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Dial In Current Limiting



9/14/96

I am currently building a 120/240v single phase solid state variable 
voltage/current regulator for my disruptive TC.  My current project is 
to limit current to the primary of my TC transformer with solid state 
devices eliminating hardware ballasts.  It's a whole lot easier to 
control waveform and power going into the primary than coming out of 
the secondary. 

This regulator resides after my 120/240 v 28 amp variac and before the 
transformer.  It is on the low voltage 60 Hz side, so safety 
precautions are mandatory.  I have obtained a couple of heavy duty 400 
amp 2500 v bipolar transistors.  These BPTs are designed for switching 
motors and heavy inductive loads.  Following the regulator, the BPTs 
are protected by two Navy surplus 100 amp EMI filters and several plain 
old heavy duty 470 v 20 kAmp peak varistors.  I also have some very 
heavy duty Navy surplus diodes for BTP protection and also a crow bar 
for overvoltage safety.  This regulator will drive my current system of 
two 12kv 60 amp neons.  I know no current limiting needed in 120 v 
neons, but they are just a test platform for now.  All the hardware has 
cost less than $100 so far.

I am designing both voltage and variable current limiting.  If first 
all goes well with current limiting, resistive and inductive ballast 
may be eliminated for noncurrent limited transformers.  I originally 
designed the regulator for current limiting, but this regulator is far 
more flexible than this simple task.   Voltage, dwell and rectification 
should be easy add ons.  Also, high power SCRs are cheap and available. 
Simple half wave and full wave bridges are easy to construct with these 
SCRs and these bipolar motor driver modules.

R Hull's DC pulsing might easily be accomplished by either full or half 
wave rectification and shunting any secondary back EMF, perhaps with 
his thyratron.  This deserves more thought.

It may invite changing the types of transformers we use.  Perhaps a 
smaller power/pulse transformer will be more appropriate and efficient. 
Perhaps tank caps may be eliminated or downsized.  Maybe the sparkgap 
eliminated (Malcolm).  Perhaps eliminate the TC primary and directly 
drive the secondary at the appropriate feedback frequency.  A DC coil 
would be a good option here also.   Over all the technology is straight 
forward and simple.  Better yet, it's inexpensive and it's here.

The bottom line is controlling and even getting away from the 120/240 v 
incoming sine wave main.

All comments and suggestions welcomed.

RWW