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Re: DC TC

Tesla List wrote:
> 
> Subscriber: gweaver-at-earthlink-dot-net Sat Feb 15 15:43:29 1997
> Date: Fri, 14 Feb 1997 20:43:14 -0800
> From: Gary Weaver <gweaver-at-earthlink-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: DC TC
> 
> Has anyone built a DC powered Tesla Coil other than a tube type Tesla
> Coil?
> 
> A tube tesla coil is basically a pulsating DC Tesla Coil.
> 
> A car ignition coil is DC and operates from points that open and close.
> But it's not TC.
> 
> I found an artical where someone built a large auto transformer that
> produced 42" sparks from a battery power supply.  It's not TC either.
> 
> Many years ago I built a crude Tesla Coil using a DC power supply an 810
> tube and a signal generator to drive the control grid.
> 
> It seem to me if a DC power source could be pulsed at resonate frequency
> it would work fine for a Tesla Coil.
> 
> The next question is what is the best way to pulse a DC power supply
> with out a tube?  How about a DC power supply with an adjustable charge
> rate for the capacitor bank. The discharge rate threw the spark gap
> could be adjusted to find resonate frequency?  The capacitors would have
> to be sized correctly for the proper charge rate.
> 
> With a Tesla Coils operating on a DC power supply it seem like the
> toroid or sphere on top of the secondary would charge up in one polarity
> like a Van De Graff generator.  Would the Tesla Coil produce longer and
> more powerful output sparks at resonate frequency from DC?
> 
> For a TC operating on an AC 12K transformer the peak to peak voltage
> swing would be 19,968.volts.  For the DC power supply to be equal to the
> AC example the DC supply would have to be 19,968. VDC.
> 
> Assuming it would be possible to pulse the DC power supply without a
> tube would the output sparks at the secondary be longer or more
> powerful?  The same?
> 
> Gary Weaver

Gary,

A DC coil is typically made using a "charging choke" which connects a
large power storage capacitor to a standard rotary-gap Tesla Coil
circuit. The Gap, C2 and L2 make up the standard TC primary circuit.
C1 is sized to be significantly larger (typically 20X or more) than C2,
and acts as a main energy storage element. L1 is the charging choke,
which is a relatively high inductance (multi-Henry), oil-immersed high
voltage choke, capable of withstanding at least 3X the incoming DC
voltage. When the gap fires, the charging choke limits the amount of
current that will flow through the gap, which MUST be a forced quenching
gap (typically a rotary. Once the gap is quenched, the tank cap is
rapidly recharged through the charging choke before the next firing.
These type chokes are used on radar systems, and can sometimes be
obtained off the surplus market at reasonable cost.

BTW, this is _not_ a trivial circuit to design, build, and make work,
since the rotary speed, dwell-time, and element sizing all need to be
balanced - it _is_ a resonant charging system. It's also somewhat
unstable when used with a rotary gap's natural variation in breakdown
voltage. Most modern high-power systems tend to use AC multi-phase
resonant charging systems instead to overcome these disadvantages.


   DC                            ||
    ---------00000000------------||------
         |    L1        |        ||     |   0 
         |              |               |   0
         |              |        C2     |---0
       -----            X GAP               0 
       ----- C1         |                   0
         |              |                   0 L2 
         |              |                   0 
    -----------------------------------------

This circuit will NOT produce DC on the secondary output, even though
the tank capacitor is initially charged only to one polarity. Once the
gap fires, the ringing of the primary circuit creates only RF in the
secondary. Since the secondary coil base is grounded, you will never
build up an ES charge in the toroid like in a VDG generator, although
you can see various high E-field electrostatic/ionic effects in the area
around the toroid. While its true that the voltage peak at the secondary
MAY be of one polarity once the secondary coil "rings up", the
previous/next half sinusiod is "relatively" close in amplitude and
opposite in polarity. 

One advantage of a DC system is uniformity of "bang size" (assuming the
capacitors fully recharge between "bangs"). Also, it is possible to get
more "bangs"/second since we don't have to wait for the AC voltage to go
through the voltage zero-points each half-cycle. Under these conditions,
a DC operated coil would give better output performance.

Safe coilin' to you, Gary!

-- Bert --