# Re: ballast core (Linearized choke)

```Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Chris,

I'd recommend chopping the core material in half to double the core area.
With this sized core, you can design your choke to use somewhere between
200 and 300 turns of #6 or #8 AWG THHN wire (from the local Home Depot) -
I'd go with the #6 to carry heavier currents without overheating since
you've got plenty of winding room.

If you want to "linearize" your taps (make the inductance change in roughly
equal steps so that your current limiting would approximately do the same),
the following approach should work. Because of the large open magnetic
circuit
with the linear core, you can safely assume that your core's permeability
will not change by a large amount in the current range of interest - under
these conditions, you can use the following formula to estimate where you
want
to place taps to make for approximately linear inductance changes per tap.

L is proportional to Turns squared.
So:
Let L = X*(N^2)  where X = a constant of proportionality based on
geometry and core material

The maximum inductance occurs when you use all the turns (Nmax):
Lmax = X*(Nmax^2)

Now suppose we want to tap at a point that gives us some factor (Y%) or the
maximum inductance. We can show this as:

Ly(at tap) = X*(Ny^2)
where
Ly/Lmax = Y%

Solving for the number of turns at this point:

Ny = Nmax*SQRT(Ly/Lmax) = Nmax*Sqrt(Y)  where Y is the percentage of
total inductance desired at the given tap.

If we now solve for Y = 10% thru 100%, we can determine the tapping points
as a function of maximum number of turns on the core. Multiply the factor
shown by the total number of turns to arrive at the turns at each tap. The
example below assumes a 250 turn ballast and a 10:1 current range in 10%
increments.

Multiplying   Turns
Factor:      at Tap
Nmax      1.00        250    Total number of turns (Maximum inductance)
N(90%)    0.95        237
N(80%)    0.89        224
N(70%)    0.84        209
N(60%)    0.77        194
N(50%)    0.71        177
N(40%)    0.63        158
N(30%)    0.55        137
N(20%)    0.45        112
N(10%)    0.32         79

Hope this helps...

-- Bert --
--
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
>
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<CTCDW-at-aol-dot-com>
>
> Hello all!
>
> OK, here goes.. I now have silicon steel laminations. they are 3.75" X 26.5"
> and the whole stack comes to a bit under 1.25". Two questions: Will this sq
> inch area be enough to wind a ballast inductor? (for a 10kVA transformer).
> should I cut these in half and make it half as long, but twice as thick?
> Thanks as always,
>
> Chris W

```