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Re: Capacitive Ballasting



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

Hi Alfred,

On 24 Feb 2002, at 22:26, Tesla list wrote:

> Original poster: "Alfred Erpel by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <alfred-at-erpel-dot-com>
> 
> 
> Howdy all,
> 
>  I was wondering about the merits of capacitive ballasting a transformer
> with electrolytic photo capacitors since you can get them for free from
> disposable cameras and they have high enough capacitance in a small enough
> footprint to be practical.
> 
> ASCII art warning -- make sure you have on a fixed width font. I use Courier
> New.
> 
> 
> 
>                    electrolytic
>          diodes     capacitors
>    -------->|--------|+ -|-------------o  o-----
>   |     |                   |          o||o
>   |     |                   |          o||o
>   |      --|<--------|+ -|--           o||o
>   |                                    o||o
> |A C|                                  o||o Xformer
>   |                                    o||o
>   |      -->|--------|+ -|--           o||o
>   |     |                   |          o||o
>   |     |                   |          o||o
>    --------|<--------|+ -|-------------o  o------
>                    electrolytic
>          diodes     capacitors
> 
> 
> Would the above circuit have any problems? Is this a stress for a good
> electrolytic?
> X(c) = 1 / (2*pi*f*capacitance)
> At .000160 F and f=60 Hz, a typical photo electrolytic yields a reactance of
> 16.578 ohms. Putting 4 in series would yield 66.315 ohms which would limit
> 115 volts to 1.734 amps. I know one thing to worry about would be that you
> aren't around resonance with the transformer's primary. How would each leg
> of the circuit "see" the capacitance? For instance, if the circuit above
> just had four 160 uF capacitors (one for each diode), for calculating
> reactance would you use 80 uF in the reactance equation or 320 uF? Would it
> even work?

An interesting question. Personally I wouldn't use electrolytics in 
AC operation. But before commenting further, there is a serious 
problem with your circuit. You will end up with DC charged capacitors 
and no circuit current once they've charged. The correct way to do 
this is:
        +            +
---------||--------||----------- 
    |         |         |
    |         |         |
    ----|<----+---->|----

    The diodes are to prevent serious reverse polarity charging once 
the caps are emptied.

    I have a reservation about using photoflash caps based on their 
physical size. I don't think they are good for high RMS currents. One-
off high peak currents, yes. I imagine they would be the standard in 
filter caps if they were up to it as everyone would like to shrink 
high value, high voltage caps to zero size.

Regards,
Malcolm