Can I measure AC vampire current this way?

[Bob Monsen]

"George" <gh424NO824SPAM@cox.net> wrote in message
news:Vcrtj.801$QC.294@newsfe20.lga...

I'd like to measure how much current various plugged-in devices draw
when they are "off". I have a digital multimeter that will measure
AC voltage. So I thought maybe I could place a small-value resistor
in series in one of the AC lines, and measure the voltage drop
across it. And then just calculate the current. But I don't know
if the results would be at all accurate.

Has anyone tried that? Is there a better way?

Bob Pease has a good article about how a watt meter works, and a working
schematic.

http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=2190

OTOH, I concur with the folks who say a Kill-A-Watt is the way to go. Much
more reliable, less dangerous, and quite fun.

Regarding how it works, all you need to do is to get a very low value shunt
resistor, and then multiply the voltage across the resistor with the voltage
across the device in series (in real time). That gives you instantaneous
power. You then integrate that to get the result over time, which is of
course energy. You need to scale the result to get it in kW hours. His meter
given in the link above does it with an analog circuit, which is pretty
cool. One can also do the math with a microcontroller, which is probably how
the Kill-A-Watt does it.

Electricity is dangerous, of course, so if you do this, keep one hand in
your back pocket at all times when you are near any live wires. The thing
that kills is AC across your heart, which can induce fibrillation.

Regards,
Bob Monsen

 

[Rich Webb]

On Sun, 17 Feb 2008 15:24:24 -0600, George <gh424NO824SPAM@cox.net>
wrote:

Rich Webb says...

Nevermind. They want cookies.

Download Opera and set it to discard ALL cookies at the
end of a browser session.

Hmmm. Just tried the link and it counted *past* zero but
eventually got to the article. Another MSIE-wired site?

I had session cookies enabled in IE7, but that wasn't
enough. But when I added it to the allowed sites for hard
drive cookies, it went right through. What was confusing
was that usually a site will say at some point if cookies
are required. This one didn't.

I don't want to discard all cookies. Some of them keep
track of where I left off, and are useful. But I allow the
specific sites I need, and just allow 1st party session
cookies on everything else.

What I do in that case, and for sites where the cookie contains login
credentials, is click to the cookie list (tools | advanced | cookies)
and set the ones that I want to keep to be retained.

If you haven't tried Opera (or Firefox) give it (them) a whirl.

--
Rich Webb Norfolk, VA

 

[George]

Rich Webb says...

Nevermind. They want cookies.

Download Opera and set it to discard ALL cookies at the
end of a browser session.

Hmmm. Just tried the link and it counted *past* zero but
eventually got to the article. Another MSIE-wired site?

I had session cookies enabled in IE7, but that wasn't
enough. But when I added it to the allowed sites for hard
drive cookies, it went right through. What was confusing
was that usually a site will say at some point if cookies
are required. This one didn't.

I don't want to discard all cookies. Some of them keep
track of where I left off, and are useful. But I allow the
specific sites I need, and just allow 1st party session
cookies on everything else.

 

[John Fields]

On Sat, 16 Feb 2008 09:58:59 -0600, George <gh424NO824SPAM@cox.net>
wrote:

David L. Jones says...

Drop $20-30 on a Kill-A-Watt. It's inexpensive, and
very good at what it does, which is what you are
trying to do (and a few more things).

That would violate the Prime Directive, and take all
the fun out of it.

Yeah, but it is the right tool for the job. Cheap,
accurate, safe, and feature rich. Get one, seriously.

If you really want to DIY then you can get an energy
meter kit:

http://www.altronics.com.au/index.asp?area=item&id=K4600
http://www.siliconchip.com.au/cms/A_102045/article.html

Well, the problem is that the cost of the kit is so high
that it would take a lifetime to recover the cost from
whatever savings I could get from it. Furthermore, the cost
to the planet in terms of raw materials and energy to
manufacture the device also exceeds any related benefit I
might get from it in measuring low-power devices. It also
looks like I have to pay $8 even to read the rest of the
Siliconchip article.

The Kill-A-Watt is certainly more reasonable. I just
wondered if there's a home-brew way to get similar
information about power consumption for esentially no money.

---
Try one of these and your multimeter:

http://www.analog.com/en/subCat/0,2879,773%255F866%255F0%255F%255F0%255F,00.html

First, use one to measure the RMS current into the DUT, then the RMS
voltage across it, then multiply the two readings and you'll have
the power your DUT is dissipating.
---

Let me try to get back to the original idea. Using my
multimeter and other things I already have, which are free,
is there any measurement I can make that would give me *any*
useful information about the power consumed by electronic
devices which are turned "off"?

---
If you don't know the impedance the device presents to the mains,
then the information you get from your measurements will be largely
irrelevant.
---

If the measurements wouldn't be correct in absolute terms,
would they at least be relatively correct in comparing
several devices?

---
Not unless the devices' inputs were identical in terms of the
impedance they presented to the mains.

--
JF

 

[Rich Webb]

On Sun, 17 Feb 2008 09:34:28 -0600, George <gh424NO824SPAM@cox.net>
wrote:

Nevermind. They want cookies.

Download Opera and set it to discard ALL cookies at the end of a
browser session.

Hmmm. Just tried the link and it counted *past* zero but eventually
got to the article. Another MSIE-wired site?

--
Rich Webb Norfolk, VA

 

[petrus bitbyter]

<snip>

Try one of these and your multimeter:

http://www.analog.com/en/subCat/0,2879,773%255F866%255F0%255F%255F0%255F,00.html

First, use one to measure the RMS current into the DUT, then the RMS
voltage across it, then multiply the two readings and you'll have
the power your DUT is dissipating.
---

<snip>

How do you account for phase errors, for instance when your DUT is a
transformer?

petrus bitbyter

 

[John Fields]

On Sun, 17 Feb 2008 15:18:03 +0100, "petrus bitbyter"
<pieterkraltlaatditweg@enditookhccnet.nl> wrote:

snip

Try one of these and your multimeter:

http://www.analog.com/en/subCat/0,2879,773%255F866%255F0%255F%255F0%255F,00.html

First, use one to measure the RMS current into the DUT, then the RMS
voltage across it, then multiply the two readings and you'll have
the power your DUT is dissipating.
---

snip

How do you account for phase errors, for instance when your DUT is a
transformer?

---
Aaarghh!!!!

What _was_ I thinking???

Thanks.

--
JF

 

[George]

Bob Monsen says...

OTOH, I concur with the folks who say a Kill-A-Watt is
the way to go. Much more reliable, less dangerous, and
quite fun.

I Googled:

kill-a-watt accuracy

and this Anandtech thread came up first:

http://forums.anandtech.com/messageview.aspx?catid=84&threadid=2147352
&enterthread=y

which compared Kill-a-Watt readings for various things with
readings on the same items from a Fluke 1735, which I gather
is a VERY expensive watt meter, with not particularly good
results. Included was:

Its not accurate at measurements of small wattages.
Things like the power usage of a dvd player in standby
mode are not accurate. Killawatt said dvd player was
using about 5 watts, fluke 2.64 watts.

Of course, even if it's wrong, it could still be useful if
it's consistently wrong - if I could apply a fudge factor of
some kind.

Anyway, I accept that the answer to my original question -
can I get good measurements with a simple home-brew ciruit -
is no, that doesn't work.

 

[George]

Nevermind. They want cookies.

 

[George]

Bob Monsen says...

Bob Pease has a good article about how a watt meter
works, and a working schematic.

http://electronicdesign.com/Articles/Index.cfm?AD=1&Arti
cleID=2190

Thanks, Bob, but I'm unable to get to the site. I just get
an unending series of ads. It says "Click here to go to
electronic design, or wait 20 seconds." But no matter what
I do, I just get the same ad and another countdown, and
another "AD=1" added to the end of the address.

 

[David L. Jones]

On Feb 17, 2:58 am, George <gh424NO824S...@cox.net> wrote:

David L. Jones says...

Drop $20-30 on a Kill-A-Watt. It's inexpensive, and
very good at what it does, which is what you are
trying to do (and a few more things).

That would violate the Prime Directive, and take all
the fun out of it.

Yeah, but it is the right tool for the job. Cheap,
accurate, safe, and feature rich. Get one, seriously.

If you really want to DIY then you can get an energy
meter kit:

http://www.altronics.com.au/index.asp?area=item&id=K4600
http://www.siliconchip.com.au/cms/A_102045/article.html

Well, the problem is that the cost of the kit is so high
that it would take a lifetime to recover the cost from
whatever savings I could get from it. Furthermore, the cost
to the planet in terms of raw materials and energy to
manufacture the device also exceeds any related benefit I
might get from it in measuring low-power devices.

You've missed several important points:
- Doing it manually can be dangerous.
- Doing it manually takes a lot more time and effort.
- Doing it manually has many traps
- The Kill-A-Watt gives you a LOT more useful info that is hard to get
with just manual measurements.

How much energy have you wasted discussing this on this forum?

It also
looks like I have to pay $8 even to read the rest of the
Siliconchip article.

The Kill-A-Watt is certainly more reasonable. I just
wondered if there's a home-brew way to get similar
information about power consumption for esentially no money.

Geeze, just buy the Kill-A-Watt meter, they are under $20:
http://www.google.com/products?q=kill-a-watt&btnG=Search+Products&scoring=p

Let me try to get back to the original idea. Using my
multimeter and other things I already have, which are free,
is there any measurement I can make that would give me *any*
useful information about the power consumed by electronic
devices which are turned "off"?

I believe some others have discussed that already.

If the measurements wouldn't be correct in absolute terms,
would they at least be relatively correct in comparing
several devices?

Possibly not. Manual measurements may have a high degree of both
absolute and relative error. Unless you know all the factors involved
and understand exactly what's happening, you'll probably get caught
out. For under $20 you can do it correctly and get a LOT more useful
info safely.

Dave.

 

[gearhead]

 Phil Allison >     Hiding a giant red herring.

In his pants.

 

[John Popelish]

George wrote:

John Popelish says...
(snip)

... That
takes a true watt meter that averages the instantaneous
product of voltage and current.

And does the electric company's meter on the side of my
house measure things correctly? Aside from various "green"
considerations, I think my meter should read the same as
theirs, whether it's technically right or wrong.

Yes, the watt hour meter on your house does a quite accurate
job of totaling the actual energy you use from your power
service. The old ones have a motor whose drove torque is
produced by the instantaneous multiplication of voltage and
current, while it is braked in proportion to its rotational
speed. It may not handle microsecond pulses, but it does
take phase shift and non sinusoidal waveforms into account.

More modern ones have microprocessors that total the product
of high speed samples of voltage and current.

Neither will charge you for inductive or capacitive current,
nor miss the energy a rectifier and capacitive filter draw
from the service at the peaks of the voltage waveform.

--
Regards,

John Popelish

 

[George]

David L. Jones says...

Drop $20-30 on a Kill-A-Watt. It's inexpensive, and
very good at what it does, which is what you are
trying to do (and a few more things).

That would violate the Prime Directive, and take all
the fun out of it.

Yeah, but it is the right tool for the job. Cheap,
accurate, safe, and feature rich. Get one, seriously.

If you really want to DIY then you can get an energy
meter kit:

http://www.altronics.com.au/index.asp?area=item&id=K4600
http://www.siliconchip.com.au/cms/A_102045/article.html

Well, the problem is that the cost of the kit is so high
that it would take a lifetime to recover the cost from
whatever savings I could get from it. Furthermore, the cost
to the planet in terms of raw materials and energy to
manufacture the device also exceeds any related benefit I
might get from it in measuring low-power devices. It also
looks like I have to pay $8 even to read the rest of the
Siliconchip article.

The Kill-A-Watt is certainly more reasonable. I just
wondered if there's a home-brew way to get similar
information about power consumption for esentially no money.

Let me try to get back to the original idea. Using my
multimeter and other things I already have, which are free,
is there any measurement I can make that would give me *any*
useful information about the power consumed by electronic
devices which are turned "off"?

If the measurements wouldn't be correct in absolute terms,
would they at least be relatively correct in comparing
several devices?

 

[Eeyore]

George wrote:

And does the electric company's meter on the side of my
house measure things correctly?

Yes.

 

[George]

John Popelish says...

Thanks very much. Since I'm only going to be measuring
phantom current, which I assume would be a few
milliamps of AC, I think it should work ok so long as
the readings are valid.

They are valid in the sense that they provide an
indication, but not an accurate measurement of power
being consumed. If the current is passed in narrow
spikes (as it is with rectified supplies) or is phase
shifted , as it is with reactive loads, then it does not
indicate exactly what power is being consumed. That
takes a true watt meter that averages the instantaneous
product of voltage and current.

And does the electric company's meter on the side of my
house measure things correctly? Aside from various "green"
considerations, I think my meter should read the same as
theirs, whether it's technically right or wrong.

 

[Eeyore]

George wrote:

But I don't quite know how to calibrate it using rectifiers.

You can't as the waveforms may vary.You need a true RMS meter for this.

Graham

 

[Eeyore]

George wrote:

I'd like to measure how much current various plugged-in devices draw
when they are "off". I have a digital multimeter that will measure
AC voltage. So I thought maybe I could place a small-value resistor
in series in one of the AC lines, and measure the voltage drop
across it. And then just calculate the current. But I don't know
if the results would be at all accurate.

Unless you use an RMS reading meter the result will very likely be
somewahat inaccurate due to the non-sinusoidal current likely being
drawn.

However there is another point. It will NOT tell you the WATTS being
used which is surely what you want to know ? For non-sinusoidal or
phase-shifted waveforms the watts are not equal to volts times amps.

Graham

 

[David L. Jones]

On Feb 16, 3:30 pm, George <gh424NO824S...@cox.net> wrote:

Ecnerwal says...

Drop $20-30 on a Kill-A-Watt. It's inexpensive, and very
good at what it does, which is what you are trying to do
(and a few more things).

That would violate the Prime Directive, and take all the fun
out of it.

Yeah, but it is the right tool for the job. Cheap, accurate, safe, and
feature rich. Get one, seriously.

If you really want to DIY then you can get an energy meter kit:
http://www.altronics.com.au/index.asp?area=item&id=K4600
http://www.siliconchip.com.au/cms/A_102045/article.html

But I wonder how that device works.

See the above silicon chip article.

Dave.

 

[Phil Allison]

"Ross Herbert"
"Phil Allison"

:
:> It's still a great device for people interested in finding out more
info
:> on> their power consumption though.
:
:
:** The bill tells you that.

Yes, but well and truly after the event...

** Yawn.....

:> Both you and I know that the average electronics
:> enthusiast who can read circuits and build stuff knows how to clip a
:> current
:> sensor around the phase wires in the meter box and wouldn't bother
getting
:> a licensed sparky in.
:
:
:** You presume far too much.
:

And you didn't answer the question...

** I see only a series of dubious presumptions.

Hiding a giant red herring.



....... Phil