Re: Micro coil help Re: Micro coil help

Tesla List wrote:
> >From dknaack-at-rdtech-dot-com Sun Dec  8 11:54:25 1996
> Date: Sun, 8 Dec 1996 00:59:13 -0600
> From: David Knaack <dknaack-at-rdtech-dot-com>
> To: "'tesla-at-pupman-dot-com'" <tesla-at-pupman-dot-com>
> Subject: RE: Micro coil help  Re: Micro coil help
> >From bert.hickman-at-aquila-dot-com Sat Dec  7 08:45:42 1996
> >Date: Sat, 07 Dec 1996 09:24:47 -0800
> >From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Re: Micro coil help  Re: Micro coil help
> >No problem - glad to help! I re-read your previous post, and it's not
> >clear why the output of the transformer would have been directly shunted
> >with a 0.01uF cap if there wasn't an internal rectifier of some type. It
> >also sounded like the unit may have been used to drive an electric
> >fence. Electric fence drivers usually have some associated pulse
> >electronics to drive the fence with intermittent high voltage pulses. Do
> >you have any other information about the unit?
> Very astute, this is a continuous current output electric fence transformer.
> 10 watt max, 0.01 uF 3KV cap on the output, runs at 1.2KV.  Thats all I
> know about it.  I assume that it is designed for pretty low current output
> most of the time, with intermittent high current operation ( when livestock
> brushes up against it ). The only things inside the case are the transformer,
> the cap, and the two wire nuts connecting it to the input power. No rectification
> diodes or pulse circuitry.

No, not very astute... you had mentioned in an earlier post that it had
the 0.01 uF cap on there as protection from sparking because it was a
fence transformer... :^) 

> >Anyways... back to your questions:
> >Assuming your transformer outputs about 1200 Volts at 6 MA (from your
> >previous post), and you may be able to push it to 10 MA if you only use
> >it intermittently. This means that you need to restrict the primary so
> >that the transformer will not draw more than about 10-12 VA. Although
> >connecting an inductor in series usually results in better performance,
> >it may be simpler to use a resistive ballast in this case, since the
> >inductor size will be quite large.
> >Since you're running off the 120 volt mains, you want to restict the
> >input current to the transformer to about 0.1 Amp or so, which will take
> >a series inductance of about 2.5-3.0 Henries, or a series resistor of
> >about 1000 - 1200 Ohms rated at 15 Watts or better.
> When I was building my big speakers, I used a 3 H choke coil in the crossover,
> the coil was wound on a 1.5 inch, air core form with 20 or 21 AWG wire.  They
> are pretty cheap, would one of these work as an inductive limiter?  It wouldn't
> be good for high voltages or more than 200 watts, but I could run it at 120V with
> no problem.

This should work fine as an inductive ballast.

> I also have just been playing with a resistive ballast, made from two 5/8 x 7/8
> inch steel sheets in tap water ( no electrolyte added ).  I can adjust it from
> about 0.9 to 1.2 KOhms.  I would imagine that this will dissipate quite a bit
> of power with no problems.
> Which should I use?  I prefer the resistive version, since its free, I have to
> spend a little for the choke coil, but if it would not be a good idea to use
> it, i can get the choke.  Also, Storey offered to provide some transformer
> secondarys that would probably work too, although I'd have to measure
> the inductance on them.

Try out the resistive ballast to see if you can get any response. The
inductive ballast will probably perform better, however. 2-3 Henries
should do the trick. I'd migrate to a wire-wound resistor for
simpliccity rather than use the water resistor. It'd be a little safer
as well...

Storey's transformers sound a little too heavy for what you're looking
for, unless you re-wound them as high inductance chokes.

> Here are the revised specs for my coil.
> ---------------------------------------------------
> Transformer:
>   Voltage        : 1.2 Kv
>   Current        : 6 mA
>   Impedance      : 200000
> Primary Cap:
>   Capacitance    : .0133 uF         
>   Reactance      : 10 ( -at- 1.2 MHz )
>   Max Voltage    : 15KV ( Saftey factor = 3 )
>   --Constuction
>     Plates       : 60
>     Dielectric   : Poly ( K=~2.2)
>     Plate Size   : 6.4 x 6.4 x 0.02 inch
>     Overall Size : 7.5 x 7.5 x 6.75 inch

I susppect you've calculated the primary cap size of 0.0133 uF based
upon the current capability of the input power transformer. While this
is OK for maximizing the size of a coil, it does NOT apply when trying
to tune for a specific secondary/toroid frequency. If you're using only
0.020" thick dielectric, with a plate size of 6.4" x 6.4" plate size,
your capacitance will be closer to 1000 pF per dielectric sheet. 

If you stacked 60 of these, this would total to an even larger value of
about 60,000 pF (0.06 uF). Either size is MUCH larger than what you need
to properly tune your system for 1.2 MHz. See the calculations below. 

> Secondary:
>   Turns          : 650 ( 36 AWG )
>   Height         : 4.5 in
>   Width          : 1 in
>   Inductance     : 2.133 mH
>   Self Cap       : 1.8 pF
>   Self Res Freq  : 2.6 MHz
>   Target Freq    : 1.2 MHz

The target frequency (Fo) assumes a toroid on 1.5" x 6" per your
previous post... OK.

> Primary:
>   Turns          : 13
>   Inner radius   : 0.75 in
>   Outer radius   : 2 in
>   Width          : 1.25 in
>   Req Inductance : 1.326 uH

For a 13-turn spiral primary with inside diameter of 1.5" and outer
diameter of 4", the turn-turn spacing will be about 0.1", and you'll
need to use insulated wire with an outer diameter of less than 0.1". For
a spiral inductor of this size, the estimated inductance at various
turns would be as presented in the table below. This table was derived
from the pancake spiral inductance formula:

     L = (Rinner^2*N^2)/(8*Ri+11*W) (uH)

  where Rinner = Inner radius of spiral coil (inches)
             N = Turn number
             W = Width of Coil = Router-Rinner, where Router = radius at
turn N

With a tank cap, C, the resonant frequency of the primary inductance and
tank cap will be:  

     F = 1/(2*Pi*SQRT(LC))  Hertz
     where L = in Henries
           C = in Farads

If you want to resonate at about 1.2 MHz, we can solve the above for the
value of tank cap required:

     C = 1/(L*(2*Pi*Fo)^2) = 1/(L*(2*3.1416*1.2e6)^2) = 1/(L*5.685e13)
     C = 1e5/(5.685*L) pF where L is in uH.

                             (for 1.2 MHz)   Estimated 
  Turn            Inductance     Tank      6.4"x6.4"x0.02"
(from inside)        uH           pF        Plate Caps:        
    5                1.9         9499          10-
    6                2.6         6648           7-
    7                3.6         4895           5-  
    8                4.7         3741           4-
    9                6.0         2941           3  
   10                7.4         2364           3-
   11                9.1         1935           2
   12               10.9         1609           2-
   13               13.0         1354           2- 

Let's assume you tapped at about turn 11 to obtain 9.1 uH. For the tank
circuit to resonate at about 1.2 MHz, you only need about 2000 pF of
tank capacitance, which would equate to a plate cap made up of only 2
layers of 0.020" LDPE with matching 6.4" square plates on either side. 

Remember: you are setting up the tank circuit to resonate with the
secondary/toroid irrespective of the maximum cap value that COULD be
charged by the power transformer. Because of the tiny size of your coil,
you don't need a large amount of tank capacitance. You can fine "tune"
the primary cap by simply changing the amount of foil area overlapping
on one of the layers. Thus if you need 3- layers, make 2 full caps and
make the third adjustable so that you can use less foil area to tune it. 

> ---------------------------------------------------
> As I work out construction details for the cap, I have come up with several
> designs.  The first uses 1/8 inch plexiglass, but it takes so much plexiglass,
> it would be too expensive.  I suppose I will make a typical rolled cap, since
> I already have the plastic sheeting, just have to get the flashing.  How do I
> calculate the capacitance of the spiral type cap?  Can I just use the plate
> type calc and use 2 plates?

Yep! But for the small value you need, I'd go with a plate cap. Stay
with polyethylene dielectric - plexy is very lossy in comparison. 

> I will be out for the next week, until monday ( MCSE classes in Austin ), so
> I probably won't be able to respond until I get back.  Unless I can figure out
> how to make a mac work.... ( I am NOT a mac person ).  I'll check the list
> tomorrow ( sunday ) morning.
> Later!
> David Knaack
> South Texas
>   [Part 2, Application/MS-TNEF  5.5KB]
>   [Unable to print this part]

Hope this gets you closer!

Safe Micro-coilin' to ya!

-- Bert --