Re: DRSSTC Additional questions

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

> 1. The tuning of the system is much broader than any
> TC that I have
> ever built... Is
> this typical of
> DRSSTCs?

Yes. However I would emphasise Terry's point, that the
tuning may not affect the spark length much, but it
affects the primary current a lot more. On my coil, I
tune the primary lower than the secondary. I start
very low which gives a low primary current, and then
raise the primary frequency closer to the secondary
till I get the current I want.

That only works if the system is running on the upper
split frequency, which will be the case if you're
using secondary base current feedback. Changing to
primary feedback with the primary tuned below the
secondary will shift you over to the lower split
frequency.

The tuning characteristic is now reversed- as you tune
the primary down further, the current increases and
the sparks get fiercer. So if you're using primary
current feedback, it's probably best to start with the
primary tuned just a tad bit below the secondary, and
lower it till you get the desired spark length or blow
something.

I use a PLL driver circuit that allows me to choose
whichever split frequency I want, no matter where I
take the feedback from and how I tune the coils. My
findings are that the upper split frequency gives more
stable and predictable behaviour, but that may come at
a cost in spark length.

The tuning that I settled on uses the secondary tuned
to 198kHz, the primary tuned to 177kHz, and the driver
operating on the upper split frequency which works out
at 220kHz. I found that taking the primary frequency
higher just sends the primary current sky high without
much increase in spark length.

The lower split gives me awful primary-to-secondary
flashovers so I can't really test it at high power
until I've made a new primary.

Steve Conner
http://www.scopeboy.com/