[Amps] Re: Filament Current Limiting

[SteveM]

hat those like me,
> who are justing getting this news, might like to see a little serious bit
of
> explanation.
>
>        Imagine placing a small inductance in series with the filament.

 ** How about the filament choke for the inductance? I know very little, but
I seem to remember that an inductor resists rapid current change ? **

73,
Steve  wd0ct

 For
> definiteness, let the filament hot resistance be 4 ohms and let the
> inductance have an X(sub)L of 1 ohms. The effect of the 1 ohm of  X  will
be
> to raise the total filament load by only a few percent (Z=4.1<17 degrees)
and
> the power factor will become .97 (vs 1.0) which will cause a small
increase
> in the dissipation of both primary and secondary. So far, nothing earth
> shaking has happened. Supposedly, the transformer primary could be tweaked
a
> few turns to keep the load voltage precise.
>
> Now suppose we get a dead-short across the filament (approx. to the cold
> start), and the secondary will see a load of X=1ohms and a Z of pure +j1.
The
> result will be, neglecting the R's at first, a secondary current of 4
times
> the normal running current. It is a purely reactive load (nearly) so there
> won't be much real power dissipation, for what that's worth. But the
current
> is safely limited to 400%, and could probably run like that for a while if
> needed.
>
> So with a little basic circuit strategy, the addition of some L, the
current
> limit is achieved. The next step is tricky and I am not going to go into
> detail here. I state without proof (it is proven in books on transformer
> design) that one of the effects of leakage inductance in a transformer is
the
> appearance of parasitic inductances  in series with primary and
secondary -
> exactly as conjured up above. In fact, by strategically designing the
> transformer and core to be less than perfect, an appropriate amount of
> effective series inductance can be synthesized.
>
> Sometime I would like to see the detailed construction of that
transformer,
> because it is not a totally trivial job to achieve exactly the desired
amount
> of leakage. Apparantly, there are several applications in the world where
> this is common (e.g. neon transformers, AC welders,  and oil burner
ignition
> transformers.)
>
> 73
> Eric von Valtier K8LV
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