Hallo José,

theoretically, a galvanic connection to mains as you pointed out, is possible. But with your design there are some pit-falls!

1. If mains voltage is specified as 230VAC SINUS, e.g., then this is only a theoretical value. May be mains voltage looks like pure sinus, when it leaves power station. But when travelling from household to household, you will hardly see a real pure sinus any longer. So, methode of simply measuring the amplitude (peak detector) and from this calculating RMS will heavily fail in many many situations.
Only a direct RMS measurement will give reliable results, if you are interested in RMS voltage.

2. Mains voltage can be contaminated with so much interference, like glitches, surges and bursts, that situations of extreme overvoltages must be taken into account!
RIFA, a manufacturer of capacitors, has made some anaylsis some years ago and found out, that occuring overvoltage spikes can be catogarized into four groups:

A = up to 0.1µsec lasting spikes of up to 6kV, from atmospheric interference like lightnings, etc.
B = up to 60µsec lasting spikes of up to 1.2kV, from mistakes in power station or near equipment, damaged fuses, etc.
C = up to 200µsec lasting spikes of up to 800V, from switching loads like motors, contacts, etc.
D = up to 1000µsec lasting spikes of up to 400V, from thyristors, triacs, amplifiers, welding machines, etc.

80% of all spikes last for 1µsec to 10µsec and are up to 1200V high. They happen more often than ten times a day, and their slew rate is high, about 200-2000V/µsec!

Well, everything you connect to mains must safely withstand these spikes!! But a small 1/2Watt resistor will not! It will only withstand a few hundreds of volt!
One remedy is, to exchange 296kOhm resistor by a series circuit of several resistors. But this will increase needed volume. And galvanic connected voltage divider will no longer result in considerably saving of volume.

3. Your methode of direct galvanic connection has the consequence, that everything, which is connected to the divider, will be contaminated with mains voltage! So, you will need to provide double isolation for the whole circuitry!! This can be especially problematic, if you want to read out the stored data via some cable link...
If you use saftey transformer, on the other hand, then double isolation is only needed for the connections to mains voltage. Means, the secondary winding is allowed to be directly touched! So, the whole microcontroller circuitry may also be touched and cable links to outer world do not present any problem.


So, why not using a safety class II transformer (9V/0.35VA), which provides a 5kV isolation (double isolation) between primary and secondary winding?? Volume of such transformer is only 22mm x 23mm x 15mm!!! Connect at output a resistive load and convert secondary voltage to RMS value.


By the way: Safety standards are not made only for commercial purposes!!! If you are an electrical engineer and you fabricate something seriously violating safety standards and somebody touching it is killed, you will directly be sent into prison!! Even in Brazil...

Good luck,
Kai