Re: why 1:5 and max 1000? (fwd)
---------- Forwarded message ----------
Date: Sun, 10 Oct 1999 09:22:11 EDT
Subject: Re: why 1:5 and max 1000? (fwd)
In a message dated 99-10-10 05:31:43 EDT, you write:
> What bothers me is that i donīt understand the max prractical ratio of
> 1:5 and the max turns of 1000. Are thes practical values or can it be
>calculated with which formulas???
> Thanks, Arwin Holland
The 1:5 max ratio seems reasonable to me, because if you make the
secondary too tall and narrow, the coupling will be too loose. If you
make the secondary too short, the sparks will hit the primary and the
coil will break down too. Also,
tall narrow secondaries will have less inductance for a given amount
of wire than a shorter wider one. This result can be seen using
the formulas for inductance such as wheeler's formulas. There are
also computer programs that can be used to calc the inductance
and coupling for various coil designs. Practical considerations also
enter into the statement, based on coilers results over the years.
Regarding the 1000 turns max, I often use 1500 turns with good results
(64" sparks using 1570 watts), and with few turns such as 400 turns, I
generally get poor results. Others have found different results. The
inductance of the secondary is the real
key, not so much the number of turns. For instance if the secondary is
very wide, this will increase the inductance, so fewer turns can be used.
For instance TC builder Skip Greiner, used about 300 turns on his
secondary and got good results, but his secondary was about 36" high,
and 17" wide. If you use many turns, such as 1500, the concern is that
the resistance will become too high, due to the thin wire, and result in
large losses. However there are always other factors. For instance using
thin wire on the secondary will permit more turns to be used on the
primary, which may reduce the spark gap losses, and actually increase
the efficiency of the TC.
There is still a certain amount of controversy that surrounds some of
these coil geometry issues.