[AMPS] Screen protection WHAAAATTTTTT!!!!

[Tom Rauch]

> Rich wrote
> 
> >>Transistors that C/E short offer zero protection.
> 
> Ok so sense the voltage drop across a resistor across the screen supply,
> feed the transistor via an opto isolator,make the transistor trip a fast
> relay in the primary of the supply transformer.This is better and more
> reliable than a fuse surely.

Better still, use a screen supply at least two times the desired 
voltage. Get that voltage from the same transformer that supplies 
anode voltage, and use a shunt regulator. Use the bias and blower 
voltage (with an air pressure switch) to turn on the HV system. 
Sense the regulator current with a trip system that turns off the PA 
if regulator current exceeds a safe margin over the quiescent 
current.

1.) Under any condition of positive screen current, you can never 
have more dissipation than the quiescent dissipation set in the 
regulator.

2.) Negative screen current will be limited to a lesser value than 
positive screen current, preventing overdissipation from negative 
screen current in most cases.

3.) Everything is interlocked.

4.) Protection is instantaneous.

5.) If any semiconductor shorts or if HV or bias dumps, the system 
fails safe.

Then you can use the fuses to prevent wire and transformer fires, 
which is a job they are suited for.
73, Tom W8JI
w8ji@contesting.com

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