Re: Capacitive Ballasting

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Al,

Even if the diodes and caps were polarized properly, the circuit will
not work because the diodes will only allow current to flow through the
capacitors in one direction. This charges them but does not allow them
to discharge. Once all the capacitors are charged, you'll get little
further current flow except for capacitor leakage current. 

However, the way that the diodes and caps are polarized in your
schematic results in two of the caps being charged with reversed
polarity. This is guaranteed to be exciting with polarized electrolytic
caps, since reversing polarity "unanodizes" the capacitors' internal
dielectric layer, causing it to break down, and letting the smoke
(actually steam) out... :^)

A capacitor must be able to freely exchange energy with the surrounding
circuit, meaning that current must be allowed to flow into, AND out of,
the cap. Adding a series diode (to protect the cap) also blocks current
flow in the discharge direction, preventing the cap from working as
desired.

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

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?
> 
> Regards,
> 
> Al Erpel