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Re: Optimal Quenching

Subject: 
        Re: Optimal Quenching
  Date: 
        Mon, 17 Mar 1997 18:23:40 -0500 (EST)
  From: 
        richard hull <rhull-at-richmond.infi-dot-net>
    To: 
        Tesla List <tesla-at-pupman-dot-com>


At 12:07 AM 3/17/97 -0600, you wrote:
>Subject: 
>            Re: Optimal Quenching
>       Date: 
>            Mon, 17 Mar 1997 12:45:41 +1200
>       From: 
>            "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
>Organization: 
>            Wellington Polytechnic, NZ
>         To: 
>            tesla-at-pupman-dot-com
>
>
>Hi John (Freau), all,
>                        Firstly, John requested a description of my 
>MOSFET gap setup. Briefly, I used a system resonant at about 180kHz 
>(from memory - I don't have my notes here) coupled at k=0.1. The 
>primary coil had a rather low inductance of just a few uH and Cp was
>about 0.5uF. Explanation: with low primary voltages around 30 or so, 
>I chose a rather large cap to get some energy in to make measurements 
>easier. The gap consisted of 4 MTP3055E MOSFETs wired in parallel and
>the primary alone gave a measured Q of 9. I was able to use the 
>standard decrement formula because all losses were linear-resistive.
>The low Q mirrored the appalling L/C ratio and L/R ratio. It was a 
>substantial improvement on a single MOSFET alone though. That gave a 
>Q around 4 if I remember correctly. Secondary output was monitored 
>using a short piece of wire dangling from a scope probe several feet 
>away from the coil. The primary was scoped directly across Lp on some 
>occasions, and the "gap" on others. The "gap" was placed across the 
>supply.
>     The MOSFETs were turned on/off using a precision pulse generator 
>with a 50 Ohm output impedance (10V capability) and the output was 
>terminated at the gate-source with a 50 Ohm resistance to get good 
>risetimes. Using this setup, I was able to investigate the effects of 
>different quenchtimes, resonant charging of Cp using a choke 
>(sometimes with a series diode) between the DC power supply and 
>primary circuit, transfer times, class C operation (I could dump the 
>cap at the resonant frequency by carefully adjusting generator 
>frequency and dwell (pulse) time - very difficult). This was 
>facinating though. With an impulse of constant (*not decrementing*) 
>amplitude transferred to the secondary evey cycle with the right 
>phasing, the output soared incredibly due to the high output 
>impedance of the non-sparking secondary as the energy accumulated
>on each cycle. There is no magic about where this kind of rise comes 
>from. Because of the very low distributed Csec, any movement near the 
>coil afffected this output greatly. Bringing a hand close reduced it 
>to near zero as the secondary tune and hence relative phase wrt the 
>primary changed.
>
>     A disclaimer - the astute will note that there are parasitic 
>substrate zeners present in the MOSFETs. This in no way affected the 
>amount of energy transfer - they did affect low frequency primary 
>behaviour with the charging choke as the primary cap and choke rang 
>at a low frequency after the gap was cut off. It also meant that 
>"quenching" the primary was less than clean on a negative half cycle
>of oscillation, but the DC bias presented by the power supply 
>overcame this effect substantially, esp. with an inductive feed from 
>the power supply. I also used the power supply current limiting 
>(current source characteristic) to good effect on occasions.
>
>    All this is real - I took two rolls of film off the storage scope 
>which showed everything captured in graphic detail complete with 
>timebase and amplitude settings. I'd be happy to send some annotated 
>photos to John if he wishes. I can't do this for everyone because of 
>the cost sorry. There may be some of these on ftp sites as I have 
>already sent some overseas.
>
>    I am composing a piece to describe the physical detail of how the 
>sidebands arise in the overcoupled system but this will take several 
>days.
>
>    A final note about the MOSFET experiments: anyone (everyone) who 
>uses either transformers with leakage inductance built-in (neons) or 
>near perfect transformers (pigs) with "current-limiting" inductors 
>attached should know that current-limiting applies *only* to limiting 
>the transformer current in the gap. It has exactly the opposite 
>effect when charging the primary cap because the two form a resonant 
>circuit of rather high Q. In many ways it is a Catch-22. There is an 
>obvious need to reduce transformer discharge current in the gap (to 
>zero if possible), and yet let the reactance of the primary cap at 
>mains frequency control its charging current and hence final voltage.
>The relevance of all this to my experiments? At low rep rates and 
>with a charging choke between the supply and primary cap, the energy 
>stored in the choke was able to charge Cp to near 60V from a 6V 
>supply!!
>
>Malcolm
>
>Good job Malcolm!  There are lots of catch 22's in coiling.  No free lunches.

I have limited current for the gap only and realized that the limiting
with
a resistor-inductor mix really hardly affected the charge time at all. 
My
inductance limiter is about 1mh while my resistive element at current is
.25
ohms.

Richard Hull, TCBOR