Hi, I’ve been reading through the posts and building resources section to test some basic power measurement schematics. My hardware is basically an instrumentation amplifier that converts the stepped down voltage and current to a single ended output which I then feed to my MCU’s ADC (TI’s CC3200, 12-bit ADC, 1.467V max voltage on the ADC pin). I used the Emonlib on Github library and made a few modifications such as hardcoding the SupplyVoltage to 1.467V and set the ADC_BITS to 12. I ran the basic sketches and they work fine, except when the mains AC is switched off, I still see Vrms showing values of around 51. Why is this? Also the power factor is being shown as negative. Here is the link to the data being shown after connecting a 40W incandescent bulb as the load. Any help on this would be highly appreciated.
Sorry, I don’t look at external sites. If your question is likely to be interesting or useful to others at some time in the future, all value is lost when the external site goes away.
My first guess would be stray pickup. Second would be your amplifier is oscillating at some high frequency, third would be digital noise getting into your ADC from the processor.
That can happen if real power is negative, and that happens if the relative phase of voltage and current is wrong according to whatever convention you adopt. Our convention is imported power is positive.