On 11/2/21 5:39 PM, Gary Lau wrote:
> <snipped>
>
> For setting the spark gap width (both main and safety), this must be done
> experimentally, with the primary tap disconnected. You want the spark gap
> to *just* fire when the Variac is at maximum. Don't do it with the cap and
> primary connected because mains resonant rise is at play, and voltage can
> easily ring up to many tens of KV if the gap allows it. You want the gap
> to fire at 12KV*1.414 V. There is no rule of thumb for total gap distance.
Indeed - many of us has killed a NST with capacitors attached.
This is why I like the configuration where the main gap is across the
NST, and the primary and cap are in series across the gap.
You can disconnect the primary tap, set the gap to "just not fire" at
your "not quite full tilt" on the variac. (or, set it to "just fire")
and then you're protected if the line voltage goes up, you get
enthusiastic if you boost the Variac to 11, etc.
In general, the NST is more fragile than the MMC. As Gary said, the MMC
just dies a little. The NST fails hard.
(and then you can browse through all the posts from decades of "how do I
resurrect my NST" involving barbecues, melting tar, etc.)