AC Measurement

[bbarney]

have a look at this

[Acetronics2]

Hi, Atom

The safe way would be :

1) to use a Voltage to frequency converter , LM 331 i.e., on the HV side. ( after applying a, say 100v, offset to your mains ...)

2) to transmit the freq. via an Opto-coupler ( 1Kv min rated !!! ... ) or an optic fiber to the measuring unit.

It will be much easier to do a freq counter with the Pic ...

Alain

[Atom058]

Hello All - Thanks for the replies. Using the RMS to DC converter chip as suggested by bbarney seems like the easiest way to go, but the chip costs $15 US which significantly inpacts the overall cost of my project. SO, at least for now, I am going to investigate some other options. I may come back to this if nothing else works.

I am considering Mister_e's suggestions - the most interesting of which is the modification of the transformerless power supply (Melanie's design):
(http://www.picbasic.co.uk/forum/atta...1&d=1139474240)

Steve: would you care to further elaborate on that approach? Is it possible to get the 120 AC voltage down to the 5V DC range and be able to detect 1V changes in the incoming AC? The transformerless power supply has a Zener in it - if I am not mistaken, that essentially chops off voltages above it's rating (in this case, 12V).

Acetronics: If I gather your response correctly, it sounds like you are addressing frequency measurement. At this time, I am interested in voltage measurement. Please let me know if I misunderstood...

Again, Thanks!

[Atom058]

Melaine - Since this is (hopefully) a modification of your design, I thought I would invite you to participate and maybe take a break from the somewhat heated debate over fusing in the other thread... (BTW, I agree with you wholeheartedly. I was an electrical safety officer at a national laboratory here in the States, and I shut down operations at an entire facility because they used some equipment fused N:G). Aaaannywaaayy.... that's not the topic here... Can you give any comments on my objective?

Thanks!

[Acetronics2]

Hello All
Acetronics: If I gather your response correctly, it sounds like you are addressing frequency measurement. At this time, I am interested in voltage measurement. Please let me know if I misunderstood...

Again, Thanks!

Hi, Atom

YES, you are ...

the goal is to convert the mains dangerous voltage to a proportionnal frequency and THEN to use an opto coupler to transmit the info to a low voltage ( = safe ) part of the meter.

so, you count a freq which is proportionnal to your voltage. C.Q.F.D.

Alain

[Atom058]

Alain - My apologies! After looking at the spec sheet, this does sound like a promising prospect after all. It looks like I need to get the voltage down to 10V or thereabouts to input into this chip. Is that correct? I tried searching for LM331 to see if any others have used this but could not find any reference to it. I even tried the Google search, too. Can you recommend a good way to get my incoming AC down to a level acceptable for input into this chip?

Thanks!

[bcd]

I have measured 240v using this IC. I used two 240v minature transformers in series across the mains to measure the voltage. This became ~6v on the (series connected) secondary side that I then fed to the NSC RMS to DC convertor IC.

I used the 'two transformer in series' trick as I had once read about it in Elektor as a way to improve the linearity of the small cheap transformers I was using.

It worked real well, only issue was the cost of the ICs. I also worked on doing various half wave and full wave recification schemes, but after smoothing found the lag was an issue.

Bill

[Atom058]

Bill - Thanks for your reply. I searched a little through the Elektor Forum and was not able to find anything like what you were describing. Can you point me in the right direction? Also, after looking at the spec sheet for the LM331, can you tell me if the circuit that they have diagrammed is necessary in it's entirety? Did you have a simpler design?

Thanks! Atom058