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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.

>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.

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.

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
    
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

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

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?

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

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