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How do I make my HV chokes?
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From: Gary Lau 19-Feb-1998 0851 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Thursday, February 19, 1998 8:32 AM
To: tesla-at-pupman-dot-com
Subject: Re: How do I make my HV chokes?
>From: D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
>to: Matt
>If running a NST then run a 500-1000 Ohm, 50 watt resistor in series with a
>2 inch dia PVC tube on which is wound approx 60-80 turns #18 AWG magnet or
>PVC insulated wire. Use this protective system off each bushing. Also a
>good idea to have a simple "bent wire" sparkgap near each bushing so
>excessive potential can flash over the 3/8 inch gap before frying your NST.
>
>DR.RESONANCE-at-next-wave-dot-net
The spark gap advice is right-on and should always be used. However,
I must respectfully disagree on the inductor and resistor advice.
An air core inductor as described above would yield only about 0.14 mH
and there's no need to use wire as large as 18 AWG.
More importantly, adding this meager inductor in series with the
HV xfmr, whose secondary is hundreds of HENRIES just by itself, can do
nothing to keep the high frequencies on the outside of the xfmr. What's
needed is a bypass capacitor to form a low pass filter, IN ADDITION to
the series damping R and a MUCH larger inductor. To have a low pass
filter, there must be some circuit element routing the HF energy to
ground, and a series-only L-R circuit has none.
I have seen advice advocating L-and/or-R-only protection networks posted
to this list many times but have not seen any rationale for using these
configurations beyond anecdotal "I've been using this and had no failures
yet". Can anyone defend series-only protection networks in a more
analytical manor?
Gary Lau
Waltham, MA USA