I'd think that 6" would be a pretty optimal secondary diameter for a ~2 kVA Tesla coil. If you can't find a sufficient length of it at Lowe's of Home Depot (try asking them if they have it or can get it, if you can't find it), I would try a local plumbing or irrigation supply. #26 AWG wire would be pushing the envelope in wire thinness for a coil this size, but it would work. And a 24" secondary winding length would be sufficient for this power level, assuming that you topped it with a properly sized toroid topload (~6"x 24"). You could probably get away with a 4" diameter coil, but 6" would be better and would likely give you longer sparks with a given amount of power throughput, compared to a 4" coil. So, in conclusion, I would aim for a 6" (or even 8") diameter by 24-26" winding length of about #24 AWG magnet wire (or 30" to 32" long for an 8" diameter form) for my finished secondary. If the PVC doesn't pan out, you can also use one of those cardboard
concrete former tubes. Lowe's and HD both carry these in 8" and 12" diameter by 48" long. The 4:1 to 5:1 is a good aspect range to aim for but it isn't really that critical, so long as you keep your dimensions within reason ( >3:1 and < 7:1 would probably be just fine). If you go the cardboard concrete former tube route, you'd just need to make sure that you completely evacuated any stray moisture from it (heat it or place it out in the sun for several hours on a warm, dry day) and then soak and coat it with several coats of polyurethane spar varnish (to keep moisture out), as cardboard is notorious for soaking in atmospheric moisture, especially in humid ambient conditions.
David
On Sunday, February 22, 2015 1:44 PM, Tim Gilmore <tdg8934@xxxxxxxxx> wrote:
I went to Lowes and found a greenish 6" pvc? Tube BUT it's only 24" and the only 6" pipe they carry. Also went to Home Depot and bought a 4" standard pvc whitish tube but only 24". They do carry longer 4" pipe but it would need to be cut from a 5' length.
Since I'm using 2 15000v 60mA NST I'm thinking I will need to use the 6" pipe but for 1200 turns Ill need 26 gauge for 22" but leaves me with a 3.5:1 ratio. Is this good enough not being at 4:1 or 5:1 ratios - or is it better to just use the more common 4" pipe (2' or buy a taller one) with the 2 NSTs?
Thanks, Tim
> On Feb 21, 2015, at 3:22 PM, Futuret via Tesla <tesla@xxxxxxxxxx> wrote:
>
>
> Tim,
>
> The secondary does not need to be as tall as the spark is long. For example
> my TT-42 coil has an 18" tall secondary and gives 42" sparks. However sparks
> are more likely to arc down from the toroid and strike the primary if the secondary
> is too short. I often place a smaller toroid below the main toroid to raise up
> the main toroid some to keep sparks away from the primary. The length
> of the sparks has a lot to do with the current in the arc and the growth
> of sparks over successive gap firings. It seems to me that a high spark
> gap break rate coil could tolerate a smaller secondary than a low break
> rate coil for a given input power, because the "bang size" (joules per
> gap firing) is lower in the high break rate coil. As far as I know no one
> has ever really researched this. It was the next thing on my list but
> I never got around to it. I usually prefer low break rate though because it can
> be more efficient in the sense of producing longer sparks for a given
> input power in a properly designed coil. High break rate has it's good
> points too though.
>
> Generally speaking, when the power is increased, the coil is made larger
> in proportion. If you use 4 times the power, then the coil should be twice
> as large in all proportions including the toroid (assuming the coil was sized
> correctly in the first case).... square law sizing concept. As you said,
> the wire should generally be somewhat thicker for higher powers.
>
> The 22 gauge plastic insulation wire will work, but may result in the use
> of fewer primary turns, which increases the spark gap losses somewhat
> due to the non-linearity of the arc resistance. Plastic insulated wire will
> also run at a higher frequency which generally results in shorter sparks
> also. Generally the magnet wire is a better choice. Very large coils
> may better tolerate the plastic insulated wire however.
>
> Cheers,
> John
>
>
> All,
>
> If I use both 15kv/60mA NSTs (yes they were identical and phased the same
> with my jacobs ladder - bigger flame spark), then the sparks could be 4-5
> feet in length so shouldn't the secondarys also be 4-5 feet tall. All of
> the designs I see are about 2 feet for the secondarys but are wider in
> diameter like 4" to 6" or more. This doesn't make sense to me. Why go wider
> in diameter and not taller in the secondary? I would think you would want
> thicker wire like 22 or 20 gauge wire and not 24 or 26 gauge. Also does it
> have to be magnet wire. Radio Shack 22 gauge wire has plastic like
> insulation. Is that ok?
>
> Thanks,
>
>
>
>
>
>
>
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