Prev: Strategies for temperature-compensating differential pairs
Next: Flow Sensors revistied
From: John Larkin on 10 May 2010 12:21
On Mon, 10 May 2010 17:07:18 +0100, Tony <Nospam(a)nospam.com> wrote:

>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.

Whether the meter uses a current transformer or a shunt, it will be
somewhat sensitive to external line-frequency magnetic fields. Try
plugging in a transformer-type wall wart next to your power meters; it
would be interesting to see if there's any effect. Just a square cm of
pcb trace loop area can be a problm here, or any asymmetry in a CT
core or winding.

John



From: Jan Panteltje on 10 May 2010 15:30
On a sunny day (Mon, 10 May 2010 17:07:18 +0100) it happened Tony
<Nospam(a)nospam.com> wrote in <4be82f49$0$3697$afc38c87(a)read01.usenet4all.se>:

>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.

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.

>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.

As 4 W over say 24 x 365 = < 8 kWh, x 25 cent = < 2$,
you would need 200 years to break even on that more expensive meter.
Not me :-)

>Tony
>
>
>
>
From: John Larkin on 10 May 2010 15:49
On Mon, 10 May 2010 19:30:55 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>On a sunny day (Mon, 10 May 2010 17:07:18 +0100) it happened Tony
><Nospam(a)nospam.com> wrote in <4be82f49$0$3697$afc38c87(a)read01.usenet4all.se>:
>
>>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.
>
>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.

It's usual to dither the current signal before digitizing it. That
washes out the ADC quantization error. There are more subtle problems
on the low end, like ground loops, mag field pickup, ADC mux hangover,
and bad code.

John


From: Tim Williams on 10 May 2010 18:04
"Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message
news:hs9mtm$6ci$1(a)news.albasani.net...
> 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

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: John Larkin on 10 May 2010 18:36
On Mon, 10 May 2010 17:04:25 -0500, "Tim Williams"
<tmoranwms(a)charter.net> wrote:

>"Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message
>news:hs9mtm$6ci$1(a)news.albasani.net...
>> 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

2 W

John


First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Strategies for temperature-compensating differential pairs
Next: Flow Sensors revistied