Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!comp.vuw.ac.nz!canterbury.ac.nz!phys169
From: phys169@canterbury.ac.nz
Newsgroups: sci.electronics
Subject: Re: How to make 600VDC from 800VDC (errata)
Message-ID: <1990Oct31.092352.9585@canterbury.ac.nz>
Date: 30 Oct 90 20:23:52 GMT
References: <1990Oct18.134252.4635@mlb.semi.harris.com> <1990Oct23.151030.5176@engin.umich.edu> <1990Oct29.152553.9568@canterbury.ac.nz>
Organization: University of Canterbury
Lines: 27

In my article <1990Oct29.152553.9568@canterbury.ac.nz>, phys169@canterbury.ac.nz writes:
>                            >    | V    
>                    R6=    <       |   
>               R3*550/200   >      +----+-----------> 600V+
>                            |  C1__|__  >
>                            \ |  _____ < R4 ~ 100K/5Watts
>                             \|    |    > ( <<R3*HFE2*579/200)
>                              |----+----|
>                             /|         |
>                            V |         < 
>                            |            > R5=R4 * 579/21
>                          -----         <
>                            ^ ' 20v     |
>                           /_\ zener*   |

Whoops! Sorry, folks, R5= R4 * 21/579. (Thanks, Curt, for pointing that out). 
Us antipodean people tend to get things upside down. 

By the way, you may need a small variable resistor in series with R5 to provide
fine adjustment of the voltage, and to compensate for base current spoiling the
output voltage calculation. This basic circuit is good where the voltage drop
required is relatively large; there is a nice modification of the circuit for
situations where the voltage drop required is small (or a wide range of output
currents is envisaged), which involves having the power transistor's collector
at earth potential (good for heatsinking many power transistors where the case
is connected to the collector)... ask me if you want the circuit.
Mark Aitchison.