Hi Michael,
On Tue, Feb 25, 2014 at 6:09 AM, Michael Barr-David <
michaelbd@xxxxxxxxxxxxxxx> wrote:
> Hi everyone
>
> This touches on some issues that I was intending to raise on 4hv.org if I
> could manage to complete registration, but as it has come up here ...
>
> Why would one want a DRSSTC to 'quench' after the first 'notch'? (By which
> I assume that what is meant is adjusting the interrupter's pulse width so
> that the burst ends at the next primary zero current crossing after the
> first 'notch' would otherwise occur. At which point, the half- or full-
> bridge IGBTs are not driven so that the excess energy in the primary
> capacitor promptly feeds back into the DC storage capacitor via the
> freewheeling diodes anti-parallel to the IGBTs.)
>
Why? because this mode of tuning theoretically offers the highest
efficiency energy transfer from primary to secondary. Notice i didnt say
"best sparks from a given system" or "best use of IGBTs" or "best use of
coil size"...
>
> The arguments against that occur to me are:
>
> 1. A DRSSTC is not like a SGTC (where all the energy must be injected into
> the primary capacitor before the spark strikes) in that not 'quenching'
> allows one to continue to pump energy into the primary tank circuit.
>
Exactly, we can get better component utilization by running longer than 1/k
cycles and tuning for a notch.
> 2. Because of the freewheeling diodes anti-parallel to the IGBTs, leaving
> the IGBTs undriven might not isolate the energy in the secondary -
> presumably, after the end of a burst, the mutual inductance of the primary
> and secondary still allows energy to be transferred to the primary and then
> to the DC storage capacitor.
>
Also agree, so its not quite the same as a spark gap clearing an arc.
Basically the energy will go to both the arc and back to the DC bus
simultaneously. You can control an H-bridge to "clamp" its output (to
nearly 0V, really 2 junction drops) which will keep it from recharging the
DC bus. This would be like a spark gap that doesn't quench, but is also
very, very, low loss.
> 3. According to Dan McCauley in the miniBrute reference design book, his
> idea is to detune the primary tank circuit so that notching does not occur.
> (I assume until streamer loading on the secondary circuit reduces its
> resonant frequency.)
>
Right, this allows it to transfer more energy. The primary current usually
builds slowly and then falls quickly once a large spark forms, and either
brings the coils into or out of tune. Viewing the primary and secondary
currents on an oscilloscope can be very useful to get some intuition on
tuning.
>
> An argument in favour that occurs to me is that, if (contrary to 3 above)
> notching occurs, the energy transfer back to the primary from the secondary
> may well cause the primary current limit to be exceeded thus causing the
> burst to be terminated before any additional benefit is achieved from
> sustaining the burst.
>
Yeah that can happen too.
>
> This is my first post, I'm relatively new to DRSSTCs and not an EE.
> Therefore I will apologise in advance for whatever misconceptions are
> buried
> in the above.
>
>
I didn't notice anything that sounded "wrong" in your description.
Steve
> Michael Barr-David
>
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
> Of Scott Bogard
> Sent: Tuesday, 25 February 2014 11:14 AM
> To: Tesla Coil Mailing List
> Subject: [TCML] First notch quenching?
>
> Greetings folks,
> A bit of a theoretical question, I'm a bit rusty so thanks for your
> patience, is it still impossible to quench on first notch? I was trying to
> explain the whole "notching" concept to someone and it occurred to me, if
> an
> IGBT or other switch was inserted to bypass the tank capacitor enitrely and
> physically disconnect the primary immediately after first notch, it would
> greatly impede the tendency to transfer energy back into the primary, as it
> would no longer be at the same resonate frequency. I feel like I might be
> missing something, or maybe not, or perhaps even it doesn't matter as such
> a
> feat is impossible with current semiconductor technology. At any rate just
> felt like putting this out there, help me to remember if you could folks.
>
> Scott Bogard.
>
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