# Why 47,000 ufd?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 8, 2010.

1. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I have been looking at several designs for propeller clocks. All the ones I have seen that incorporate super caps use a a value of 47,000 ufd.

I fully understand that the purpose of the super cap is to supply power to the PIC while it is spinning or in the case of a clock that uses a generator to generate the PIC power, power it while it is not spinning.

I would like to know why 47,000 ufd? Why not 45,000 or 50, 000?

And is the fact that a cap is 47,000 ufd make it a super cap? Or is there something special about supercaps?

Jun 1, 2009
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47u is a common capacitor value.
I can't provide details but long long ago various values for capacitors and resistors were seemingly plucked out of thin air at random, mostly to allow for various ratiometric combination's for common voltage divider/filter networks.

You could probably study this stuff for years and never know find out exactly how the value's were chosen. In general the exact value especially of larger capacitors isn't important, but the ratio between that value and another component in the circuit might be important, so while there is no specific need the various values be exactly what they are for a given application electronics components are designed to provide multiple uses in general, and the wider a variety of uses it can provide the better, even if that leads to a seemingly arbitrary value.

3. ### Wendy Moderator

Mar 24, 2008
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Agreed. My theory is they fit well on a logarithmic graph.

4. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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So a 50kufd or 45kuf would probably work just as well?

Is a regular 50k ufd a super cap just because it is so large?

5. ### tom66 Senior Member

May 9, 2009
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The values approximately follow the series of 10^(n÷12) where n is 1 to 12. Plot this out, you get 1.21, 1.46, 1.77, 2.15, 2.61, 3.16, 3.83, 4.64, 5.62, 6.81, 8.25 and 10. They are then apparently corrected such that each has a ±10% tolerance band and no bands overlap. That is E12; it's similar for the E24, E48, E96 and E192 series.

Apr 7, 2010
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7. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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If you look at the standard series, such as here: http://ecelab.com/list-capacitors.htm, it seems clear that each value, starting at 1.0 is approximately 10% greater than the previous value. There is rounding along the way, so the value that should be (1.1)^16 = 4.6 is actually 4.7 and so forth. I would guess it is no more complicated than that. Had a different starting number been picked, the series might be different. But, picking an arbitrary value of 1.0 to start seems simple enough.

One runs into a similar situation in the biological sciences where serial, two-fold dilution schemes are common. It makes a difference whether one starts at 1 or 100. That choice has actually be subjected to arbitrary standardization by panels of "experts."

John

8. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Thanks I get why caps are sized a certain way, But I still did not get an anwer to why most people that use a super cap to power their pic chose 47,000 ufd. There are other comon large valued caps. Why not one of those?

Also is there something special about a super cap or is it just a really big cap?

9. ### beenthere Retired Moderator

Apr 20, 2004
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The construction is considerably different from conventional electrolytics. 47,000 mikes isn't that big, by the way. Supercaps come in ratings of 2 - 25 Farads, not microFarads. Try supercap as a search term.

Apr 5, 2008
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Jun 1, 2009
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I have a couple 1F 5V supercaps. That's 1,000,000 uFarads. Bought them a couple years ago for playing with, impressive bit expensive, I replaced the battery on a hand crank flashlight with one, I should have replaced it with 3-4 but the ones I got were 12 bucks each.

Nothing makes a supercap a supercap it's a buzzword in the industry nothing more. The term is generally applied to capacitors that have large Farad ratings relative to their physical size, generally by using construction that allows for enormous surface area to exist between the plates of the capacitor, there are different types not all are used for the same purpose so one construction type may be better than another, it's all highly application dependent.

One theory as to why you see such a common value for the capacitor in the propeller clock's see is that people tend to copy schematics when found and then expand on them for their own uses, or perhaps simply use what someone else built and just claim the schematic as their own (either intentionally or not) when posting the project that they made.

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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You can get 20V/1 Farad caps For \$50-\$100 at car audio shops. They are used to decouple the supply at the sub's amp terminals, giving bass a better "kick" on large transients while not losing any fidelity on other amps during the voltage sag due to the sub amp slurping up 60 Amps.