Film capacitor as power-supply filter

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 ...

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 Thanks, 
    - Win

Leakage current going high & the cap reforming at power-on is not always failure or imminent failure.

NT

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

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.

Yes, and sometimes the unit itself provides that. And sometimes it doesn't but C survives & reforms.

'what about' is always a too-vague question

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

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

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I should try that.

--
 Thanks, 
    - Win

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...

I meant an OLD instrument.

Back in the '60 and '70s we would apply 12 to 24 volts to old equipment to warm the power transformer to drive out moisture. The tubes were pulled, fi rst and it was left on this setup for 24 hours. It seemed like every old pi ece of test equipment was store in a dank basement.

I had a Sprague TO6 capacitor analyzer that could reform to 500VDC, and it metered the leakage current. If the oxide layer was bad, you could slowly r amp it up until it had acceptable leakage at the rated voltage. The Militar y had a set schedule to test and reform NOS electrolytics that were in ware houses. I used to see a lot of surplus caps with a label showing the dates.

Are modern electrolytics any better than the old ones, from the aspect of un-forming themselves?

Or maybe the old ones are, well, just old.

it metered the leakage current. If the oxide layer was bad, you could slowl y ramp it up until it had acceptable leakage at the rated voltage. The Mili tary had a set schedule to test and reform NOS electrolytics that were in w arehouses. I used to see a lot of surplus caps with a label showing the dat es.

The oldest still working lytics I have are 1930s. I doubt any modern lytic will last that long. OTOH olduns were quite leaky.

NT

to warm the power transformer to drive out moisture. The tubes were pulled, first and it was left on this setup for 24 hours. It seemed like every old piece of test equipment was store in a dank basement.

it metered the leakage current. If the oxide layer was bad, you could slowl y ramp it up until it had acceptable leakage at the rated voltage. The Mili tary had a set schedule to test and reform NOS electrolytics that were in w arehouses. I used to see a lot of surplus caps with a label showing the dat es.

I have some NOS Sprague that I will try to dig out. They are Axial, and ove r 20 years old. They are still in sealed plastic bags of 100. I will do som e tests on them, when I can dig them out. I have a Sencore LC53 Digital LC Meter to use for the testing.

As far as time differences in how long the formed surface lasts, most of th e newer types haven't been around long enough for a real comparison. How ca n you compare recent parts to ones that are approaching 100 years old?

The older Sprague Capacitors had better seals. Their aluminum cans were sea led with hot tar. They held so well that some I threw into the trash were i ncinerated. The formed can was almost spherical, before they blew out the s eal. This was 50+ years ago, and some were 20 years old at that time..

Sprague was the best, with Cornell Dubilier second and Mallory third, as fa r as being well sealed. Other brands failed more often, especially unbrande d ones used by low end OEMs of consumer electronics.

Sprague was always our first choice for replacements. The suppliers were al ways pushing lower grades, because they made more money on them.

The highest failure rates for Electrolytics in TVs in the '60s and '70s wer e the 160uF@250VDC High Ripple Current rated parts desiged for the input ca pacitors used in the voltage doubler in transformerless chassis. People bit ch about the doubler concept, yet it is still used in computer power suppli es.

The Electrolytes have improved, which allow the same ratings in smaller pac kages but those can't dissipate as much heat as the larger cans did.

Most small Electrolytics these days appear to only have a rubber plug that is crimped into the can. That's why they end up crooked when they overhead and push part or all of the seal out of the can. I had one HP desktop on th e bench when one failed in the power supply. The can shot out through the r unning fan, followed by foot long flames.

A company I worked for near Orlando 30 years ago was hired to do asset reco very when Sprague closed their Orlando facility. Their manager told us that very few customers were buying quality Electrolytics anymore. All they car ed about was a really price, and just enough quality to make it through the ir warranty to their end customers.

I'm certain that the Military did enough testing to justify their scheduled reforming. It gave them more than enough of a part number to outlast whate ver they were made for.

This is just a guess, but I'd guess that two things have happened to the making of electrolytic capacitors in the past 50 years, and they have opposite effects:

(1) Improvement. Higher specs, longer lifetime at high temperature, etc. This should make newer capacitors last longer than old ones.

(2) Cheapening. With mass demand, there's a desire to make capacitors as cheaply as possible. Not everybody will pay for 105 C temperature rating or name-brand reliability. Look at the unrecognizable brand names coming out of Asia.

So... they've gotten better... except some of them... maybe?

Perhaps one economic factor is that, as the cost of other components has fallen, people expect the electrolytic capacitors to be very cheap also. The capacitors now stand out as the expensive components. And that's an incentive to lower standards.

fallen, people expect the electrolytic capacitors to be very cheap also. T he capacitors now stand out as the expensive components. And that's an inc entive to lower standards.

Some of them are approaching zero value. Older equipment was expected to la st 10 years or more. A lot of today's junk is lucky to last a year or two. In that respect, the costs are going up, not down. Lower grade caps are a v ery high failure item. I had one C-band receiver on the bench a few years a go. It had about 85 electrolytics, and every one was bad Most had no brand name, and some weren't marked for temperature. It belonged to a bar, and th ey needed it right away, so they paid a large bill to have it working the n ext day. I generally use Panasonic and Rubicon for repairs and my projects, these days.

Near the turn of the century a Missouri man named Chris started badcaps.net Chris' "master list" enumerates the capacitors that he trusts:

Chris also hosts a lively forum at

Although its more apropos to repair.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

I visited that site several times in its early days, when most people didn't believe that the industry had been duped with substandard parts. I wanted to see what brands were causing problems, and which ones were being faked. I left manufacturing in 2001.

Well, the variac trick is for old-old stuff--linear power supplies, maybe tube rectifiers....

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com