#NEMA-Style #SplitPhase is a really #cursed electric system.

• Seriously, why does anyone bother with 120V Split-Phase??

https://www.youtube.com/watch?v=u0SsejDCVkU

I am eternally reckoning with trying to understand #Transformers and #SplitPhase in particular. Current progress:

(Leaving out 3-phase.)

There's a high voltage hot and neutral that must never touch, except that if they touch by coiling around a magnetic core, and there are two other wires doing the same on the other side of the core, that's fine.

Not only is it fine, but you can vary amperage/voltage, and you can even stick a third wire there in the middle. The voltage difference between the middle and the edges will be half the difference between the edges.

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Tricking a Smart Meter into Working on the Bench

When the widget you're working on is powered by a battery or a USB charger, running it on the bench is probably pretty safe. But when the object of your reverse-engineering desire is a residential electrical meter, things can get a little dicey.

Not that this elevated danger level has kept [Hash] from exploring the mysteries presented by smart meters. Still, with a desire to make things a little safer, he came up with a neat trick for safely powering electrical meters on the bench. [Hash] found that the internal switch-mode power supply on the meter backplane was easy enough to back-feed with a 12-volt bench supply, rather than supplying the meter with the full 240-volt AC supply it normally gets when plugged into a meter base (these are meters for the North American market, where split-phase 240-volt is the norm for residential connections.) But that wasn't enough for the meter -- it powered up, but stayed in a reset state without fully booting. Something more was needed to bring the meter fully to life.

That something proved to be a small AC signal. Normally, a resistor network divides the 240-volt supply down to about 3 volts, which is used by the sensing circuit in the meter. [Hash] found that injecting a 60-Hz, 600-mV sine wave signal with about a 3-volt DC bias into the sensing circuit was enough to spoof the meter into thinking it's plugged into the meter base. The video below has a walkthrough of the hack, and some nice shots of the insides of the meters he's been working with.

[Hash] has been working with these meters for a while now, and some of the stuff he's learned is pure gold. Be sure to check out his 2021 Remoticon talk on meter hacking for all the fascinating details.

#reverseengineering #ac #residentialmeter #smartmeter #smps #splitphase #spoofing