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Cherrytree
Regular Contributor Username: Cherrytree
Post Number: 48 Registered: 082005
Rating: N/A Votes: 0 (Vote!)  Posted on Monday, 17 October, 2005  12:22 am: 

} 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 R1=4K7 R2=10K 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 thanks from CT

Ron_h
Valued Member Username: Ron_h
Post Number: 12 Registered: 072005
Rating: N/A Votes: 0 (Vote!)  Posted on Monday, 17 October, 2005  03:26 am: 

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: F1*C1=F2*C2=F3*C3... So, 58*1uF=25*Cunknown Solving for Cunknown, Cunknown=58/25*1Uf Cunknown=2.32uF 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) 
