Hello. I am trying to build device like emontx but with ESP8266 so directly communicating via wifi. And I am interested in idea of just one transformer to provide power as well as voltage measurement source. Specifically in design with two diodes rectification and ground from centre tapped transformer as described on https://learn.openenergymonitor.org/electricity-monitoring/voltage-sensing/why-cant-i-use-a-single-transformer . But I am afraid that mentioned design is too much tied to specifics of atmega because
- suggested 120 uF capacitor is probably too small to safely hold as regulator source to handle any larger load (which ESP8266 can easily require)
- my bigger question is common to all emonTX design where AC input is biased to DC usually via resistor divider 470k + 470k backed by 10 uF capacitor. Unless that capacitor is fully charged there is not expected DC bias and attached AC voltage then goes nearly full halfwave into negative. Charging 10uF cap through 470k can take quite long. Then all that time are all ADC inputs exposed to negative voltages which is usually bad. Did you think about any solution preventing this? Or just relying on that atmega should be tolerant to -0.5V on inputs and probably can handle even deeper excursion (which under full load on sensor in the moment of powering on can probably happen) shortly as current is limited by relatively not so low impedance on input (which iself is for ADC also not good thing). Unfortunately I am considering using MCP3208 ADC which is not sub zero voltages tolerant and even with adding so callled negative tolerant single ended opamp (LM358) to provide some low pass filter and low impedancy ADC will be still exposed to short times of small negative voltages. Maybe it will survive, but maybe not . Any sugestions? Thank you.