Re: Energy storage in primary?

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

 > Original poster: "Jolyon Vater Cox by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 >
 > Is it possible to build a TC where energy is stored initially as high
 > current in an inductor (the primary) rather than high voltage in a 
capacitor?

Yes. An induction coil operates in this way. Ignition coils are the
same thing.

 > I am thinking of a setup where current ramps up slowly through the inductor
 > before being abruptly switched off (by semiconductor switch or similar)
 > after a predetermined current or period of time has been exceeded; the
 > current in the primary rising and falling as "saw-tooth" waveform.

The classical approach is to use a mechanical interruptor, but an
electronic switch works perfectly.

 > As primary input power for this would be largely determined be current
 > rather than the voltage of the PSU
 > how high would the current have to be/ how low could the voltage be for
 > decent spark output say, a minimum of 6 inches or more?

Consider the input energy per bang. In this case 0.5*L1*i1^2, where
L1 is the primary inductance and i1 the maximum primary current.
You will soon see that for high output power, or i1 is too high,
or L1 is quite large. This causes L2 to be very high too, and we
end with a coil that has several mH of primary inductance and
several H of secondary inductance, operating at a few kHz at most.

 > For the control logic would this likely need an exotic switch-mode power
 > supply IC with PWM and dead-time control or could a simple astable like a
 > 555 do the job?

A 555 astable driving a suitable power transistor is ok. But be careful
with the construction, or it will retrigger at each spark. A fast diode
in series with the transistor may be required, to prevent voltage
reversals over the transistor during the energy transfer.

 > For the high-current, high-speed switch would bipolar transistors (e.g.. TV
 > line-output power devices) or MOSFETS be suitable or would IGBTs be 
necessary?

TV transistors work well enough.

 > Would it not be necessary to connect a capacitor across the switch to
 > absorb/slow down the high-voltage transient produced when the switch opens/
 > would necessary voltage rating of switch and capacitor be comparable to
 > that of the primary capacitor in a conventional spark-gap TC?

This capacitor is necessary, not only to limit the voltage over the
switching element, but to tune the transformer in order to achive good
efficiency. The design can be done so a 400 V capacitor is enough.
The tuning is different from the Tesla coil case. Primary and secondary
shall be tuned to slightly different frequencies for perfect energy
transfer.

My program mres4, at http://www.coe.ufrj.br/~acmq/programs, can
design these systems (for awhile you will need a simulator to see
the waveforms). The program multires can design transformerless
versions, and plot the waveforms.

Antonio Carlos M. de Queiroz