Re: New type of TC RFI filter
Hi Terry and all,
This is very interesting as I have been fighting to get rid of those
primary bursts for a long time with little success so far. This sounds
like a good possible solution. It sounds like you will get chance to try
it out before me. My coil is currently undergoing a re-build.
Thanks for posting these findings, and keep us informed.
- In sunny Newcastle.
On Thu, 5 Aug 1999, Tesla List wrote:
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> Hi All
> Tonight while trying to get my spark gap model to fit real measurements
> (not quite there yet :-0), I think I found what causes those giant current
> spikes. Also, I think I have a way to eliminate them now.
> It appears that the primary coil has self capacitance just like the
> secondary coil. The space around the primary becomes charged and stores
> energy in the form of capacitance. For my coil, I measured this
> capacitance as 38.9pF. At 21kV that works out to only 0.0086 joules of
> stored energy.
> However, that energy is basically shorted by the spark gap and is
> dissipated in about 500nS. So we have a pulse with a burst power of about
> 17 kilowatts. This pulse appears to be responsible for ringing every stray
> L an C in the primary circuit and generates all kinds of RF noise up into
> the GHz region.
> Modeling shows that a small inductor (100nH) in the primary path would
> easily stop this pulse. Unfortunately, a very small capacitance (5pF)
> across that inductor would simply restore the high frequency path again...
> Thus, it would almost be impossible to make the inductor with low enough
> parallel capacitance to work. At thousands of amps, GHz frequencies, and
> 20+ kV a noise filter is not basic.
> However, one may be able to make the high frequency filter with just two
> pieces of heavy (like RG-8) coax. The circuit would look like:
> Two x 2-foot long sections of RG-8 coax would be placed in-line with each
> lead from the spark gap to the primary coil. The coax shield would be
> grounded on the spark gap end and left open on the primary end. The ground
> connection between the two coax cables would need to be very good! They
> would also need to be grounded to the local system ground but with less
> This may really help those that are having problems with RF noise...
> It will take me a day or so to test this out, but I think it will work.
> Any comments or suggestions are welcome...