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Re: Ballast



Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jim, all,
             Referring to this post and Mark's note on MOT current 
draw, it's as well to note that whichever transducer is used (CT, 
resistor etc.), a meter is *not* going to give you an accurate 
picture of what is going on. You need an oscilloscope to do that. For 
example, MOTs appear milder than they really are when a current meter 
is used. The scope shows that the primary current is far from 
sinsoidal with peaks beyond 16A. I checked this out with an isolated 
scope and 0.1 Ohm sense resistor in the neutral lead. The primary 
current waveform looks something like this:

      *
     * *
    *   *
   *     *                       *
*            **             **
                 *       *
                   *   *
                    * *
                     *

Regards,
malcolm

 
On 6 Nov 2001, at 14:29, Tesla list wrote:

> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> But watch out with that 100A across 1 ohm, because it will dissipate 10 kW!!
> The best overall solution is to use a current transformer.  The usual
> current transformers are rated for a 5A secondary current and 50,100,200,
> etc. amps through the hole in the middle. That is, their turns ratio is
> 10:1, 20:1, 40:1, etc. They run about $20-25 BRAND NEW, and around $5 used
> surplus.
> 
> You can get standardized shunts (50 mV = rated full scale) fairly easily,
> or a short piece of copper wire can serve as an admirable shunt.  AWG10 is
> 0.001 ohms per foot.  Make your shunt, then calibrate it with a known
> voltage and load (I wouldn't use a 100W light bulb, by the way, because
> their resistance isn't all that constant).
> 
> Say you coil up 10 feet of AWG10 (for 0.01 ohms).  You get a good quality 1
> ohm resistor, hook it in series, hook up a car battery (so about 12 Amps
> will be flowing, dissipating 150W in the resistor!), measure the voltage
> across the whole thing, measure the voltage across the 1 ohm (it will be
> around 12V), measure the voltage across your shunt (should be 120 mV, or
> so), and now you have a calibration.
> 
> You can use finer wire, or NiChrome, also.
> 
> 
> 
> Tesla list wrote:
> > 
> > Original poster: "R Heidlebaugh by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> > 
> > on 11/05/01 4:29 PM, Tesla list at tesla-at-pupman-dot-com wrote:
> > 
> > > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <A123X-at-aol-dot-com>
> > >
> > > I'm planning to make a gapped core ballast with an MOT core. I'm
looking to
> > > get
> > > around 16amps, or 8amps if I use my variac to take the voltage to 240v.
> > > Anyway,
> > > can I just use the original primary as the winding for the ballast?
Also I
> > > don't have anything to measure the current with. Will radioshack have
good
> > > enough meters?
> > >
> > > Mark
> > >
> > >
> > If you measure the voltage across a known resistor you have the current
> > equiv. I like to use a 1 ohm or a o.1 ohm resistor so I dont nead to think
> > about conversion. If you use less than 0.1 ohm you nead good meters and
> > calibrated standards. 100v across one ohm is 100 amp
> >     Robert H
> 
> 
> 
>