This was all discussed here in the late 1990s,
(see archive) but i will repeat as there are
probably a few different hams reading and
contributing since then.
Resonant elements in power supply filters were
discussed in Terman's Radio Engineers Handbook.
The point of using them was to allow using a
higher bleeder resistor value (less heat), a
smaller choke, and still prevent soaring when the
power supply was unloaded. Normally the resonance
only gets close at the low or zero current
inductance, because at any load current,
including zero signal idling current, the power
supply has enough load.
Resonant choke HV filter circuit was used in the
Harris HFL1000 amplifier, the Galaxy 300 SSB
transceiver, the BE FM1.5A and FM3.5A broadcast
transmitters, and probably a few other
transmitters with high voltage supplies.
The Harris used 7 Hy in parallel with 0.25 uF. A
single 6 uF shunt C followed this, and a 100k
bleeder. It supplied 2.7 kVDC.
The Galaxy had 4 Hy in parallel with 0.1 uF,
followed by a shunt string of three
electrolytics, 200 uF each in series. They had a
15 k bleeder across each, and it supplied 800 VDC.
The BE used a 5.06/3.5 Hy choke in parallel with
a pair of series connected 0.97 uF 2500 VAC
capacitors. This was in a 2 section filter, first
a shunt 4 uF capacitor, then a series 3.5 Hy and
a shunt 4 uF. Bleeder was 200 k. It supplied 4100
VDC at up to an amp. The chokes were both in the
negative lead.
I designed it for the BE transmitters in 1982. I
tested many junkbox chokes using a setup with an
audio oscillator, a DC supply, a General Radio
1650 bridge, and an audio voltmeter. Feeding the
choke in series with a decade box, I dialed in
the value of R which caused the voltage division
to be 1/2. This gave me the 'impedance' that the
choke exhibited at the audio frequency, which
resulted in XL. These days we can use a small
meter. I also tried to bias the chokes with an
Amp of DC, to measure how much they changed under
current. At the time BE had a great collection of
old chokes, so i tested a 1.9, 6.8, another 6.8
and a 6 Hy choke, each with a 0.27 uF capacitor
in parallel, to determine what sort of Q each
resonant circuit had.
Several things became apparent to me, to build it
at such high voltages. The capacitors could not
be standard paper/oil similar to what Plastic
Capacitors and many others make. I used
polypropylene film dielectric capacitors in oil,
lower loss at high frequencies. These were made
for use in microwave oven (MO) power supplies,
for the HV supply. I found hundreds of these at
one company, for an excellent price, so we bought
a large batch and tested all to get an average
value of 0.97 uF. With two in series, or 0.48 uF,
it worked with the choke i designed. The voltage
rating of MO power capacitors was 2500 VAC, not
enough for this 4.1 kV DC supply. So with two in
series, it was adequate. I measured the voltage
across the resonantor differentially, and found
that at full load, the ripple viktage was 6 kV
p-p. I still have a photo of the waveform. Not
sinusoidal. At zero load, the ripple became a
nice 120 Hz sinusoid, with 4 kV p-p.
For the choke, I had SNC transformers in Oshkosh
design a custom unit with the two taps, one for
US and one for 50 Hz power overseas. I sent them
capacitors to use and they individually tested
the chokes with them until they got it right. The
late Carl Seivers at SNC did the work, he was a
great man to work with and knew his stuff. At SNC
he had an old guy who had designed a resonant
choke circuit for some ham rig, maybe it was a
prototype Heathkit or something, i remember the
discussion about that.
BE used the same choke in the smaller
transmitter, with a different power transformer
for lower voltage.
I compared ripple, inrush current, transient
response, and other measurements, and concluded
that it was a decent circuit for a class C FM
transmitter. Of couse, you would wonder why do it
at all in a constant power transmitter? The
reason was that the exciter can and would be
removed, turned down, leading to no current, as
the class C tube would be cutoff. At this point,
the plate meters would peg unless we used a very
huge L and a very hot bleeder. Stored energy in
the power supply was also considerably less than
the alternative 20 uF C version without the
resonator. However, one thing to remember is that
the resonator only notches out 120 Hz, and higher
frequency harmonics of the power line, like 240,
360....720 Hz are still present. This was the
reason for the second L and C, as a smoothing
filter section, the transmitter had exceptionally
low AM noise from ripple on the HV.
I never had them blow up, and it was a well
behaved circuit. However, I would also recommend
that hams consider other options, like a hefty C
input filter with step start and high surge rated
diodes these days, since the transformers are
typically designed for that. As others have said,
you can get some high circulating currents in the
LC trap at 120 Hz, and this leads to capacitor
failure if you don't account for it. Oil and
paper caps will eventually fail if not careful.
73
John
K5PRO
>
>Message: 3
>Date: Fri, 11 May 2007 13:21:12 -0400
>From: "Jim Tonne" <tonne@comcast.net>
>Subject: Re: [Amps] Power factor and choke vs resonant-choke input
> supplies
>To: <rbonner@qro.com>, "'Amps Amps'" <amps@contesting.com>
>Message-ID: <002101c793f0$c75ccb90$2602a8c0@jim177093b3dd9>
>Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
>
>This resonant-choke discussion is crying for someone
>to do a Spice analysis so we can see the various
>waveforms.?
>
>If someone will give me a schematic with relevant
>values I'll volunteer to give it a shot.
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