More Mini Coils
Subject: More Mini Coils
Date: Mon, 16 Jun 1997 10:36:16 +1200
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: Wellington Polytechnic, NZ
Here are some results from the weekend's playing around.
Since I have taken no photos, I am presenting sufficient information
for you to duplicate my results. I have achieved 4" attached single
channel streamers (with 4.5" at odd times but not often enough to be
convincing) using a power input of 20W.
Power supply: The flyback supply I published on the HV list. Some
measurements on throughput and frequency I took last night indicate
it is drawing a bit over 30W from the mains but I have yet to measure
that exactly. Will post when I have done so. If true, efficiency is
around 67% which is right in the ballpark for a typical switcher. In
claiming 20W, I am measuring by: Ecp x BPS.
Coils used: Twin secondaries, one driven by a primary.
Powered secondary: Hs = 7.5", Ds = 2", closewound with 0.3mm copper
diameter wire. Top load : 3" sphere 1" above the windings.
Counterpoise secondary: Connected to common earth with powered
secondary. Hs = 7", Ds = 2", closewound with 0.3mm dia wire with
a 4" x 3/8" toroidal top load mounted 1.5" above the windings.
Primary for secondary #1: 3.5 turns 1.2mm copper diameter wire, Dp =
4", Hp = 0.5 - 0.6". Cp = 7.8nF mica doorknobs.
The coils were placed so that the secondary windings started
about the same height off the bench. Both secondaries are wound in
the *same* direction (sorta pseudo half-wave), and where positioned
as described, the secondaries were exactly 5.5" apart (winding to
winding) with the terminal 4" apart at their closest points. I
omitted to disconnect the earth to the two secondaries but will do so
next time and just let them work one against the other.
I set the gap to 6kV and progressively closed it up until the
streamers connected in a single channel. Best for this distance was
Vgap = 4.5kV at 250 BPS (scoped). Ecp was about 80mJ. BPS roughly
doubled over the 6kV setting.
I checked out max distance the "twins" would play with
each other at. This turned out to be about 7" with a shower of wispy
streamers connecting them at Vgap = 6kV, even at very low BPS
showing voltage counted most where ionization was lacking. No doubt,
they would connect single-channel at this distance if BPS went up to
the 250 mark (around double the supply power/current).
By separating the coils considerably (minimizing mutual
coupling) both emitted their own streamers showing that they are
mirroring each other via a common base connection (currents in
opposite phase) and not working through mutual inductive coupling
(which would have made them oppose each other).
I am now convinced that there are records there for the taking,
and that matching the supply to the coil is where it's all at and
this was supported by my earlier single mini-coil experiments. One
important thing I noted was that the cap has to be large enough to
match the supply current characteristics for good quenching (i.e.
supply current must not be too great for the capacitance). A distinct
lack of quench at high BPS was noted when I tried using around 2nF and
power arcs were the result at low Vgap settings. I wouldn't like to
say that 7.8nF was optimum for this setup either.
You should be able to calculate all frequencies, capacitances,
inductances etc. from the information given.
I think the results speak for themselves. I will find time to
photograph this soon. BTW, the banjo effect was very noticeable to
the eye as the streamers rose.