Re: Gap turn off at zero current and TVI
Hi Terry, Malcolm, Gary, and all,
Thanks for the suggestions about how to eliminate the VHF bursts when
the tank current rings through zero. I have spent the last few days
building an aluminium screened box with a tank cap and coil, along
with an RQ style spark gap. HV is fed in from a neon trannie
through two 1/4" holes in the case. Another 1/4" hole allows a scope
probe to be connected.
First I tried running the circuit with only the RQ gap inside the box,
and the L and C outside in the room. The interference was bad. I then
put the whole tank circuit inside the box and fed in HV from outside
through two wirewound resistors. The box was bolted along all seams,
but the interference was just the same. I couldn't believe it.
I am building a battery powered HV source so I can seal the whole lot
in the box. Seriously.
Unfortunately the scope I had used before went back to my workplace,
and I have now changed jobs ! Now I am stuck with my 20MHz type which
is not storage, so I can just about see the VHF bursts if the scope
is on full brightness in a pitch black room !
Connecting a snubber across the gap or the primary winding (maybe both)
is an interesting idea. I have quite a few poly caps and bits from
MMC's around but need to get some good HV resistors. I tried
connecting various caps across the gap or across the primary coil but
they did not seem to make much difference. (The resonant frequency did
drop slightly when connected across the coil.) This should have a
similar sort of effect as using a ribbon primary, Gary ?
I was dissapointed that I could not "shift" the VHF bursts to a less
anoying frequency by adding capacitance. I must try adding some
resistance to make an RC network as this should actually absorb some of
the energy better. Some PSpice sims make this look very promising. It
may indeed give better quenching !
This RC snubber across the gap is similar to what many use already to
protect neons. Has anyone actually had an improvement in TVI when
employing such a filter and removed it again to check ?
In a discussion with an EMC Eng where I work, he pointed out that stray
capacitances in the order of pico-farads would still result in
oscillations well below VHF TV frequencies. He could not think what
would cause oscillations at such very high frequencies. He also said
that if I could remove enough energy from the bursts then the gap may
not re-ignite after the current zero. It may be only the terrific
voltage produced in the bursts that keeps the gap lit through the
zero crossings !
This is certainly weird, and I am learning as I go. I will keep the
Tesla List posted on any developments.
Awaiting further comments and suggestions.
All welcome. (I need all the help I can get)
- In sunny Newcastle
(except when Alan's in town :-)