[Collins] Re: Need 516F-2 replacement choke source

[Dr. Gerald N. Johnson]

On Sun, 2008-08-17 at 10:53 -1000, pete wokoun, sr. wrote:
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> 
> 
> I built up a 516F-2 'equal' power supply a while back and had concerns 
> also with that resonant choke circuit and limited choke breakdown voltage.  
> I don't think that original resonant choke filter works all that well; I
> found a swinging choke worked much better.  You can read about my design 
> considerations as well as voltage comparisons here:
> 
>  
> 
> http://www.qsl.net/kh6grt/page4/516powersupply/516powersupply.htm
> 
>  
> 
> pete
> 
>  
Without looking at your web page, I can agree. The resonant choke could
be good at notching the primary 120 Hz ripple frequency at the expense
of passing more of the higher harmonics to the filter capacitors. If it
was resonant at 120 Hz. My Shure reactance rule says 8 Henries and .05
mfd is resonant at 250 Hz. Not much 120 Hz filtering there, unless the
low current inductance is really 34 Henries. But it can only work if the
choke's inductance is constant and that is hard to accomplish with an
iron core and varying DC in the choke. It may well be that all it
accomplished in the 516F-2 was to limit peak currents into the filter
capacitors, a requirement to keep the 5R4 from dying prematurely.
Silicon rectifiers tend to accept much larger peak currents and not like
the inductive kick from turning off the load so choke input filters are
not used as much with them. In the low power power supplies in the
821A-1, each input choke was equipped with a selenium plate transient
absorber to protect the diode strings.

In my own 516F-2 replacement, I used a single high voltage winding with
a bridge rectifier. I wanted lots of capacitance on the high voltage
output to maximize the voltage and the voltage regulation. I took the
low voltage off the transformer winding center tap with a choke input
because I wanted to lower that voltage. It works just fine. Silicon
rectifiers, of course.

A swinging choke should nearly always give better voltage regulation
than a fixed or tuned choke or a capacitive input filter, unless the
capacitance is very large, then peak currents wipe out tube rectifiers.

Fact is, given choices in single or double high voltage secondaries,
silicon or vacuum rectifiers, oil or electrolytic filter capacitors, and
fixed or swinging chokes, there are many possible circuits that will
work perfectly fine. Its sure that some will show a little better or
worse efficiency, power factor, voltage regulation (efficiency, pf, and
regulation may  not optimize at the same time), cost, and physical size.

OK, now I've read it. Just a couple comments.

100 mfd 450 volt capacitors were not common, maybe not available at all, and 
for sure were not small enough to fit under the original 516F-2 chassis
in 1959. One needs to watch their ripple current rating in this application.
Physically small electrolytics may not hack the applied ripple current leading
to higher operating temperatures and shorter life.

The insulated choke core will float at a voltage determined by the voltage division
of leakage Resistances coil to core and core to chassis and the distributed
capacitances of the same components for the AC component of the voltage. With 
differing dielectric properties you might want to control that float voltage 
by using a couple resistors or a near center tap on the three resistors 
providing the bleed (and minimum load for regulation) and voltage division on the
HV filter capacitor. E.g. about half supply. Then you might want to encase the
choke in a housing made of 1/16" clean fiberglass to be sure fingers or tongues
can't reach that floating core even if little tongues from a kitten or puppy.

Otherwise, except that it is a bit bigger than a 516F-2 I think your supply is 
a good design that will run a very long time. It is much smaller and prettier 
than the one my dad (K0CPN, now silent key) built. I call his the 516F-2000 but
no ham at a hamfest has given it a second look. It runs cold though at full load.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer