[Boatanchors] Antique Radio

[Jim Wiley]

Joe -


I would probably start with a "brute force" DC supply delivering about 
36 to 40 volts, feeding that to a LM-317 series 3-pin adjustable 1-amp 
regulator working together with a additional pass-transistor rated for 
15 to 20 amperes collector current.  Add a couple of tantalum caps to 
stabilize things against oscillations, a  fixed resistor of 240 ohms 
from the output to the regulator adjust terminal, then a 1k ohm pot 
(wired as a rheostat) from that same terminal to ground, and shazam! (a 
technical term :-) )   - an you have an adjustable regulator that will 
go from about 3 volts to about 36 volts, and deliver 6 to 10 amps  
depending on that actual components chosen. 


For a fixed voltage regulator, replace the pot with a fixed resistor, 
with the value to be determined by the formula in the data sheet for the 
desired output voltage.  


The primary DC supply can be a transformer delivering perhaps 28 volts 
AC to a bridge rectifier, feeding a 50-volt rated electrolytic capacitor 
in the 10,000 to 20,000 uFd range.   These values are  typical, and not 
a result of actual engineering calculations.   You can get fancy and do 
all the numbers, but  I have always just used what was at hand, and 
operated using the "this looks about right" principle.  Only rarely have 
I had to make a change to improve performance. 


The ARRL handbook, and many other sources,  will have typical circuits  
showing how to connect an external current boost transistor to a 3-pin 
regulator.    


If all you need is a 32-volt fixed supply, you can use a fixed voltage 
3-pin regulator as a starting point.  Unfortunately,  there does not 
appear to be a 32-volt version of the LM78XX series.  So, in that case, 
you can start with a lower voltage version and "trick" it into thinking 
it is a higher-voltage unit.  For purposes of discussion, start with a 
LM7812.  This is a 12-volt fixed-voltage regulator, capable of 1 amp 
output current.  Then, do the "trick" of floating the ground pin above 
ground up with a resistive divider.  This will allow you to make the 
7812 into a  32-volt fixed voltage regulator.  Add a current-boost 
transistor (perhaps a 2N3055),  and you're done.  


Again, the handbook (or the manufacturers data sheet for the LM7800 
series regulators) will have circuit information.  You can download the 
data sheet from Mouser's web site.  I would still add a couple of  10 uF 
50-volt tantalum caps across the regulator input and terminals (hot to 
ground in each case) to bypass any tendency to oscillate.   Of course, 
you will need suitable heat sinks, insulating washers, and so on that 
are typical for this kind of work.  For the heatsink, a person can often 
find something from a scrapped (whatever) that will do the job, and may 
already be drilled with the proper hole pattern for the output 
transistor.  Use mica insulating washers for the pass transistor, 3-pin 
regulator, and bridge rectifier plus a THIN application of heat-sink 
thermal grease. 


Mouser will have what you need, and they will sell single quantities - 
but these parts are so cheap (less than a couple of bucks each)  I would 
probably get a few extras for spares and future projects.  


- Jim, KL7CC



Joe Walden wrote:
> Thanks for the email Jim....  I will gladly build a supply and I all ready
> have the power transformer..  I lookef for a regulator that would carry 3
> amps but didn't find one..  Maybe I didn't look far enough..  I probable
> even have the filter cap and the other parts..  Just need a regulator and a
> schematic..  I would like for it to be adjustable...  I will certainly be
> obliged for any help..  Is there any parts you need for a project..  Joe
> w5jdy
>
>   
>