AC Measurement

[mister_e]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1185&stc=1&d=116343790 0">

On a PIC with ADC and comparator on the same pin it could be possible.

Comparator interrupt to detect the zero crossing, you wait 'till 0.707 Vpeak - x offset delay, then take an ADC reading.

OR use 2 I/o, one INT (let's say INT0) + ADC.

But i think the poor psu may suffer if you need many mA.... Could be handy to use high efficiency LED display.

EDIT: another maths approach using samplings... process the long RootMeanSquare stuff... or by 0.707*Vpeak.

[Atom058]

Steve - Thanks for the diagram. It looks to me to be a modification of the transformerless power supply with the addition of a resistor divider network to provide the input to my PIC. Is that correct?

I don't intend to power my PIC or anything with this - so the current capability of the circuit is not important. I just need to know how to condition the AC so that I can read the voltage.

I like your idea of triggering an interrupt (like for the AC Lamp Dimmer circuit you helped me out with), waiting a few uS and then start measuring the voltage on an ADC input. I would then find the highest value and from that I can calculate the voltage. However, I believe that I need to get the incoming voltage to the ADC pin to 5V or below. Isn't that correct? That seems to be the $64 question... how do I get the 120V AC input to some level that is acceptable for an ADC input? Is this accomplished with the resistor divider circuit that you presented?

I appreciate your help!

Thanks!

[mister_e]

I believe that I need to get the incoming voltage to the ADC pin to 5V or below. Isn't that correct?

In fact the Vpeak must be lower than 5 volt. So a Vac(rms) of 5*.707=3.535 Vac(rms)

how do I get the 120V AC input to some level that is acceptable for an ADC input? Is this accomplished with the resistor divider circuit that you presented?

Yes. Maybe a dual secondary transformer (like the one you used for the DImmer) could work. This will also provide your insulation. So you use one output for your PIC and plah plah, the other for your reading.

Maybe it will work good... maybe not... in theory yes... but i feel some potential bugs... interesting to know your results.