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Re: Solid-state TC - transformer design



Tesla List wrote:
> 
> >From 100624.504-at-CompuServe.COMWed Oct 23 22:14:53 1996
> Date: 23 Oct 96 20:00:51 EDT
> From: Alan Sharp <100624.504-at-CompuServe.COM>
> To: Tesla List <tesla-at-poodle.pupman-dot-com>
> Subject: Re: Solid-state TC - transformer design
> 
> Greetings All,
> One dumb question - leakage induction. I'm assuming that the primary
> acts partly like a pure transformer perfectly coupled to the secondary.
> But partly like a pure inductor blind to the secondary. And of course
> partly as a pure resistor boiling off the enamel :(. So the effect of the
> leakage induction is then to resist the rise in current and to give
> back EMF at switch off. Is this right?
> 
> Roderick wrote:
> 
> >   I guess I've made a few mistakes already! I'm using a toroid core
> >instead of a bobbin core and I'm also using solid #10 guage house wire
> >for the primary wound for push pull operation. I've coated the core with
> >epoxy resin, wound the primary, then coverd the primary with scotch 130c
> >high voltage spicling tape. The tape has voltage rating of 69KV .The
> >secondary is wound in the same direction with #22 guage magnet wire 50
> >turns.OOPS primary 20 turns.
> 
> >   Core has a AL value of 2725
> 
> >  I am also attaching the driver I will be using.
> 
> I've made much much bigger mistakes!
> A toriod will work - from the data you sent it looks like its usable at the
> frequency. I don't know what is cross sectional area is (A in cm^2) but if
> you measure it and plug it in the formula you'll find out if you are in the ball
> park. You'll may need either more primary turns - or less voltage.
> I use E-cores because its much easier to wind the coils on a bobbin.
> The heavy wire is fine too - Harri and I are working at 1000w plus and
> at these power levels you have to squeeze out every advantage going.
> Short runs - check the temperature - you should be OK till say 200w.
> 
> Dont start it at 165v! My first attempts were at 24v and I was delighted
> to get a 1/2" spark.
> 
> One snag with your circuit is that there is no current limiting -
> so if your transformer saturates bang! :(. (You may have seen
> my post on how I'm planning to impliment current limiting).
> At lower powers you should get away with it.
> Start low - push it gradually - it may fry eventually :(
> 2A ? fuse advised.
> It could well work at 165v with a 2.5:1 ratio.
> 
> the irf841 isn't the best choice FET, irf740 is beefier. Higher
> gate capacitance though.
> 
> As you know - you want AC - not DC from the output, one end
> to ground, the other end to the coil. Its very helpful to put a pair
> of LED's back to back into the coil feed line - they'll glow when
> you're in tune (or ar a harmonic).
> 
> So Roderick don't throw away your toriod - do the calculation - if your
> happy that it won't saturate, try at 24v and work up. 2.5:1 is OK to
> start with.
> 
> Harri and Jim spoke about winding layers:
> 
> >On the other hand, if you wind 1st layer from left to right, then bring
> >the wire back to the left side somehow, then wind the second layer again
> >from left to right there will be voltage difference equal to the
> >voltage induced in one layer. A great advantage! If you calculate
> >the interlayer capasitance (through energy balance) you will find
> >you the interlayer capasitance is 1/4 of the capasitance you did get
> >with the fist method!
> <<<<
> 
> I'm trying this - but I'm taking the wires from each layer out of
> the coil, I'll connect outside. This will also enable me to have
> 3 taps - 1:7,14,21. The potential problem is that I'll get
> electrical breakdown around the connections.
> 
> Thanks Harri for the ideas on interleaving layers. I'll may well
> try this once I've tested the double core former.
> 
> >Altering LC-tank freq. is not easy. The best way I can think of is
> >to take advantage of the nonlinearity of ferrites. That is, feed
> >some syncronous current to the coil, therefore altering its effective
> >permeability, therefore altering the frequency. That, however, is not
> >an easy task to do. Using adjustable cores with high power devices
> >is not a good choise either.
> 
> >If anyone figures out a good solution for that one, I'd certainly
> >like to hear about it!
> 
> So would I - but the capacitor can be done - its easier conditions
> than a capacitor encounters in the conventional spark gap circuit.
> But I saw the tuning problem as well - I could only think of altering
> the cap - switching in smaller values or building a large variable
> capacitor with moving plates.
> 
> I am planning to get a 3' arc though - but I'll do it with a pair of
> identical coils and drive them out of phase. One PWM board,
> two driver boards, two output transformers, two magnifier coils.
> Alas first I have to set up my lab in the garage - its just too
> much for the house!
> 
> Alan Sharp (UK).


   I'm going to use a 10 ohm 225w power resistor placed before the
filter capacitors in the circuit to limit the rate of charge to the
capacitors. In previous circuts I blown the bridge rectifier even though
it was a high current, high voltage type! The filter caps are two 4500uf
in parallel. I have also recently acquired a surplus 120vac variac rated
at 45amps to control the voltage. You can disregard the filter network
on the secondary of the output transformer because it was taken from a
manufacturers application circuit. The mosfets I will be using are
Harris semiconductor IRFP450s.