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Thread: RMS vs Average voltage?
www.piclist.com/techref/power.htm?key=voltage
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face BY : David Cary email (remove spam text)



Hey Bob,

I like to get a general idea of the "whole system" before I let my EE training
kick in and fixate on one tiny little part.

You have something plugged into the wall. There's a triac (perhaps a lamp
dimmer) between that power cord and a transformer. There's ... /something/ ...
on the other side of that transformer.
You want to measure /something/ ( RMS voltage into the transformer ? RMS voltage
out of the transformer ? RMS current ? average power ? ) and get that data into
a PIC, and you've already figured out you need a (hopefully simple) circuit to
buffer between the (typical) 120 V(RMS) wall power and the PIC's analog input.
Then some software on that PIC takes that data and does ... /something/ with it,
within /a small/ time.

Am I anywhere close ?

Is /a small time/ within 1 cycle of the (60 Hz) power, i.e., about 16 ms ? A PIC
can do a lot of stuff in 16 ms.

If you measure the average voltage on any AC line, you will get (very close to)
0 Volts. That's why people use RMS volts rather than average volts to describe
"120 VAC" or "220 VAC" power outlets. (Given a square wave signal +7 -7 +7 -7 +7
-7 +7 etc. forever, what is its average value ? What is its RMS value ?).

If you have a resistor R between your signal source and the PIC analog input,
and a capacitor C from the PIC analog input to ground, then you get a time
contstant T = R*C. One rule of thumb is to make C big enough so that time
constant T is at least 5 sampling times. (Software grabs a new value from the
ADC every 1 sampling time).

With a resistor divider made of 2 resistors R1 and R2, the effective R to plug
into the above formula is the "parallel equivalent resistance",
 R = 1/( 1/R1 + 1/R2 ) = R1*R2 / (R1 + R2). That's also the "effective
impedance" that must be below 10 KOhm (preferably below 1 KOhm) to meet
Microchip's specs.

That's about all I can help without filling in /some/ blanks.

Has anyone played with using a SA612A or SA602 analog multiplier to multiply
current and voltage in realtime to get (real) power, then filtering and
digitizing the power signal ? Is there a better chip for this application ? (I'm
looking for very low cost).

--
David Cary

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