Re: A few questions

-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, June 18, 1999 9:41 AM
Subject: A few questions

>Original Poster: christopher boden <chrisboden-at-hotmail-dot-com>
>A couple questions from the Geeks.
>1. What is the effect of capping the ends of the secondary? Is it really
>nessessary? Why?
To keep the arcs from going down the inside of the tube.... If a 24" long
coil can make, e.g. 36" sparks, you have to provide some means to prevent
the sparks from going inside...

>2. Has anyone ever thought of using MOV's instead of a safety gap? Would
>this be possible? Why?

MOV's are not an infinite life device.  Every hit they take degrades them a
little bit. They are also not adjustable, and, you'd have to make a HV MOV
by stringing a bunch of lower voltage ones in series.
An adjustable gap is awfully easy....

>3. If the secondary is capped, is it good practice to mount studs in either
>cap for electrical connections to the ground strap and toroid?

If the stud goes through the cap, then it provides a path for the HV to get
to the inside of the tube.

>4. We have 2 15' ground rods with VERY heavy ACSR wires connected to each.
>We had them placed espically for TC use. They're 2' apart with less than 1
>ohm between rods.
>How do we connect them to the TC? Should we use the ACSR or copper strap?

ACSR = Aluminum Core Steel Reinforced = what they use on HV power lines
ACSR is probably massive overkill and not appropriate anyway, although you
probably got it for free.... You don't really need the strength of the steel
(in it's normal application, with multi hundred foot spans, you do need the
strength of steel), nor the low density of Aluminum (Al is best on a
resistance per pound basis, also suitable for long spans).

I'd run a bunch of parallel copper wires, say, #14 or smaller house wire.
Cheap, easy, and very low impedance.  Unless the run is very(!) long, you
don't have to worry about resonance effects, but you do want low L and low
R, both of which are nicely obtained by a bunch of parallel insulated wires.
In general, you would worry more about L than R, BTW..  100 ft of #14 has a
resistance of about 1/4 ohm. But, that same 100 ft has an inductance of
about 30 uH, which, at 200 kHz, is an impedance of 37 ohms.  If you were to
run, say, 10 strands of #24 instead of the #14, the resistance would be the
same, but the inductance would be 1/10, so the inductive impedance would be
about 4 ohms.  The wires will be in close proximity, so you don't get quite
that much improvement (the magnetic fields interact) but it will be close.
And, ideally, you should braid the strands (i.e. Litzendraht wire), but as a
practical matter, you can just bundle them together. I think the old NBS
circular 74 says the extra L from just bundling is 10-15%.