RE: Micro coil help

>From bert.hickman-at-aquila-dot-com Sat Nov 30 10:12:30 1996
Date: Fri, 29 Nov 1996 22:43:17 -0800
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Micro coil help

Tesla List wrote:
> >From dknaack-at-rdtech-dot-com Fri Nov 29 19:59:25 1996
> Date: Fri, 29 Nov 1996 19:41:20 -0600
> From: David Knaack <dknaack-at-rdtech-dot-com>
> To: 'Tesla List' <tesla-at-poodle.pupman-dot-com>
> Subject: Micro coil help
> Here is my design for the micro coil I am building.
> I have a few questions tho..
> Thanks for any input you have!
> ---------------------------------------------------
> Transformer:
>   Voltage        : 3.5 Kv
>   Current        : 3 mA
>   Impedance      : 1166666
> Primary Cap:
>   Capacitance    : 0.002274 uF
>   Reactance      : 48.6
>   --Constuction
>     Plates       : 2
>     Dielectric   : Glass ( K=~6)
>     Area         : ~1.7 in square
> Secondary:
>   Turns        : 650 ( 36 AWG )
>   Height       : 4.5 in
>   Width        : 1 in
>   Res Freq     : 1.44 MHz
> Primary:
>   Turns          : 13
>   Inner diameter : 0.75 in
>   Outer diameter : 2 in
>   Width          : 1.25 in
>   Req Inductance : .00537 mH
>   Ind. -at- turn 9  : .00533 mH
> ---------------------------------------------------
> 3) Do I need to protect the xfrmer from inductive
>    kickback or something?RF choke? Safety gap?
>    There is a ceramic disk cap across the output
>    of the secondary on this xfrmer, what is it for?

This sounds like a small DC power supply, in which case the capacitor is
a DC filter cap. Depending upon the breakdown voltage of this cap and
the series diode (if a DC supply), you may run into problems trying to
use this in a Tesla Coil application. If possible, remove the diode and
output filter cap so that you're running directly off the transformer
secondary. This will also allow your tank cap to discharge safely when
you shut power off. Another option is to use a small neon transformer
(3.5 or 4.5 KV at 30 or 60 MA) which should be a little "tougher" to

It is an AC supply.  It is simply a largish transformer with a 3KV
cap across the terminals. It appears to be rated for 0.01uF

I made a mistake, the output voltage is only 1.2KV( 300 volt drop on 
one of four 1 meg ohm resistors in series), but that ups the current to
about 6ma. My first clue was that a 3KV cap would not be across a
3.5KV output... The impedance goes to 200K, and drops my primary
capactiance to around 0.013 mF

Should I still remove the output cap?  I would guess it is some kind
of protection against sparking ( somthing that happens alot with
fence transformers i suppose ).

I am currently looking for a neon transformer, but I haven't had
much luck yet.

> 4) The res freq calc is based strictly on the length
>    of wire used in winding the secondary, is there
>    a better way to estimate the value( without the
>    use of a signal generator )?  I have a 'scope.

A signal generator is really useful, although it is possible to build
one from linear IC's. There is a rough-cut formula for estimating the
self-resonant frequency of a air-core solenoid, and when I plug your
coil's size and number of turns, I get about 2.6 MHz.

I would like to build a signal generator like this, If anyone has
any construction details, I would like to see them if possable. 
Even some general ideas would be nice.  

> 5) When I put on a discharge terminal, how do I
>    determine what the new res freq is? Just move
>    the tap around and watch the output?

Adding a top terminator will significantly lower the frequency, since
the effective self-capacitance of you coil is very low (about 1.8 pF).
If you add a small toroid (say 1" x 4"), you'll add about 3+ pF, which
will take you down to about 1.5 MHz. Your best bet is to provide a way
to tap the primary for best performance. The primary you've described
would have a width of only 5/8" if you've got an inner diameter of 0.75"
and an outer diameter of 2". Did you actually mean radius?? In any
event, for about 1.5 MHz, this would be somewhere between turn 9 (if the
inner RADIUS of the primary is 0.75") and 11+ (if the inner DIAMETER of
the primary is 0.75") as measured from the inside.

I am working on a top terminator, it is toroidal, 6x1.5 in, it will probably be
something like foil covered paper mache ( I made a plaster form to cast
various materials in, paper seems to work the best).  How do you calc the
frequency drop? Is this a linear LC circuit?

oops, yes, I did mean radius.  The 2.6 MHz frequency is pretty high, but
with the toroid I think I can make it work.  I was planning on varing the 
capacitor to reach the res freq, since my primary is pretty small, and 
hard to tap.  I have it tapped a little above turn 9, and built a capacitor 
that I can adjust

I broke down and wrote myself a windows program to do the plate cap
calcs.  I can enter any three of the four variable values ( number of plates,
plate spacing, plate area, or capacitance ) and calculate the missing value.
The cap I came up with seems a bit small, but the calcs say it will work,
I want to get some opinions.  The capacitance is variable from 0 to 0.5 uF,
by the equations.  The cap is constructed with two strips of steel 7/8x1 in,
spaced 0.25 in apart, air dielectric.  Should be good for 4KV ( safety factor
of 3 ).  The calcs put this at 0.56 uF at max, but this is more than most
of the big rolled caps, this dosn't make sense to me, am I missing something?

Unfortunatly, I don't know how to measure such a small capacitance.
Can I measure DC current flow thru a large resistor until it drops to a
certain percentage of the max ( thirtysome percent? )  I did something
like this with some monster caps in high school physics class, but
its been a long time, and this cap is much smaller.
Or should I go buy some small caps of know rating and put them in
a circuit and measure the voltage across them?  I should be able to
calc the relative values by the voltage ratio correct?

> 6) This is the tank config I was planing on, should
>    I wire it exactly as shown, or do I need some other
>    components( xfrmer protection )?
>      120v   3500v             O
>      60Hz   60Hz              O Secondary
>      ----O||O--------o o--  O O ( fired on
>          O||O     |      |  O O   counterpoise )
>          O||O     |      -->O O
>          O||O     =         O O   -o o-  : Spark gap
>          O||O     |         O O   =      : Primary cap
>          O||O     |         O O   O      : Coils
>      ----O||O---------------O O

You need to swap the spark gap and the tank cap, such that the gap is
directly across the transformer output. This will reduce the magnitude
of high voltage RF that will get into your power transformer during
You could also try using series inductors in the HV leads from the
transformer, but these are probably _not_ necessary with your small

Its good to hear that, I don't think that this transformer would stand up
to much punishment. 
> 9) If I do need some kind of RF choke, what can I
>    use?  Will the secondary on a dead transformer
>    work?  How can I tell if a homemade RF choke is
>    effective without risking my xfrmer?

There's no magic answer here... With a small system, you may actually
create more problems by adding series chokes! Just make sure that you
don't open the gap wider than that which will jump from your supply
WITHOUT the tank cap/primary connected. For a 3.5 KV supply this is
probably around 1/8" or so...

ok, so I want the gap set so that when the supply is turned on, it will arc
over.  then I can add the cap and primary.
Is drawing a spark off of the output of the transformer hard on it? Should
I have some kind of current limiter so I won't burn up the transformer?
( After all, its only good for 10W. )

Thanks for the input!
David Knaack
South Texas

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