UPS Inverter Waveform Distortion: Wow!

You’ve never seen a “modified sine” inverter before, Will? These are built solely to very cheaply power switchmode power supplies that don’t really care about power quality. All low-end UPSes are like that since you barely even need a feedback circuit to put out usable AC.

They regulate voltage by adjusting the pulse width of that square wave. The higher the load or the lower the battery voltage, the wider the pulse becomes, and the closer to a square wave you get, while the RMS output voltage remains reasonably on point. It’s simply pulse width modulation, except every other pulse goes negative (the comparison diagram on the LTT article is horribly incorrect lol). It’s a perfectly reasonable way of temporarily powering switchmode or transformer based power supplies, but completely unacceptable for anything that spins or uses capacitors straight on the mains.

They become extra fun if you connect white goods to them, since the power supplies in stuff like fridges, wall socket power meters, remote outlets and other small, mains-powered devices like that are of the capacitive dropper type. That type of power supply is entirely dependent on a fairly clean sine wave for regulation, and they’ll literally just go on fire if you hook them to a “modified sine” inverter, lol.