PIC 16f877 A/D conversion Allegro ACS712

[DavyJones]

Oops, that would be a TLV274.

I knew I should have checked that.

TLV274 is a rail-rail version of the LM324.
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Wouldn't you know it Digikey doesn't stock any.... argh

[DavyJones]

Wouldn't you know it Digikey doesn't stock any.... argh

How about the TLV271 I only need one for my test. I think I will ultimatley need the TLV272 because I am controlling and trying to read the current from 2 pumps.

[Darrel Taylor]

Yeah, only available in the TSSOP package.

I agree, those are good alternates.
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[DavyJones]

Darrel, I think I might understand now I know that we are off subject from a general pic discussion but I'm going to keep going in this thread it does pertain to working with a pic ultimately so....

I think I am picturing this correctly now. If I take a thousand readings from the PIC and just keep track of the max at some point I will get a peak let's say it's 12 amps. But like you are pointing out that is truly a peak if we were able to get a sample at the top of each waverform over time it would average out to 9 amps which is the true power. The fact that when I sample the A/D I have know idea were in the wave form I am at, at any given point in time, makes it impossible to do just sampling the ACS712 by itself. If I could get a reading form the ACS712 at the top/bottom of each wave then it could be done but then that is what the RMS to DC is going to do for me give me a nice straight DC line.

Did I get it?

[dhouston]

If I could get a reading form the ACS712 at the top/bottom of each wave then it could be done but then that is what the RMS to DC is going to do for me give me a nice straight DC line.

You are still making an unwarranted assumption that you have a sine wave. Darrell is suggesting the RMS chip and I suggested looking at the output of the Allegro chip with an oscilloscope (to see the waveform) because we don't think you have a sine wave. 0.707 x the peak will work with a sine wave and averaging ALL of the readings will work with any waveform (if the readings can be taken fast enough to get a good sampling of the wave form).

There are at least three ways to do this with non-sinusoidal waveforms. See the web page I referenced earlier (via email)...

• http://www.pge.com/includes/docs/pdf...idal_power.pdf

I vaguely recalled a somewhat similar thread - it took me a while to find it.

• http://www.picbasic.co.uk/forum/show...=motor+current

[DavyJones]

Dave,
I can see were I made it sound like I thought I still have a pure sine wave thats not what I meant. What I was actually trying to say was what you just stated that because it was not a pure sine wave if I took thousands of readings and averaged those it would work.

I'm looking at the document I saw that link in the email but it came acrossed somewhat broken I did not get the entire link as a complete link in the email.

Thanks
David

[dhouston]

I can see were I made it sound like I thought I still have a pure sine wave thats not what I meant. What I was actually trying to say was what you just stated that because it was not a pure sine wave if I took thousands of readings and averaged those it would work.

It's a bit more complicated - it requires some complex math. See Digital True RMS Converters on page 3 of the referenced pdf.

If you look at the waveform and see it is more or less sinusoidal, then 0.707 x the peak output of the Allegro should get you close enough for what you're trying to do.

[DavyJones]

Dave, Thanks I appreciate all you and Darrel are teaching me and not getting to stressed with me while I try to learn this. I'm going to work on capturing the wave this evening and take a look at what I see. I do have the components that Darrel suggested on order as well I want to hook those up in any event to help me understand. Things are so much easier when the device is DC like they were with the pressure transducer.

thanks again
David