[AMRadio] Smoothing Choke


Jim candela jcandela at prodigy.net
Thu May 4 09:59:32 EDT 2006



Rick,

   Sorry but I am going to throw a wrench into this discussion. Those
swinging choke power supplies, although useful, have been out of favor for
years, especially when the loading is transient such as for a CW rig. Sure
if the critical inductance versus minimum load current are correct, the
output voltage will not soar under a light or no load. But what happens when
the load is applied suddenly such as with CW? The answer is arguable of
course, but I think the answer is a sudden significant and momentary dip in
B+ which all depends on the L-C-L-C values chosen.

   My choice in situations like this is to use choke input with a smoothing
choke (not swinging), and resonate the choke at the ripple frequency (say
100 or 120 hz) with a suitable oil filled parallel capacitor, and then use a
large value capacitor from output B+ to ground. With today's inexpensive
"snap-in" electrolytics, we can easily have significant energy storage in
the output filter capacitor. Back when those old handbooks were written,
chokes were easy to make, and capacitor technology was difficult to get high
'C' in a small area for low cost. Today capacitors are dirt cheap, and the
prices of chokes are sky high. Aiming for no more than 1% full load ripple
is doable with today's components, and without high cost.

   If the turn on transients are too high for your rectifiers, maybe
consider a two step start circuit to limit peak current surge, and then
apply full voltage to the primary. This can often be done with a relay and a
few other parts.

   For audio use, whether for low level audio, modulator B+, or unmodulated
B+ to the final PA, the supplies must be bypassed well (low reactance) at
the lowest audio frequency used. If not the power supplies themselves will
get modulated progressively worse as the audio frequency drops. This leads
to all kinds of instabilities such as unwanted feedback, or maybe
"motor-boating". Vintage supplies were often notorious for being modulated
unless the audio frequency was limited to something no lower than the power
line ripple frequency that the filter was designed for.

   Can those in this group with ex-broadcast transmitters such as the 20V2
describe what types of filters these transmitters use?

Regards,
Jim
JKO


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Rick Brashear
Sent: Wednesday, May 03, 2006 8:06 PM
To: bcarling at cfl.rr.com; Discussion of AM Radio
Subject: Re: [AMRadio] Smoothing Choke


Let me be sure I am correct in my calculations.  Choke input (5 - 25 H)
with 4 mfd filter will allow about 5% ripple.  Now, when I add the
second choke (5.5 H) and the second filter (2 mfd) is this now
calculated as a capacitor input for this last stage?  If so, then it
would result in another .15% reduction or about 3.3% total ripple.  Is
this correct?

Rick


Brian Carling wrote:

>Yipes - 5% ripple on a 400V supply - won't that give you about
>20V of AC buzz on your carrier?
>
>
>


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