Re: Mains Current Monitoring

To: tesla-at-pupman-dot-com
Subject: Re: Mains Current Monitoring
From: Jim Lux <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Mon, 22 Mar 1999 18:51:53 -0700
Approved: twf-at-verinet-dot-com

Tesla List wrote:
> 
> Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
> 
> > Original Poster: "jim lux" <jimlux-at-jpl.nasa.gov>
> >
> >
> > -----Original Message-----
> > From: Tesla List <tesla-at-pupman-dot-com>
> > To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> > Date: Friday, March 19, 1999 10:23 AM
> > Subject: Mains Current Monitoring
> >
> 
> <snip>
> > That is exactly what a current transformer is.... You'll need to select an
> > appropriate toroid core material (i.e. ferrite won't do).
> 
> Why?
> Malcolm
> <snip>

Ferrite saturates at a very low flux which is ok in RF designs where the
frequency is high, and the flux isn't too high, but not so good at power
line frequencies. Tape wound silicon steel would be preferred, possibly
a powdered metal.

I like someone elses idea of cannibalizing an old power transformer. Get
one of those little 10-20VA PC mount transformers with the windings on
separate bobbins. Then, rip the low voltage bobbin off and use the
original line side winding as your output.  You could probably rip open
a wall wart or almost any junk electrical equipment manufactured in the
last 20 years to get a suitable transformer.

Calculate the max primary  winding current based on the transformer
rating: 12VA at 120V = 1/10 Amp. Run a single wire through the core gap
where the secondary used to be and run a known current through it.
Measure the current through the original primary. Now you know the turns
ratio, and you can adjust accordingly. Say it had 100 turns on the
primary: you could measure up to 10 Amps with this (10 amps/100:1 turns
ratio = 1/10 amp on the measurement winding)