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Author Topic: Resonant power supply filters again..  (Read 3129 times)
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Patrick J. / KD5OEI
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« on: March 23, 2019, 06:51:12 PM »

Time to get back to some HV filtering stuff. Still have not given up on break-in operation for the amp project.
Two obstacles - B+ swings and TR switches. So about the B+ swings the resonant filter looks pretty good.

I hope to have a reasonable sized 2.5H 2A inductor at some point. The resonating C would be 0.685UF for 125Hz and 0.7UF for 120Hz. Advice was to go for 125Hz since the L will drop with current.

I'd appreciate any help on what type of capacitor technology to use choose from.
Several example transmitters and amps use the resonant filter circuit.

The conundrum has been the circulating current through the capacitor (see images).
In LTspice, with a 2.5H choke, it's just shy of 1.8A peak to peak. Like a sawtooth with a risetime of about 8ms and a fall time of just about 100us. Heck of a di/dt there.

In the simulation I set the 0.685UF resonant cap series resistance above to 1 Ohm.
No way to guess what it usually is. Those old parts are not made any more.

There are several posts or articles where this has been discussed, and pictures of what has been used in those positions One shows an assortment of those "plastic capacitors" type of cylindrical units. Not sure if many of them actually answer the question definitively. Those links are at the end of this post.

--

In old commercial/ham gear of good size the resonating cap looks like a little oil cap in these manuals:
Collins KW-1
2500VDC
T503 Plate trans 2900-0-2900 700mADC 7500V insul.
L501 6H 700mA 10KV
C505 cap paper 0.15uF p/m 20% 5KV. (looks like an oil cap)

collins KWS-1
2000VDC
T503 Plate 2000-0-2000 pn662-0155-00
C501 cap paper 0.15uF p/m 10% 5KV. (looks like an oil cap)
L501 8H reactor 55 Ohms 120CPS 668-0153-00

Collins 20V-3
T107 Plate 3200-0-3200 Raytheon F-2833 collins 672-0385-00
L113 10H @ 10VRMS 69Hz 1ADC 50 )hms Chicago 19069 collins 687-0625-00
C181 paper 0.08uF 5% 2400V sprague 92460 collins 930-0467-00 (looks like an oil cap)

--

This is all well and good, but on web sites for modern oil caps such as 1uF/10KV, ratings are not quite clear.
How is this to be interpreted?
highenergycorp.com, on their oil caps:

AC Ripple Voltage: The sum of the DC voltage and peak AC ripple should not exceed the capacitor's VDC rating.
Additionally, the AC component should not exceed:
20 % of VDC at 60 Hz
10 % of VDC at 400 Hz
5 % of VDC at 1 kHz
1% of VDC at 10 kHz

Does this mean that the peak ripple can't exceed the DCV across the cap? In the resonant filter, it's 100% ripple, no DC to speak of.

--

I'd appreciate any help on what type of capacitor technoloty to use choose from.


###########
other posts/aricles about this
https://www.qsl.net/i0jx/supply.html
https://www.eham.net/ehamforum/smf/index.php?topic=86974.0
http://lists.contesting.com/_amps/2007-05/msg00112.html
http://lists.contesting.com/_amps/1998-02/msg00030.html
https://www.diyaudio.com/forums/tubes-valves/99577-design-resonant-input-choke-psu.html
https://electronics.stackexchange.com/questions/211440/resonant-choke-filter-revisited
https://www.vintage-radio.net/forum/showthread.php?t=91550
http://www.turneraudio.com.au/audiofilterchokes-page2.html
http://jrkyn.esy.es/13de078/dc-power-supply-handbook.pdf
https://antiqueradios.com/forums/viewtopic.php?f=12&t=230988
https://dalmura.com.au/static/Choke%20measurement.pdf


* power supply resonant filters.png (181.1 KB, 1920x1425 - viewed 277 times.)
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« Reply #1 on: March 27, 2019, 12:20:06 PM »

First off, imho, the way to do this is in situ. Meaning, in the actual power supply.

There are things that the SPICE simulation will not take into account. The dynamic
resistance of the power transformer, and the actual load Z and phase angle.

Also, elliptic filters, which is what this is are, are somewhat hairy in that the depth of the
notch (depth of the null) and the frequency is reasonably critical WRT to the actual
inductance and the actual capacitance. So, it's not really predicated by the simulation.

Think ur going to need a HV cap, depending on the required value, oil filled cans have
been used historically. Since it's a low frequency application, how they perform at RF
isn't terribly relevant, it would seem. A bunch of series'd film caps might fly?

I'd check the mfrs specs for various caps, some are AC rated, if not for a specific
cap, then for the type - say metalized polyester for example.  

Some time back I found some large value surplus motor start caps that were actually
polypropylene, they'll handle large ripple currents at 60 and 120Hz... not useful for this
but just an example.

Oh, unless I am upside down on this, if the L decreases, you need more C to keep the
frequency constant.

If you look at the curve, you'll see that after the notch the attenuation more or less is
a flat amount, maybe 20db... before the notch the attenuation is reduced to zero.

As far as instantaneous current draw? Maybe a fast cap after the choke? Cheesy

Btw, neat idea to simulate the QSK with the circuit you put in there!!
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« Reply #2 on: March 27, 2019, 11:47:19 PM »

In Situ. That's why so much research. It's a little dangerous to say the least.

My concern is oil capacitor explosion, but that ought not happen until it heats up?
A plastic unit won't spew oil. I have a bunch of NOS 1uF 6KV paper in oil units and some smaller.

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« Reply #3 on: March 28, 2019, 08:48:56 AM »

In Situ. That's why so much research. It's a little dangerous to say the least.

My concern is oil capacitor explosion, but that ought not happen until it heats up?
A plastic unit won't spew oil. I have a bunch of NOS 1uF 6KV paper in oil units and some smaller.

Metal cans for the paper in oil?
Unaware of them exploding... but, "I'm not aware of too many things, I know what I know, you know
what I mean..." Edie Brickell & The New Bohemians (before she married Paul Simon).

Not terribly dangerous, unless you forget to use your shorting stick, turn the unit off, and maybe unplug it
before doing cap trimming...

But, try your simulation with a cap (various values) to ground after that resonated choke, and see what that does to the
waveform?
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« Reply #4 on: March 28, 2019, 09:01:42 PM »

The 1uF/6KV ones are in metal cans with ceramic posts, very well insulated. There are a few of those 'plastic capacitors' around as well.

It already includes a 31.5uF capacitor after the resonator. Did you mean to change that?
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« Reply #5 on: March 29, 2019, 01:26:27 AM »

Here are some 'plastic capacitors' type of glass enclosed ones i0jx said are suitable for trimming. and also discusses a swinging choke for resonance, and a Henry 4K Ultra.
https://www.qsl.net/i0jx/supply.html
scroll down to where resonant filters are discussed.

I have some like that, not sure what values etc.

This statement he makes, I find interesting. Did not know these chokes swing??
" Resonant Choke Power Supply (RCPS)  ...  RCPS approach (see Fig. 3) has been proposed at least since the late fifties, and adopted in high-end linear amplifiers, such as the Collins 30S-1 and most Henry Radio amplifiers."


He also shows a "Hay bridge" for measuring inductance with current flowing.
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« Reply #6 on: March 29, 2019, 10:05:08 AM »


Oh, sorry, missed the cap after the choke - only did a quick look.
So, skip that. Except messing with that value will have some effect.

Pretty sure I have that site up, he missed something, in that he did not check the
response curve at the end, iirc, I noted that after reading the webpage with some
interest. Think it is still open on this machine... so he got part of the way there...
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