as everything ran OK from its output. Again, my intent on posting this was to share my horrible experience so others might not suffer the same fate. Yes, anger was a parial motivation too.

As an aside my wife was very pleased that the microwave took far less time to do its thing with the Aims inverter. It sure seems to make a difference in that regard.