Re: power supply questions

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

Original poster: dhmccauley-at-spacecatlighting-dot-com 


 > Hello!
 >
 > I have two questions about changes I want to make to my power
 > supply:
 >
 > 1st: I only have one variac to controll the main HV transformer.
 > Currently I use it in standard configuration and additional
 > inductive ballast to limit the current. By means of that I
 > experience a lack of output voltage due to the voltage drop in
 > the ballast / variac. Can one use the variac to controll the
 > current ( in series ) WITHOUT using one to controll voltage?
 > Would that give a voltage gain?

You can use a variac as a variable inductor.  However, inductance will vary
from variac to variac and so will the current ballasting.
Also, you will not get a gain in voltage from a variac used like a ballast.
You will get a voltage drop across the winding (IR drops).  Depending on how
small your variac is and what the current draw is, this could be
significant.

  > 2nd: I thought about using DC to feed my coil by rectifying the
 > xformer output with a series setup of 1N4007 diodes under oil and
 > charge a 30nF cap for cmoothing purposes. The main C1 would be
 > 60nF. Can this be done or has the smoothing cap to be bigger than
 > tha primary cap?

You don't need to put the diodes under oil if you are just rectifying an NST
(<15kV).  Also, in a smoothed rectifier arrangement, the amount of
capacitance you use as a smoothing
filter will depend on how much allowable ripple you have and by what the
load is on the rectifier.  For a 15kV NST running about 1KW of power
(average), you would need a good amount
of capacitance for a good DC output with minimum ripple- probably greater
than 50uF.  Typically, you would use an LC type filter with some series L to
help with ripple and smoothing.  This allows
you to use a much smaller smoothing capacitor.

Dan



 > Any suggestions welcome
 >
 > Sincerely
 >
 > Christoph Bohr
 >
 >
 >
 >
 >