Reading mVAC with PICs

[Atom058]

Hello All - First time poster - long time reader! Up until today, I have been able to solve all my PIC-related questions by surfing through this awsome site. But I have finally hit a roadblock. I can find info almost related to what I am looking for, but not exactly.

Here's my problem. I have a client that works at a manufacturing facility that uses furnaces (some draw up to 2000A). He has some 500A furnaces that he wants to monitor the current of. He has transducers that output 1 mVAC per Amp. This means that I am looking at a range of 1 - 500 mV. I am OK with reading and dealing with mV DC stuff, but I am unsure as to how to handle mVAC. He wants to see the low end (i.e., 1mV / 1A). If I use a simple bridge rectifier circuit (BRC), I loose the low end. Should I amplify the AC and then send it through the BRC? If so, can anyone recommend an op-amp (or something) that can amplify an AC signal to a point that it will not be affected by the BRC? The best thing would be something that would just do a straight conversion.

Any help will be appreciated!

[Dave]

Atom058, Is the millivolt measurement Isolated? If so I would use a 10x amplifier stage and then an amplifier configured as an active rectifier. With this arrangement you should be able to read the peak current (ac) and then multiply it by .707 to get the RMS value or .637 to get the average value. At 500 mv. x 10 = 5.0 v you will be using the fullscale 10 bit capability of the a/d on board the microprocessor.

Dave Purola,
N8NTA

[Atom058]

Dave - Thanks for your quick reply! I am not sure if the measurement is isolated, but you blew me out of the water with "a 10X aplifier stage and then an amplifier configured as an active rectifier". My knowledge of AC electronics is limited (as you might have guessed). Can you provide a little more info or point me to some reading material (spec sheets, etc.)?

[bbarney]

I don't know if this will help but have a look at these
http://www.analog.com/en/subCat/0,2879,773%255F866%255F0%255F%255F0%255F,00.html

http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4701

[Dave]

Atom058, Have a look at these circuits.

Dave Purola,
N8NTA

[Atom058]

bbarney, Dave - Thanks for all the great information! Let me digest it and get back to you.

Thanks again!

[precision]

You can use hi precision current measurement ac or dc with this chip.

ACS706
you can order free sample upto 4 nos.




http://www.allegromicro.com/sf/0706/index.asp

http://www.allegromicro.com/datafile/0706-05C.pdf

[Ron Marcus]

Would there be a problem using an internal comparator on the PIC for the rectifier op amp? I did this for an S meter for a tracking receiver monitor.I used a germanium diode in series with the feedback resistor. As I understand it, before the diode conducts, the op amp gain is very high. After it forward conducts, the op amp gain becomes the ratio between the two gain setting resistors, effectively reducing the diode voltage loss to a very low value. Even though the comparators are not meant for high frequency work, the circuit worked quite well.But...What if the AC voltage goes below Vss?

Ron

[Atom058]

Hello All! Sorry about the lack of communication this last week - Had an emergency project that needed all of my attention. Anyway, thanks for all of the information. I had no idea that this was going to be as complicated as it has turned out to be. Just trying to keep this as simple as possible. If I can't come up with a simple solution, I may have to cut my losses. Maybe I can convince my client to bite the bullet and change transducers to ones that output DC voltage (he didn't seem too excited when I mentioned that before).

Anyway, thanks again! And if anyone has any new ideas, please let me know!

Atom058