Film capacitor as power-supply filter

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Is there any reason not to use a 20-uF 900-volt film capacitor as a power supply filter?

I'm updating a vintage Heathkit capacitor checker that I use regularly for re-forming electrolytics, and its power supply filter is currently two 40-uF 450-V in series, without balancing resistors; applied voltage is nominally 600 V DC.

This hasn't given any trouble, but I'm uneasy about its future.  Now that film capacitors are available with suitable ratings, is there any good reason not to use one?

(P.S. This may be one of the few instruments in active use that have both a tuning eye tube and an LED.  It's already an anachronism.)

Re: Film capacitor as power-supply filter
On Sunday, October 6, 2019 at 9:19:57 PM UTC-7, snipped-for-privacy@uga.edu wrote:
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Sounds like a good idea, to me.   If there's no limit resistor, it might help to put ten ohms
in series (old electrolytics weren't low-ESR, and you don't want to find out that the inrush to
a good capacitor blows your rectifier).

Re: Film capacitor as power-supply filter
Thanks.  The rectifier is a vacuum tube.  If I change it to a diode, I'll add resistance.

Re: Film capacitor as power-supply filter
On Sun, 6 Oct 2019 21:19:52 -0700 (PDT), snipped-for-privacy@uga.edu wrote:

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The film cap should be fine.

I don't think that electrolytic caps in series need balancing
resistors. They take care of themselves. If you need a bleeder, you
may as well split it in two, but otherwise don't bother.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Film capacitor as power-supply filter
snipped-for-privacy@highlandsniptechnology.com wrote:

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 ** It's highly amusing that while electros generally self protect when used in series, film caps do not.  

It is a near certainty that poly caps in series will fail from overvoltage if no balancing Rs are used.  

Direct opposite of what so many assume.


...   Phil  



Re: Film capacitor as power-supply filter
Phil Allison wrote...
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 I'm not comfortable with self-balancing for any type.
 Electrolytic leakage currents must create small carbon
 pathways, that's not good.


--  
 Thanks,
    - Win

Re: Film capacitor as power-supply filter
Winfield Hill wrote:

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** Win's alleged discomfort is his problem.  


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** Paranoid nonsense.  

Leakage current with high voltage electros is normal, adding " balancing " Rs to a series pair only increase it by forcing one cap to a higher voltage.  

BTW Win :  

Got any more silly crap to post about noise in BJTs used or mic-pres?  

Or about pretending Toyota Hybrids and full EVs are the same ?  

Or the loopy notion that what you chose to do and believe is normal ?  



.....  Phil  







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Re: Film capacitor as power-supply filter
Phil Allison wrote...
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 Excuse me!  Where do you think those currents through
 an insulator are coming from?  Any forming processes
 aside, high-voltage leakage,** over-voltage, or over
 anodization, which we're talking about, involves series
 current pathways, carbon formation, corrosion, oxygen
 loss, and eventually, failure.  The currents increase
 exponentially with modest over-voltage.  Proper usage
 is spelled out in the Guideline for use of aluminium
 electrolytic capacitors, DIN 45811, and elsewhere.

 ** Low-voltage applications don't use series capacitors.


--  
 Thanks,
    - Win

Re: Film capacitor as power-supply filter
Winfield Hill Posts another Fuckwit OT reply :

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** Is this meant to be a reply to MY post ?  

There was another poster who disputed Win's hypothesis re "carbon pathways".  

Hmmmmm.....  

Might be another one of those AKW / CO2 paranoid, fear mongering stories.  

Greta would be amused.  



.....  Phil  

Re: Film capacitor as power-supply filter
wrote:

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That is precisely the charm of wet aluminum caps. The series string
optimizes itself for minimum possible leakage current.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Film capacitor as power-supply filter
On Tuesday, 8 October 2019 02:05:56 UTC+1, Winfield Hill  wrote:
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Leakage is a normal part of electrolytic operation.


NT

Re: Film capacitor as power-supply filter
snipped-for-privacy@gmail.com wrote...
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 I understand the normal leakage.  But excess leakage one
 sees after over anodization, etc., high leakage that only
 gets worse with time, that's got to be part of a failure
 process.  Once the formed oxide layer has been thoroughly
 penetrated, new processes can take place ...


--  
 Thanks,
    - Win

Re: Film capacitor as power-supply filter
On Wednesday, 9 October 2019 19:25:08 UTC+1, Winfield Hill  wrote:
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Leakage current going high & the cap reforming at power-on is not always failure or imminent failure.


NT

Re: Film capacitor as power-supply filter
snipped-for-privacy@gmail.com wrote...
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 Yes, right.

 Classic old-capacitor reforming is done with a forgiving
 environment, like a 1k series resistor, etc.  What about
 an instrument that's powered up after having been off for
 a decade or two?  What about excessive currents, are they
 really only forming currents?

 I purchased a really useful instrument, a HANEG** HM2814A,
 for electrolytic capacitor leakage tests.  The instrument
 works to 500V, has multiple test settings, current limits,
 meter readouts and diagnostics.  My designs use lots of
 450V electrolytic capacitors, some in series, and I often
 check them on this instrument.  I've encountered new-stock
 but maybe older* 450V caps, designed for use at 380V, but
 with massive excess leakage at only 350 volts.  From the
 increasing current levels, I'm convinced that these parts
 would fail, if forced to operate at their design voltage
 (note: 18% safety margin).   I didn't test to destruction.

 From what I've seen, none of the manufacturer's lengthy
 design notes deal with this failure mode.  I've had lots
 of experience with carbon pathways that behave this way.
 The leakage currents continue to increase without limit.

 * Lots of my new inventory is 15 to 20 years old, arrgh!

 ** The name is HANEG, but its HM part number implies HAMEG,
 so I placed its info in my HAMEG folder.  A made-in-China
 thingie, Google doesn't have much info about HM2814A, but
 gives a name Jinyitong Electronics.  It came with a good
 manual, and very useful accessories, but no mention of a
 manufacturer's name.  But it's a very useful instrument.


--  
 Thanks,
    - Win

Re: Film capacitor as power-supply filter
On Wednesday, October 9, 2019 at 10:14:57 PM UTC-4, Winfield Hill wrote:
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Exactly why I'm fixing up this vintage Heathkit IT-11.  It's great for re-f
orming and for testing leakage.  I am going to calibrate the leakage tests  
so I know how many microamps is the pass/fail threshold for each of the thr
ee.  And probably change them to whatever I think is useful with modern cap
acitors.

Re: Film capacitor as power-supply filter
On Thursday, 10 October 2019 03:14:57 UTC+1, Winfield Hill  wrote:
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Yes, and sometimes the unit itself provides that. And sometimes it doesn't but C survives & reforms.

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'what about' is always a too-vague question

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Excessive currents are by definition excessive, though nothing is said there about the definition that makes it so.
Some caps reform, some don't. A variac or limited current dc psu is useful.  
The need for a reform does not always mean a cap is faulty or nearing failure, sometimes it does.


NT

Re: Film capacitor as power-supply filter
wrote:

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I've heard of people using a variac to slowly bring up an old
instrument.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Film capacitor as power-supply filter
snipped-for-privacy@highlandsniptechnology.com wrote...
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 I should try that.


--  
 Thanks,
    - Win

Re: Film capacitor as power-supply filter
On Thursday, October 10, 2019 at 7:50:51 AM UTC-7, snipped-for-privacy@highlandsniptechnology.com wrote:

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A switchmode regulated supply, though, takes maximum current input
when voltage is low-to-borderline.   So that mainly is good for  
unregulated power (or power-wasting series or shunt regulated).
It's also questionable if there's a DC fan, or ferroresonant circuit...

Re: Film capacitor as power-supply filter
wrote:

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I meant an OLD instrument.




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