Re: Problems & DC drive solutions

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

Original poster: "Kevin Ottalini by way of Terry Fritz <twftesla-at-uswest-dot-net>" <ottalini-at-mindspring-dot-com>

Steve:
	A reservoir capacitor does nothing to help to protect the
diodes, and dramatically lowers the overall performance of the
coil.  I have tried this a number of times using different
values and hookups, and in all cases it was worse than useless.

Also, a 1uFd to 10uFd at 18+ Kv is very expensive, especially
compared to four resistors! (and much better used as a tank cap!)

Although minimizing ripple is desirable in most DC supplies,
the reality is that (as usual!) Tesla Coil applications
don't behave in a "typical" manner.

The resistor is called a "softening" resistor and is strictly
intended to limit the surge currents both from the diode and
in reflection to the diodes. Ramping up power slowly from
a variac into the system has no role here .. the diodes
must be protected at all times from overload and reverse
bias conditions ... for example when the gaps fire.

The resistors actually waste very little power ... at 4kw
of average power, assuming 14.4kv, the average current is
~278 ma.  .278A across 5Kohms give a Vdrop of 1390 volts,
which gives a total power loss of 386 watts, or about 9.7%.

Very reasonable considering how expensive high voltage &
high current diodes cost!

The tank capacitor actually acts as a filter cap as
much as one is needed.  The only observed side effect
is that there are some slight 60Hz and 120Hz performance
"peaks and troughs" relative to the speed of the async
rotary spark gap ... sort of an interesting extra with
a little acoustic presence as well.

Best,
	Kevin

----------
> From: Tesla list <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re:  Problems & DC drive solutions
> Date: Sunday, January 07, 2001 5:36 PM
> 
> Original poster: "Steve & Jackie Young by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <youngs-at-konnections-dot-com>
> 
> Kevin & other DC coilers,
> 
> I had a look at your schematic, referenced below.  Have you considered
> connecting a reservoir capacitor, of 1-10 Mfd at 18+KV at the output of
your
> bridge to ground?  This would then feed the rest of your circuit, except
you
> could then get rid of the power wasting power resistor.  This would
insulate
> your HV diodes from the large current when your gap fires because the
> reservoir cap would supply the large instantaneous power and your diodes
> only have to handle the average current to the reservoir cap.  The only
> thing to watch out for is blowing the diodes by not slowly ramping up the
> power to your potential transformer.  (Note, the reservoir cap makes the
> circuit even more potentially deadly - be very careful if you try this)!
> 
> Your thoughts?
> --Steve
> ----- Original Message -----
> From: Tesla list <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Sunday, January 07, 2001 3:04 PM
> Subject: Re: It's Me Again, After a Long Recess, and I've got problems...
> 
> 
> > Original poster: "Kevin Ottalini by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <ottalini-at-mindspring-dot-com>
> >
> > Ryan:
> > An easy solution would be for you to convert to DC drive ...
> > then you can use any size capacitor that you can balance with
> > your primary inductor (to get to the secondary resonance).
> >
> > You can also eliminate all the ballasting you are using for
> > the pole transformer (you still need the main variac).  The
> > system will be self-limiting and the current draw will be a
> > function of the BPS rate and your cap size.
> >
> > You will need three additional components to implement this:
> >
> > 1.  You need a rectifier that can handle ~1amp at ~18kv
> >     (1.414*12,400) and should be able to handle 50 to 150amp
> >     peak surges.  Microwave oven diodes are an inexpensive
> >     source, although I know of several people who have used
> >     arrays of small diodes like 1N4007.
> >
> > 2.  You need a large wattage, non-inductive limiter resistor in the
> >     range of 5Kohms to 10Kohms and about 800watts (I use 4ea 160watt
> >     20Kohm ceramic resistors in parallel, cooled a little extra with
> >     a muffin fan). The resistor is there to prevent the diodes from
> >     suffering too large of a surge current (not to limit the power).
> >
> > 3.  You need to build or convert your SRSG to asynchronous (any
> >     speed) operation.  This can be as simple as adding a variac
> >     to your existing SRSG, or changing the AC motor for a DC and
> >     a small rectifier and a small variac.  I use a small 5400RPM DC
> >     motor and 8 poles with a range of about 2BPS to a max of about
> >     770BPS.
> >
> > A complete schematic is here:
> >  ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/DCCOIL/DCSCH2A.PDF
> >
> > Although I use a 4-diode full wave bridge, you could use a simpler
> > two-diode full-wave rectifier as well.
> <SNIP>
> 
> 
>