Reading mains voltage issue... Requesting assistance...

[aq_mishu]

Guys,
I'm looking for a solution to read AC 220 mains voltage. Basically i have around 50 places where i have to read the voltage. Please consider the following:

1) i'm well aware of the mains and isolation issues.
2) I am using a full bridge transformerless psu for uC (mega8). Means N is not grounded.
3) for voltage reading, I just need the rms volt, and i dont need to know 220 or -220 (means a DC measurement is fine)
4) i can not use transformer due to cost and circuit physical dimentioning issues.
5) incase of op amp based reading, i can use 741 or such, with single pwr as 5v only.
6) i can not tie N to gnd as i will use RS232 and that may make trouble to interface with PC.

I already have searched a lot. One i found was a enegrymeter. Bt that will not work for me as N is tied to gnd. Also, i can not use advanced pwr measuring ICs from AD or such as they are not available at market. Also price is an issue. So guys, can somebody help me out?

Also, if i use a cap after live and a resistor of 100ohm, (like transformer less psu), then will it up/down according to change in 220?

I can not make a resistor devider as that will make it real complex, when im scaling 220v down to some 2.2 v. Ideas??

Mishu.

 

[FvM]

At least two points in your post sound confused:

- You say you have a transformerless power supply, in other words your circuit isn't isolated from mains power. O.K. But how do you think to make a serial connection to a PC without isolation?

I can not make a resistor devider as that will make it real complex, when im scaling 220v down to some 2.2 v.

Conversion of 230V to OP/µC level can be either performed by a transformer or resistor divider. I guess, you'll want to make friends with one of the two methods...

 

[crutschow]

If you Google "isolated RS-232" you will find a number of chips that can do that isolation.

How far apart do these 50 measurements need to be taken?

 

[FvM]

Isolated RS232 can be a solution for the problem. But the confusion with point 2) and 6) remains.

To adjust the points from my view:
- the unisolated power supply requires an isolated serial interface in any case
- the bridge rectifier psu design implies that circuit ground can't be tied to N. That's possibly not well considered because it increases the voltage measurement effort. Differential amplifiers are required instead of single ended voltage dividers. There's a reason why usual energy measurement designs are using N connected ground.

 

[aq_mishu]

At least two points in your post sound confused:

- You say you have a transformerless power supply, in other words your circuit isn't isolated from mains power. O.K. But how do you think to make a serial connection to a PC without isolation?

Well, i found in internet that when psu is made with neutral tied as ground for the ckt, then that makes risky for pc connection. The psu idea i got from transformerless for fullwave rectification is using a bridge rectifier at first for rectification, that makes ground seperate from neutral wire. Thus i can get a safe GND for uC and PC.

The only RS232 available here in my market is max 232 and 233. Also im gonna use rs485 and max 485 is the only available.

And YES!! Got this point finally, and right FvM, that is the reason they use N hooked up as GND. Im going to change the plan a bit, for few of the case, i will use voltage measurement, the vital places, and there i will invest for transformer. But for mass case of my prat, i have finally set up with sensing 220VAC (mains) only. But it is not the same, means the device will be powered up from mains with transformerless psu for cheap, and will sense presense of mains of another device, whether that device is having power or not... (Remote monitoring whether a device is getting power or not, kind of a bms). Thus i will read the voltage per floor. But i have several rooms and sections, where i dont need to read volt (as from same DB board) but need to know if they are UP (like say the breakers are ON). Now its all about sensing mains for the same issue.

Ideas??