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Re: My FET T.C.



On Tue, 16 Nov 1999 20:53:10 -0700 Tesla List <tesla-at-pupman-dot-com> writes:
>Original Poster: Alan Sharp <AlanSharp-at-compuserve-dot-com> 
>
>Ken,
>
>You're doing very well with solid state coiling.
>
>>2.  Input current (so far), about 4A -at- about 50 sparks/sec.  Spark 
>rate
>>variable from a hand-held "wand" from 1 at a time on up.
>
>I take that this is some kind of interupter so that you can switch it 
>on
>and off at 50Hz

>From KCH:  No, the wand (from an old computer joystick) contains only 2
pushbuttons (1, "arm" & the other, "go"), a pot to vary the spark rate,
and an extensible rod for the handy teasing of sparks from the toroid.  I
can vary the spark rate from 1 every few seconds to several
hundred/second, with the max. rate only limited by the current I care to
pass thru the MOSFETs.

>>3.  Driver-- 24, 85A, 500V power MOSFETs electrically in push-pull
>>parallel, together driving the equivalent of 1 primary turn (in a
>>patent-applied-for circuit).  Soon to be increased to 48, then--if 
>my
>>endurance holds out--to 72.  That will double, then treble the spark
>>length (and the input current for a given spark rate).  The primary 
>is
>>untuned.
>
>Do you mean 85A or 8.5A mosfets - why so many? they will handle
>a peak current many times greater than the continuous current rating.

>From KCH:  Eighty five.  When I fully populate the machine with 72 of
them, each one will have to pass perhaps 60 A, on alternate half-cycles,
during the pulse-burst preceding each spark.  The 85 A is the Idsm
rating.  The primary current in that case will be some 600 A during the
pulse burst.

>I thought that 8 was a lot. There seems to be some flexability about
>the way that the term "push pull " is used by folks - I take it that 
>you
>mean that
>both sides of the circuit are idendical low side switches driving a 
>center
>tapped primary.
>

>From KCH:  I'll decline to go into details about the circuit until my
patent appl. is decided.  Suffice it to say that half my transistors
conduct for 1/2 cycle & the other half, for the other.  And when I
incorporate more than 24 of them, they become connected--for rf--in
series-parallel.

>>Soon to be increased to 48, then--if my
>>endurance holds out--to 72.  That will double, then treble the spark
>>length (and the input current for a given spark rate).  The primary 
>is
>>untuned.
>
>you will increase it - you may not double it - there is a law of
>diminishing
>returns. Also FETs can be killed by over voltage. I tried this and 
>managed
>to fry quite a few FET's.  The problem is that when the FET's switch 
>off
>and
>the magnetic field collapses a very high voltage is induced on the 
>primary
>- it could easily exceed 500V. Also if by resonance we achieve a very 
>high
>voltage on the secondary some of that is reflected back to the 
>primary.
>

>From KCH: I'm counting on my circuit to a) double the primary current
when I double the qty of transistors, etc. and b)  protect each
transistor against an over-voltage from any cause exceeding around 3x the
applied dc voltage of 160 V.  Since the secondary "knows from nothing"
about primary voltage but only responds to applied magnetic flux, I
figure that doubling the flux by doubling the current will double the
spark output--if the minimum curvature radius of the top electrode allows
for that, that is.

>>4.  Maximum voltage, other than at the ~110 KHz output frequency, 
>present
>>in the primary apparatus at any time with any quantity of MOSFETs & 
>at
>>any rep. rate, 160V.  That's one hundred and sixty volts.  Safer for 
>the
>>kiddies.
>
>And the FET's

>From KCH:  You bet.  I've blown many of them at $4-5 a pop ("pop", for
sure!).
>
>>5.  Excitation frequency, always the instantaneous self-resonant
>>frequency of the secondary.  
>
>Which is drops when a spark is produced.

>From KCH:  Let it drop; I track it.  It also changes significantly
depending on what conducting surfaces are around & about.  That's no
problem, either.
>
>>6.  Secondary, about 1400T "pie"-wound -at- ~11T per pie.  Turn-turn
>>spacing, n.l.t. 0.1".  Wire, mostly18 AWG tinned Cu with some 14 AWG
>>phos.-bronze spring-wire for outer turns.
>
>please expain pie wound? 
>
>From KCH:  See my response to another query.

>>7.  Top electrode, currently 4" c.s.d. x 14" mean diameter.  In the 
>next
>>few days to be increased to 5" c.s.d.  Material, Al flex-duct.
>
>Bigger sparks as long as you have enough voltage to break out

>From KCH:  Quite so.
>
>>I hope before too long to peddle my plans for a pittance.  In the
>>meantime, I'd value hearing from anyone making something similar.  
>But
>>post it to the List!
>
>You can see my stuff at 
>http://ourworld-dot-compuserve-dot-com/homepages/alansharp/

>From KCH:  I'll take a look!
>
>Alan Sharp (UK)
>
>

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