Re: DC Tesla Coil

From: 	Alfred C. Erpel[SMTP:aerpel-at-op-dot-net]
Sent: 	Saturday, January 03, 1998 10:09 PM
To: 	Tesla List
Subject: 	Re: DC Tesla Coil 

Antonio and All,

>>     If my tank capacitor is charged with DC only, it seems to me that it
>> would be much easier to get a solid resonance occuring in the tank
>> with a fixed spark gap (well quenched)...

>With resonance frequencies in the range of 100 kHz, 60 Hz AC or DC is the
>same thing. AC is just easier to obtain at moderately high voltages and
>high power.
>In a normal Tesla coil circuit, the spark gap works as a switch, that keeps
>the primary circuit connected, without interruptions, for time enough for
>the complete transfer of energy from the primary circuit to the secondary
>circuit. This may take a good fraction of a milisecond, after several tens
>of cycles of oscillation. After the complete transfer, the gap shall open,
>leaving the energy in secondary circuit to be gradually dissipated in
>RF corona, sparks, and other losses. With the gap open, the primary
>is charged again from the low-frequency supply, and the cycle is repeated.
>Antonio Carlos M. de Queiroz

    I would like to discuss an example.
    Let's assume a capacitor of .000000084 Farads and a primary of .00003
Henrys. This circuit has reasonable values for a TC and will resonate at
100,000 hz. Let us assume a transformer of appropriate size feeding the
capacitor and a helical primary coil of 5 inch radius and 10 inch height. A
13 turn coil will give an inductance of .00003 Henrys. This primary coil
will be about 34 feet long. If I use number 8 stranded wire at .0006 ohms
per ft., the total resistance of my primary will be .0204 ohms.
    This inductor (primary) has a time constant (L/R) of .00147 seconds
(assuming no resistance in the capacitor). This would be the amount of time
it takes the magnetic field to grow to 63% of its final potential value
assuming there is enough current available. The amount of time for 1 cycle
of 100,000 hz is .00001 so if their was enough current available for 1 time
constant it would take 147 times longer than 1 cycle.

    Some thoughts I have about the above that I would like commented on:

1) It seems that maximum power will resonate thru a tank circuit when the
potential energy storage of the capacitor equals the potential energy
storage of the inductor. i.e. when .5CV^2 = .5LI^2
Is this true?
If so, then to resonate at 100,000 hz would require a capacitor of
.000001591 F and a primary of .000001591 H. This primary inductance value
does not appear to be practical in a tesla coil.

2) I don't believe in the example circuit above that there is even close to
enough energy to fully *charge* the primary. How would you calculate this
value and the amount of time it would take to transfer into the primary. Is
this the reason it doesn't matter that a time constant (above) is 147 times
longer than 1 cycle?

3)How does the fact that as the primary is establishing its magnetic field,
it is also inducing current in to the secondary affect this scenario. Aren't
we theoretically hoping that most of energy being transferred into the
primary won't go into building a field, but will be induced into the

Alfred Erpel