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:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 17 October, 2005 » Equations « Previous Next »

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Equation again Cherrytree17/10/05  12:46 am
Corner Frequency!!Cherrytree17/10/05  12:34 am
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Regular Contributor
Username: Cherrytree

Post Number: 48
Registered: 08-2005

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Posted on Monday, 17 October, 2005 - 12:22 am:   Edit Post Delete Post Print Post

Hi all,
here's my first question.. it's from a book I got from maplins. by K.Brindley

I would like to know what answer you get when you do this equation on a calculator he goes

I have to type the equation in words cos i have not got the special characters here
its 1divided over 0.7 times it by 10 the times it by ten to the negative power of six,times it by(4700+20,000) hope this is correct.. the answer should be 5.8
THE bits to this is a timer in astable ok
C1=10uf electrolitic cap..
and a 1.5k resistor on pin 3 of the output..

Now I've measured that and its correct according to my fq(counter).. then keep the R1+R2 values the same but change C1 to 1uf electro then the output measures 58hz.. 100nf =580hz
10nf= 5.8khz
1nf=58khz.. ok

thats that bit BUT SAy I want a output of 25hz what cap would be?? I make it half of the 1uf elec, is that right?? or wrong? as I gather if I half the 10uf to say 4.7uf then surely you get half of 5.8hz..
Yes Im not good at maths much ..
as you can tell.
ok thats it for this bit.
any input to this and any helpful replys very welcomed
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Valued Member
Username: Ron_h

Post Number: 12
Registered: 07-2005

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Posted on Monday, 17 October, 2005 - 03:26 am:   Edit Post Delete Post Print Post

You have already discovered that the frequency is inversely proportional to the capacitance, or, looking at it another way, the product of the capacitance and the frequency is constant:


So, 58*1uF=25*Cunknown

Solving for Cunknown,



The closest standard value is 2.2uF, and the tolerances of polarized caps are generally pretty sloppy. You might want to use a nonpolarized cap if you need good accuracy.

(Message edited by ron_h on 17 October, 2005)

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