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Old 17th Sep 2020, 9:32 pm   #1
G6Tanuki
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Default Electrolytic capacitor can-indents.

By which I mean the longitudinal 'dents' or 'crimpings' that are sometimes seen on the cans of larger electrolytic capacitors (see attachment).

What were these for?

The engineer in me thinks they were perhaps to improve contact between the can and the capacitor-itself, to better remove ESR-generated heat from the 'workings'.

The production-engineer/accountant in me ponders that it was to support the physically-smaller innards of lower-rated capacitors when fitted in larger cans, so saving money by allowing one standardized size of can to be used across a range of capacitors whose internals varied in size.
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Old 17th Sep 2020, 9:59 pm   #2
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Default Re: Electrolytic capacitor can-indents.

Maybe to increase cooling surface area too.
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Old 17th Sep 2020, 10:49 pm   #3
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Default Re: Electrolytic capacitor can-indents.

I thought it was just to hold the guts in place, saves having to fill the can with bitumen Like they did with older ones.

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Old 17th Sep 2020, 11:06 pm   #4
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Default Re: Electrolytic capacitor can-indents.

I thought it was to hold the rolled-up sandwich in place under vibration/jolting as you tended to see it on arduous-duty types- it seemed to be most common on Philips types and others that looked somewhat similar to Philips. I also recall a Philips data-sheet comment on their big, high-ripple PSU types that removing the characteristic blue shrink sleeve would increases the ripple rating by around 20% through improved dissipation to surroundings. It was noticeable that these types were also available in dual format with values such as 2x 2350uF and 2x 3400uF that were obviously a 4700uF or 6800uF sandwich with a slit down one foil.
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Old 18th Sep 2020, 6:01 am   #5
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Default Re: Electrolytic capacitor can-indents.

Good question, I have a few of those and presumed they were the equivalent of go-faster stripes on cars, IE a cosmetic design feature, thinking about it though those dents mean you can fit a smaller cap in a bigger can, the cynic in me is reminded of Mars bar packets and breakfast cereal boxes; big box not much inside.

Andy.
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Old 18th Sep 2020, 6:46 am   #6
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Default Re: Electrolytic capacitor can-indents.

As it tends to be in larger sizes, could it just be a way of strengthening the cylinder / allow use of thinner material?

TTFN,
Jon
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Old 18th Sep 2020, 9:30 am   #7
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Default Re: Electrolytic capacitor can-indents.

As these were usually only seen on larger capacitors I always thought this was so they could be physically held / supported in clips. The 'dents' prevent the capacitor from rotating within the clips...?
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Old 18th Sep 2020, 10:39 am   #8
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Default Re: Electrolytic capacitor can-indents.

I have also been under the impression that they were there to help provide secure mounting. I could well be wrong!
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Old 18th Sep 2020, 11:04 am   #9
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Default Re: Electrolytic capacitor can-indents.

I think I remember reading that the indents were intended as a deliberate weak point in the capacitor can that would expand outwards if the cap failed and produced a lot of gas in the process. Much like the cross pattern scribed into the top of modern axial electrolytics.

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Old 18th Sep 2020, 11:09 am   #10
M0FYA Andy
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Default Re: Electrolytic capacitor can-indents.

One clue might be if we could find out when in the manufacturing process these indents are made - before or after the can is 'filled'?

Andy
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Old 18th Sep 2020, 12:05 pm   #11
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Default Re: Electrolytic capacitor can-indents.

my money is on it being a way to ensure a snug fit of the innards without squashing them.
In a previous life we used to repair motor inverters that were strapped to the motors themselves, with all the ensuing vibration-caused problems. A common fault was the chinese electrolytics with the internal connecting foils severed and the swiss-roll innards free to rattle around in the can.
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Old 18th Sep 2020, 4:35 pm   #12
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Default Re: Electrolytic capacitor can-indents.

One series of capacitors which use this technique is the Philips/Mullard 106/107 range.

An article from Philips Technical Communications Volume 11, No. 106, July 1970 has an article about the design and manufacture of these components.

It would seem that several of the reasons for the can indentations proposed are correct.

Two paragraphs from the article are:

"The 106/107 range of high-capacitance electrolytics offers the end user a high-grade, long life component to meet those needs. It can handle approximately three times the ripple current of its predecessor, and it offers up to one farad.volt capacitance-voltage product, very low effective series restance, and a working temperature range extended to 85 deg. C. The mechanical properties are also significantly improved; the capacitors will stand up to severe bump and vibration conditions".

"Vacuum impregnation is carried out to impregnate the cell with electrolyte. Then the cell is assembled and sealed into the can by a technique which clamps the winding in the can and ensures that the cell is in extremely good thermal contact with the can. This is a contributing factor in the high ripple ratingspecified for the 106/017 series. A ring of e.p. rubber forms an extra sealing element between the top cap and the can. Compression is applied during the spin-sealing process and gives excellent proof against the vibration and bump tests experienced in some severe specifications. The can is then insulated by a plastic sleeve".

There is a cut-away drawing of the construction of the capacitor supporting this description.

If anyone would like to read the whole article, which is very informative, I can scan it.

PMM
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Old 18th Sep 2020, 9:53 pm   #13
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Default Re: Electrolytic capacitor can-indents.

Quote:
Compression is applied during the spin-sealing process and gives excellent proof against the vibration and bump tests experienced in some severe specifications.
A company I worked for used a custom-specified large non-polarised electrolytic in some products that we made to order in small batches.
We had a few of the products from one batch returned as faulty and discovered said capacitors had gone open-circuit. This had happened either in transit to our customers or soon after putting them into service, where they were expected to withstand moderately rough treatment.

On inspection, the capacitor windings were loose in the can, obvious on shaking them, and when dismantled the lead-in wires to the terminals were found to be broken.

I had a rummage in the stores and pulled out the entire stock for 100% QC. A worrying proportion of them had loose innards so I sent the whole batch back to the manufacturers after expressing my, er, considerable disappointment, in a phone call. (This was pre the e-mail era).

The suppliers (a well-known UK manufacturer) cured the problem, satisfactorily, by running them all through their crimper followed by 100% testing.
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Old 19th Sep 2020, 7:20 am   #14
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Default Re: Electrolytic capacitor can-indents.

Quote:
Originally Posted by pmmunro View Post

If anyone would like to read the whole article, which is very informative, I can scan it.

PMM
I would be very interested in the complete article!
Thank you in advance for the scanning.

I have used several of this type of capacitor and they performed very well.

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Old 19th Sep 2020, 10:56 am   #15
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Default Re: Electrolytic capacitor can-indents.

Jac,

I have scanned the article in *.PDF format but it is too big to attach here.

If you would like to send me a private message with an email address, I will sen it to you that way.

PMM
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Old 19th Sep 2020, 11:51 am   #16
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Default Re: Electrolytic capacitor can-indents.

Thanks!

pm sent.

Jac
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Old 19th Sep 2020, 1:41 pm   #17
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Default Re: Electrolytic capacitor can-indents.

Quote:
Originally Posted by pmmunro View Post
Then the cell is assembled and sealed into the can by a technique which clamps the winding in the can and ensures that the cell is in extremely good thermal contact with the can. This is a contributing factor in the high ripple ratingspecified for the 106/017 series.
This is exactly the reason that I'm so dubious about the practice of re-stuffing smoothing capacitors. Personally, I never do it because these capacitors were never designed to be fitted inside the case of another one without any ventilation. I think that it's just the fact that in most cases where re-stuffing is carried out the set doesn't get the continuous use that it would once have had when new. However, looking at it from another angle, you could argue that the body of the replacement capacitor is shielded from the radiated heat from surrounding valves and hot droppers, again due to short term use, the heat would never have time to eventually penetrate the outer casing to affect the capacitor within.

I'm not saying that I would never re-stuff, just that I would rather either do a 'historical' straight replacement of the original part or leave the original part in place and fit a replacement nearby or under the chassis out of sight.

I too have wondered about the crimp like dents manufactured into some capacitors. You would have thought that the pressure of the crimped dent would stress the insulation between the foil plates of the capacitor and eventually cause a short, but evidently not.
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Old 19th Sep 2020, 3:04 pm   #18
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Default Re: Electrolytic capacitor can-indents.

Modern capacitors have a high maximum ripple current that is well above the original rating.
I used 33uf capacitors rated at 300ma ripple to re-stuff a can that had been running 80ma.
The de-rating is well and truly enough that local ventilation will not be a problem.
Some old capacitors used to have the ripple rating printed on them.
Modern quality capacitors have data sheets that show much higher ratings.
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Old 21st Sep 2020, 1:41 pm   #19
G6Tanuki
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Default Re: Electrolytic capacitor can-indents.

Fascinating insights, thanks all! I guess the indents serve a multitude of purposes!

[Reason I asked in the first place was that the blue electrolytic in my original posting - which came from a Pye AC200 mains-power-unit - was dead-short. Cutting the end off I couldn't pull-out the 'gubbins' because they were held by the indents. The replacement Kemet capacitor is significantly smaller but lacks the can-indents].
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