Spark Gap Losses And Thoughts...
Taking many observations, measurements, models... into consideration, I
think there are two general types of gap that one should try to shoot for.
The first is the quench gap. These gaps try to transfer the energy from
the primary into the secondary and trap it there by opening the gap while
the energy is in the secondary system. They have the advantage of reducing
stress on the primary cap and components, and being easy to make. However,
they tend to have much higher resistance and will loose a lot of system
power if they fail to quench. Since higher currents tend to be difficult
to quench without making a really high loss gap, quench gaps tend to be
better on low power (~1kW) systems. The multi-gap types made from many
pipe sections seem to work best.
The other type of gap does not rely on quenching. They depend on providing
a purely low resistance path for current to flow while loosing as little
energy as possible. They are terrible at quenching but since they loose
less power, they can accommodate many primary to secondary energy transfer
cycles without becoming a major loss to the system. Rotaries and simple
gaps fall into this category. They have the advantage of being able to
handle very high currents and are thus suited to high power systems.
I also suspect that if one doubles the current through a spark gap, the
resistance is half of what it was. In other words if you have twice the
current you dissipate twice the power and not four times as the I^2 rule
would suggest. If the I^2 rule were true, as you doubled the primary cap
size, the system arc power would not change much. But if you assume the
resistance of the gap drops in half, the system arc power about doubles.
This is gleaned by observation, measurement, and computer modeling. It
appears such basic rules must hold true for our coils to exhibit the
behavior they do.
The gap losses account for around half of the power that goes into a TC.
Reducing this loss can have a dramatic effect on system performance. So
far, the best gaps I have used are sync rotaries with only two gaps with
close spacing and large cool running electrodes. Such a gap does not even
try to quench but rather provide as low of resistance as possible. I
suspect that a single gap rotary would be better but that would require a
500+ amp AC slip ring assembly...
In my coil's primary, my best guess as to the primary resistances are as
Primary cap AC resistance 0.060 ohms (10 x 5 EMMC)
Primary conductors including primary inductor 0.488 ohms
These were measured with my HP meter and from poly cap data sheets and seem
The gap resistance should be about 2.5 ohms.
So, the primary inductor (0.25" tubing) and cap are not loosing too much
power. A heavier primary may help a little bit. But the big loss is the
gap accounting for about 80% of the primary loss or 325 watts out of 860
into the coil and 273 watts into the output arcs (also, 130 watts in the
neon and filter system and about another 130 into other things).
So, the gap is where to extra power is. At full power, a TC does not have
that many energy transfers so I do not think quenching is that big of deal
for a system that puts out pretty good arcs. I think one needs to work on
making the gap as low resistance as possible...
Thoughts and comments always welcome.