If your line frequency is well known you can just go by time.  But if
the frequency varies you may have more or less than one cycle.  If you
have 110% of a cycle the voltage you get depends on where in the cycle
you started.  If the extra 10% was near a zero crossing you will read
low.  If it was near a peak you will read high.

If your product doesn't trust the line voltage, can it trust the
frequency?

Sherpa Doug

> -----Original Message-----
> From: michael brown [mailto:n5qmg@AMSAT.ORG]
> Sent: Wednesday, February 27, 2002 10:59 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [pic] Math Question - Measuring AC Voltage
>
>
> Doug says:
>
> > There are many ways to measure AC voltages.  The most common is RMS
> > which is the usual 220VAC (110VAC USA).  RSM voltage is
> litterally "Root
> > Mean Square", so you square your readings, average them
> over one cycle,
> > and take the square root.  It would help if you could sync
> your readings
> > to the zero crossings of the power line.
>
> Why would syncing to the line make any difference.  As long
> as he samples
> one complete cycle why would it matter where he started and
> where he left
> off?
>
> michael
>
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