# [TCML] Ballast choke- can someone check my math?

David Dean deano at deanostoybox.com
Sun Nov 18 17:05:08 MST 2007

```Hi

I am preparing to build a ballast for a polemount distribution transformer.
I have acquired 500 feet of 11 AWG magnet wire and a suitable former, a piece
of G10/FR4 fiberglass tubing 2.25 inch O.D., 2.125 inch I.D. I had prepaired
a spreadsheet to work out the geometry of the coil, but made some mistakes
when entering Wheelers formula for a multi turn solenoidal coil. I apparently
did not have enough parentheses and got results that were of by an order of
magnitude or more. I have reworked the spreadsheet and think the problem is
fixed, but before I build the thing and find out I still have it wrong, I
would appreciate a fresh set of eyes to check the results and see if they are
in the ballpark.

There will be an iron slug to slide down the tube to increase inductance, but
that is not reflected in the spreadsheet. The coil is supposed to limit
current to a maximum level depending on a tap point which will be selected
according to the application at the time of use. The slug will be withdrawn
to reach this maximum level.

The spreadsheet can be found at

http://deanostoybox.com/temp/ballast.ods

for the Open Office version, and at

http://deanostoybox.com/temp/ballast.xls

for one that is supposed to be in Microsoft Excel 95 format.
In that one the Greek symbols for "micro" "nano" and "Ohm" have been replaced
with "?" so you will have to pick the right one for the location and
substitute mentally.

Row 43 in the spreadsheet is representative of my proposed geometry. The last
tap will be at 390 turns with taps at 360, 330, 300, and 270. If you look at
the I^2*R dissipation in column  "Q", fear not for I am planning to sink the
thing in a pot of oil.

Thanks
deano

```