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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).