Re: Digital power meter for audio amps
From: Country Loon (fitlikemin_at_mearns.fairmers.co.uk)
Date: 10/12/04
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Date: Tue, 12 Oct 2004 15:51:07 +1300
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:6gklm0lujf96j9f27hf7rs9dakbql9vot2@4ax.com...
> On Mon, 11 Oct 2004 08:28:24 GMT, the renowned "Ban" <bansuri@web.de>
> wrote:
>
> >Greg K wrote:
> >> I need some help with the schematic/design of a digital power meter or
> >> watt meter if you prefer. The objective is to be able to measure the
> >> power output from an amp driving a speaker. I understand I need to
> >> measure current, voltage, and cos(phi). The current and voltage should
> >> be obtainable through A/D converters. Measuring the phase angle I am
> >> not sure about yet. The output of the final result will be displayed
> >> on 7-segment LEDs.
> >>
> >> The way I see it is that a DMM can do voltage and current but can't
> >> multiply them together and give you a power reading. I don't know what
> >> chips a DMM might use to do this though. Right now I'd be happy with
> >> some circuitry to measure voltage and current; multiply them and get a
> >> display.
> >>
> >> If you need any clarifications about this, please ask.
> >
> >Greg,
> >you do not need to measure cos_phi, because it is contained in the other
two
> >measurements. If you multiply the momentary values of voltage and current
> >with each other, the output will be the momentary power, which you will
have
> >to process for RMS with the appropriate time constants. You could also
use
> >an analog multiplier and feed its output to the true RMS DMM. Or(best)
you
> >could use your soundcard to sample voltage and current and use some
software
> >like matlab for processing.
>
> I like this meter:
> http://www.aqdi.com/PowerMeterLarge.jpg
>
> It reads in "RMS watts", not those nasty regular watts.
>
RMS watts? Is there such a thing? Well there is average power and
instantaneous power but rms power is back to the old chestnut of Hi-Fi buffs
that have no engineering degree!
Tom
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