Re: Solid State Voltage and Current Regulator (fwd)

From: 	Richard Wayne Wall[SMTP:rwall-at-ix-dot-netcom-dot-com]
Sent: 	Thursday, January 08, 1998 4:16 PM
To: 	Tesla List
Subject: 	Re: Solid State Voltage and Current Regulator (fwd)

You wrote: 


>> Harri's observation about these series pass transistors are 
>> to large variable resistors is partially correct.  Resistor are 
>> fully on and are incapable of functioning in a cut off mode.  These
>> transistors dissapate virtually no power in the full on mode or 
>> mode. 
>Cut off mode I'll believe, but no power dissipation in full on mode?
>That's hard to believe.  What is their forward drop?  Since any BPT is 

>essentially two back-to-back diodes, the forward drop is usually at
>least a few tenths of a volt.  That equates to a lot of dissipated 
>when they are fully conducting.  Are these by any chance Insulated 
>Bipolar Transistors?  I hear that IGBTs have _very_ low forward drop, 
>and I see them being used in some of the higher-end lighting dimmers 
>place of SCRs these days- their ability to turn on at any desired 
>eliminates the need for a filter inductor.
>Following this thread with considerable interest,


When the transistor is cut off it's easy to see there is no heat 
dissipation.  When the transistor is fully conducting, CE voltage drop 
is small, as you point out.  Since transistor dissipation is 
proportional to CE voltage times CE current it is also easy to see 
there is "virtually" little dissipation at saturation.  But, you gotta 
control the current.  Between cutoff and full saturation the voltage 
drop across CE is significant and as Harri has pointed out, the 
transistor acts like a variable resistor.  Significant heat may be 
dissipated in this mode.  

These are bipolar transitor modules.  Not IGBTs.  Although I wish I had 
a few IGBTs to play with, they are big bucks at these power levels.