Safety issues when sensing AC voltage

[Marco]

Is there a standard warning label for circuit boards to indicate any part of the circuit is likely to be live?

[Terry Bites]

Something like this
You can bias up the attenuated AC to keep it in range for your ADC.
Most resistors are rated for a max of 250V so you need a string to protect against a short. A series cap limits the current in the chain if all else fails.
A Zener, a MOV, a fuse and a Class X cap will protect you from most nastys.
Always check for a single fault condition that will either kill you or your hardware.

[wraper]

Something like this
You can bias up the attenuated AC to keep it in range for your ADC.

You don't want to place a zener diode directly on the signal though as it will affect the measurement due to leakage current below breakdown voltage or at least it should have quite a bit of headroom above actual voltage. 3.3V zener is certainly not suitable for 2.5V signal if you look into datasheet. 10n Cx is completely counterproductive here, what it will do the best is screwing up waveform and passing fast transients right through. 20mA fuse has no meaningful function too.

[David Hess]

There is nothing wrong with the circuit or idea if it is implemented properly.

Are there special safety considerations also on the component selection? E.g. the 300K resistor?

The series dropping resistor should be a high voltage and fusible type.  Better would be to use two resistors in series so one failing would not cause excessive current.

[wraper]

high voltage and fusible type.  Better would be to use two resistors in series so one failing would not cause excessive current.

Wrong, there is no current possible that would ever trip fusible resistor. Although resistors extremely rarely (like almost never) fail with some resistance reduction, they never fail short. Not to say fusible resistors do not even exist with such resistance values to begin with.

[Siwastaja]

Wrong, there is no current possible that would ever trip fusible resistor. Although resistors extremely rarely (like almost never) fail with some resistance reduction, they never fail short. Not to say fusible resistors do not even exist with such resistance values to begin with.

This. Fusible resistors are used when small resistance values are needed for current limiting (e.g. cheap-than-NTC-ass inrush limiting); they basically combine resistance and fuse in one package, fuse part of which sucks compared to a real fuse, but is just barely acceptable and offers lowest price and smallest size.

OP simply needs to decide between a high voltage rated part, or putting more easily second-sourceable standard parts in series. Enough of them, assuming worst-case voltage rating, which for resistors (ignoring the absolute smallest packages) is 200V. Sometimes designers specify a physically larger part, by specifying higher power rating than they actually need (and getting larger body as a side effect), because they are often rated to 300V or even more, in which case two in series would (probably, maybe) suffice.

But legislation is different everywhere and so is grid quality, and if your TRIAC or whatever dies anyway after some peak voltage, it makes little sense to make the resistor divider better than that. Lightning strikes cause overvoltage spikes on countryside, which may be or may not be a huge problem.

[PCB.Wiz]

Hello guys,
I was asked to upgrade an AVR (AC voltage regulator) because it is using a very old freescale microcontroler wich is hard to find out, besides more expensive.
This AVR is very cheap, to make it affordable, so I have been told, new design can not go further than 10% above of the current production price.
The way it senses the AC voltage is attached. It is basically and half wave rectifier and resistor divider, then the signal go to the ADC pin. Here,  Neutral and micro controller GND are connected. I know, for safety it is better to have microcontroler gnd isolated from mains, but in this case I see no problem because people is not going handle the circuit. The pcb is going inside a box. Right?

Yup, if everything is 'hot' there is no need to isolate mains sense.
This is measuring  the voltage, you do not need to be as low as 330k, which is 160mW at 230V.
You did not mention SMD, but with new designs in SMD, it is common to use resistors in series to spread the voltage, and heat. Surge rated parts are better.

For replacement MCU, almost any small 8b (or better) MCU with enough pins, can manage measure mains and wiggle outputs decisions.