I am just starting to collect parts to build a House power usage system.
I find that there three common methods to convert and read the 120 vac with regular analog input.
A. Step-down transformer (9vac) to voltage splitter and full sign wave placed at + 2.5 v offset. Advantage - able to read full wave.
B. Step-down transformer to full wave bridge, cap to average, voltage splitter and 5.1 v zener to protect.
advantage -- average RMS DC delivered.
C. Step-down transformer to one diode, voltage splitter. Measure + peak only. Advantage low part count. easy arithmetic to account for .2 v diode drop and 1/2 wave.
I don't see how measuring the voltage tells you anything about usage. So you have 120 V. so what ?
How does that tell you about usage ?
Where were you planning to connect your monitoring circuit ?
Yes, you are correct,
good details that match my listed 'A' method. Good to know about the phase error rates.
I will have to search to see if like devices are available for 120v mains. At my location I will need two.
Imbalance in the split phase service is common problem here, Costa Rica.
raschemmel:
I don't see how measuring the voltage tells you anything about usage. So you have 120 V. so what ?
How does that tell you about usage ?
Where were you planning to connect your monitoring circuit ?
Yes, I will also have two current transformers at the service entry point on the split phase service. That is an additional issue large enough to have a unique thread.
The bottom line of my question: Is the peek level of just the + side of the AC sufficient for this application or do I need to measure the full sign wave? I can compute the RMS from either but are there errors introduced when I move to a power computation with the current measurements?