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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 1st Dec 2017, 10:51 pm   #21
Maarten
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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Originally Posted by astral highway View Post
I do suspect it's a clone. It's a very high rep name on there, and I don't even want to mention it.
The brand name itself is in fact not as important as people make it out to be (as long as it is reputable). However if you don't want to mention it, it might be advisable to at least look up and post all relevant parameters as they are very relevant here. Or just link the data sheet of course, which can mostly be done without mentioning the brand name yourself, even if it is obviously in the datasheet.
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Old 1st Dec 2017, 11:02 pm   #22
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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Originally Posted by HamishBoxer View Post
Talking of caps ,remind me where can I find suitable 4.7uF 500Volts for stuffing smoothing caps?
I don't know exactly what you need (axial, radial, size, ....?) but I found some "small" 4.7uF 630v axial polypropylene ones on Ebay for restuffing a DKE38. If size and price aren't a problem a search will turn up various physically larger ones intended for cross-over networks.

Mods, am I allowed to mention dunkuk?
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Old 2nd Dec 2017, 12:29 am   #23
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Default Re: Electrolytic capacitor fails after 1 hour in service!

I agree with bluepilot.

If you can eliminate electrolytic capacitors where possible. It is not too difficult in valve gear now as often there are many poly caps of the same capacity and voltage that can be used for re-stuffing.

I have now gone to other types of capacitors to replace the filter electros in my valve gear restorations, they are shown at the end of this thread:

https://www.vintage-radio.net/forum/...=140570&page=2

The problem remains though in vintage transistorized equipment where the common capacitances can be over 30 to 50uF, often into the 100's or 1000's of microfarads and there is no room to fit other capacitors. In this case my practice is now to fit Tantalums, preferably mil grade for lower range values and I remain forced into an electrolytics for the high values, but I always strive for the higher temp rated parts.

In all my home design projects now I avoid electros by design if possible.

There is a field where electronic designers felt obliged to avoid electrolytics. This was in the design of electronic modules for under the bonnet use in cars (and of course in spacecraft).

If you look at vintage circuits created by companies like Motorola in the 60's and 70's, for various ignition and charging circuits there is not an electro in sight.

One company tried to subscribe to this view in the 70's and made an add on CDI unit, the Delta 10. Their design succeeded in eliminating nearly every electro except one, they had to have one high capacity filter cap. These units are nearly always still working because of this. They used a special 125 degC rated part (photo attached). It is interesting today that these caps are nearly always perfect over 40 years on and they are the only electrolytic cap I'm fond of.

These electros are made by Rifa and last much longer than conventional electros and they also have much better physical seals. They are still available last time I used one, you can nearly 100% guarantee they wont fail or leak over your pcb. They are the only electroltic cap that I trust now.

What this really shows is that it is possible to make good electrolytics, but they cost a lot more $, that Rifa cap is not cheap. So we are looking at a situation where electros are being made down to a price, not up to a standard. With all the different competing companies making them, trying to undercut each other on price, it is not really surprising that there are so many poor quality electros around. A simple case of you get what you pay for perhaps.
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Last edited by Argus25; 2nd Dec 2017 at 12:52 am. Reason: add remark
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Old 2nd Dec 2017, 10:41 am   #24
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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The problem remains though in vintage transistorized equipment where the common capacitances can be over 30 to 50uF, often into the 100's or 1000's of microfarads and there is no room to fit other capacitors. In this case my practice is now to fit Tantalums...
Yet in this Form there are threads decrying Tantalums!

I actually like them. They have to be used with care (minimise spikes, so I would hesitate to use in the OP's application), but they don't lose capacitance at low temperatures, and they don't have the wear-out mechanism of aqueous electrolytics. They aren't forgiving of momentary overvoltages though, whereas aluminium aqueous types are, which is useful when bread-boarding circuits and something goes wrong (if that has happened, the capacitor still goes in the bin in the fullness of time as a precaution).

Quote:
Originally Posted by Argus25 View Post
...these caps are nearly always perfect over 40 years on and they are the only electrolytic cap I'm fond of... are made by Rifa and last much longer than conventional electros and they also have much better physical seals.
I've used just TWO (ever) of these, but it is good to hear of Argus's experience! I'll be back for more. It's curious that Rifa persist in absolutely rubbish X capacitors, but make super electrolytics!
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Old 2nd Dec 2017, 11:44 am   #25
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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Yet in this Forum there are threads decrying Tantalums!

I actually like them.
Yes but that is because there are Tantalums and there are Tantalums.

Many early MFR tants are problematic, shorting out and other antics that stink out a room, crash many 1980's vintage computer mother boards and fail in test equipment and make many people hate them, but these problems ultimately got ironed out for most types of Tants over the last 20 years.

However the main Tants I'm referring to are the axial mil spec types, that I might dare to say "never fail".

Have a look at all the axial mil spec tants in this monitor that was made in 1987, every one of them is just as perfect as the day they were made, just scroll through the article to see the images of the PCB's, they are the kind of Tants you will never have any problems with. They are still available these days:


http://www.worldphaco.com/uploads/Th...o_monitor..pdf
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Old 2nd Dec 2017, 11:50 am   #26
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Default Re: Electrolytic capacitor fails after 1 hour in service!

Hmm. Not sure they never fail. I had an expensive wet tantalum axial one blow up on me quite spectacularly in a Tektronix 465B. The only one in it! It took out everything around it requiring some rather extensive board repair.

They were mainly picked up because they were hermetically sealed and had a relatively high working voltage compared to solid tants.

They are also very expensive. Up to 100 per capacitor in low volumes.

Really the new Kemet/AVX tantalums are pretty good. I can't blow them up. I've tried. They just get hot and fall off the board, even if you reverse bias them at 2x their rated voltage.
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Old 2nd Dec 2017, 12:33 pm   #27
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Default Re: Electrolytic capacitor fails after 1 hour in service!

My money's on it being incorrectly marked, either voltage or polarity-wise.
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Old 2nd Dec 2017, 1:06 pm   #28
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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Really the new Kemet/AVX tantalums are pretty good..
I've used quite a few of these and have not had a single problem with them, yet.

And Kalee20, it is odd that Rifa make those notorious X2 caps, perhaps they have redeemed themselves with the 125 deg C electrolytics.
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Old 2nd Dec 2017, 4:36 pm   #29
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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An X2 capacitor may be severly underrated or otherwise unsuitable for decoupling noisy DC power supply lines. It was designed to satisfy a very specific set of conflicting requirements found in mains filters and the like.
Absolutely right. I think it would be closed down pretty quickly in a pulse application (not that this is actually being used as a pulse capacitor, but it is repetitively running big currents up a DC bus and X2's aren't designed for that at all).
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Old 2nd Dec 2017, 4:41 pm   #30
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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My money's on it being incorrectly marked, either voltage or polarity-wise.

Andrew, I think so, too. It seems to be behaving like a 25V or 16V device. I think even 25V rated would blow quickly on the DC bus, feeding that chip, quite a noisy environment, so probably a 16V mislabelled as 63V.

I may contact the manufacturer to see if this has happened before. I still don't feel it's appropriate to name them as the provenance is murky, not from what we would recognise as a trusted electronics wholesaler but an EBay business. They were described as low ESR, 105 degree C rated.

Quote:
Originally Posted by Argus25 View Post
What this really shows is that it is possible to make good electrolytics, but they cost a lot more $, that Rifa cap is not cheap. So we are looking at a situation where electros are being made down to a price, not up to a standard. With all the different competing companies making them, trying to undercut each other on price, it is not really surprising that there are so many poor quality electros around. A simple case of you get what you pay for perhaps.
Hugo, a very interesting point. And indeed, since individual electrolytics, even the lower voltage ones, are so expensive, there's an incentive to clone them at a dirt cheap production cost, charge top dollar and make 8 profit a pop. Far more, obviously, for the 380V big ones that are sought after by audiophiles.

I'll take a look at Rifa's from now on. I haven't used tants for years, and I didn't know that they could be good enough for mil service, so that's both interesting and reassuring. I have seen them as decoupling caps on many DC buses on big old boards with multiple chips on them, from the early 80s, but have never tested them to see if those particular ones still work to spec.
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Old 2nd Dec 2017, 4:42 pm   #31
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Default Re: Electrolytic capacitor fails after 1 hour in service!

I'm led to believe that 'super capacitors' are extremely robust and reliable so it begs the question why they don't use that technology to make electrolytics? Wouldn't the finished product, give the capacity/density ratio make them extremely compact too?

Or doesn't super capacitor technology work that way?
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Old 2nd Dec 2017, 4:45 pm   #32
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Default Re: Electrolytic capacitor fails after 1 hour in service!

IIRC, super-caps don't have good ripple current capacity and are good only for 'holding up' memory circuits. I read this probably 20 years ago so things may have changed since then.
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Old 2nd Dec 2017, 5:22 pm   #33
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Default Re: Electrolytic capacitor fails after 1 hour in service!

Supercaps are interesting. They can handle silly ripply currents now in the hundreds of amps territory!. The thing with capacitors is ESR vs energy storage. They have crazy low ESR and huge energy storage.

A couple of AA's have 30KJ or so of energy sitting there, which is quite a lot. But the series resistance is ridiculous. It takes a lot to drag that energy out.

A huge top end EDLC, say 2600F (yes farads) has 8KJ of energy which it can shift pretty quickly due to 1.8 or so milli-ohms of ESR.

Only problem is the voltage and price: https://uk.rs-online.com/web/p/elect...itors/0271316/

10F ones are affordable.

Eaton sell banks which are balanced so you can use them up to 48v or so. There are two of them in the front end of our UPS banks. According to the spec, they can deliver 2000A (!) for a second without failing. I wouldn't want to be on the end of that!
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Old 2nd Dec 2017, 5:48 pm   #34
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Default Re: Electrolytic capacitor fails after 1 hour in service!

Thanks for the update, Mr Bungle. I can clearly recall warnings about the early supercaps not being suitable for reservoir caps in PSU's as they couldn't take the ripple. My, how things change!
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Old 2nd Dec 2017, 5:56 pm   #35
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Default Re: Electrolytic capacitor fails after 1 hour in service!

Super caps come in various types.
The high ripple ones are often sold with a digital display on them for use for decoupling on automotive "axle bouncer" units with all those speaker cones that bob up and down in the boot.
Memory types will never work with more than a trickle charge. They will charge in a minute or so and hold memory for several days.
We have not yet discovered the mark/space ratio of the SMPSU that blew up the capacitor in the OP.
2 amps is 4 amps pulsed at 50%. What was it?
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Old 2nd Dec 2017, 6:06 pm   #36
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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Quote:
Originally Posted by kalee20 View Post
Just a thought - it was the right way round wasn't it? 18V is about what I'd expect to gradually destroy an electrolytic, climaxing in an hour or so.
I did place it with the correct polarity.
A place where I worked had a whole batch of electrolytic caps (from reputable supplier / manufacture) delivered labelled with the wrong polarity - that was fun (and a load of work to get all the caps taken out again delivered on that GRN)
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Old 2nd Dec 2017, 6:52 pm   #37
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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A place where I worked had a whole batch of electrolytic caps (from reputable supplier / manufacture) delivered labelled with the wrong polarity - that was fun (and a load of work to get all the caps taken out again delivered on that GRN)
Woah, Chris!! How does 'consequential loss', /liability work when this happens on such a scale, business-to-business??
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Old 2nd Dec 2017, 8:22 pm   #38
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Default Re: Electrolytic capacitor fails after 1 hour in service!

I don't know what went on on the legal side, but it was pain for Test and Production.

In mitigation it was a high end product line, so relatively low volume (from what I can remember the quantities were in the tens, not hundreds or thousands) and there was no chance of anything duff getting out to clients because of the immediacy of the ramifications.
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Old 2nd Dec 2017, 8:34 pm   #39
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Default Re: Electrolytic capacitor fails after 1 hour in service!

Counterfeiting has certainly extended down to smaller and less expensive components/assemblies in recent years, and very much like the proverbial bump in the carpet- squash it down and it pops up elsewhere. It would not surprise me at all to learn that discrete components such as these are now being counterfeited. The packaging and external appearance has become VERY convincing in many of these. I was shown some convincingly faked crabtree and MK packaging and also injection moulded circuit breakers cases some years ago, and the detail on the latter was up to the standard of a revell or airfix model- but it was simply a switch, with nothing else inside. On this occasion they had come out of China, but to be fair they are also coming from elsewhere.
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Old 2nd Dec 2017, 8:37 pm   #40
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Default Re: Electrolytic capacitor fails after 1 hour in service!

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I may contact the manufacturer to see if this has happened before. I still don't feel it's appropriate to name them as the provenance is murky, not from what we would recognise as a trusted electronics wholesaler but an EBay business. They were described as low ESR, 105 degree C rated.
Please do!

You say they are a reputable brand, well, in that case if they are mislabelled they'll want to know. If someone 'out there' is making counterfeits, again they'll want to know.

It's even possible that they may be genuine, but known internal rejects, but a dishonest employee has intercepted them en route to the skip to flog them and make some easy money. Again, they'll want to know.
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