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 Post subject: 8 MFD
PostPosted: Mar Tue 06, 2018 3:31 pm 
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Something I've noticed in electrolytics.

Particularly the early days of them in the early 30s, so many are rated at 8 uf.

Why this "Magic" number? Paper caps would usually be in the range of 1, 2 or 5. such as .05 uf, .1 or .002 uf
You rarely saw "8" in the value.

Granted this was before the IEC standards we use today. 10, 22, 33, 47, 68, 82, 100 etc.

There must have been a reason behind the common use of "8 uf"

Was there something in the makeup of the electrolytic favored this value? I'm curious.

-Steve

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 4:13 pm 
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azenithnut wrote:
Was there something in the makeup of the electrolytic favored this value?
-Steve

I think we can rule out that option.....any capacitor technology I am aware of is equally useful across a range of values. Different ranges of course favor one type or another. At the extremes you might chose mica for 10pF, where electrolytic might be the better choice for 10F.

Here's a GUESS: When they first started making electrolytics, they started with a value that would be maximally useful. (I don't know who "they" was....)

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 5:59 pm 
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pixellany wrote:
(I don't know who "they" was....)

That was the guy who said four or even six MFD isn't enough capacitance.. :wink:

Filter capacitance necessary varies with current draw, most radios of the early era had similar requirements...

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 6:08 pm 
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I suppose it could have to do with paper filter caps of the late 20s. 1 uf, 2 uf, 4 uf were common.

Double it to 8 uf for an electrolytic. There were also 16 uf.

I don't know, maybe that is all it was?

a value that was maximally useful?
Hmm.

well, I guess I was hoping for an interesting reason :lol:

-Steve

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 6:54 pm 
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Gosh, I thought some of the postulates were absolutely fascinating.......:)
OK--try this:
" 2, 4, 6, 8---who do you appreciate?"

Remember-----capacitance is everywhere--our job is to organize it.

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 8:44 pm 
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Well, Mark I certainly do appreciate your input on the matter, as well as your contributions to this forum! :D

You are a great asset here!

-Steve

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 Post subject: Re: 8 MFD
PostPosted: Mar Tue 06, 2018 9:54 pm 
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The problem was that the old wet electrolytics had extremely wide tolerances. The capacitance was all over the map due to manufacturing variations, temperature, and age. Values were typically anywhere from 80% above marked value to 20% below marked value. There are some replacement grade electrolytics which still have tolerances like that today.

Since the numbers were so fuzzy to begin with, they soon realized that it didn't pay to make intermediate values. For example, a 4-uF cap that is 80% over would be 7.2-uF, which might be more than a 6-uF cap that was 20% under (4.8-uF). Doubling the value for each step made sense, i.e. 2-uF, 4-uF, 8-uF, 16-uF, and 32-uF. So a 1-uF cap could be as much as 1.8 uF, a 2-uF cap could get you from 1.6-uF to 3.6-uF, a 4-uF cap could fall from 3.2-uF to 7.2-uF, an 8-uF cap could fall from 6.4-uF to 14.4-uF, and so forth. With the introduction of so-called "dry" electrolytics in the mid-1930s, it became possible to control tolerances more closely so radio companies could order other values as needed from OEM suppliers. But the 2/4/8/16/32 system lingered for a long time afterward.

Today's E (exponential) series of "preferred numbers" are based on the same concept as the old electrolytics. The numbers are logarithmically spaced so that the next number in the series is just beyond the tolerance of the next. The E-3 series (1.0, 2.2, 4.7) is seldon used any more but it fits parts with greater than 20% tolerance such as replacement grade electrolytics. The E-6 series (1.0, 1.5, 2.2, 3.3, 4.7, 6.8) is used for parts with 20% tolerance. E-12 numbers are used for parts with 10% tolerance, E-24 numbers for 5% tolerance, and so forth.

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