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Re: Fw: Primary dummy-loading

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

Hi Chuck, all,

On 5 May 01, at 12:40, Tesla list wrote:

> Original poster: "Charles Hobson by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>
> From: Tesla list <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Friday, May 04, 2001 11:46 PM
> Subject: Re: Primary dummy-loading
> > Original poster: "Dr. Duncan Cadd by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> >
> > Hi Gary, All!
> >
> > >While I really can't be running my coil in my basement due to
> > >streamer-strike concerns, I would like to be able to do some level
> > >of live indoor testing to scope gap timing.  If I simply remove the
> > >secondary, there can be no quench and the gap and cap activity will
> > me
> > >much more stressful than I'd like to impose on them.
> > >
> > >I was wondering if anyone has come up with a way to dissipate the
> > bulk
> > >of each primary bang in something less hazardous to my basement
> > >(and
> > me
> > >and my scope) than streamers.  I was thinking of either some kind
> > >of dummy load resistor in series with the primary.  Or perhaps
> > >placing a sheet of aluminum slightly above the primary to dissipate
> > >the bangs
> > as
> > >eddy currents?
> >
> >
> > I think both of these will work, the eddy current load is
> > particularly innovative, but I think your problem is going to be
> > substituting a suitable load without changing the very quench/timing
> > characteristics you want to measure.  That is going to be the tricky
> > bit.  You can couple a resistive load (e.g. a "cantenna" - oil
> > filled ham transmitter load) with the same coupling constant as the
> > normal secondary which you can obviously determine in the usual way,
> > but can you be sure that a purely resistive load produces the same
> > quench & timing characteristics as the secondary when sparking, and
> > what exactly is the right value of load resistance at that same
> > coupling constant anyway?  Problems I have often mused on, with no
> > useful outcome :-(
> >
> > I feel your best bet is to take the empirical approach and try
> > connecting a variety of resistive loads, and maybe add a bit of
> > inductive and/or capacitive reactance to the loads, and see if the
> > quench & timing characteristics do vary according to the value of
> > load resistance and reactance.  Maybe the variation will not be
> > large, but this could be useful data to measure in any case, I'm
> > sure someone on the list will appreciate it.  It's a moot point
> > whether you can successfully equate this to a sparking secondary
> > though.  The other option is to have a grounded rod close enough to
> > the topload to ensure that all sparks go to the grounded rod and not
> > the other items in your basement.  For scope protection I have used
> > a small neon bulb directly across the probe, in parallel with a low
> > value (1 ohm) resistor, and this seems to work quite well (and yes,
> > the neon did light up with a 1 ohm resistor across it :-)
> >
> > Hope this helps.
> >
> > Dunckx
> > Geek#1113 (G-1)
> >
> Hello all, This is an intriguing problem, but have hesitated jumping
> in on this thread until the notion of a "ham" dummy load was
> mentioned. Most antenna dummy loads have a resistive impedance of 50
> ohms. It appears that you would need to substitute a secondary coil
> with another coil to match the primary coil impedances to the dummy
> load and use the same coefficient of coupling. However, by doing this,
> (no mean task)  there would be  high peak RF currents and voltages to
> deal with. . For example, consider 4 joules per bang, and the
> estimated duration of energy dissappation being 50usec (a conservative
> estimate), the peak power would be 80kW. With a 50 ohm load, this
> works out to 40 amps rms during this 50usec interval. The RF voltage
> during this same interval would be 2000V rms. Based on this, I think a
> dummy load rated at  a kW will probably handle this. The
> interconnecting coaxial cable needs to be considered also for these
> kinds of currents. If there was a mismatch between the Tesla Coil
> primary and the simulated load, the 80kW of peak power would be
> ringing in and out and back and forth until it is all dissipated.  I
> hope this helps.
> Chuck

There is a serious problem with using resistors in this way. The 
transient nature of secondary loading under spark conditions is being 
overlooked. If one were to match the secondary such that the load 
absorbed all energy, you would end up with a critically damped 
secondary response. In practice, the secondary normally rings up 
until a load begins to appear. Or have I misunderstood the aims of 
the proponents of this idea?