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Re: Ballast Form and Function



Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

By "9/30", are you referring to a neon sign
transformer?  Most folks don't ballast NSTs because
they've got a "shunted" core (basically, a built-in
ballast).  Just plug 'em in and go.  In certain
situations with NSTs a variac might be prescribed, but
not as a ballast.  Ballasting is generally employed
only with higher-power or otherwise with non-shunted
transformers, such as MOTs, PTs, and "pole pigs".

As for what makes a good ballast, it really depends on
how much money, time, etc., you want to spend, and how
much you care about efficiency.  Resistors (such as
heating elements) are fine and convenient, but do
literally "burn up" a lot of power before it gets to
your transformer.  In theory, inductors avoid this by
limiting current magnetically.  In practice, because
inductors use a lot of wire, you get a little bit of
both worlds.  The more wire, the more resistance, and
the more heat.  A spool of wire off the Home Depot
store shelves is a time-tested method of ballasting
large transformers, but because there's no steel core,
the inductance is achieved via a large number of turns
and a correspondingly long length of wire.  This
translates into a high resistance.  As a result, while
much more power-efficient than pure resistors, such
ballasts tend to need a "cooling off" period after
relatively short periods of use.  By introducing a
steel core, as in the popular "sliding choke" ballast
designs (search the list archives or Google and you'll
get plenty of hits), fewer turns of wire are required
to obtain the same inductance.  This, in turn, makes
more convenient the use of heavier-gauge wire.  Both
of these things tend to reduce resistance and keep the
ballast cool-running.

All that said, my first ballast (which I still use) is
a plastic garbage can full of baking soda-doped water
with two copper pipes immersed in it.  Basically, a
big resistor.  I put a piece of PVC over one pipe
which can be slid up and down to vary the resistance.
Cost about $20 to build and tolerates 100+ amps for
several minutes before needing to cool off.  Some
people are all about efficiency, but I tend to operate
mainly on "convenience factor" :-)  Only by recent
discussion and revelations about saturable reactors
have I started to think about building an inductive
ballast.

Regards,
Aaron, N7OE

--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: Gregory Morris <gbmorris@xxxxxxxxx>
>
> Hey folks,
>
> Sorry if this is trivial, I generally like to search
> the archives
> before asking questions like this, but it seems
> somewhat specific.
>
> First of all, what I think I know: ballast is for
> current limiting,
> it is basically a length of wire wrapped up in a
> volume-efficient
> bundle--essentially a big resistor--and sometimes is
> wrapped around
> an iron core to get the added effect of the
> impedance of a  good inductor.
>
> And what I'd like to know: what range of resistance
> is good for a TC?
> Is the iron core necessary or just beneficial?
> Specifically I got a
> little heating element from dismantling a broken
> 1500W kettle. It is
> about 2" - 3" in diameter and a quick calculation
> tells me it has a
> resistance of about 10ohms. Is this applicable to
> the new 9/30
> powered coil I am building, or should I just scrap
> it with the rest
> of the kettle? If so, do you have any advice or
> links for when I make
> my own ballast?
>
> Thanks,
> Greg
>
>
>