Fw: Primary dummy-loading

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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