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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
>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
>of each primary bang in something less hazardous to my basement (and
>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
>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.

Geek#1113 (G-1)