Analog multipliers and watt measurement

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurement/ade7763/products/product.html

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <bv_at_NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurement/ade7763/products/product.html

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem....

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

J. Watkinson, The Art of Digital Audio, 2nd ed. (Focal, Stoneham, MA
and Borough Green, Sevenoaks, U.K., 1988) gives a detailed review of
the literature, in his discussion of A/D conversion, amongst a lot of
other useful stuff.

However, with typical modern loads, the load current is far from
sinusoid and require a significantly higher effective sampling rate.
This might still require increasing the sample rate once more by a
decade or more.

On Mon, 19 Dec 2011 22:03:38 +0200, upsidedown_at_downunder.com wrote:

However, with typical modern loads, the load current is far from
sinusoid and require a significantly higher effective sampling rate.
This might still require increasing the sample rate once more by a
decade or more.

_If_ it can be assumed that the current waveform is repeated
identically at several cycles and the measurement values needs to be
updated, say once a second, a lower sample rate would do. Sampling
(with a short gate time) at a sampling frequency slightly off the
mains frequency (say 204 Hz/244 Hz for 50/60 Hz), the waveform can
reconstructed quite accurately in 1 s (just as in sampling
oscilloscopes), when the sampling moves around the waveform.

Thus multiplying the current and voltage samples at this low sampling
frequency and averaging the power for a whole second (instead of a
single mains cycle) might be sufficient.

Wondering if this approach would be compatible with dithered
oversampled ADC conversion ?

But if the cycles are not identical and a higher display update rate
is required, this approach does not work.

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

On Mon, 19 Dec 2011 09:17:23 -0800 (PST), Bill Sloman
bill.sloman_at_ieee.org> wrote:

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

J. Watkinson, The Art of Digital Audio, 2nd ed. (Focal, Stoneham, MA
and Borough Green, Sevenoaks, U.K., 1988) gives a detailed review of
the literature, in his discussion of A/D conversion, amongst a lot of
other useful stuff.

Playing with dithering requires quite high oversampling rates.

For example in properly dithered CD chains (44.1 kHz sampling
frequency, 16 bits hence 96 dB theoretical SNR), in the "fade to
noise", an audio test signal to the system can be lowered well below
-96 dB and the tone is still detectable, but only well below the
maximum audio frequency (20 kHz). At 1 kHz, you might be able to
detect the tone perhaps at -105 dB.

In the current channel of a TrueRMS meter it might be sufficient to
sample the signal at say 200 Hz, if we could assume that the current
waveform is sinusoid. Adding dithering noise and sampling at a few kHz
will get some extra bits to the averaged low frequency samples.

However, with typical modern loads, the load current is far from
sinusoid and require a significantly higher effective sampling rate.
This might still require increasing the sample rate once more by a
decade or more.

Thus, the sampling rate might have to be over 20 kHz, first to get rid
of quantization error of the low bit count ADC with ditheration and
downsample down to say 2 kHz and use these samples to multiply the
corresponding voltage samples at 2 kHz to correctly represent the
harmonics of the current measurements.

A very much doubt that a very small controller would handle it.

On Mon, 19 Dec 2011 10:14:54 +0200, upsidedown_at_downunder.com wrote:

On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <bv_at_NoSpam.com> wrote:

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a
SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few
mA, so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known
load, 10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC
onboard an 80 cent uP, like an HC05 class part. I've done it with
a 7-bit single-slope ADC, resolved a few watts out of 20,000.

On Mon, 19 Dec 2011 09:17:23 -0800 (PST), Bill Sloman
bill.sloman_at_ieee.org> wrote:

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

It only needs to be uncorrelated to the line frequency and span a
couple of ADC bits. One meter I did, I used a triangle wave from a
schmitt-gate oscillator, which has a pretty flat PD. We sold over 50K
channels of that one.

Another meter, designed for residential metering in India, I used the
uP code itself as a random noise source, just one bit of each opcode,
out a port into an RC lowpass. That worked too.

I don't do meters any more. There's no money in it.

John

John Larkin <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 09:17:23 -0800 (PST), Bill Sloman
bill.sloman_at_ieee.org> wrote:

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform a
computed watt function, along with true RMS current, voltage, VARS, and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

It only needs to be uncorrelated to the line frequency and span a
couple of ADC bits. One meter I did, I used a triangle wave from a
schmitt-gate oscillator, which has a pretty flat PD. We sold over 50K
channels of that one.

Another meter, designed for residential metering in India, I used the
uP code itself as a random noise source, just one bit of each opcode,
out a port into an RC lowpass. That worked too.

I don't do meters any more. There's no money in it.

John

Using the code itself as the source for analog dithering sounds like a
brilliant hack, but I suppose you would have to be careful about WHICH bit
you used, depending on how the opcodes were constructed for that particular
microprocessor.

"John Larkin" wrote in message
news:bllue757ev8175qkd94ek4g2p9nfbe6a78_at_4ax.com...

On Mon, 19 Dec 2011 10:14:54 +0200, upsidedown_at_downunder.com wrote:

On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <bv_at_NoSpam.com> wrote:

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or a
SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage measurement,
but how about the current measurement in a general purpose instrument?

This may have to handle tens of amps of peak current or just a few
mA, so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known
load, 10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC
onboard an 80 cent uP, like an HC05 class part. I've done it with
a 7-bit single-slope ADC, resolved a few watts out of 20,000.

The transducers I mentioned were basically 1% accuracy class, and they only
needed to work for voltage variations of about 20% (100-140V), and current
from about 5% to 125% of nominal (usually 5A). For higher voltages and
currents, PTs and CTs were used. They generally drove analog switchboard
meters, which had 270 degree round dials and about 2% accuracy. Mostly for
visual indications on a panelboard.

The power analyzers were also about 1% or perhaps as accurate as 0.25%, so
an 8 bit or 10 bit ADC would be sufficient if a microcontroller were to be
used. Since such power analyzers sell for $5000 and more, cost is not really
a major factor, so a higher precision 12 bit ADC would be overkill, but the
PIC18F26K80 is only $3, so why not? Even more important may be simultaneous
sampling, but apparently not available on PICs. But external devices could
be used, or else just interpolate between readings. And most analyzers have
multiple ranges of current and voltage.

On Tue, 20 Dec 2011 00:48:02 GMT, Ralph Barone
address_is_at_invalid.invalid> wrote:

John Larkin <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 09:17:23 -0800 (PST), Bill Sloman
bill.sloman_at_ieee.org> wrote:

On Dec 19, 4:25 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 19 Dec 2011 10:14:54 +0200, upsided...@downunder.com wrote:
On Mon, 19 Dec 2011 16:03:57 +0800, "Bruce Varley" <b...@NoSpam.com
wrote:

Analog Devices has an IC that computes watts, VARS, and other
quantities for
power analysis:
http://www.analog.com/en/analog-to-digital-converters/energy-measurem...

For power line frequencies, probably a PIC with ADCs could perform
a
computed watt function, along with true RMS current, voltage, VARS,
and even
frequency and phase angle or power factor.

Paul \

Indeed, a 12F675 provides 4 10-bit ADC channels, and enough flash to
accommodate all these calcs, you can bit bang to a serial output or
a SPI
input DAC. All in 8 pin DIP.

The 10 bit resolution might be sufficient for the voltage
measurement,
but how about the current measurement in a general purpose
instrument?

This may have to handle tens of amps of peak current or just a few
mA,
so apparently some range selection would be needed.

For an instrument that is going to monitor a more or less known load,
10 bits might even be sufficient.

If you add a little noise to the current signal before you digitize
it, you can do a utility-grade meter with an 8-bit mux'd ADC onboard
an 80 cent uP, like an HC05 class part. I've done it with a 7-bit
single-slope ADC, resolved a few watts out of 20,000.

It helps if you are picky about the probablity distribution of the
noise you add.

It only needs to be uncorrelated to the line frequency and span a
couple of ADC bits. One meter I did, I used a triangle wave from a
schmitt-gate oscillator, which has a pretty flat PD. We sold over 50K
channels of that one.

Another meter, designed for residential metering in India, I used the
uP code itself as a random noise source, just one bit of each opcode,
out a port into an RC lowpass. That worked too.

I don't do meters any more. There's no money in it.

John

Using the code itself as the source for analog dithering sounds like a
brilliant hack, but I suppose you would have to be careful about WHICH bit
you used, depending on how the opcodes were constructed for that
particular
microprocessor.

Right. The MSB turned out to have an ugly waveform, so I used the next
one down. I think... it's been a while. It drove a port pin every IRQ,
and that drove an RC lowpass to make the dither. The result was a
really ugly ratty triangle sort of thing. It worked.

A real pseudo-random sequencer would be sensible on an ARM or
something, but the HC05 is a dog for many-bit math.

John