Oversizing capacitors (audio and power)

[john12ax7]

Would you say that the resistance of a wire is misleading because it has some frequency dependence?

In some cases yes. Although part of my background is RF so I tend to view everything as resistive and reactive. Ultimately it depends on what matters at the frequency of interest.  So I would agree it's not necessarily the words but rather what matters when, knowing what you can ignore and what you shouldn't.

 

[abbey road d enfer]

In some cases yes. Although part of my background is RF so I tend to view everything as resistive and reactive.

Skin effect is not reactive; however it often happens at frequencies where the inductive element also becomes significant, so both must be taken into account in the description of a frequency-dependant resistance.

Ultimately it depends on what matters at the frequency of interest.  So I would agree it's not necessarily the words but rather what matters when, knowing what you can ignore and what you shouldn't.

On that I wholeheartedly agree. Often, the need for simplification obscurs the more complex underlying reality.
Going back to the OT, ESR is a convenient benchmark for preliminary combing, but it's not the alpha and omega of the suitability of a capacitor.

 

[user 37518]

Ohms at 100k is spec data sheet speak for ESL (series inductance) in series with ESR (series resistance). Showing it as ohms at a (relatively high) spot frequency makes comparison easier (but only if everybody specs their caps that exact same way...).

ESL and ESR are the actual specifications. The ESL will continue rising above 100 kHz. A typical combined capacitor impedance vs frequency plot will look like the familiar bathtub curve, so it will first fall with increasing frequency as capacitance dominates, then flatten out (while ESR dominates) , then start rising as ESL dominates.

JR

But why does it change with the voltage rating of the capacitor?

 

[Rob Flinn]

Skin effect is not reactive; however it often happens at frequencies where the inductive element also becomes significant, so both must be taken into account in the description of a frequency-dependant resistance.
On that I wholeheartedly agree. Often, the need for simplification obscurs the more complex underlying reality.
Going back to the OT, ESR is a convenient benchmark for preliminary combing, but it's not the alpha and omega of the suitability of a capacitor.

What I understood was that the skin effect only comes into play at well above audio frequencies.  With that in mind is it something you believe it's necessary to consider for audio gear ?

 

[JohnRoberts]

What I understood was that the skin effect only comes into play at well above audio frequencies.  With that in mind is it something you believe it's necessary to consider for audio gear ?

Skin effect has to do with crowding current flow out of the middle of conductors and onto the skin or outer layer.

Skin effect can be seen at mains power frequency in very large diameter high current conductors. Some use a poor conductor like steel for the center core, with better conducting aluminum wrapped around to form the outer skin where the current actually flows and the resistance matters.

In audio practice we typically think of it as a HF phenomenon but it isn't only HF.

JR

PS: My signature line on another forum says "On the internet people tell you everything "they" know, not the answer to "your" question."  Perhaps that explains this weer....

 

[JohnRoberts]

But why does it change with the voltage rating of the capacitor?

I am not sure which "it" you are asking about but higher voltage capacitors will differ in several mechanical aspects to support that higher breakdown voltage (and higher energy storage).  It is easy to imagine a higher voltage capacitor having a higher ESR and ESL because the conductive path is longer, but there can be other variables that make it less simple to predict.

As i tried to explain the resistance number at 100 kHz is a marketing number to sell capacitors and not a real specification, that are more complex. While you can draw generalities from such data, be careful about trying to extrapolate across other cap families and technologies.

There are lots of moving parts inside capacitors as these posts should already reveal.

JR

 

[user 37518]

Skin effect has to do with crowding current flow out of the middle of conductors and onto the skin or outer layer.

Skin effect can be seen at mains power frequency in very large diameter high current conductors. Some use a poor conductor like steel for the center core, with better conducting aluminum wrapped around to form the outer skin where the current actually flows and the resistance matters.

In audio practice we typically think of it as a HF phenomenon but it isn't only HF.

JR

PS: My signature line on another forum says "On the internet people tell you everything "they" know, not the answer to "your" question."  Perhaps that explains this weer....

Going off-topic, I never understood silver plated copper cables used as microphone cables and so on, if my calculations are correct, a 20KHz signal has a skin depth of 467 micrometers in a copper cable and 448 micrometers in a silver cable,  so I dont understand how can a few micrometers of silver coating on a copper cable can make everything sound  (allegedly) more hi-fi and justify its price, perhaps im missing something?

 

[JohnRoberts]

I never understood silver plated copper cables used as microphone cables and so on, if my calculations are correct, a 20KHz signal has a skin depth of 467 micrometers in a copper cable and 448 micrometers in a silver cable,  so I dont understand how can a few micrometers of silver coating on a copper cable can make everything sound  (allegedly) more hi-fi and justify its price, perhaps im missing something?

Audio technology is pretty mature so the cable/wire industry has to work hard to invent merchantable differences they can use to win sales and maybe even charge more...  Many of these may have a sliver of truth/utility and a pound of audiophoolery.

There are many funny wire sellers who made millions selling customers features they don't need or don't even work.

JR

PS: In my last phono preamp kit I used gold plated phono jacks. Not because I thought they would work better, but because my customers thought they would.  :

 

[sahib]

Skin effect can be seen at mains power frequency in very large diameter high current conductors. Some use a poor conductor like steel for the center core, with better conducting aluminum wrapped around to form the outer skin where the current actually flows and the resistance matters.

JR

Indeed. Generally 1" diameter and above. That is why in cases like this a hollow copper tube is used instead of a solid core.

 

[moamps]

Skin effect can be seen at mains power frequency in very large diameter high current conductors. Some use a poor conductor like steel for the center core, with better conducting aluminum wrapped around to form the outer skin where the current actually flows and the resistance matters.

Aluminium conductor steel-reinforced cables (ACSR) are used in overhead power lines mostly because of mechanical properties. For underground cables this technology isn't used, AFAIK.

 

[moamps]

Indeed. Generally 1" diameter and above. That is why in cases like this a hollow copper tube is used instead of a solid core.

This technology is used for high power RF cables (usually above 1kW at 50ohms or in small power systems at very high frequencies, above 1GHz), not for mains power cables, AFAIK.

 

[abbey road d enfer]

What I understood was that the skin effect only comes into play at well above audio frequencies.  With that in mind is it something you believe it's necessary to consider for audio gear ?

Agreed; however, the ESR figure given by manufacturers is supposed to cover all the components of ESR. Low-ESR caps are not used only in audio.

 

[sahib]

This technology is used for high power RF cables (usually above 1kW at 50ohms or in small power systems at very high frequencies, above 1GHz), not for mains power cables, AFAIK.

Not correct.

Just have a look at high voltage high current busbar systems.

Skin effect is part of the gig. There are other factors such as temperature rise and heat dissipation which directly affect the current carrying capacity of a busbar.

 

[JohnRoberts]

Agreed; however, the ESR figure given by manufacturers is supposed to cover all the components of ESR. Low-ESR caps are not used only in audio.

And the veer goes on.... 

+1 low ESR caps were developed for switching power supplies to replace more expensive dielectrics with cheaper aluminum.

Low ESR is only a concern  for audio in passive loudspeaker crossovers.

JR

 

[moamps]

Just have a look at high voltage high current busbar systems.

I never saw round high power mains bus bars.  Especially with hollows. They are usually rectangular in shape ...

Skin effect is part of the gig.

partially because of skin effect.  Mainly because of good heat transfer and simple installation.

 

[sahib]

I do not mean to get into peeing contest but what I understand that there is this notion that somehow skin effect is not an issue at 50Hz.  In small signal applications such as audio, of course. But when you have a couple of thousands of amp circulating it is an issue. And if you read my post carefully you'll see that I did not generalize. But somehow you base your conclusion on the fact that you have not seen an example of it around you. I have.

 

[moamps]

.....And if you read my post carefully you'll see that I did not generalize.

Yes, you did it here
"Indeed. Generally 1" diameter and above. That is why in cases like this a hollow copper tube is used instead of a solid core."
Go to google and you will find thousands of pictures of high power cables with solid core and high diameters.  And very few with a hollow or steel core. Mainly because for mechanical integrity or oil cooling.  Skin depth of 8-9mm for cooper at 50, 60 Hz doesn't mean that there is no current at all in farther parts of  a core.

 

[sahib]

Yes, you did it here
"Indeed. Generally 1" diameter and above. That is why in cases like this a hollow copper tube is used instead of a solid core."
Go to google and you will find thousands of pictures of high power cables with solid core and high diameters.  And very few with a hollow or steel core. Mainly because for mechanical integrity or oil cooling.  Skin depth of 8-9mm for cooper at 50, 60 Hz doesn't mean that there is no current at all in farther parts of  a core.

Well. You may have a point. That was in support to John's comments and was short hand. What I meant  was that in high current applications the skin effect is generally considered 1" diameter and above.  However, my wording can be interpreted wrongly and I take responsibility for that.

When it comes to using large current carrying cables  I had enough of my share even at the age of as early as 15. My (late) father had an electrical engineering business and we are a family of electrical engineers (well I have gone back to do my degree now at the age of 56).

I have also not suggested that there is no current flow in the areas nearer the core (not the farther).  But hollow tubes are used in applications where efficient cooling is necessary, and the skin effect is taken into account.