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Topic: Capacitors; 16V 25V 50V What's the difference? (Read 19180 times) previous topic - next topic

marine_hm

I purchased an "assortment" of capacitors from ebay.  Being a newb, I figured that would get me started in my project.  For the most part, that's probably true but....

What's the difference of a 47UF Capacitor in 16V, 25V and 50V?
I hate to assume that that's the max voltage it's rated for.  I'm only running off a 9V battery.  It calls for a 10Uf capacitor, but the assortment came with only 10UF 50V.  Would this still work?  I know size is a consideration as well.

Thanks.  Nick

Grumpy_Mike

#1
Jan 26, 2016, 05:17 am Last Edit: Jan 26, 2016, 05:17 am by Grumpy_Mike
Yes it is the capacitor's voltage rating.
Using a higher voltage rating capacitor has no effect.

russellz

Yes, you can use the 50V version.  The only difference will be bigger can size and a very slightly improved reliability which you won't notice.

Russell.
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

krupski

#3
Jan 26, 2016, 09:41 am Last Edit: Jan 26, 2016, 09:42 am by Krupski
I purchased an "assortment" of capacitors from ebay.  Being a newb, I figured that would get me started in my project.  For the most part, that's probably true but....

What's the difference of a 47UF Capacitor in 16V, 25V and 50V?
I hate to assume that that's the max voltage it's rated for.  I'm only running off a 9V battery.  It calls for a 10Uf capacitor, but the assortment came with only 10UF 50V.  Would this still work?  I know size is a consideration as well.

Thanks.  Nick
Electrolytic capacitors such as the kind you described are made by taking two long, thin sheets of aluminum, separated with paper saturated with an electrolyte and rolled up into a cylinder. Each foil sheet is one terminal.

Now, obviously, the more surface area the aluminum sheets have, the more capacitance you get (and the larger the capacitor physical size).

To make a high value, small capacitor, you have to minimize aluminum foil thickness and paper insulator thickness.

Now, when the paper insulator is very thin, it won't take much voltage to break it down and short out the capacitor.

That's why they have a voltage rating. The same capacitance, but higher working voltage of course has a thicker paper insulator (and because the aluminum plates are spaced further apart by the thicker paper, the capacitance per unit size is lower.

So you need more aluminum area and thicker paper for a higher voltage capacitor, therefore it will be larger.

Hope this explains it.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

Paul__B

Hope this explains it.
It might to the uninitiated, except that (as you know,) the paper is not an insulator so its thickness is not the directly defining factor

The actual explanation is a trifle more complex than that, isn't it?

MarkT

The dielectric thickness in an electrolytic capacitor is very tiny - measured in nanometers or micrometers
and is a layer of Al2O3 built up on the Al foil by applying a voltage to "form" the capacitor.  This layer is what
breaks down, not the paper separator (actually soaking in electrolyte and conducts very well).

A thicker layer of this oxide has to be grown for a higher voltage, and eventually this means thicker
foil to start with.  Higher ripple current capacitors need thicker foil too.

The energy stored in the capacitor (in the dielectric layer) is 0.5 C V^2, and generally the volume of
a capacitor is roughly proportional to C V^2 for any given type.

This means that doubling the working voltage for the same capacitance value means 4 times as large
a device, so it usual to pick the minimum working voltage you can get away with for large capacitors
to reduce size/cost.

Using a capacitor near to its max working voltage does mean more leakage current however.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

krupski

#6
Jan 26, 2016, 01:57 pm Last Edit: Jan 26, 2016, 02:01 pm by Krupski
It might to the uninitiated, except that (as you know,) the paper is not an insulator so its thickness is not the directly defining factor

The actual explanation is a trifle more complex than that, isn't it?

Explanations are meant to convey understanding. All too many teachers and professors are so impressed by their own brilliance that their attempts to "explain" a subject by throwing in every detail and every applicable formula only serve to further bewilder and turn off the student.

I'm sure you've had professors that filled the blackboard with complex equations, then rambled on and on about every detail, then asked "any questions" and you were so overwhelmed that you didn't even know WHAT to ask.

I strive NOT to teach that way.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

krupski

#7
Jan 26, 2016, 01:59 pm Last Edit: Jan 26, 2016, 02:00 pm by Krupski
The dielectric thickness in an electrolytic capacitor is very tiny - measured in nanometers or micrometers
and is a layer of Al2O3 built up on the Al foil by applying a voltage to "form" the capacitor.  This layer is what
breaks down, not the paper separator (actually soaking in electrolyte and conducts very well).

A thicker layer of this oxide has to be grown for a higher voltage, and eventually this means thicker
foil to start with.  Higher ripple current capacitors need thicker foil too.

The energy stored in the capacitor (in the dielectric layer) is 0.5 C V^2, and generally the volume of
a capacitor is roughly proportional to C V^2 for any given type.

This means that doubling the working voltage for the same capacitance value means 4 times as large
a device, so it usual to pick the minimum working voltage you can get away with for large capacitors
to reduce size/cost.

Using a capacitor near to its max working voltage does mean more leakage current however.
Ever take apart an electrolytic cap? The insulator is quite thin... a lot like tissue paper... but certainly thicker than "nanometers" or even "micrometers".

Concerning the rest you wrote... 100% correct, but way beyond a beginner's level.

A good teacher judges what his student knows SO FAR and teaches from that point upward.  "Throwing the book" at the student gets him nowhere.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

MarkT

Did you not read what we said - the paper is a separator, it is not an insulator it is soaked
in electrolyte.  The Al2O3 layer formed by electrolytic action (hence the name) is the insulating
layer and it is very very thin (which is why you can get values of capacitance 1000's of times
larger than plastic film of the same size).

https://en.wikipedia.org/wiki/Aluminum_electrolytic_capacitor
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

krupski

#9
Jan 26, 2016, 02:20 pm Last Edit: Jan 26, 2016, 02:25 pm by Krupski
Did you not read what we said - the paper is a separator, it is not an insulator it is soaked
in electrolyte.  The Al2O3 layer formed by electrolytic action (hence the name) is the insulating
layer and it is very very thin (which is why you can get values of capacitance 1000's of times
larger than plastic film of the same size).

https://en.wikipedia.org/wiki/Aluminum_electrolytic_capacitor
Did you not read what I wrote? I was attempting to keep the explanation SIMPLE... to convey the reason for a voltage rating on a cap and NOTHING MORE.

If you were to explain to a rank beginner how a car engine worked, would you get into various cylinder head designs, the reasons for multiple intake and exhaust valves, how electronic ignition works, why spark plugs are plated with platinum or iridium, the meaning of "octane rating" and how it's measured, how valve overlap helps scavenge the exhaust out of the cylinder, how a supercharger works, the difference between a supercharger and a turbocharger, what the main bearings are made of, the advantages of four bolt mains vs. two bolt mains, .... ad-nauseaum... or simply say "the air and fuel go in, the spark ignites it and the hot gas pushes the piston down, generating torque, then the burned fuel comes out as exhaust"?

We have too many "professors" here.......
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

russellz

To make a high value, small capacitor, you have to minimize aluminum foil thickness and paper insulator thickness.

Now, when the paper insulator is very thin, it won't take much voltage to break it down and short out the capacitor.
Simplifying an explanation for beginners is admirable.  Making false statements like the above is not a simplification.

Russell.
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

ChrisTenone

#11
Jan 26, 2016, 07:23 pm Last Edit: Jan 26, 2016, 07:26 pm by ChrisTenone
I applaud teaching at a correct level - it is the most valuable kind of teaching, and yes - I HAVE had teachers that gave WAY too much information in an introductory lecture.

However, when simplifying it is important to either be correct, or to note what parts are modeled for simplicity.

Krupski, I like your description of a capacitor made with aluminum foil and paper. In fact, my students actually make such a capacitor to use with a van de graaff generator. That way they can see the mechanism for breakdown with their own eyes.

However when you say "Electrolytic capacitors such as the kind you described are made by taking two long, thin sheets of aluminum, separated with paper saturated with an electrolyte and rolled up into a cylinder." confusion can result. This is because the student may recognize from other studies that paper saturated with an electrolyte conducts electricity. (My students also make a battery with saturated paper bridges.)  Your simplification would have been more correct if you had just left electrolytes out. Explaining the use of electrolytes in a capacitor requires a greater level of detail.

Quote
We have too many "professors" here.......
Actually, it's good to have such learned folks here. Through constant peer review, one can be relatively certain that posted information is correct.
What, I need to say something else too?

krupski

#12
Jan 26, 2016, 07:40 pm Last Edit: Jan 26, 2016, 07:42 pm by Krupski
I applaud teaching at a correct level - it is the most valuable kind of teaching, and yes - I HAVE had teachers that gave WAY too much information in an introductory lecture.

However, when simplifying it is important to either be correct, or to note what parts are modeled for simplicity.

Krupski, I like your description of a capacitor made with aluminum foil and paper. In fact, my students actually make such a capacitor to use with a van de graaff generator. That way they can see the mechanism for breakdown with their own eyes.

However when you say "Electrolytic capacitors such as the kind you described are made by taking two long, thin sheets of aluminum, separated with paper saturated with an electrolyte and rolled up into a cylinder." confusion can result. This is because the student may recognize from other studies that paper saturated with an electrolyte conducts electricity. (My students also make a battery with saturated paper bridges.)  Your simplification would have been more correct if you had just left electrolytes out. Explaining the use of electrolytes in a capacitor requires a greater level of detail.

Actually, it's good to have such learned folks here. Through constant peer review, one can be relatively certain that posted information is correct.

Thanks for backing me up!  :)

I said "electrolytes" to prevent our resident one-upmanship-engineers here from "reminding" me that an electrolytic capacitor contains electrolyte... but I got hammered anyway for the insulator thing.....  :)

Can't win.

And, when I said "professors", I specifically put it in double quotes... snidely meaning that some people THINK they are EE professors and go out of their way to "prove" how much they know (by copying stuff from Google I'll bet)... LOL!
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

marine_hm

OK, now I'm confused...  Capacitors have an octane rating?


Nah, just kidding.  I got it (ALL)!  My local "Shack" just got in the 0.1UF Poly capacitors I was looking for.
Thank you all for the lessons.  I got my answer first, then the lesson to follow.  Good teaching method.

Grumpy_Mike

Note that poly capacitor's are no good for decoupling, which is the main use of a 0.1uF capacitor with the Arduino. They need to be ceramic.

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