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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?
 > >
 >
 >
 >
 >
 >
 >
 >