Re: Ballast Puzzle

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

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

Hi Mike,

On 12 Aug 2001, at 18:16, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Hollmike-at-aol-dot-com>
> 
> In a message dated 8/12/01 4:46:38 PM Pacific Daylight Time,
tesla-at-pupman-dot-com 
> writes: 
> 
> 
> >
> > Ballasts like this can work well, and can be wound on wood, but don't wind 
> > them on any solid piece made of steel or iron!! The rod will act as a 
> > shorted turn and generate ALOT of heat, and rob you of much of your power, 
> > making your coil vastly innefficient. I saw what happens when you put a 
> > steel bolt inside one of these, and then try and take the bolt out after a 
> > few second run, OUCH! 

Which is why you'd use laminations if you were going to use a core. 
 
> 
> Pardon my ignorance, but I thought transformers will 'reflect' the impedance 
> of the secondary back to the primary.

They do. A cap of say, 0.1uF on the secondary side will appear as 
10uF on the primary side of a 1:10 step up transformer.

  Can one not ballast the secondary side 
> of the the transformer to effectively limit the current of the primary? 

Yes and no. If the ballast resonates with the cap at mains frequency 
and is allowed to ring up, primary and secondary currents will also 
climb (to core saturating levels). 

 I 
> know this impedance matching is done when driving a speaker from a tube 
> amplifier, but how could this be done for TC's(thinking pole pig or
similar)? 

The impedance match in that case is effecting a match between the 
power supply and load while keeping the tubes operating within their 
ratings.

>   It seems like a resistor placed on the transformer output could ballast
the 
> transformer without dissipating too much of the power(thinking I^2R
losses).   

Good chokes are far less lossy than resistors. It's worth remembering 
that inductances required on the primary side of the transformer 
(typically 10's of mH) translate to an equivalent inductor in the 
Henries range on the secondary side. That is a lot of wire. 

>   Another question springs to mind as well:   If one uses inductive 
> ballasting to limit the current(primary of transformer), would power factor 
> correction be of use here?   If so, where would  one place the capacitor?   
> between the ballast and transformer or 'in front' of the ballast inductor? 

A PFC cap is connected directly across the mains to cancel any 
inductive component the mains sees at the input terminals of whatever 
is connected to it. The primary cap itself should correct for the 
ballasting inductance in a resonant charging system.

Regards,
malcolm