Newsgroups: sci.electronics.misc,sci.electronics.design
Subject: Re: Newbie Question: Measuring electrical power: PICs and Clamp probes
References: <[email protected]> <[email protected]>
"Luhan"  writes:

> [email protected] wrote:
> > Are there any designs out there for an electricity meter capable of
> > very accurately measuring electrical power consumption, non
> > intrusively?
> >
> > Basically what I'd like to do is either build or buy some sort of clamp
> > probe the could be attached (or wound??) around the insulated live wire
> > of a domestic power cable, then by (presumably?) measuring the strength
> > of the electrical field generated by the current flowing through the
> > wire, determine and log both voltage and current, and hence electrical
> > power.
> >
> > This seems like an ideal application for a PIC and data logger.  The
> > only part I know nothing about is the clamp probe to the mains cable,
> > and how you would convert the measured electrical field into voltage
> > and current.  How do you stop the presumably equal and opposite
> > electrical field in the neutral wire from interfering with the
> > measurement, if the wires are close together and can't be separated?
> >
> > Is this idea feasible?  Has anyone got a circuit diagram?  I don't want
> > to buy a complex and expensive power analyser, or have to touch the
> > wires in the circuit under measurement.
> 
> You can use a magnetic clamp-on probe to measure current, but it must
> go on only one of the two wires to prevent the fields from cancelling.


This is what is needed usign traditional current transformer and other 
clamp meter constructions. 

But there are also now (just became available wuite recently) 
clamp current meters that work with multi-core cables. 
Meaning you can clamp such meter over a normal 2 or 2 wire mains 
cable, and still get reading. The magnetic fields two wires on mains 
cable pretty well cancel each other, but not entirely to all 
directions. Those multi core cable clamp meters use some sort of 
special magnetic sensors to sense those small fiels.  

Some technical description how this is done:
http://www.suparule.com/technology_accurrent.htm

Here are some links to products that use this technology:
http://news.thomasnet.com/fullstory/465571
http://www.conpro.be/megger_multicore_digital_clamp_meter_MMC850_en.pdf
http://www.davis.com/showpage.asp?L3ID=2525
http://home.nestor.minsk.by/build/news/2005/07/2203.html

> Measuring voltage needs some kind of direct connection.

Usually you need direct connection to get reliabl readings. 
It is possible to get some less accurater results using capacitive 
sensing methods though the electrical wire insulation 
(maube suitable for some applications).    
 
> >From the level of inexperience implied in your question, and the level
> of danger in doing this kind of project, you should have your life
> insurance policy made out to..
> 
> Luhan Monat
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/