AC Measurement

[Atom058]

Hello - I want to monitor and display AC voltage (120V 60Hz). I have scoured all the posts that I could find that dealt with AC and was not able to come up with exactly what I am interested in. I found some that got close but then fizzled out...

I am interested in monitoring my line voltage over the a range of 120V +/- 5V (115 - 125V) and I only need a resolution of 1 V. I am not interested in voltages out of this range.

If I feed my 120V through a transformer, rectify it and get it to a voltage (<5VDC) to feed into a PIC ADC (10 bit), will this work?

The math works: Let's say I have 5V DC output that represents 125V AC input. At 10 Bits ADC resolution, that means that 1 bit (step) = 4.9 mV DC or 122.5 VAC. A drop of 1 V AC input to 124V AC would equate to 40 mV DC drop from 5 V to 4.96 VDC. This easily gives me the resolution that I need.

I guess my question is if the circuit as described above will give me the response that I need. Is there a way to do this without a transformer (like the transformerless power supply)? I realize the safety implications but I want to keep this as small as possible.

Thanks!

[mister_e]

You could read the whole thread bellow
http://www.picbasic.co.uk/forum/show...t=degree+60hz\

Check the last one. I like the idea of 0.707 Vpeak but never experimented is as now. In theory and if your timings are good, it works.

If properly design, something around the transformerless PSU idea may also works.

[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