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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 mF
> 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
> ---------------------------------------------------
> 1) Why are the transformer impedance and cap
> impedance supposed to be equal?
For neons, this is to maximize the amount of power that can be
transferred to the tank cap for a givent power output from the neon. A
smaller cap can be used with no problem. A larger cap will load down the
neon so that it won't develop full output voltage.
> 2) Did I get the current correct on the xfrmer?
> the output appears to be 3.5 Kv and it is rated
> for 10 Watts.
Looks OK.
> 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
damage.
> 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.
> 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.
> 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
operation.
You could also try using series inductors in the HV leads from the
transformer, but these are probably _not_ necessary with your small
system.
> 7) Using this design, how can I check that I am
> getting the primary to 'ring'? My concern is that
> instead of getting the primary oscillating, I will just
> have a short circuit thru the gap and primary.
If you are charging the primary cap and firing the gap properly the
circuit WILL ring. The trick is to get the primary circuit tuned to ring
at the same frequency that the secondary wants to ring at.
> 8) Do I need more than one gap in series? What
> should I set the gap spacing to?
At the low power you'll be running at a single gap between carriage-bolt
heads should work just fine.
> 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...
>
> Thanks again!
> David Knaack
> South Texas
Your welcome. Good luck on your micro coil!
-- Bert --