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Old 24th January 2008, 11:07 PM   #1
nhuwar is offline nhuwar  United States
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Default Formula question

Ok just have a quick one.

The formula 1/2piRC= -3db point

The question I have is do I put in the capacitance is the value in farads or microfarads.

If the value is farads is the answer to the formula in hertz ???????



Thanks for the help.


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Old 24th January 2008, 11:14 PM   #2
Sherman is offline Sherman  United States
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The C is in farads and the answer is in hertz.

The formula I have in my "useful formulas" spreadsheet-

1/(2*Pi*R*C)*1,000,000

That way I can put the value in in microfarads since the spreadsheet is designed to use "real world" values and the formula converts it for me.
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Old 24th January 2008, 11:28 PM   #3
SY is offline SY  United States
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C in farads, R in ohms. Or R in megohms, C in microfarads. Both give you the answer in Hz.
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Old 25th January 2008, 12:12 AM   #4
nhuwar is offline nhuwar  United States
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Gracias



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Old 5th February 2008, 12:12 AM   #5
nhuwar is offline nhuwar  United States
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Wait a sec I missed something before if you use microfarads and megohm is your out come in megahertz?

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Old 5th February 2008, 12:44 AM   #6
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Hi

As SY said, the answer is still in Hz.

The F unit is divided by 1000000 to obtain microF. This 1000000 is multiplied with the R, to obtain the megaR.
100k is 0.1M (ohm).
100nF is 0.1microF.
and so on...

Good luck, Erik
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Old 5th February 2008, 12:45 PM   #7
nhuwar is offline nhuwar  United States
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Ok I was just making sure. I was getting some werd results thars all. I think my problem was I was thinking high pass instead of low.

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Old 5th February 2008, 02:46 PM   #8
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Reminds me of the early days when a 1mfd marked cap was actually 1uF in disquise ! some dreadful mistakes were made. A true 1mfd cap is quite a different animal.

SY remember this ?


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Old 5th February 2008, 04:43 PM   #9
SY is offline SY  United States
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Yes, indeed. Especially on cheesy paper caps (which are now all the fashion rage).

Sadly I'm also old enough to remember mmf. And cps. I remember that QST, in one of their April issues (maybe 1967, 1968?), published a nomograph for cps to Hz conversions.
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Old 5th February 2008, 05:02 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by SY
Yes, indeed. Especially on cheesy paper caps (which are now all the fashion rage).

Sadly I'm also old enough to remember mmf. And cps. I remember that QST, in one of their April issues (maybe 1967, 1968?), published a nomograph for cps to Hz conversions.
I was about 9 when Batman first hit TV (1966?) and I too still remember cps, mmfd, and the physical components themselves - the electrically leaky wax covered paper things that often passed for capacitors, and those terrible carbon composition resistors with a tolerance of 20%, which always seemed to be way off the mark..
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