[TCML] OLTC midi problem

[lightningfor@xxxxxxxxxxxxxxxxxxxxxx]

Hi All,
A small problem that I would like to present to more experienced 
builders...

I have been building a small OLTC that is still in testing phase.
The coil produces about 2.5 feet long arcs and runs great with the basic 
PWM controller I have made to give the 0-500bps.

IGBT is a 1200v 600A, and is driven by a simple 30A driver IC and 
circuit, with isolated PSU, and opto coupler isolates between the driver 
and the PWM control box.
PWM control box is metal, negative to case, powered by battery and 
connected via a coax cable with the shield connected to mains supply 
earth at the driver end.

So with this set up, the coil runs great...

I then bought a basic midi interface from EVR, and assembled, tested etc 
the board and set pulse output to suit the needs of my setup. (also 
running off battery...)
Connected in place of my manual controller and runs great upto about 75% 
power input to the coil.
Even a fraction above the 75% mark and suddenly, nothing but the 
occasional splutter...
Moved controller further away... still faults at the same 75% mark.

Scope across output of midi controller is clean and runs continuously 
even when spluttering, so I ruled out the controller.
So I put scope accros the Gate of the IGBT... and what do you know, the 
problem went away!
Next I removed the scope and connected the Emitter side of gate to 
ground via a 1000pf capacitor (i cant ground it directly due to 
circulating currents) and it also solved the problem, running upto full 
power no problem...

So my question is, what was the real cause of the problem?
Any idea's?

Thanks,

carlos
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla