John,
Thanks for this post. I am keeping it as a reference.
I have a couple of questions:
How can the Harris filter (7 H * 0.25 uF = 1.75) and the Galaxy filter (
4 H * 0.1 uF = 0.4 ) be both resonant at the same frequency, 120 Hz?
Also, in the BE filter, where was the resonant filter connected? before
or after the 2 x 4 uF and 3.5 Hy pi-filter?
And what do you mean by a 5.06/3.5 Hy choke?
Thanks a lot
Angel Vilaseca HB9SLV
John T. M. Lyles a écrit :
> 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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