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24th January 2008, 11:07 PM  #1 
diyAudio Member
Join Date: Jan 2007
Location: Cypress Texas

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. Nick 
24th January 2008, 11:14 PM  #2 
diyAudio Member
Join Date: Feb 2004
Location: Chicago area

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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Sherman 
24th January 2008, 11:28 PM  #3 
diyAudio Moderator

C in farads, R in ohms. Or R in megohms, C in microfarads. Both give you the answer in Hz.
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"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." 
25th January 2008, 12:12 AM  #4 
diyAudio Member
Join Date: Jan 2007
Location: Cypress Texas

Gracias
Nick 
5th February 2008, 12:12 AM  #5 
diyAudio Member
Join Date: Jan 2007
Location: Cypress Texas

Wait a sec I missed something before if you use microfarads and megohm is your out come in megahertz?
Nick 
5th February 2008, 12:44 AM  #6 
diyAudio Member
Join Date: Mar 2003
Location: Zürich

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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my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY! 
5th February 2008, 12:45 PM  #7 
diyAudio Member
Join Date: Jan 2007
Location: Cypress Texas

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.
Nick 
5th February 2008, 02:46 PM  #8 
diyAudio Member
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.

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 ? r: 
5th February 2008, 04:43 PM  #9 
diyAudio Moderator

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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"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." 
5th February 2008, 05:02 PM  #10  
diyAudio Moderator

Quote:
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"To argue with a person who has renounced the use of reason is like administering medicine to the dead."  Thomas Paine 

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