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*To*: tesla-at-pupman-dot-com*Subject*: Re: Lower secondary cself => better performance?*From*: "Tesla list" <tesla-at-pupman-dot-com>*Date*: Tue, 13 Feb 2001 06:38:11 -0700*Resent-Date*: Tue, 13 Feb 2001 07:12:17 -0700*Resent-From*: tesla-at-pupman-dot-com*Resent-Message-ID*: <lsw67D.A.mHC.-CUi6-at-poodle>*Resent-Sender*: tesla-request-at-pupman-dot-com

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <paul-at-abelian.demon.co.uk> Marco wrote: > If I recall correctly, in the past it has been suggested to build a > secondary coil with a certain H/D ratio (also) in order to minimize > its self-capacitance. The coil's effective capacitance does show a minimum in the region of unity h/d, as suggested in Medhurst's figure 9. This is easy to understand in terms of the separate contributions of internal and external capacitance. As the diameter is reduced, both Cint and Cext are reduced. As the length is reduced, Cext is again reduced but Cint increases (since points on the coil with a large PD between them are brought closer together). Thus there will be a minimum capacitance, at a certain h/d, below which, Cint dominates and above which, Cext dominates. The modest h/d at which this minimum occurs is probably less than ideal for impulsed TC applications due to voltage breakdown considerations. > Was therefore believed that a lower self-capacitance results in > better performance? Based on energy conservation, the output voltage of the secondary for a given bang size net of primary losses is inversely proportional to sqrt( Cee) where Cee is the equivalent energy storage capacitance of the resonator. Thus, selecting the optimum h/d ratio and using the smallest possible coil length, along with the smallest possible topload, is the advice I would give to achieve the maximum output voltage for a given input energy. However, the experienced coilers on this list all seem to advocate using the largest possible topload, reporting that performance improves as the toroid size is increased. Why should this be? There must be some other factor(s) which in practice are more important than energy storage considerations. The conditions for obtaining efficient primary circuit operation and an optimum coupling to the secondary may demand a larger topload. The effective inductance Les of the secondary coil does increase as topload is applied, and it could be that an over-sized (from the point of view of energy storage) toroid is of benefit by enabling use of a higher primary L/C ratio and a lower f1, both of which may lead to greater primary efficiency. Also there is the matter of obtaining an optimum power-transferring impedance match between the toroid and the breakout loading, and it may be the case that a large toroid provides the appropriate shunt matching - the optimisation is then for power transfer rather than top voltage. Anyway, for what it's worth, those are my speculations on the subject. Cheers, -- Paul Nicholson, Manchester, UK. --

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