Re: DC secondary components

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

[DC blocking cap in coil base]
Jim wrote:
> Say at 100 kHz, you'd want the impedance of a few ohms, so the
> voltage didn't rise too high.  C = 1/(6.28E5*X)... Ball park
> it at around 0.1 uF...  

Yup, wonder what DC rating it would require?

> this would make an excellent science fair project

Indeed.  Glad to see that just building a TC is not enough to qualify
for a science fair, otherwise it would be just an 'engineering fair'.

[DC pedestal under the coil base]
Bert wrote:
> Greg Leyh actually did this experiment a couple of years ago using
> one of his large coils. He used a high energy capacitor bank
> charged to a high potential to elevate the base of his secondary.
Antonio wrote:
> If I remember correctly, the capacitor was charged by the DC
> current that appears in the secondary coil as consequence of
> the rectifying action of streamers. 

Oh, so we're not clear which of two different experiments were going
on here.  Have to do some digging for the original stuff...

Bert wrote:
> I suspect that you'd need to elevate the secondary DC potential to
> a significantly higher potential  (many 10's of kilovolts positive)
> before you'd actually begin to see changes in streamer behavior. 

Thinking about it, I'm inclined to agree.

It might be that inserting a blocking capacitor into the base would
be a good method of measuring the *average* DC component of the
streamers, since it integrates it.  Seems to me that the charge
fetching up on this blocking capacitor would equal the total of
the +/- charge 'imbalance' achieved within the streamers.

Robert Heidlebaugh wrote:
>  When I adjust my grid leak resistor to pure sign wave with a
> sphere on top I get a smooth blue carona and max plate  current at
> about 250Kv. when I adjust my grid leak bias to unbalanced sign
> wave my current drops slightly and stringers start to form as a
> charge builds on the sphere, AC and DC together to form stringers. 

There's no mechanism whereby the DC biasing of your VTTC can insert
DC into the secondary.  The effect you describe is no doubt due to
the tube(s) being biased further back into cut-off, thus increasing
the efficiency and output power of the exciter.

The plate current may be very 'unbalanced', but the TC transformer
doesn't couple the DC component through to the secondary.

** I think I may have dropped a list post - did someone say they'd
seen a couple of kV across a DC blocking cap in the sec base?
--
Paul Nicholson
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