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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 13th Mar 2018, 2:26 pm   #1
Flummoxed
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Default Capacitors. Is physical size important?

I'm resurrecting a 1973 Goodmans One-Ten tuner amp, which I have owned from new.

I would like opinions whether I can replace two failed power supply capacitors with physically smaller capacitors that have ostensibly the same specs. The original capacitors are showing signs of leakage and are Erie brand, spec'd as 6800uF, 40V. The cans are covered in blue vinyl, have two solder tags (+&-) are 35mm dia by 102mm long. (Presumably 1 3/8" x 4" in old money!) The 35mm dimension is important for the existing mounts, but it's proving tough to find 35 mm dia allied to ~100mm length without paying a King's Ransom.

So, what's the downside in using readily available 6800uF, 100v units that are 35mm dia item but only 50mm long? (Goodmans stated that the duty of these items is in the 'supply reservoir' ).

Thanks
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Old 13th Mar 2018, 2:38 pm   #2
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Default Re: Capacitors. Is physical size important?

The modern ones will be fine. Today's electrolytic capacitors are much smaller than their vintage counterparts because the materials and construction techniques have improved a lot.

The only time that the physical size of an electrolytic capacitor really matters is when it's being worked hard with a high ripple current at high frequency, and it has to dissipate the resulting losses as heat. Then bigger is better.

The reservoir capacitors in a 1970s audio amplifier have, by today's standards, a very easy job, so smaller capacitors will be fine.

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Old 13th Mar 2018, 2:45 pm   #3
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Default Re: Capacitors. Is physical size important?

The specs are all that matters. The physical size just varies with the technology used in the construction.
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Old 13th Mar 2018, 2:52 pm   #4
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Default Re: Capacitors. Is physical size important?

Over the last 40 years or so, electrolytics have become tiny in comparison to their 70's precursors.

There are still physically large types, low ESR 400VDC types available for e.g. decoupling low impedance high-speed switching circuits, taking big gulps of power out of a DC bus (pulse circuits), and these cost substantially more, but a component rated 40V and 6,800uF will be very small compared to its ancestor.

Small, but absolutely fine! As usual, you will find people trying to persuade you that a capacitor for an 'audio' role has to cost 40, but it's just profiteering!
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Old 13th Mar 2018, 3:03 pm   #5
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Default Re: Capacitors. Is physical size important?

I wouldn't use higher voltage rated electrolytics than the original specs called for. They need to see voltages they are designed for to retain their integrity. Something relating to silver oxide inside that's maintained through use. I'm sure someone else will put it more eloquently than I. This doesn't apply to other capacitor types, just electrolytics.

Don't worry about the size, I've replaced loads with modern tiny ones, providing you use decent brands you will find they're better made. (EDIT - not that I've owned stuff long enough to have witnessed that!)
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Old 13th Mar 2018, 3:11 pm   #6
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Default Re: Capacitors. Is physical size important?

Hey, just to be clear, my post about high voltage types is in no way a suggestion to use them. It’s just an observatIon on size and price, as background info. I am illustrating the difference between an electrolytic in w stressful role vs. one in the OP’s amplifier.
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Old 13th Mar 2018, 3:36 pm   #7
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Default Re: Capacitors. Is physical size important?

Quote:
I wouldn't use higher voltage rated electrolytics than the original specs called for.
I do not think this is a problem, except to bear in mind that electrolytics gradually adopt the working voltage that they are exposed to so don't go and swap it out into something else without reforming it.

I have just encountered this issue as a design fault of the Farnell E350 PSU. The voltage on the main smoothing is switched with the output ranges. This creates the dangerous problem that if the highest range is not used for many years, and then it is selected the result will not be good.
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Old 13th Mar 2018, 6:49 pm   #8
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Default Re: Capacitors. Is physical size important?

I have to admit that (after making sure ESR, temperature rating are equivalent) I usually choose for the best size match even though that probably means the modern replacements will be 35V instead of the original 16V, etc.
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Old 13th Mar 2018, 7:27 pm   #9
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Default Re: Capacitors. Is physical size important?

It is important to look at ripple current rating as well if they are filter caps. If this is under specified they tend to heat up pretty quick and turn into a salute instead.

As for working voltage, doesn’t matter as long as you don’t approach it. I’ve used 250v caps in 10v circuits fine.
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Old 13th Mar 2018, 7:34 pm   #10
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Default Re: Capacitors. Is physical size important?

Modern electrolytics are vastly better than their predecessors: it's all about internal chemistry. I have no problems fitting physically-smaller but electrically-equivalent modern ones in place of older bulkier items.

Some may worry about heat buildup in physically smaller ones: my way of thinking is that heat buildup from within the capacitor is due to the thing being inefficient [it has internal-resistance] and modern capacitors generally have lower internal-resistance (that chemistry-thing again) so they should heat less than older ones.

If a modern 'can' capacitor is smaller so ends up loose in its clamp I've used a length of plastic waste-pipe with a slit cut down the side as a packer.
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Old 13th Mar 2018, 8:31 pm   #11
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Default Re: Capacitors. Is physical size important?

Quote:
Originally Posted by PsychMan
Something relating to silver oxide inside that's maintained through use.
Aluminium oxide, maintained by leakage current.

As others have said, OK to use electrolytics of a higher voltage rating - but they will gradually adjust to the lower voltage and so should not then be exposed to their official rated voltage without first reforming them.
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Old 13th Mar 2018, 10:12 pm   #12
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Default Re: Capacitors. Is physical size important?

I usually try (if at all possible) to keep to the original diameter for capacitors secured by clamps, saves drilling etc & otherwise modifying. And yes, I too have found trying to keep like for like physically, really expensive.

Funnily enough, I'm currently repairing a Coutant PSU, mainly smoother gone winky (compressed smoke escaped!)

The original cap was enormous for a 9000uF 50V item, the new 10,000uF 63V looks a little lost, only problem is, where the new cap is much shorter, I need to replace some wires as they won't reach.

Trialing the new part as proven it'll be a successful repair.

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Old 14th Mar 2018, 12:18 am   #13
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Default Re: Capacitors. Is physical size important?

I have in the past turned some nylon bushes to act as spacers to allow original clamps to grip smaller modern capacitor cans (the bushes are slotted)

General advice is not to run aluminium electrolytics below 10-20% of their rating.

When made, the capacitors have two strips of aluminium foil separated by two strips of paper or fibrous lastic soaked in conductive jelly. Both sides of both foils are slightly oxidised (fresh aluminium is very reactive and oxidises immediately in air.)

Once the thing has been rolled up like a swiss roll, the thing is driven electrically. The oxide on both sides of one strip is broken down, so the strip connects across its full area with the conductive goo, while the oxide on the other strip is built up. When this has been completed with thick enough oxide, the capacitor is finished. The foil with the oxide is one electrode, the conductive goo is the other electrode, and the naked aluminium strip is just wide area contact to connect to the goo.

The prupose of using goo is that it can intimately coat the texture of the oxide, so there can be a lot more area than you'd expect... simply because the oxidised strip was textured beforehand.

So electrolytic capacitors are made by electrolytic activity. And what has been made that way can be unmade that way too.

Modern capacitors have got better conductive goo. Their losses and self-heating are better, sealing to prevent loss of water vapour from the goo is better too.

Check ripple current ratings and check ESR specs. Also look at the life expectancy figures for different temperatures.

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Old 14th Mar 2018, 10:37 am   #14
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Default Re: Capacitors. Is physical size important?

Quote:
Originally Posted by G8HQP Dave View Post
As others have said, OK to use electrolytics of a higher voltage rating - but they will gradually adjust to the lower voltage and so should not then be exposed to their official rated voltage without first reforming them.
The thing I love about this forum is I learn new things all the time, I therefore stand corrected
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Old 14th Mar 2018, 5:41 pm   #15
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Default Re: Capacitors. Is physical size important?

So electrolytics and their voltage rating is something of an issue for variable power supplies...


If we have 350V DC rated reservoir caps and we have our variable power supply turned down supply 80VDC for a few hours, and then we abruptly turn it up to 180VDC, our electrolytics are now 'formed' for the lower voltage, right?

What's the workaround? Switchable reservoir caps?
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Old 14th Mar 2018, 6:40 pm   #16
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Default Re: Capacitors. Is physical size important?

It takes months (or perhaps a few years?) for an electrolytic oxide layer to deteriorate significantly, even when sitting on a shelf unused. Any leakage current will help preserve the oxide. Don't worry about a few hours or a few weeks at less voltage. The problem is most likely to show itself when a cap is re-used in a new circuit, having been recovered from a scrapped circuit which used it for years at a lower voltage.
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Old 19th Mar 2018, 1:03 am   #17
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Default Re: Capacitors. Is physical size important?

Quote:
Originally Posted by GMB View Post
Quote:
I wouldn't use higher voltage rated electrolytics than the original specs called for.
I do not think this is a problem, except to bear in mind that electrolytics gradually adopt the working voltage that they are exposed to so don't go and swap it out into something else without reforming it.
I think it's even less of a problem. Generally when being used between a certain minimum and maximum percentage of their rated WV they will stay formed. I use 20% and 80% as a rule of thumb, but that's probably on the safe side.
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