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Bicycle Wheel for toroid?



Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>


Bicycle wheel for toroid.

 

Last Thursday I set up an 18-inch BMX bicycle wheel as a "toroid" on top of my
10 7/8" x 3" 440 turn secondary, connected 2 parallel saltwater caps (a couple
of 2-litre pop bottles) in the primary tank and tuned my coil for best spark at
6 1/2 turns on the primary.

I repeated this using one saltwater cap, and tuned for best spark at 8 1/2
turns of primary.

Then I tried 2 saltwater caps in series. Best spark was obtained with 11.3/4
turns of primary but ran out of turns- probably would have been better if there
were more turns on the primary!

So far all sparks were about 2" except with the 2 caps in series when the spark
length was shorter, but this to be expected coil was probably incorrectly tuned
due to lack primary turns and smaller total capacitance of the capacitors when
they were connected in series. Spark are brighter and hotter (and more
noticeable!) than with the small toroids I have previously used (e.g.. 1 toroid
1" high by 4.625" across, or the same plus 1 to 2 toroids 1.125" high by 5.75"
across on top, with or without an additional 9" pie tin!)

On Friday I repeated experiment with 2 foot bicycle wheel and I am now getting
3" or so sparks -hot and blue- with 2 saltwater caps in the primary, which is
tapped at the 

6 3/4 turn.

This is best performance so far with this coil although the 2 foot "toroid"
makes it enormous (volumetrically inefficient or what!). I am still using my
ignition-coil based HV driver (powered from 13.8 VDC 5 Amp PSU) to DC-charge my
primary cap via 60 x 1N4007 diode string; charging gets increasingly slower
with higher and higher values of primary capacitance! which would need to be
increased even more if toroid size were to be further increased (!) but for
space reasons alone doubt if it would be practical to go much further down this
route! 

That said I think that using bicycle wheels as toroids may have advantages;
after all the rim is suspended on spokes which should interact very little with
the magnetic field and the small surface area afforded by the spokes should
mean there will be little extra capacitance between the hub and the rim and to
be added to the "toroidal" capacitance at the rim.

Are my assumptions correct? Has anyone used a bicycle wheel or similar as
toroid like this before and finally, is a 3" spark good performance good for a
coil of this type and size?