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Re: Energy storage in primary?
Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
So the primary oscillation will not be quenched when the switch turns on
again -short-circuiting the primary capacitor?
Could quenching be accomplished by means of saturating ferrite core in
series with the TC primary- these have a high impedance to current changes
at low voltage but low impedance to current changes at higher voltage -when
the core saturates.
Jolyon
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, January 28, 2003 7:20 PM
Subject: Re: Energy storage in primary?
> 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?
> >
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