[AMRadio] Transformer help

[Donald Chester]

>From: Gary Schafer <gschafer at mediaone.net>
>I seem to remember from the early days that a swinging choke was
>not recommended as power supply dynamic regulation would suffer badly

Gary,

It depends on how much bleeder resistor and filter capacity you use.  The 
bleeder resistance in ohms should be no more than 1000 times the filter 
choke inductance at no load (other than the bleeder resistor). This might 
require some trial and error.  Try different values of bleeder resistance 
until the DC voltage drops to about 0.9 times the rms voltage of the 
transformer winding (one side of CT if you are using full wave CT circuit).  
Reducing the resistance below the critical value should cause relatively 
little additional voltage drop. If you key and unkey the final (as in CW 
operation) with the power supply on, the value at full  load should drop to 
no less than 90% of the no-load value.  You can use less resistance, but you 
start using up current capability.

Use as much filter capacitance as you can muster, without blowing a fuse 
when you key the power supply.  I use about 30 mfd. in mine.  That maintains 
good dynamic regulation with the swinging choke.  In the old  days, filter 
capacitance was hard to come by, so most power supplies used 1 to 4 mfd.  
You could get the ripple down to an acceptable value with multiple filter 
sections, but the dynamic regulation was terrible.  It is better to use as 
large a capacitor as practicable, with one section of filter using a 
swinging choke.  I have found that with more than 50 mfd, you have to use 
some kind of step start circuit to avoid popping fuses when you key up.

Don K4KYV

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