Re: solid-state TC, transformer design

To: Tesla List <tesla-at-poodle.pupman-dot-com>
Subject: Re: solid-state TC, transformer design
From: Alan Sharp <100624.504-at-CompuServe.COM>
Date: 21 Oct 96 07:30:45 EDT
>That method is called "push-pull" and does produce an AC
>waveform [square wave] at the transformer output. It's advantage: only
>2 switch elements are needed. It's disadvantage: the non-conducting
>switch is subject to twice the applied voltage.

Thanks Jim - the push pull definately produces AC at the output.
The point that I was making is that there isn't AC at the input side
where I measure the current. The twice the voltage is a bummer
I needed to build first a 140v and then a 180v PSU. Also the snubbers
capacitors charge up to 2*V on the off cycle - time for a full bridge.

> I hate to butt in but I wonder if you fellas could answer my question?
>Can you wire more than one layer of secondary winding over the primary
>without frying something?

butt in any time - I've used 3 or 4 layers of secondary windings but the heat
is a problem. Modulating on/off at about 50hz 0-50% duty cycle helps a lot
(and seems to increase spark length). Short runs initially and then check the
heat in the transformer. (Capacitance will charge an unearthed core - ouch
as I discovered - I now switch off then test.). 

Basic design method:

1) Get hold of a core that will deal with the frequency and power you are using.
Generally transformers about 2"*2" will handle about 500-1000w at 200kHz.
ie ETD49, bigger for 100kHz ie EC70. (2.7"*2.7").

2) Work of the number of primary turns from:
N=V*T*10^8/B*A
N- primary turns, V voltage, T time in seconds that V is applied (1/2 period)
A effective cross sectional area of central core in cm^2.
B max allowable flux density in guass. Typically 3000 but 1000 is a more
reasonable value.

3) Twist together multiple enameled wire to form the primary.
use 0.3mm diameter wire for 200kHz, 0.4mm diameter for
100kHz. (Skin effect makes thicker wire less efficent)

4) Insulate very carefully. I use clear silicone sealent. Or an
epoxy. Don't take the secondary windings all the way to
the ends. (400v transformer, 15:1 ratio means there is 6kV
in the transformer). Insulate all the wires coming from the
transformer former. Silicone can be ripped down easily
if you want to reuse bobbins. 

5) Use transformer bobbins and buy extra bobbins 
for experimental purposes.

6) Start with a very conservative ratio - say 5:1 and work
up. Use as thick a wire as you can get in the space, expect
to wind 3 or 4 secondary layers.

7) Fit the 2 half cores tightly together. (All right I confess I
have used strong rubber bands)

8) Pray that the laws of physics wont look too closely at the 
interlayer capacitance, the leakage induction and that spot in the
middle of the transformer where the insulation is too thin.

9) If the laws of physics are being scrupulously applied restart at 1.

The above is based on what I've gleaned from others on this list -
the mistakes are all my own.

Have fun,

Alan Sharp.