[AMRadio] Transformer help

[Gary Schafer]

I understand about avoiding resonance with the choke and the filter capacitor.

I saw an article about a Harris SSB amplifier that used a swinging choke in its
supply. However most others that employ chokes that I have seen have a "tuned
choke".

The idea of using a swinging choke has been discussed and there seem to be
differing opinions. I have seen some that say that the choke can not keep up
with the current fluctuations because the choke tends to be a constant current
device rather than a constant voltage device like a capacitor. So what
supposedly happens is a dip in output voltage of the supply when current is
first drawn. This seems to make sense but it would also seem that with a large
enough capacitor it could be compensated for. After all it does take current to
recharge the capacitor.

Of course the critical load must be maintained on the supply to keep the voltage
from soaring at no load. That is the main reason for using a swinging choke. To
have a much higher inductance at low loads so that less of a minimum load is
required so that the voltage does not soar.

I wonder how in an AM transmitter where swinging chokes are often used, things
work out ok? The class B modulator has a current demand very similar to an SSB
amplifier. The class C amp is sometimes powered from the same power supply (some
times not) which would provide a high minimum load on the supply but then most
of the time the power supply is turned off to shut down the RF. So if there is
no changing load it would seem to be useless to use a swinging choke. And if the
swinging choke comes into play in supplying the varying load of the modulator,
how is it different from an SSB amplifier?

73
Gary  K4FMX



Bill Smith wrote:

> Part of the design criteria for a power supply is to make sure the
> inductance of the filtering network does not match a "critical" value, at
> which the filtering network approaches resonance.  Of course at resonance
> high voltages and currents are generated in the network, which is quite
> opposite of the intended effect from a power supply.
>
> A swinging choke's inductance varies with load.  The reason they are not
> used with SSB gear is because of the extreme current fluxuations with SSB
> driven amplifiers, especially when there is little modulation (no current
> draw).  At this point, the critical value of a supply using a swinging choke
> becomes very hard to avoid.  Swinging chokes work much better in a AM
> transmitter with a relatively constant current draw.
>
> 73 de Bill, AB6MT
> billsmith at ispwest.com
>
> ----- Original Message -----
> From: "Gary Schafer" <gschafer at mediaone.net>
> To: <amradio at mailman.qth.net>
> Sent: Saturday, February 16, 2002 2:56 PM
> Subject: Re: [AMRadio] Transformer help
>
> > Thanks Russ & Jim.
> >
> > That Is what I remember from long ago but now I am wondering if it would
> work ok
> > with a large filter cap like around 25 mf.  In the days when swinging
> chokes
> > were used a lot large values of capacitance were hard to come by.  I would
> not
> > be depending on the choke for any filtering, only voltage regulation.
> > Any thoughts ??
> >
> > 73
> > Gary  K4FMX
> >
> >
> > Tommye & Jim Wilhite wrote:
> >
> > > Russ is right about the swinging choke, Gary.  Good for AM, not good for
> ssb
> > > for the reason you cited.
> > >
> > > Jim
> > > de W5JPW
> > >
> > > >
> > > > Speaking of choke input, I wonder if a swinging choke only, will cause
> > > > problems on SSB. The supply that the transformer is going into has a
> 20-4
> > > > henry swinging choke. It was used that way before but I am not sure it
> is
> > > a
> > > > good idea. I seem to remember from the early days that a swinging
> choke
> > > was
> > > > not recommended as power supply dynamic regulation would suffer badly
> ?
> > >
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