RE: Solid State Voltage and Current Regulator (fwd)

From: 	Richard Wayne Wall[SMTP:rwall-at-ix-dot-netcom-dot-com]
Sent: 	Monday, January 05, 1998 2:48 PM
To: 	Tesla List
Subject: 	Re: Solid State Voltage and Current Regulator (fwd)


Harri wrote: 

>>beware that if the transistors fail they will (typically) short, so
>>you will
>>suddenly burst up to full power - this may not be a good thing.
>>also, you said "dissipates" 3 KW - did you really mean that it will
>>3KW???  I would hope the dissipation is much lower than 3KW.

>The configuration that has been described has a transistor in the
>series-pass configuration.
>The transistor acts like a variable resistor! It will indeed
>dissipate a lot of power. That's one
>of the reasons I'm not fond of that approach even though it is very
>simple one to build.
>Some pwm approach would be much better.



A couple of good points.  Yes, that's 3.1 kW power dissapation that a 
single module can handle, if necessary!  These guys are brute force 
solid state devices.  Not for the faint of heart.  

If the current limiting BPT fails (short) then the base drive to the 
series pass BPT is depleted and the transistor is shut off.  In the 
very unlikely event the series pass module fails (shorts) the potential 
current in a noncurrent transformer is unlimited.  It's also fairly 
simple to put a crowbar circuit in to shut down the main circuit if 
this unlikely event occurs.  Also, note these modules are well 
protected with multiple reverse diodes built in and are designed to 
control large inductive loads and motors.  Right up the Tesla Coiler's 

Harri's observation about these series pass transistors are analogous 
to large variable resistors is partially correct.  Resistor are always 
fully on and are incapable of functioning in a cut off mode.  These 
transistors dissapate virtually no power in the full on mode or cutoff 
mode.  As they turn on, the power they dissapate is proportional to CE 
current through the transistor times the voltage drop across CE.  So, 
depending on the set current and voltage, power dissapation may vary 
widely.  All the small solid state voltage regulator devices opporate 
with this same series pass configuration.  And, current limiting of 
large noncurrent limited transformers now employs various inductive, 
capacitive and resistive schemes.  Often dissapating power in other 

Please let us know more about your pwm approach for controlling 
voltage, current and driving HV transformers.