[AMRadio] Swinging chokes

Tue Feb 19 06:55:50 EST 2002

Hi Gary,

The benificent gods of internet routing finally allowed me to reach the I0JX site.  This tutorial describes the situation pretty much as I see it.  http://www.qsl.net/i0jx/supply.html

As Don has pointed out, you need significant output capacitance to avoid transient dips on application of load with a choke-input filter.  But you will find similar transient dips with any choke filter.  I think Measures is mixing apples and oranges when he implies that resonating the input choke makes the transient voltage dip go away.  http://www.vcnet.com/measures/Rebuttal.html

An input choke is resonated simply to improve static regulation at the low current end of operation.  It is definitely best not to overdo this resonation with too much choke tuning capacitance, or resonance will occur at higher current, and excessive output voltage will appear at low current.  It takes high output capacitance to avoid load-transient current dips when a choke filter is used.

I also find very little information on resonated choke-input filters.  And if a reference even goes into the swinging choke idea, it usually just tells us to use a heavy bleeder resistance load to produce enough minimum supply current to hold R(load) to limit the required critical inductance at low currents.

By the way, I have found that the L(Hy) = R(load)/1000 equation for critical inductance is only approximate.  For calculating critical inductance, Radiotron says that at 50 Hz, L(Hy) = R(load)/940, and at 60 Hz, L(Hy) = R(load)/1130.  Terman agrees that for 60 Hz, L(Hy) = R(load)/1130.

If you have access to old QSTs, check F. S. Dellenbaugh, "The First Filter Choke - Its Effect on Regulation and Smoothing" from QST, March 1932.  Dellenbaugh goes into air gap and core saturation and some very bad things that can happen when using too small of a gap in a swinging choke (oscillating overloads).  Unfortunately, he does not mention tuning the choke.

Also the AC and DC voltages on the choke must be considered.  The exploding choke story may be describing a choke that arced over internally when powered up.  Perhaps it was still moist from storage.  We know how that can destroy a power transformer.  Or perhaps it was designed to be used on the ground side between the transformer center tap and ground, but was instead connected on the hot side and its case grounded.  Perhaps too much resonating capacitance was applied, reducing the effectiveness of the choke at low currents and causing a system failure due to the resulting voltage surge toward 1.4 x RMS, which the rest of the system could not withstand.  By themselves, I seriously doubt that the extra currents resulting from resonance would cause the incendiary results described, but a good arc to ground with the full supply power applied would certainly blast the choke into oblivion, especially with a big bank of charged caps contributing to the event.

  Bacon, WA3WDR

Gary Schafer wrote:
> 
> Bob,
> 
> I guess you noticed that Measures advocates that any choke other than a tuned choke is bad news in a power supply because of the poor transient response. Although he does hedge a little when he says that the problem is not as great with a larger filter capacitor.
> 
> It seems to me that with a tuned choke, once you draw some current and the inductance drops a little and goes out of the resonant mode, you are back to having a regular choke input supply. And at that point I would think that the same transient problems would exist as if the choke was not tuned.
> Or maybe a regular choke (not a swinging choke) that is tuned will stay in resonance longer as more current is drawn?
> 
> There must be some critical value of filter capacitor that needs to be used when a choke is involved to eliminate the transient problems.  I have never seen anything written on that. I also have seen very little written on the transient problems either. Power supplies are usually looked at in a static mode rather than dynamically.
> 
> 73
> Gary  K4FMX
> 
> Bob Bruhns wrote:
> 
> > Hi Gary,
> >
> > I can't get to the qsl.net link tonight, but I checked out the measures/rebuttal.
> >
> > Inductance will cause a transient voltage drop when a load is switched in, if there is not enough capacitance.  But the time frame is not seconds, it is 1/120 second.  And because inductance is always used in this kind of filtering, this effect will exist whether inductive input or capacitive input is used.  In fact, the reduced inductance of the swinging choke under load might be expected to reduce this effect compared to a non-swinging choke in a choke-input filter.  And the high peak currents of a capacitor-input filter will cause a constant voltage drop as you increase load current.
> >
> > The reason that low-current swinging-choke resonation helps is that it reduces the current drawn by the 120 Hz ripple at low current, which effectively increases the inductance of the choke at low current, and this keeps the filter output closer to the 0.9 x RMS instead of riding up to 1.4 x RMS.  At higher current, the inductance is reduced, so the resonance is off-frequency and therefore no longer effective.  But at that point, the load current is high enough so that the inductance of the choke is high enough to keep the output at 0.9 x RMS.  So the resonating cap helps at the low-current end of operation.
> >
> > At the high-current end, heavy saturation of the swinging choke core causes the inductance to drop more than the current rises, so you start to drop below critical inductance.  With proper component selection, the resulting tendency to increase from 0.9 x RMS toward 1.4 x RMS will tend to cancel the voltage drop caused by line, transformer and rectifier resistances.
> >
> > The approximate critical inductance for a choke input filter is L(Henries) = R(load)/1000.  With a swinging choke, you should figure R(load) from the supply voltage and current at the low-current end of operation.  The inductance value may be reduced somewhat if the tuning cap is used.  You'll have to experiment.
> >
> >   Bacon, WA3WDR
> >
> > Gary Schafer wrote:
> > >
> > > Hi Bob,
> > >
> > > I understand the things that you have stated.  From what I have seen there seems to be some difference in the characteristics between a "tuned" choke and a non tuned swinging choke. I have attached a couple of links that I found on the subject. One is written by Richard Measures. Look down the page to the part of the article on POWER SUPPLIES, filters.  Interesting, but I am not sure it really happens with the swinging choke the way it is described
> > >
> > > The other discusses tuned chokes and also a swinging choke.
> > >
> > > http://www.qsl.net/i0jx/supply.html
> > >
> > > http://www.vcnet.com/measures/Rebuttal.html
> > >
> > > Go down to POWER SUPPLIES in the article.
> > >
> > > 73
> > > Gary  K4FMX
> > >
> > > Bob Bruhns wrote:
> > >
> > > > Hi Gary,
> > > >
> > > > A swinging choke is meant for varying loads.  It provides a degree of system regulation with a varying load.  If we could not use it for a class B amp meant for SSB, we could not use it for a class B modulator either.  However, in an AM system, a choke input filter is easier to design if the supply powers both the modulator and the RF amp, because supply current will be much higher during the resting modulator condition.
> > > >
> > > > A swinging choke is designed to saturate gradually as more DC current flows through it.  This is natural for ferromagnetic cores, only requiring some optimization in the design of the core (generally under-sizing).
> > > >
> > > > A choke-input supply requires a minimum inductance versus current.  If there is enough inductance, the choke averages the current during conduction, and the DC output is about 0.9 times the RMS AC applied between the rectifier diode and ground.  But if the minimum amount of inductance (critical inductance) is not present, the choke is not effective, and the filter operates more like a capacitor-input filter (DC out equals 1.4 times the RMS AC applied between the rectifier diode and ground; DC rides the crest value of the AC waveform).
> > > >
> > > > If the choke inductance falls as more current is drawn, it will eventually reach the point where there is not quite enough inductance for it to operate purely as an inductive-input filter.  What happens?  When you are drawing more current, you get an increase in output voltage over what you would have gotten with more inductance.  BUT - when you are drawing more current, you get more voltage drop in the AC mains, from power transformer transformer resistance, etc, and the available raw voltage drops somewhat.  So, if you match the swinging choke to the maximum current and the voltage drop from other parts of the system, these two effects approximately cancel, and you can actually get better regulation.  I consider this a natural.
> > > >
> > > > Below a minimum current, the choke will also have too little inductance, and the output voltage will rise.  But you have amplifier resting current and power supply bleeder current, and you can improve the  operation of the swinging choke by adding a small parallel capacitor across the swinging choke.  This capacitor is chosen to resonate with the choke at two times the mains frequency when the minimum current is drawn.  The Collins KW-1 does this, and it works...  BUT, remember that the capacitor sees a large voltage swing, so use one that has enough voltage rating to be a filter cap in the circuit!
> > > >
> > > > To prevent damage in the event of a choke or resonating capacitor failure, make sure the rest of the system can withstand short term 1.6X overvoltage.  Uncle Ed (WA3WPUN) had the resonating capacitor fail in his KW-1, and the resulting overvoltage caused damage in the rest of the transmitter.
> > > >
> > > >   Bacon, WA3WDR
> > > >
> > > > Gary Schafer wrote:
> > > > >
> > > > > Thanks for the response Bill. What I was trying to find out was if the
> > > > > particular transformer was rated  220 or 120 volts. It has no markings and
> > > > > only a single wound primary. I did find out how to check. By applying voltage
> > > > > with a variac and monitoring the primary current for a rapid increase which
> > > > > would indicate core saturation. If it made it to 220 volts without saturation
> > > > > (about 250 ma current draw) then it was ok for 220 volt operation. If the
> > > > > primary was intended for 120 volts it should saturate at around 150 volts.
> > > > > Current would rapidly rise to several amps. I tried this with a small filament
> > > > > transformer (rated for 120 volts) also and indeed found that it would saturate
> > > > > at around 150 volts.
> > > > >
> > > > > 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 ?
> > > > >
> > > > > 73
> > > > > Gary  K4FMX
> > > > >
> > > > > wwd at netheaven.com wrote:
> > > > >
> > > > > > Resend: Did this get out to Gary? Anyone? (testing, testing) .   .   .
> > > > > >
> > > > > > Putting 240vac on the full primary will get you via a solid-state bridge
> > > > > > (choke input) about 3400 vdc. Which is probably just what you wanted for a
> > > > > > 'big' rig. I'd insulate the core from the chassis with sturdy standoffs.
> > > > > > Also caution signs since a secondary short to core will leave it hot to
> > > > > > chassis! Always  go in first with your hv probe! It will probably last
> > > > > > forever. Good luck. Bill N2KQA
> > > > > >
> > > > > > > Gary Schafer <gschafer at mediaone.net> said:
> > > > > >
> > > > > > >I wonder if anyone can help identify a transformer for me.
> > > > > >
> > > > > > >It is a plate transformer with what looks like a hypersil (sp) core. It
> > > > > > >has a single primary with 3 taps. Not sure if it is rated for 120 or 240
> > > > > > >volts. With 120 volts I get 1900, 1770 or 1685 volts out. No center tap
> > > > > > >on secondary.
> > > > > > >The only numbers on it are:  AA 3883  and S179027. Do those numbers mean
> > > > > > >anything to anyone ?
> > > > > >
> > > > > > >It is probably mid 70s vintage.
> > > > > >
> > > > > > >Also does anyone know how I can tell if it is ok to run on 240 volts
> > > > > > >rather than 120 volts other than amount of smoke?
> > > > > >
> > > > > > >Thanks
> > > > > > >Gary  K4FMX
> > > > > >
> > > > > > --
> > > > > > -----------------------------------------------------------
> > > > > > wwd at netheaven.com
> > > > > > -----------------------------------------------------------
> > > > > >
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