Afroman And The Case Of The Suspect Inverter

If you search the internet for 12 volt to mains AC inverter designs, the chances are you’ll encounter a simple circuit which has become rather ubiquitous. It features a 4047 CMOS astable multivibrator chip driving a pair of MOSFETs in a push-pull configuration which in turn drive a centre-tapped mains transformer in reverse. Not a new design, its variants and antecedents could be found even in those pre-Internet days when circuits came from books on the shelves of your local lending library.

[Afroman], no stranger to these pages, has published a video in which he investigates the 4047 inverter, and draws attention to some of its shortcomings. It is not the circuit’s lack of frequency stability with voltage that worries him, but the high-frequency ringing at the point of the square-wave switching when the device has an inadequate load. This can reach nearly 600 volts peak-to-peak with a 120 volt American transformer, or over a kilovolt if you live somewhere with 230 volt mains. The Internet’s suggested refinement, a capacitor on the output, only made the situation worse. As he remarks, it’s fine for powering a lightbulb, but you wouldn’t want it near your iPhone charger.

If this video achieves anything, it is a lesson to the uninitiated that while simple and popular designs can sometimes be absolute gems it must not be assumed that this is always the case.

We’ve featured surprisingly few mains inverters here at Hackaday. There is this Arduino powered one that uses a PWM drive, but that’s about it. [Afroman] however has provided us with plenty of entertainment. Here he is on the subject of boost converters, for example, or more recently building an FM bug.

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