AC voltage step down to get its rms without using step down tranformer

[afridii]

Hi all
i need to get RMS voltage of AC main. the specifications are
5oHz and maximum RMS expected is 230volts.
kindly help in detail. i studied a bit using OP AMP but i am not sure of safety for the resistors Ri and Rf
the link is **broken link removed**

kindly help if someone has better idea

 

[Audioguru]

The opamp and microcontroller are both connected directly to the 230VAC which is a deadly danger. Isolation is needed.
Voltage spikes on the 230VAC will destroy an opamp. An input protection circuit is needed.
The input resistor needs to be made to use high voltages.

 

[FvM]

A mains connected measurement circuit will be designed according to a proper overvoltage category. Usual CAT II requires the circuit to widthstand a test voltage of 1500 to 2500 (depending on the applied standard) for 1 min. Using standard 0.5 or 1 W resistors, multiple resistors have to be connected in series.

A dual voltage divider and differential amplifier might be useable instead of galvanic isolation.

 

[afridii]

should i use step down transformer which is safer and ensure isolation ??

 

[Audioguru]

Yes, a transformer is recommended.

 

[FvM]

A transformer is a simple and safe solution. It's however difficult to achieve high accuracy with a transformer. Standard transformers have considerable nonlinearity, a dedicated metering transformer is large and expensive.