Calculations.....the dark art.

Fenris

Joined Oct 21, 2007
288
Hi all

I am trying to get my head around this formula. It is to calculate the values of the resistors and capacitors needed to set a specific frequency of a wein bridge oscillator.

the formula is f = 1/2PiRC

Well it seems simple enough and with a bit of study I found one area I was screwing up was using the correct format for the values IE;

68K = 68000ohms 0.1uf = 100nF

Now the thing is I can do the sum but I am still not getting the answer I expect. The circuit has had its frequency adjusted to 30Hz from the original and the guy has calculated the values as;

R = 53.1K C= 0.1uF (100nF)

The sum is done as follows there may be an error due my calculator lacking the 'to the minus 9' option which I believe adds 9 zero's to the end of the 100nF so it looks like the 100*10,000,000,000 in the sum below.

1/(2*3.141592654)*(53,100)*(100*10,000,000,000) =

8,451,127,477,076,154.380388999 which is no where near 30Hz

So I am not doubting the guys got it right. Mainly because I know I am a total thick *********** when it comes to this sort of thing. So what am I doing wrong. Keep it simple because I really am thicker than thicky McThicky a very thick person.

regards

Fenris

thingmaker3

Joined May 16, 2005
5,073
100nF = .0000001F.

Divide "100" by 10^9 rather than multiplying.

http://www.simetric.co.uk/siprefix.htm Fenris

Joined Oct 21, 2007
288
Hi there

OK I did this on my calculator;

1/(2*3.141592654)*(53,100)*(100/10^9)

= 0.000845113

I/m still doing something wrong I'm guessing. I have bookmarked that page BTW Wendy

Joined Mar 24, 2008
21,840
F=1/(2π(53e3)(100e-9))

Fenris

Joined Oct 21, 2007
288
Hi there

Well I dug out my old faithful Scientific calculator as the PC based offering is also part of the problem I think.

The calculator presents the calculations exactly as you would write it. The only thing I did prior to entering the data was cancellations of 0's. So the sum became;

1/(2∏*53)*(100e-6)

0.282274804 or 0.28 to 2 decimal places.

So is this correct? It's not 30Hz, if it is, but it is very close.

A resistor of 47K and a 100nF (0.1uF) cap would give 0.318 (0.32) or 32Hz if I have it right.

regards

Fenris

Last edited:

Ron H

Joined Apr 14, 2005
7,012
Hi there

Well I dug out my old faithful Scientific calculator as the PC based offering is also part of the problem I think.

The calculator presents the calculations exactly as you would write it. The only thing I did prior to entering the data was cancellations of 0's. So the sum became;

1/(2∏*53)*(100e-6)

0.282274804 or 0.28 to 2 decimal places.

So is this correct? It's not 30Hz, if it is, but it is very close.

A resistor of 47K and a 100nF (0.1uF) cap would give 0.318 (0.32) or 32Hz if I have it right.

regards

Fenris
I don't know what you are doing. 0.28 is not even close to 30.

You need to do this, with all the parentheses in the right places:

Fr=1/(2*π*53e3*100e-9)

When I do this on my RPN HP15C, I get 30.029Hz.

jpanhalt

Joined Jan 18, 2008
7,697
1/(2∏*53)*(100e-6)

0.282274804 or 0.28 to 2 decimal places.
I get 30.0 Hz. What value are you using for pi? Check you numerical entries.

John

Fenris

Joined Oct 21, 2007
288
Hi there

OK I am starting to bounce of the walls here. This is such a simple thing to do and I can't seem to get it bloody right. There is still a possibility I am using the functions on the calculator incorrectly but I am not sure. (Me and maths are like oil and water (Thanks to a secondary school maths teacher who I still hate with a vengeance 20+yrs later))

Here is a photo of my calculator. Does it have the required 'devices' to do the sum? If so which ones (I know Pi) Sorry for being so thick regards

Fenris

jpanhalt

Joined Jan 18, 2008
7,697
It is not a "sum" it is a "product/quotient". In other words, you multiply pi times 2 times the resistance times the capacitance. You will get a number like "333" (the decimal place depends on how you set it up) divide that into 1 for .003, then multiply by the exponent, in this case 10^4, to get 30.

John

Fenris

Joined Oct 21, 2007
288
I don't know how I am not even sure I am using the correct Fnc buttons on the calculator to do the math as it looks when written down.

I get it written as this;

1/(2*∏*53E3*100E-9) when it should look like this 1/(2*∏*53e3*100e-9)

The E i am getting from the Exp. To get 'e' I there is one marked eX which produces

e^ on the calc screen.

When I say I am thick I really mean thick.

regards

Fenris

jpanhalt

Joined Jan 18, 2008
7,697
Forget the calculator. 53 is close enough to 50 to at least get you in the ballpark, which is 100 divided by 2. So, do it on paper with pencil.

Alternatively, try 6.28 times 53. You should get 332.84. If you don't get that number, then you are using the calculator wrong, which has nothing to do with understanding the math.

John

recca02

Joined Apr 2, 2007
1,211
When dealing with 10^(insert any number here +ve or -ve) it is better to go ahead with the rest of the calculations and solve for the power of 10 later.

Bailey45

Joined Oct 27, 2008
18
=1/(2*∏*53E3*100E-9)
=1/(2*3.14*53000*0.0000001)
=1/(2*3.14*0.0053)
=1/(2*0.0166)
=1/0.033
=30.04

Fenris

Joined Oct 21, 2007
288
Thanks for your patience all.

Right using the 2nd Line of Baileys example I can run that on my calculator and it works. I understand that the 53000 is 53K expressed in Ω's and 100nF is expressed as 0.00000001 in 'long hand'. If I try my calc with the top line it throws an 'error 1' So I must be using the wrong 'phrases'.

The main reason I want to get this math under control is I want to set up the wein bridge such that I can have a range of settings (only 3 or 4) so I can use a rotary switch to select the frequency I want. So I will need to be able to calculate the RC values for the other frequency's I wish to have to hand. But I don't want to keep having to ask the same question with different numbers of you guys when it's patently obvious, it's a simple formula, well to some of you.

Basic math I am fine + - * / I can do. It's just this voodoo stuff I have trouble with. I will also go and google 'using scientific calculators' because I must be formating it wrongly as well.

Thanks again for your help.

regards

Fenris

Wendy

Joined Mar 24, 2008
21,840
Hi there

Well I dug out my old faithful Scientific calculator as the PC based offering is also part of the problem I think.

The calculator presents the calculations exactly as you would write it. The only thing I did prior to entering the data was cancellations of 0's. So the sum became;

1/(2∏*53)*(100e-6)

0.282274804 or 0.28 to 2 decimal places.

So is this correct? It's not 30Hz, if it is, but it is very close.

A resistor of 47K and a 100nF (0.1uF) cap would give 0.318 (0.32) or 32Hz if I have it right.

regards

Fenris
You wrote 53KΩ as 53Ω, in my problem I wrote 53e3. Be careful of the zeros you eliminate, they can bite you.

Fenris

Joined Oct 21, 2007
288
Hi Bill

Thanks for that Glad to report, having googled my calculator, I can now enter the math correctly so that it actually works. The Exp key was the one I needed but I also had to learn that using the +/- key was needed as well just prior to entering the number 9.

Yep I had also taken 0's from opposite sides of the separate figures which is wrong wrong wrong! I think I may have a handle on it now and will keep practicing thanks everyone for your help.

I have downloaded the manual for my Sharp EL-531LH calculator which is where I found out hot to use the Exp key properly and I have just realised that my calc is 25yrs old :O

regards

Fenris

Fenris

Joined Oct 21, 2007
288
To test my grasp of the workings I have done the math of the existing RC in the wein bridge diagram posted at the beginning of this thread.. Can some one say yay or nay please.

R = 68K = 68000 (68E03)
C = 0.0047uF = 4.7nF =0.0000000047 = 4.7E-9

1/(2*∏*68E03*4.7E-9) =497.98Hz

regards

Fenris

Ron H

Joined Apr 14, 2005
7,012
To test my grasp of the workings I have done the math of the existing RC in the wein bridge diagram posted at the beginning of this thread.. Can some one say yay or nay please.

R = 68K = 68000 (68E03)
C = 0.0047uF = 4.7nF =0.0000000047 = 4.7E-9

1/(2*∏*68E03*4.7E-9) =497.98Hz

regards

Fenris
I get the same answer. Fenris

Joined Oct 21, 2007
288
Good grief! Does this mean I have the grasp of it!!!!!!!!!!!

Thats one in the eye of my maths teacher regards

Fenris

Audioguru

Joined Dec 20, 2007
11,251
For many years I have calculated anything that has "1 divided by 2 x pi" as just "0.16". My result is only 0.5% wrong.