# Progress on the DC drive

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From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Sent: 	Sunday, January 11, 1998 9:45 AM
To: 	Tesla List
Subject: 	Re: Progress on the DC drive

Tesla List wrote:
>
> From:   Greg Leyh[SMTP:lod-at-pacbell-dot-net]
> Sent:   Saturday, January 10, 1998 5:58 AM
> To:     Tesla List
> Subject:        Re: Progress on the DC drive
>
> L.Robertson wrote:
>
> > I see some discussion lately concerning DC drive for coils,
> > and thought my struggles with such a drive may be of interest.
>
> snip
>
> > I got a 2 uf 60 kV Aerovox pulse cap. for the storage
> > capacitor, and constructed a diode bridge from 30 kV
> > 1 amp diodes, carefully placed in a tupperware full
> > of oil, and added quite alot of filters between the diodes
> > and the Aerovox.
> >
> > This resulted in about 12 " sparks, but the safety gap on
> > the capacitor started barking at only 25% on the variac.,
> > even when opened to 1" .
> >
> > O'scope indicated huge 5 Mhz parasitic oscillations.
>
> What are the values in your filter circuit?  A PSPICE
> simulation of the interaction between the filter and
> primary may be in order.
> If you're running DC, you can use an L-diode filter,
> which is much more effective and less prone to parasitic
> oscillations than an L-C filter.
>
> -GL

Larry,

If I remember your proposed circuit, it was the spark gap equivalent of
an H-bridge, where the tank LC circuit was the "load". The pair of gaps
G1 and G2 operated in such a fashion as to reverse the polarity of the
voltage applied to the tank cap on every alternate presentation.

<---------------------------------------------------
|              |                            |
|              |                            |
|              |                            |
To Filter &    |              o                            o
Charging       |   +           \             ||  0.018 uF
Circuit      -----        G1     o---OOOOO---||---------o    G2
----- Vdc                  Lp   ||  Cp      \
|   -          o                            o
2 uF |              |                            |
|              |                            |
|              |                            |
<----------------------------------------------------

In this interesting configuration, the 2 uF Aerovox cap is actually part
of the high-current tank circuit whenever the gaps fire. It's
significant;y different than the diode-L configuration seen in most
other "radar modulator" resonant charging configs. Are the two caps, gap
and primary physically close together and are the connections from the
Aerovox cap of the same heavy construction as the rest of the tank
wiring? Also, what is the nature of the filters you're using between the
NST's and the Aerovox cap? Sounds like you are "shock exciting" the
filters and the filters appear to actually be creating a problem for
you. Are the leads betwen the 2 uF cap and the filter relatively short,
and do you have sufficient damping resistance in the filter network?

Finally, the tank cap and gaps will be running at substantially higher
voltages due to the additive DC voltage stored on the tank cap prior to
the next gap firing. And, because of the diode chain, your 2 uF cap will
be storing near the peak value of the NST's output. Add to this the
natural |Vdc| "overshoot" from the tank circuit's oscillations, and you
could be seeing peak-to-peak excursions of as much as 3*15KV*1.414 = 63
kV across the tank cap at full input power. This may be why your safety
gap is firing at lower than expected variac settings. You may want to
try adding a set of vacuum gaps in the CP-LP path to reduce the voltage
stress on the rotary gap.

Safe coilin' to you, Larry!

-- Bert --

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