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I'm seeing 5"-long caps marked "3000F"! Did they change label units??

sf44

Jul 16, 2022
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For decades, low-voltage can-style capacitors a couple of inches long were typically around 470 uF (microfarads). Larger diameters 3 or 4 inches long would run 4700 uF. But now I'm seeing caps about the same size offered on lots of manufacturers' websites marked "3.3F" all the way to "3000F". And note that's "F" not "uF" or mFD or whatever else.
When I was in school--admittedly long years ago--we read that a "one farad" cap would be the size of a house. Now I realize technology is always improving but can anyone confirm that you can get a "3000 farad" cap just a few inches long? I'm extremely skeptical. From the pics of the cans I'm thinking they should be 3000 uF, but LOTS of manufacturer sites use just "F".
Did the industry decide to change the marking conventions so "F" is really "uF" or is there some other explanation? Thanks!
 

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CircutScoper

Mar 29, 2022
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For decades, low-voltage can-style capacitors a couple of inches long were typically around 470 uF (microfarads). Larger diameters 3 or 4 inches long would run 4700 uF. But now I'm seeing caps about the same size offered on lots of manufacturers' websites marked "3.3F" all the way to "3000F". And note that's "F" not "uF" or mFD or whatever else.
When I was in school--admittedly long years ago--we read that a "one farad" cap would be the size of a house. Now I realize technology is always improving but can anyone confirm that you can get a "3000 farad" cap just a few inches long? I'm extremely skeptical. From the pics of the cans I'm thinking they should be 3000 uF, but LOTS of manufacturer sites use just "F".
Did the industry decide to change the marking conventions so "F" is really "uF" or is there some other explanation? Thanks!

Nope. They really mean Farads. Note the phrase "Super Capacitor." These devices store so much charge (e.g., nearly an amp-hour per volt) that they bridge the difference between regular capacitors and rechargeable batteries.
 

Harald Kapp

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The potential to kill doesn't stop at low voltage. High currents can be deadly, too. If only indirectly (a mild shock, not deadly, but you fall and your head hits a sharp corner - deadly).
Although in this case I tend to agree that supercapacitors are probably rather safe components.
 

Bluejets

Oct 5, 2014
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Photo shows 3v........ description says 18v.......

Never trust Chinglish
 

Harald Kapp

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Photo shows 3v........ description says 18v.......
Look closer: This is a module with 6 capacitors in series which accounts for 18 V. At least in theory. I do miss some way of balancing like e.g. resistors in parallel.
 

Bluejets

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So you would latch on to it fully charged anyhow?

I doubt it.
 

sf44

Jul 16, 2022
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Thanks to all who replied. Now for the serious question: As y'all know, the equation for capacitance is
C= kappa * epsilon * A / d, where kappa is the dielectric constant, epsilon is a constant (the permittivity of free space), A is "plate area," and d is the distance between the two "plates" (typically aluminum foil). So to increase C from, say 30 uF to even 300F is a factor of 1E7 -- ten million.
....And they're getting that in the same size can! So I'll give ya that we can use thinner foil, and a thinner dielectric, and using some faaabulous new dielectric we might get a kappa of 20 or so, compared to7 from teflon. But none of these tweaks seems likely to allow us to get five MILLION times more foil area in the same can.
....So...anyone know how the makers did it? I'm intrigued enough to cut one open and see how!
 

CircutScoper

Mar 29, 2022
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Thanks to all who replied. Now for the serious question: As y'all know, the equation for capacitance is
C= kappa * epsilon * A / d, where kappa is the dielectric constant, epsilon is a constant (the permittivity of free space), A is "plate area," and d is the distance between the two "plates" (typically aluminum foil). So to increase C from, say 30 uF to even 300F is a factor of 1E7 -- ten million.
....And they're getting that in the same size can! So I'll give ya that we can use thinner foil, and a thinner dielectric, and using some faaabulous new dielectric we might get a kappa of 20 or so, compared to7 from teflon. But none of these tweaks seems likely to allow us to get five MILLION times more foil area in the same can.
....So...anyone know how the makers did it? I'm intrigued enough to cut one open and see how!

Google "double layer." https://en.wikipedia.org/wiki/Double-layer_capacitance
 
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