Fusor Forums

Today I was fortunate enough to measure the primary waveform of the Spellman supply, and I thought someone might find it interesting and perhaps have a stab at the topology involved.
The slides are marked by the output current, and the voltages involved, well I forgot to mark them down, but they range from 10-30kV

Tomgang, Idle
0.05mA, the supply snaps into attention, at 50kHz
At 0.2mA, the supply shifts to 100kHz
3mA, the current peaks get higher @1A/div
7mA, still higher peaks
20mA, @4A/div, the primary currents tops out at 6A
Somehow I hope this will be of some interest to someone.

Cheers, Finn Hammer

Of course, they are typically just whacking the primary of an air core coil (Tesla coil) standard damped wave. It must be tightly coupled to get any useful power out. Spellman has their act down on this effort. A special ferrite core can be made to do this as well and boost the transfer power a little better, as well. They have been doing it for years.

Richard Hull

Richard,


Actually a proper pair of ferrite UR cores, coils on separate legs, so plenty of leakage to suit a nice ressonant switching scheme. But no, there is a good old UC3825 from Unitrode, governing the switching in PWM mode, not as elegant as I had expected from reading loads of Spellman appnotes. There are two identical halfbridges, one for each transformer.

However, notice those secondary coils! No bigger than a silver Dollar, and basket woven, nice stuff. To my surprise, they have the high potential up against the core, with ground on the outside windings. I assume this arrangement provides a better field controll, the old pancake tesla coils of Kinraide fame had it that way, but I would not have guessed it.

I bought a MoReCo coil winder in an attempt to replicate those windings for my own intertainment, going to be interesting when it arrives.

Cheers, Finn Hammer