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.