# capacitor

Discussion in 'General Electronics Chat' started by Robert123, Feb 22, 2010.

1. ### Robert123 Thread Starter New Member

Feb 12, 2010
29
0
How I choose the right capacitor for low pass filter OR high pass filter
I know that I need to do it whit bode diagram
And cut the frequency after the knee
But how I choose the right capacitor

2. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
In its most basic form:
C=1/2pi*F*R
or (easier to choose the capacitor and tailor the resistor around it)
R=1/2pi*F*C

)

3. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
It works well if you express the resistance in Megohms, and the capacitance in uF. The frequency is in Hertz.

So for F=160Hz and assuming a 10k resistor, you get:
C= 1/6.28(160Hz)(.01M) = .09, or about .1uF

Works for low and high pass, first-order filters, and the frequency is equal to the -3dB point.

4. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
Try this: http://www.electronics-tutorials.ws/filter/filter_2.html
Look at "Cutoff Frequency and Phase Shift"; the formula above is there. Where they mention "70.7% output" equals -3dB, which is where the signal is half as loud as the input (it is the point that these filters are designed around).

And also, there are "E-books" links at the top of the page here; there is sure to be more information there, also! http://www.allaboutcircuits.com/vol_2/index.html

This was all confusing to me when I started, too. Try to read a lot about filters. It is not as hard as it looks, it just takes a little research and simulation on the computer!

5. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
Are you trying to reduce power supply ripple? Or are you working with IC's? .1uF decoupling caps work well with IC's, across the power supply connections.
Are you using LT Spice?
For reference, this site has some information on how to choose power supply capacitors:
http://www.valvewizard.co.uk/

6. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
Hi Robert, no problem, we like to help people here - and I also learn more by doing so!
Go to the Valve Wizard's site, and choose "Smoothing and Filtering" on the left-hand menu. That will take you to a section dedicated to power supply design, which will explain more about how it is done.

You have to decide how much ripple you will allow, and then do the calculation to find the correct capacitor:

The formula is C=(t*I)/V
-where I is average load current in A, V is ripple voltage peak to peak, t is duration between charging cycles (1/mains frequency*2)

LT Spice IV is a circuit simulation program that is available free from Linear Technologies. You can choose your circuit devices and actually SEE how well your filter works! I would download it and learn about it if I were you, it is invaluable for this kind of problem! If you have a circuit you're trying to work on, feel free to post more specific information about it, and we'll give you more insight.
Here is a link to LT Spice:
http://www.linear.com/designtools/software/index.jsp