I’m no fan of power inverters. In my experience they’re horrifically inefficient, have power appetites that make engine starter motors look like electric toothbrushes & reduce the life expectancy of lead-acid batteries to no more than a few days.
However I have decided to do a little analysis on a cheapo “600W” model that Maplin Electronics sells.
After a serious amount of metallic abuse, the bottom cover eventually came off. The sheet of steel used to close the bottom of the aluminium extrusion was wedged into place with what was probably a 10 ton hydraulic press.
As can be seen from the PCB, there’s no massive 50Hz power transformer, but a pair of high frequency switching transformers. Obviously this is to lighten the weight & the cost of the magnetics, but it does nothing for the quality of the AC output waveform.
The 12v DC from the battery comes in on very heavy 8-gauge cables, this device is fused at 75A!
Here’s the fusing arrangement on the DC input stage, just 3 standard blade-type automotive fuses. Interestingly, these are very difficult to get at without a large hammer & some swearing, so I imagine if the user manages to blow these Maplin just expect the device to be thrown out.
On the input side, the DC is switched into the pair of transformers to create a bipolar high voltage DC supply.
The large rectifier diodes on the outputs of the transformers feed into the 400v 100µF smoothing capacitors.
As mains AC is obviously a bipolar waveform, I’m guessing this is generating a ±150v DC supply.
After the high voltage is rectified & smoothed, it’s switched through 4 more MOSFETs on the other side of the PCB to create the main AC output.
The label states this is a modified-sine output, so I’d expect something on the scope that looks like this:
Modified-sine doesn’t look as bad as just a pure square output, but I suspect it’s a little hard on inductive loads & rectifiers.
However, after connecting the scope, here’s the actual waveform:
It’s horrific. It’s not even symmetrical. There isn’t even a true “neutral” either. The same waveform (in antiphase) is on the other mains socket terminal. This gives an RMS output voltage of 284v. Needless to say I didn’t try it under load, as I don’t possess anything I don’t mind destroying. (This is when incandescent lamps are *really* useful. Bloody EU ;)).
About the only thing that it’s accurate at reproducing is the 50Hz output, which it does pretty damn well.
As is usual these days, the whole system is controlled via a microcontroller.
