Power Meter accuracy

[Joe G \(Home\)]

Hi All,

What can the home power meter in the meter box read down to 1watt or less?

Have you some suggestions fo searches for power meter designs.

Thanks in advance.

Joe

 

[Tony]

It no doubt has a software cutoff, to keep zero drifts from indicating
power when there's no load. It's hard to keep electronic power meters
from having small offsets.

John

Most home meters are very inaccurate, really designed for measuring big
power (100s of watts and KW) and really no use at anything below 10w due
to noise and drift.

I have 2 mains plug power meters, 1 registers 4w with a 1w source but
has at least 1 decimal place, and 12w on a 5w source. The other one has
no decimal places but seems to be accurate to the display level and
measures down to 1w.

I've similar problems with industrial equipment, but you can at least
get a good datasheet specification with that. The datasheets I have for
these home units are plainly incorrect for at least one of them.

You can get a good professional meter for about £400 specifically with
low power directive in mind. Powertek ISW8001, there might be a cheaper
version.

 

[Tim Williams]

It is lonly logical.
If they use a 10 bits ADC, then for a max of 2 kW the miniumum is
already 2 W.

2mW*

Tim

 

[Tim Williams]

2 W

So they have an analog multiplier first?

Tim

 

[Grant]

So they have an analog multiplier first?

Did you not hear the whoosh as it flew over their heads?

Grant.

 

[JosephKK]

So they have an analog multiplier first?

Tim

How much variation in line voltage do you see before it impacts
performance of lamps, motors, and most other loads; 10%?

 

[Joe G \(Home\)]

Hi All,

What can the home power meter in the meter box read down to 1watt or
less?

Sorry, I was talking about power meters that measure power for the
utilities companies.

I know TI AD and others have power meter reference designs.

Regs
Joe

 

[Tim Williams]

A 10 bit ADC has 1024 steps
2000 / 1024 is just about 2
???????????????

Hello?

Presumably, they're converting voltage and current seperately.
1/1024 = -60dB, or 2kW/1e6 = 2mW.

Tim

 

[Tim Williams]

Did you not hear the whoosh as it flew over their heads?

Gee, I think I just embarassed John into not replying. I must be getting
*good*. ;-)

Tim

 

[D Yuniskis]

Hi Joe,

Sorry, I was talking about power meters that measure power for the
utilities companies.

"Classic" meters are nominally ~1% devices. Doesn't sound
very impressive until you think about the wide operating
range (environmental as well as electrical) that they
have to operate in as well as their long term reliability
*and* low cost.

Modern "solid state" devices usually start at 1% and get
better. E.g., there are 0.5% class devices, 0.2% class, etc.

Of course, the SS devices place some "burden" (a few W) on the
line this has to be accounted for (either in the design or
in the billing).

KWH meters are amusing devices. More effort is expended on
all sorts of insane details (register ratio, watthour constant,
etc.) that belong in a lexicon specific to just KWH meters.
And, lots of "rule of thumb" stuff that often seems
arbitrary to an "outsider" :-/

Keep in mind the types of folks who use/install these things.
They aren't typically concerned with the same sorts of metrics
that engineers would...

 

[Tim Williams]

Is the 2mW* thing some sort of joke? Please explain it.

"*" indicates a correction to an error (usually tpyographical).

*typo

Like that.

An error of 2W is obviously wrong, and was hopefully only a typo.

Tim

 

[Martin Brown]

Presumably, they're converting voltage and current seperately.
1/1024 = -60dB, or 2kW/1e6 = 2mW.

Line voltage is usually 230 +/- 20 or around ~10% variation.

So the dynamic range is limited by the current measurement. This tends
to suggest that they actually use a 12 bit ADC in the common ones over
here since they will measure 1W and max out at ~4kW.

Clamp on ones are much cruder and good to about 10-20W but with a much
higher maximum to measure the entire domestic consumption.

Regards,
Martin Brown

 

[Paul Keinanen]

It is lonly logical.
If they use a 10 bits ADC, then for a max of 2 kW the miniumum is already 2 W.
Add any offset in the current sensing, or even one bit eror,
and you are at a minium of 4 W etc etc.

What exactly are you trying to measure, real power or apparent power ?
Is it assumed that the current (and/or voltage) looks like a sine
wave?

While a constant amplitude clean sinusoid voltage connected to a pure
resistive load will produce an in-phase sinusoid current, measuring
the peak current (and assuming nominal voltage) could be used to give
some real power indication. Measuring the current with 10 bit ADC
could give a 1000:1 power range (such as 2 kW:2 W).

Any load with some reactive components would cause a phase shift
between voltage and current and the simplistic system would only
produce the apparent power. Any rectifiers would create a large peak
to average current ratio, so the 1.41:1 peak/rms ratio for a sinusoid
is no longer valid.

For real power, you must multiply the instantaneous voltage with the
instantaneous current and average out. This can be done by an analog
multiplier followed by analog averaging before the ADC or use separate
ADCs for current and voltages sampled at a high frequency and do the
multiplying and averaging in digital domain.

With rectifier loads, the peak current can be quite high, often much
higher than the fuse ratings and the harmonics on the voltage can
cause long duration peaks higher than the fundamental frequency. Thus
the signal conditioning, multiplication and averaging require a quite
large dynamic range, regardless if the most processing is done in
analog or digital domain.