While we’re 100 years past Edison’s fear, uncertainty, and doubt campaign, the fact of the matter is that DC is a bit easier to wrap one’s head around. It’s just so honest in its directness. AC, though? It can be a little shifty, and that results in some unexpected behaviors, as seen in this video from [The Action Lab].
He starts off with a very relatable observation: have you ever noticed that when you plug in a pickle, only half of it lights up? What’s up with that? Well, it’s related to the asymmetry he sees on his Jacobs ladder that has one side grow hotter than the other. In fact, it goes back to something welders who use DC know about well: the Debye sheath.
The arc of a welder, or a Jacobs ladder, or a pickle lamp is a plasma: ions and free electrons. Whichever electrode has negative is going to repel the plasma’s electrons, resulting in a sheath of positive charge around it. This positively-charged ions in the Debye sheath are going to accelerate into the anode, and voila! Heating. That’s why it matters which way the current goes when you’re welding.
With DC, that makes sense. In AC, well — one side starts as negatively charged, and that’s all it takes. It heats preferentially by creating a temporary Debye sheath. The hotter electrode is going to preferentially give off electrons compared to its colder twin — which amplifies the effect every time it swings back to negative. It seems like there’s no way to get a pure AC waveform across a plasma; there’s a positive feedback loop at whatever electrode starts negative that wants to introduce a DC bias. That’s most dramatically demonstrated with a pickle: it lights up on the preferentially heated side, showing the DC bias. Technically, that makes the infamous electric pickle a diode. We suspect the same thing would happen in a hot dog, which gives us the idea for the tastiest bridge rectifier. Nobody tell OSHA.
[The Action Lab] explains in more detail in his video, and demonstrates with ring-shaped electrode how geometry can introduce its own bias. For those of us who spend most of our time slinging solder in low-voltage DC applications, this sort of thing is fascinating. It might be old hat to others here; if the science of a plain Jacobs ladder no longer excites you, maybe you’d find it more electrifying built into a blade.
Continue reading “Pickle Diodes, Asymmetric Jacobs Ladders, And Other AC Surprises”
