Re: Micro coil help
Tesla List wrote:
> > Subject: Micro coil help
> >From dknaack-at-rdtech-dot-com Mon Dec 2 22:26:06 1996
> Date: Mon, 2 Dec 1996 18:45:33 -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
> >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?
> I said:
> 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
> You said:
> 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.
Remove the cap. Its severely detuning the primary circuit, and also may
not be able to withstand the higher voltage stress seen during Tesla
> > 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?
You can model the circuit as a series LC circuit, where C = Cself (sec
coil) plus Ctoroid. F = 1/(2*Pi*SQRT(LC)). The slightly larger toroid
you're planning to use will have a somewhat larger capacitance (about
4.8 pF) which will drop your secondary to about 1.3 MHz.
> 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?
It looks like there's an error in your capacitance calculation - you
appear to be off by roughly 6 orders of magnitude - the values you are
predicting are about right if the units are PicoFarads (1e-12F) and not
MicroFarads. The capacitance of a 2-plate capacitor can be approximated
C = 0.2248kA/t (PicoFarads)
A = plate area in square inches
k = dielectric constant (1 for air)
t = dielectric thickness (inches)
> > 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. )
You need to determine if the transformer self-limits under short circuit
conditions. My initial "guess" would be that it won't. The easiest way
would be to place a 1000 ohm 10 Watt resistor directly across the output
and measure the voltage developed across the resistor. If your
transformer "limits" current to a relatively low value (say 10 MA), you
should only see about 10 volts developed across the resistor. If you see
substantially more, then you may need to externally limit current with a
series inductor. Keep looking for that neon! :^)
> Thanks for the input!
> David Knaack
> South Texas