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From: Joe G (Home) on 9 May 2010 05:36
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


From: John Larkin on 9 May 2010 10:01
On Sun, 9 May 2010 19:36:02 +1000, "Joe G \(Home\)"
<joe.g(a)optusnet.com.au> wrote:

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

Household mechanical meters with max power of 24 KW will usually
resolve about 15 watts on the low end. They have magnetic detents (a
hole or slot in the disk) that stop them from rotating at low power,
so that people don't get upset by seeing the disk turn at zero load.

>
>Have you some suggestions fo searches for power meter designs.

There are commercial power metering chips from Analog Devices and
maybe TI. Lately people just digitize current and voltage samples and
do the math. It's not easy to do an electronic meter that approaches
the performance of a rotating disk meter, much less the reliability,
ruggedness, and cost.

I've worked with four companies who tried to do electronic meters.
Three are defunct... Metricom, Synergistic, and the Niagra Mohawk
fiasco. I hope it wasn't my fault.

John

From: Robert Baer on 10 May 2010 03:02
Joe G (Home) wrote:
> 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
>
>
Well, the Kill-a-Watt "meter" fails to register a (linear,
unregulated) wall-wart powered light that takes about 4 watts.
From: John Larkin on 10 May 2010 10:04
On Mon, 10 May 2010 00:02:16 -0700, Robert Baer
<robertbaer(a)localnet.com> wrote:

>Joe G (Home) wrote:
>> 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
>>
>>
> Well, the Kill-a-Watt "meter" fails to register a (linear,
>unregulated) wall-wart powered light that takes about 4 watts.

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

From: Tony on 10 May 2010 12:07
John Larkin wrote:
> On Mon, 10 May 2010 00:02:16 -0700, Robert Baer
> <robertbaer(a)localnet.com> wrote:
>
>> Joe G (Home) wrote:
>>> 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
>>>
>>>
>> Well, the Kill-a-Watt "meter" fails to register a (linear,
>> unregulated) wall-wart powered light that takes about 4 watts.
>
> 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.

--
Tony



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