# Re: Rebuilding an OBIT

```Original poster: "Metlicka Marc by way of Terry Fritz <twftesla-at-uswest-dot-net>" <mystuffs-at-orwell-dot-net>

maybe an explanation on how to calculate the cross sectional area of the
core would be a
great help for those trying to get a grip on winding there transformers and
inductors? is
it the sum of all the exposed areas in the cross section and to figure core
mass, is it the
sum of all iron area in the core?
thanks.
marc

Tesla list wrote:

> Original poster: "Luc by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<ludev-at-videotron.ca>
>
> Hi Ed
>
> May be I make a mistake but the formula I have for the numbers of turns  in
> a primary of a
> transformer is different.
> My formula is:  N = (E · 10^8)/ (4.44·f·A·B), where N=Number of turns;
> E=Primary Voltage;
>        f=line-frequency; A=core cross-section Area; and, B=max.flux, in
> lines. Assuming 60,000
> lines of flux...
> or N = (k·E)/ A, where k=6.256 when f=60 Hz., or, k=7.507 if f=50 Hz.
>
> My formula could be no good, I never wound a transformer myself but on a 2
> square inch cross
> section core you have for 110V 55 turns and with the formula I have 344
turn .
>
> Please tell me I'm wrong : It will be less work to wind a transformer !
>
> Luc Benard
>
> P.S. I extracted this formula from a link post last week on the tesla list.
> "Roll Your Own Power-Transformers! " I don't remember the URL I just copy
> the documentation...
>
> Tesla list wrote:
>
> > Original poster: "Ed Phillips by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>
> >
> > Tesla list wrote:
> > >
> > > Original poster: "S.Gaeta by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> > <sgtporky-at-prodigy-dot-net>
> > >
> > > Hi All,
> > >
> > > I have a 10KV, 23mA OBIT that burned out in furnace duty last Winter. I
> > > finally got around to depotting it and found that the only part of the
> > > secondary that was any good is the bobbin. I am in the process of
rewinding
> > > it with 34 AWG wire (much thicker than what was on there before!). I am
> > > designing it to give me about 5KV total. I chose the 34 AWG because it
> looks
> > > like the same diameter wire that they used on a 12/60 that I also
depotted.
> > > My goal is to get 60mA out of this thing.
> > > My question is: How much current can I get out of this thing before the
> core
> > > saturates and gives me grief.
> >
> >         Current doesn't saturate cores, voltage does!  You'll need the same
> > number of turns on the primary that you had before.  If you destroyed
> > the primary, a conservative rule of thumb is that the number of turns is
> > the voltage divided by the cross sectional area of the core in square
> > inches.
> >
> > <A lot of folks have written about knocking
> > > shunts out of NSTs, but has any one else ever tinkered with OBIT cores?
> > >
> > > Thanks,
> > > Sue
> >
> > Ed

```