Re: Energy storage in primary?

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

Original poster: "Bert Hickman by way of Terry Fritz <teslalist-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Jolyon,

In theory, yes. However, in practice, it's difficult to find an "opening 
switch" that can handle high current and then quickly be able to withstand 
the subsequent high voltage developed across the inductor. Some pulsed 
power systems have been successfully designed to use energy initially 
stored in inductors (quite desirable, since inductors can store 
significantly more energy in a smaller volume than HV capacitors). However, 
opening switches for these systems tend to be "single shot" devices 
(exploding wire, exploding foil, or using high explosives to destroy the 
conductor). Closing switches (spark gaps, thyratrons, SCR's, ignitrons) are 
much easier to find and use, especially for repetitive operation, hence 
their greater popularity.

Pulsed inductive energy storage can easily be scaled down to make it 
repetitively usable. For example, older "point and condenser" ignition 
systems stored energy in the ignition coil primary, releasing it by 
suddenly disrupting the current supply when the points opened, and then 
"ringing" it with the condenser that was across the points. Magnetos on 
small engines operate in similar fashion.

So, in principle, a small classical air core TC using inductive energy 
storage should also be feasible by using the primary of your TC as the 
energy storage inductor and connecting your tank cap directly across it or 
across your opening switch. A high current low voltage source would be 
briefly connected through the TC primary through a semiconductor switch, 
charging the primary inductor. Assuming the switch can then be quickly 
turned off, most of the inductor's stored energy (bang size = 0.5*Lp*Ip^2) 
would oscillate in the primary tank circuit to be transferred to the secondary.

No to your question: To obtain a 6" spark, we could figure that we'd need 
at least 50 watts of input power at 120 BPS (predicted via John Freau's 
spark length estimator for a 12" spark in an efficient system). This 
implies a bang size of about 0.42 Joules. The semiconductor switch we use 
must be able to withstand the initial current and also the peak voltage 
developed across the inductor. It must also be able to switch off rapidly 
enough (this may be tough if the device is heavily saturated). Assuming a 
tank inductance L and primary capacitor C, the peak voltage developed once 
the switch opens will be V = Io(sqrt(L/C). Let's plug in some numbers for a 
small system:

Let:
     L  = 50 uH
     C  = 0.2 uF
     Then Fo = 50.3 kHz
     Desired Bang size = 0.42 Joules
     Required Ip  ~ 130 Amps
     Vmax = 2055 volts... a bit on the high side

Increasing C decreases Vmax, but this also decreases Fo. Going to a 0.5 uF 
tank cap reduces Vmax to about 1300 volts and drops Fo to about 1300 volts 
- not an unreasonable value. Either an IGBT or MOSFET switch could work, 
but the challenge will be turning it off quickly enough so that most of the 
energy remains in the LC circuit.

BTW, one other problem with this configuration is that, once oscillating, 
there is no simple way to "quench" the primary circuit, and energy will 
cycle back and forth between the TC primary and secondary. A more 
sophisticated switching arrangement, using another switch, would be 
necessary to "break" the primary LC circuit for proper quenching.

Building a coil that operated in this mode would be a good demonstration 
vehicle, but its unlikely that it would be nearly as efficient as a more 
conventional disruptive coil.

Best regards,

-- Bert --
-- 
Bert Hickman
"Electromagically" (TM) Shrunken Coins
Stoneridge Engineering's Teslamania
http://www.teslamania-dot-com

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?
>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.
>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?
>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?
>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?
>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?
>