Finding reactance of capacitor

[zachtheterrible]

This is KILLING me!! I've spent an hour trying to figure this out.

Xc=1/2 pie f C

Simple enuf

The only problem is, what do I do when I have a capacitance of .27uf and a frequency of 1000hz? If it was 100MHZ, it would be fine because I could simply plug in the values. Somehow I need to convert something to get the value right but i can't get it!! HELP!

 

[evandude]

0.27 uF = 0.27 * 10^-6 Farad
100MHz = 100 * 10^6 Hz

C should be in Farad, F in Hz. when C is in uF and F is in MHz, it just happens that the 10^-6 and 10^6 cancel out.

if you really want to keep everything in uF and MHz, then you could always think of 1000Hz as being .001 MHz and plug it in that way...

it's just algebra. if you are having trouble with the powers of 10, i'm sure you can find plenty of websites that can provide you with a complete list of all the prefixes and their values. Sooner or later you're sure going to have to have a complete understanding of the conversions, so it's best to wrap your mind around it now.

 

[David Bridgen]

On your hand-held calculator (or on Google's, once you figure out the syntax) enter:

2 x pi x 1000 x 0.27 x 10^-6

then take the reciprocal.

n.b. I wrote the above and pressed the "Submit" button. Evandude posted while I was writing!

 

[zachtheterrible]

Thanx Evan and David.

David, I only have a regular calculator so its a little hard to do that.

Evan, I have a conversion table that has all of the . . . well they're not prefixes, but you know what I mean (mega, giga, micro, pico etc) and wut they mean.

Anyway I guess the simplest way to do it is to find out for example that if I'm doing picofarads, and my frequency is megahertz, I'd just divide tera (it corresponds with picofarads, pico= 10^-12 and tera=10^12) by pico and find out what my frequency is in terahertz.

Did that make sense? I do believe that it will work but just want to make sure.

 

[mstechca]

Most equations do not have units attached to the variables.

Convert xxxfarads to farads.
Convert xxxhertz to hertz.

xxx in the above cases could be mega, micro, nano, etc.

in your case, you want to convert 0.27uF to something farads.
You multiply by (10 ^ -6)

you get 0.00000027 farads.

You have already specified 1000 Hz, so that is ok.

now plug in the numbers like so:

Xc=1/( 2 * 3.141592657 * 1000 * 0.00000027)

by the way, 3.141592657 is pi on your calculator.

 

[David Bridgen]

Thanx Evan and David.
David, I only have a regular calculator so its a little hard to do that.

If you type an equation into Google's search window Google will display the result.

I haven't used it other than out of curiosity, but I believe it will accept quite complex equations.

....... I have just checked it out with the same figures I used in my previous post, above, and it works.

Oh, there's a free scientific desktop calculator download "calc98" at
https://www.calculator.org/

Try it - and the Google one.

 

[audioguru]

Hi Zach,
For many years, instead of using 1 divided by 2 pi RC, I use 0.16 divided by RC to find the frequency where the cap reactance equals the resistor. I use 0.16 divided by Rf to find the capacitance that equals the resistor at a certain frequency.
I found no point in using a pi value accurate to 9 decimal places when most caps and resistors vary by 5%. My 0.16 substitution is accurate to within about 0.5%.

On the bottom part of the equation, I cancel micro with mega and milli with kilo. If I end up with micro on the bottom, it converts to mega on top (0.16 times 1M is 160K). If I end up with milli on the bottom, it converts to kilo on top (0.16 times 1K is 160).

Example: 0.16 divided by 0.27uF times 1KHz.
1) The micro and the K partially cancel at the bottom leaving 0.16 divided by 0.27milli.
2) The milli at the bottom converts to kilo on top leaving 160 divided by 0.27.
3) The answer is 593 ohms, accurate to about 0.5 percent.

Notice that I multiply the bottom part of the equation, then store the product in the calculator's memory. Then I enter .16 divided by memory-recall to get the answer. :lol:

 

[ljcox]

This all appears to be making a simple calculation complicated.

I often use Excel for these calculations. It makes life easy as you can do multiple calculations in milliseconds. Also you can cut and paste, print the results, draw graphs, etc.

Xc = 1/(2 pie F C) can be simplified to

Xc = 0.1592/(F C) as 1/(2 pie) = 0.1592.

Hope this helps, Len

 

[stevez]

Step back and take a look at 'scientific notation' as a way of handling this stuff. It's fairly universal and helps with extremely large or extremely small values. Many calculators handle this with the 'E' key (not e).

 

[zachtheterrible]

Thank you everyone, this makes things so very much clearer!!
I figured out how to use scientific notation on my computer calculator that comes w/ windows. Ill probably have to refer back to this later though.

For many years, instead of using 1 divided by 2 pi RC, I use 0.16 divided by RC to find the frequency where the cap reactance equals the resistor.

Just so im not confused audio, you meant to say: 1 divided by 2 pi FC, I use 0.16 divided by FC to find the frequency where the cap reactance equals the resistor.

Is this correct, or did you bring a whole different equation in??

btw, this is my 1000th posting!! Now im godawful :lol:

 

[audioguru]

Hi your worship, the terrible god Zach,
f= 0.16 divided by RC.
C= 0.16 divided by Rf.
R= 0.16 divided by Cf. :lol:

 

[mstechca]

hi god
lol