RE: ballast core

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi David,

On 2 Apr 01, at 17:19, Tesla list wrote:

> Original poster: "David Dean by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>
> 
> Hi Malcolm
> 
> This is the way I want to build a ballast. I have been toying with the
> idea for several months now, well at least since July. I have found
> several formulas for the inductance of a multi layer solenoid, and
> using 1 as the relative permeability, calculated a coil. Then I wound
> the coil that I made, then I measured the coil, then recalculated. I
> can not get agreement between the real coil and the calculated value.
> I am guessing that it is because I am using THHN insulated wire for
> the test coil and the formulas must assume magnet wire. Anyway I
> always get significantly less inductance as measured than as
> calculated.

The formulae probably assume maximum packing factor for the wire as 
you say. So, all you need is a bit of "coring". One approach to 
making a good quality core is to gather old SMPS, separate the 
ferrites from their windings, and pack them into a tube which will 
slide into the solenoid. I built two this way. The ferrites were 
smashed to increase the packing density. The material was mostly 3C8 
which exhibits low losses at high frequencies. Some backfilling of 
the tube with a material that sets should dampen the noise factor.

Regards,
malcolm